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A Comparative Computational Study of Model Halogen-bonded Complexes of FKrCl Jerelle A. Joseph, and Sean A. C. McDowell J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5091262 • Publication Date (Web): 15 Oct 2014 Downloaded from http://pubs.acs.org on October 19, 2014
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A Comparative Computational Study of Model Halogen-bonded Complexes of FKrCl
Jerelle A. Joseph Department of Biological and Chemical Sciences, The University of the West Indies, Cave Hill Campus, Barbados
Sean A. C. McDowell * Department of Biological and Chemical Sciences, The University of the West Indies, Cave Hill Campus, Barbados email: [email protected]* Telephone: 246-417-4352*
Abstract Quantum chemical calculations for the FKrCl molecule at various levels of theory were performed and suggest that this molecule is metastable and may be amenable to experimental synthesis under cryogenic conditions. The FKrCl molecule forms weak halogen-bonded complexes FKrCl…Y with small molecules like FH and H2O and its computed properties were compared with analogous complexes of its precursor, FCl, and its rare gas hydride counterpart, FKrH. The cooperative effect of additional non-covalent interactions introduced at the F atom in the FKrCl…Y dimer (to give Z…FKrCl…Y trimers) showed a general strengthening of the intermolecular interactions in the order halogen-bond < hydrogen-bond < beryllium-bond < lithium-bond.
Keywords: rare gas, hydrogen-bond, sigma-hole, lithium-bond, beryllium-bond, Krypton
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INTRODUCTION
The chemistry of rare gas compounds was rejuvenated by the discovery in 2000 of the only known neutral chemical compound of Ar, FArH, by Räsänen and co-workers.1 Their seminal paper spawned a number of experimental and theoretical studies of a general class of XRgH compounds (Rg = rare gas, X = electron-withdrawing atom or fragment). Although metastable, these compounds were prepared under cryogenic conditions in rare gas matrices by UV photolysis of an XH precursor, followed by thermal mobilization of the H atoms, leading to the insertion of the Rg atom between the X and H to form XRgH.1-4 The stability of FArH derives from the strong F- [Ar-H]+ ion-pair character of this species, the Rg-H bond being mainly covalent in nature, whereas the F…Ar bond is predominantly ionic. Several reviews of this topic are to be found in refs.4-7
These XRgH compounds are capable of forming hydrogen-bonded complexes, XRgH…Y, with small molecules like N2 which exhibit the unconventional blue-shifting behaviour, a topic of much interest since it was first reviewed by Hobza and Havlas in 2000.8 Hydrogen bonding normally causes the covalent bond containing the H atom (in a proton donor molecule) to vibrate at a lower vibrational frequency (a red shift) than when it is uncomplexed.9 However, it was found that the characteristically intense Rg-H vibrational stretching frequency, which allows for easy identification by IR spectroscopy, is shifted to higher frequencies due to the interaction with Y.10-13 This blue-shifting effect has been rationalized by invoking enhanced F-[Rg-H]+ ion-pair charge separation when the complex with Y forms.12
Another non-covalent interaction that has received considerable attention over the last ten years or so has been halogen bonding, represented symbolically by R-X…Y (R = electronwithdrawing atom or group; X = halogen atom; Y contains a source of electron density). The key feature of the halogen bond is the attractive and highly directional interaction between a positive region of locally depleted electron density along the extension of the R-X covalent bond, called the σ-hole, and a negative site on the partner molecule Y (e.g. lone pairs or π-electrons).14-16
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Several halogenated rare gas species have been prepared and characterized over the years.17-20 There are also experimental and computational studies on hydrogen-bonded compounds [HRgY…HX/H2O13,
21-23
(Rg = Ar, Kr, Xe; X, Y = Cl, Br, I) and YRgH…N224] containing
halogenated rare gas subunits. The possibility of halogen-bonded complexes of rare gas compounds has been raised by a recent study of halogenated xenon cyanides which reported the synthesis and characterisation of ClXeCN, ClXeNC and BrXeCN by photolysis and thermal annealing of ClCN and BrCN in a low temperature Xe matrix.25 The experimental results for these compounds were in agreement with quantum chemical calculations and this work further suggested the possibility of synthesising molecules of general formula X-Rg-X’, where X and X’ are electronegative fragments like halogens.25
A recent theoretical study of the halogenated xenon isocyanides, ClXeNC and BrXeNC, complexed with several nitrogen-containing molecules via a halogen…nitrogen bond indicates that these weakly-bound complexes may be stable, with computed interaction energies at MP2/aug-cc-pVTZ ranging between -1.0 and -3.1 kcal mol-1 being reported.26 Another high-level theoretical study of F-Ar-X…NCY (X = Cl, Br; Y = H, F, Cl, Br, OH, NH2, CH3 and CN) also predicted stable halogen-bonded complexes, with computed CCSD(T)/aug-cc-pVTZ interaction energies ranging between -0.88 and -3.20 kcal mol-1.27 These two theoretical studies seem to be the only work reported on halogen bonding involving rare gas compounds.
The paucity of reported studies on halogen-bonded rare gas complexes prompted the present work and this contribution should complement the two previously mentioned theoretical studies. In our study, we chose to explore the stability of a model halogenated Kr compound, F-Kr-Cl, the results of which should indicate whether it is feasible to synthesise these compounds in a low temperature Kr matrix. The stability and properties of model halogen-bonded dimers with general
formula
F-Kr-Cl…Y (Y = FH, OH2, NH3) were also investigated and this allows for the comparison of the halogen-bonded complexes with their hydrogen-bonded F-Kr-H…Y counterparts.
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Furthermore, the effect of the insertion of the Kr atom between the F and Cl atoms of FCl in halogen-bonded FCl…Y complexes can also be assessed by comparison with FKrCl…Y.
An additional halogen-, hydrogen-, lithium and beryllium-bond was introduced into the initial dimer structure by attaching FCl, FH, FLi and the Be atom of BeH2, respectively, to the negative F atom of F-Kr-Cl…Y, followed by subsequent optimization to locate stable structures. This allowed us to investigate the cooperative effect of the additional non-covalent interaction on the model F-Kr-Cl…Y dimers. The changes in the topological properties computed by the Quantum Theory of Atoms in Molecules (QTAIM)28 procedure and the changes in the atomic charges computed by the Natural Bond Order (NBO) procedure29 were useful in elucidating the bonding characteristics of the monomeric, dimeric and trimeric species.
COMPUTATIONAL METHODOLOGY
Computations were performed using the Gaussian 03W suite of programs.30 These computations were mainly performed at the MP2/6-311++G(d,p) level of theory and include optimized geometries and harmonic vibrational frequencies for the monomers, dimers and trimers in our study. The rare gas hydrides FRgH are known to be metastable with respect to fragmentation via the two-body FH + Rg pathway. However, these species are kinetically stable since sufficiently high energetic barriers exist to ensure that these species have a finite lifetime at cryogenic temperatures.1-7 The inherent stability of these rare gas compounds is usually dependent on the energetic barrier to the three-body X + Rg + H fragmentation.1-7
The monomers, FCl, FKrCl and FKrH, were all optimized at the MP2/6-311++G(d,p) level. Several molecular properties of interest were obtained from this computation, including bond lengths, dipole moments, vibrational frequencies and energies. The feasibility of synthesising FKrCl may be gauged from computation of the dissociation energies of FKrCl relative to the two-body FCl + Kr and the three-body F + Kr + Cl decay channels. A transition state for the
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two-body fragmentation was located at MP2/6-311++G(d,p), from which the barrier height for this process was determined. The dissociation energies and the barrier heights were also computed at higher levels of theory using the MP2/-311++G(d,p) optimized geometries. These results were useful in establishing the metastability of FKrCl and suggest the possibility of its experimental preparation.
Complexes of these monomers may be formed by interaction of the positive regions on the Cl atom of FCl/FKrCl (or the proton of FKrH) with negative sites on Y (Y = FH, OH2, NH3). Molecular electrostatic potential plots are useful in mapping the regions of maximum charge depletion (Vs,max) and accumulation (Vs,min) on molecular surfaces. The Wave Function Analysis - Surface Analysis Suite (WFA-SAS)31 was used to compute these properties for the preceding molecules by mapping the MP2/6-311++G(d,p) electrostatic potential onto a 0.001 a.u molecular density surface.32
The FCl/FKrCl/FKrH…Y dimers were then optimized at the MP2/6-311++G(d,p) level. The frequency calculations for these systems confirm that they are true minima on their respective potential energy surfaces. Their interaction energies were computed as the difference between the energy of the optimized dimer and the isolated monomers. Other properties, such as vibrational frequency shifts were also computed relative to the unperturbed monomers.
A
theoretical
study
of
interaction
energy
and
intermolecular
separation
for
the
FCl/FKrCl/FKrH…FH dimers was carried out at MP2/6-311++G(d,p), MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ. The dimers were re-optimized using the latter two methods and minimum energy structures were located. The results were then compared to those previously obtained using the MP2/6-311++G(d,p) procedure, in order to assess the reliability of this method.
The Z…FKrCl…Y (Z = FCl, FH, H2Be and FLi ;Y = FH, OH2) trimers were optimized in order to investigate the cooperative effects due to the new non-covalent interaction introduced by the
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molecule Z. The pair-wise non-additive contribution (∆Enon-add) to the interaction energy (∆E) is a measure of the cooperativity of the interactions in the trimer and this was computed as: ∆Enon-add = ∆E – (∆Eab + ∆Ebc + ∆Eac)
(1)
where, ∆Eab, ∆Ebc and ∆Eac are pair energies – each calculated at the geometry that the pair adopts in the optimized trimer.
The electron density (ρ) and negative Laplacian of electron density (L) at the bond critical points (bcp) were computed with the QTAIM method as implemented in the AIMAll software.33 The partial atomic charges were computed using the NBO procedure as implemented in Gaussian03W.29, 30
DISCUSSION Table 1 The molecular properties in Table 1 illustrate the effect of the insertion of the Kr atom between the F and Cl atoms of FCl. There is a modest decrease in the dipole moment but the dipole moment derivative ∂µ/∂rFCl is positive, whereas ∂µ/∂rKrCl is negative. However, the most significant change is in the dipole polarizabilities, especially the parallel polarizability α║, which is substantially larger for FKrCl due to the large Kr atom. Comparing the FKrCl with FKrH, it can be seen that the latter species has a substantially larger dipole moment (which is typical for the rare gas hydrides). However, the dipole derivatives have the same (negative) sign and are similar in magnitude. The α║ is larger for FKrCl (relative to FKrH) since Cl is more polarizable than H. It is notable that the IR intensity of the Kr-H stretch is orders of magnitude larger than that IR intensity of the Kr-Cl stretch and suggests larger perturbations in the electron density in the FKrH relative to FKrCl (since IR intensity is proportional to |∂µ/∂r|2). Table 2
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The metastability of F-Kr-Cl may be assessed by computing the barrier height for the two-body F-Kr-Cl → FCl + Kr dissociation pathway, which is highly exothermic – having a ∆E of -229.8 kJ/mol at MP2/6-311++G(d,p), Table 2. A transition state for this pathway was located at MP2 /6-311++G(d,p) and is shown in Figure 1 (figure 1(a) gives the reactant, 1(b) the transition state and 1(c) the product) . The barrier height is computed to be 33.1, 107.8, 191.6 and 193.5 kJ/mol at MP2/6-311++G(d,p), MP2/aug-cc-pVTZ, B3LYP/aug-cc-pVTZ and B3LYP/aug-cc-pVQZ, respectively. It is clear that the barrier height is sensitive to the level of theory. Though the 6311++G(d,p) basis set gives a substantially smaller value for the barrier height than for the larger basis sets, all methods predict FKrCl to be kinetically stable with respect to the two-body dissociation pathway. For the three-body atomization, FKrCl is predicted to be energetically unstable with respect to the atoms at MP2/6-311++G(d,p). However, with the larger basis sets FKrCl was found to be energetically more stable and the values for the atomization energies are in good agreement. The larger basis sets should produce more reliable results and therefore we can conclude that FKrCl is more energetically stable than its separated atoms. The FKrCl molecule was also fully optimized at MP2 and B3LYP using the aug-cc-pVTZ basis set and the atomization energies were found to be 70.6 and 54.0 kJ/mol respectively. These values are in good agreement with the corresponding values of 65.2 and 49.9 kJ/mol listed in Table 2, which were computed at the MP2/6-311++G(d,p) optimized geometry. The substantial barrier height for the two-body F-Kr-Cl → Kr + Cl-F dissociation and the computed three-body F-Kr-Cl → F + Kr + Cl atomization energies, computed at both MP2 and B3LYP, indicate that F-Kr-Cl may be stable enough for synthesis at cryogenic temperatures – possibly under rare-gas matrix isolation conditions. Table 3 Table 3 summarizes the molecular surface electrostatic potentials for the monomers in this study. The maximum positive surface potential Vs,max along the extension of the F-Cl or Kr-X (X = H, Cl) bond is in the order F-Kr-H > F-Cl > F-Kr-Cl. We would, therefore, expect that the strength of the F-Cl/F-Kr-X…Y interaction would vary in this order as well. Similarly, the Vs,min for Y
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correlates with its basicity. We expect that the binding strength of the complexes would also increase in the order of the Vs,min for the atoms of Y directly bonded to F-Cl/F-Kr-X; i.e., FH < OH2 < NH3.
Table 4 The dimer results in Table 4 allow us to compare the effect of insertion of the Kr atom into the F-Cl molecule (i.e. F-Cl…Y vs F-Kr-Cl…Y). It also allows comparison of the hydrogen-bonded rare gas compound (F-Kr-H…Y) with the halogen-bonded rare gas compound (F-Kr-Cl…Y, Figure 2; figure 2(a), 2(b), 2(c) showing the optimized structures for Y = FH, OH2 and NH3 respectively). As far as we are aware, this is the first time that a prototypical halogen-bonded rare gas molecule has been compared with its hydrogen-bonded rare gas counterpart. A number of pertinent observations can be made. (a) The binding energies for the dimers are in the order F-Kr-H…Y > F-Cl…Y > F-KrCl…Y. This is also the order of the FKrX/FCl dipole moments, which indicates that the electrostatic and polarization components dominate the attraction; for the Kr-containing species, the dispersion energy can be expected to make a larger contribution to the attraction than in the F-Cl complexes. (b) The binding strength also generally increases with the basicity of Y; i.e., NH3 > OH2 > FH. (c) All dimers are nearly linear, though F-Kr-Cl…FH deviates by 13° from linearity. (d) Comparing F-Kr-Cl…Y with F-Cl…Y (Y = FH, OH2, NH3): i.
Insertion of Kr into the F-Cl bond leads to less strongly-bound F-Kr-Cl…Y complexes (relative to F-Cl…Y) presumably because of the smaller dipole of FKr-Cl and the reduced Vs,max on Cl, which results from the insertion.
ii.
Consistent with the smaller ∆E, there is a greater Cl…Y separation in the F-Kr-Cl complexes than in the F-Cl complexes.
iii.
For F-Cl…Y, the F-Cl bond extends, accompanied by a red shift of the F-Cl stretch, both of which increase as the base Y becomes stronger.
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iv.
By contrast, for F-Kr-Cl…Y, there is a small Kr-Cl bond contraction (by 0.001 Å) and negligible blue shift for the most weakly bound complex (Y = FH). However, as we go to Y = OH2, we obtain a small Kr-Cl bond extension and a small red shift. For the most strongly-bound F-Kr-Cl..NH3 complex, a large Kr-Cl extension (0.133 Å) and sizeable red shift (-55 cm-1) are obtained. For both F-Cl…Y and F-Kr-Cl…Y, the electrostatic dipole-dipole attraction and the attraction between the σ-hole from FCl/FKrCl and the lone pairs on Y, are enhanced by the polarization of the large Cl atom by the electric field arising from the dipole of Y. This explains the general tendency for FCl/KrCl bond extension and red shift observed for these complexes.
v.
Interestingly, the Kr-Cl extension in F-Kr-Cl…NH3 is substantially larger than the F-Cl extension in F-Cl…NH3, whereas the Kr-Cl red shift is much smaller than the F-Cl red shift. This may be a consequence of the much larger Kr-Cl separation (2.395 Å) compared with F-Cl (1.673 Å); i.e. it is much easier to displace the longer Kr-Cl bond than the F-Cl bond. However, the stronger non-covalent FCl bond in the F-Cl complex leads to a larger red shift of the F-Cl stretch compared with the Kr-Cl stretch in F-Kr-Cl.
(e) Comparing F-Kr-Cl…Y with F-Kr-H…Y, it is evident that: i.
The hydrogen-bonded dimers are much more strongly-bound than the halogenbonded dimers, since µ (F-Kr-H) >> µ (F-Kr-Cl), as is Vs,max (Kr-H) compared to Vs,max (Kr-Cl). Also the repulsion between the Cl lone pairs and the lone pairs on Y would tend to diminish the attraction between F-Kr-Cl and Y; note the substantially larger X…Y distances in the F-Kr-Cl dimers compared with the F-Kr-H dimers.
ii.
As expected, ∆E increases with increasing basicity of Y. However, the F-Kr-H…NH3 dimer does not optimize to a minimum, probably because the lone pair of NH3 so strongly attracts the proton on F-Kr-H that the linear orientation leads to a destabilized complex; i.e., the NH3 pulls the H away from F-Kr-H to the extent that the entire complex falls apart.
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iii.
For the least-strongly bound FH complexes, the Kr-H bond contracts and a large blue shift of the Kr-H stretch is predicted. For F-Rg-H molecules, hydrogen bonding usually leads to a blue-shifted Rg-H stretch (with concomitant Rg-H bond contraction) [6]. These molecules usually have negative ∂µ/∂rRgH values (i.e. their permanent dipole moments increase when the Rg-H bond is compressed). Table 1 shows that F-Kr-H has a negative dipole derivative, whereas FCl has a positive dipole derivative. The F-Rg-H…Y interaction is consequently stabilized by the increased dipole-dipole attraction that results from bond compression, while the F-Cl…Y interaction is enhanced by bond extension (Table 4). However, the bond contraction (in F-Kr-H…Y) is counterbalanced by the attraction of the proton to the electron density (lone pairs) on Y. The increased basicity of H2O (relative to FH) leads to the latter effect dominating so that for FKr-H…OH2, an extension of the Kr-H bond (by 0.003 Å) and a large red shift (of -108 cm-1) are obtained. The F-Kr-Cl molecule also has a negative dipole derivative (similar in magnitude to the F-Kr-H value), but its substantially larger polarizability (Table 1) leads to larger induction and dispersion energy contributions to the attractive potential in
FKr-Cl…Y, which in turn yields larger bond extensions compared to the F-KrH…Y dimers (Table 4). iv.
The bonding in the F-Kr portion of F-Kr-X is best described as being ionic; i.e. F-Kr+. The F-Kr-H…Y interaction usually causes a shift of electron density out of the covalent Kr-H σ orbital (leading to bond shortening) and into the σ* orbital of F-Kr (leading to bond elongation) [6]. This is consistent with the F-Kr bond extension and red shift in F-Kr-H…FH and F-Kr-H…OH2. However, the net electron density shift in the F-Kr-Cl…Y analogues appears to be smaller, as indicated by smaller F-Kr bond extensions (0.008 Å H2Be > FH > FCl; i.e. the lithium-bond yields the most strongly-bound, while the halogenbond yields the most weakly-bound trimers. The stronger base OH2 produces more stronglybound trimers than FH. Interestingly, the Kr-Cl…Y orientation becomes more linear as ∆E increases; i.e. the stronger the perturbation due to Z, the closer to 180° is the Kr-Cl…Y angle. For example, the Kr-Cl…F angle in Z…FKrCl…FH is 168°, 172°, 176° and 179° for Z = FCl, FH, H2Be and FLi, respectively. The molecule Z polarizes FKrCl by shifting electron density from the Kr-Cl bond towards the F atom. This results in the observed Kr-Cl bond contraction and associated blue shift of the Kr-Cl stretch. The F-Kr bond elongation and associated red shift of the F-Kr stretch are also consistent with this electron density displacement. For FCl…FKrCl…Y, there is a small change in the F-Kr
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bond length and blue shifts of the F-Kr vibration are obtained – presumably the repulsion between the Cl lone pairs on FCl and the F lone pairs on FKrCl restricts the F-Kr bond extension and increases the F-Kr vibration. The polarization of the dimer subunit by the third molecule in Z…FKrCl…FH/OH2 leads to strengthening of the Cl…F/O halogen bond as reflected in the decrease in r(Cl…F/O), relative to the FKrCl…FH/OH dimers. The Cl…Y separation in the trimers decreases as the strength of the additional non-covalent interaction increases (i.e. FCl > FH > H2Be), but there is a slight increase in going from H2Be to the most strongly-bound FLi species. The extent of polarization of the FKrCl…Y dimer, as well as the electrostatic dipole-dipole contribution to the binding strength in the trimer, follows the order of increasing FX dipole moment (FLi > FH > FCl) and is reflected in the relative interaction energies of the trimers. On the other hand, the H2Be molecule polarizes FKrCl by shifting electron density from the F atom towards the vacant 2p orbitals of the Be atom, facilitated by the short Be…F separation shown in Table 6. The separation of Z from the F atom of FKrCl…Y in the trimers is another measure of the strength of the interaction (or polarization) introduced by the molecule Z – the Z…F distance decreases from 2.582 Å (for the halogen bond) to 1.645 Å (for the beryllium bond) in Z…FKrCl…FH, However, there is an increase of the Z…F distance to 1.833 Å, going to the Libonded interaction, even though this is the strongest non-covalent bond – it is likely that the repulsion between the 1s electrons of Li and the lone pairs of the F atom of FKrCl restricts how closely FLi can approach FKrCl in the trimer. Furthermore, it should be noted that although the Li-bonded interaction produces the most strongly-bound trimer, the Be-bonded interaction yields the largest FKr and KrCl bond length changes and frequency shifts, as well as the shortest Z…F and Cl…Y separations. Table 7
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The cooperative effect of the new non-covalent interaction due to Z and the Cl…Y halogen bond in Z…FKrCl…Y trimers is evident from the results in Table 7. The pair interaction (ac) involving the terminal molecules Z and Y are negligible, except for the Be-bonded interaction, where it makes a substantial repulsive contribution. The largest non-additive contribution to ∆E (70 and 84% of ∆E for Z = FH, OH2) occurs for the Be-bonded trimer, which again is an indication of the large perturbations of the FKrCl…Y dimer caused by this interaction. The percentage non-additive contributions for the other non-covalent interactions is in the order of their relative strengths. Hence, the order of the percentage contribution of the non-additive energy to ∆E is in the order H2Be > FLi > FH > FCl. For all trimers, the Z…FKrCl pair is the dominant two-body interaction, and as expected, follows the order of the total interaction energy ∆E. Particularly, for the FLi…FKrCl…Y complexes ∆Eab is atleast 20 kJmol-1 higher than for the corresponding value for the H2Be species. For FLi…FKrCl…FH, a relatively short F…Kr separation of 2.9 Å was computed and is consistent with the structure shown in Figure 4d, which suggests an additional interaction between the F of FLi and the Kr atom. Similar results were obtained for the FLi…FKrCl…OH2 trimer. This additional interaction is probably the reason why the Li species are more strongly bound than the Be species, despite the latter having larger non-additive energy contribution. Table 8 The values for the electron density ρ and the negative Laplacian of the electron density L displayed in Table 8 indicate the mainly covalent character of the FCl bond (L > 0), while the ion-pair character of FKrH and FKrCl is illustrated by L < 0 for the F…Kr bcp. The covalency of Kr-H (L > 0) is evident, but the Kr-Cl bond appears to be a weak non-covalent interaction (L < 0 and relatively small). The covalency of the FCl bond appears to be reduced by the formation of the FCl…Y dimers (L < 0). For the Kr-containing dimers, the ρ and L values are similar to the corresponding values for the uncomplexed Kr species, though there is a significant increase in the small negative L values for the Kr-Cl bond, which probably indicates stabilization of this particular bond in the respective dimers. The L values for the Cl…Y or H…Y interactions in the dimers are negative
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and correlate well with their relative strengths; i.e. the magnitude of the negative L values are generally in the order FKrH > FCl > FKrCl. The relative strengths of the new non-covalent interactions introduced by Z in the Z…FKrCl…Y trimers are also generally reflected by the order of the L values for the Z…F bond, except that the values for the Be-bond are larger than those for the Li-bond; i.e. the negative L values are in the order Z = H2Be > FLi > FH > FCl. This is also the order of the L values for the Cl…Y halogen bond in the trimer complexes. Hence, it would appear that the L values are a good indicator of the relative strength of these intermolecular interactions, as noted in previous work by Duarte et al.35 The ρ and L values for the F-Kr bond in the trimers are similar, but there seems to be an inverse correlation between the smaller negative L values for the Kr-Cl bond and the relative strength of the Z…F interaction; i.e. the weaker Z interactions have larger negative L values, while the stronger interactions have correspondingly smaller values. For example, the L values for the weaker FCl and FH interactions in Z…FKrCl…FH are -0.093 and -0.087 a.u., whereas for the stronger H2Be and FLi interactions, the L values are -0.067 and -0.072 a.u. Table 9 The changes in the atomic charges on the FCl, FKrCl and FKrH molecules, as well as the intermolecular charge transfer, due to dimer and trimer formation can be assessed from the NBO results given in Table 9. The changes in the atomic charges are assumed to be more informative than their absolute values. For all three molecules, dimer formation results in an increase in the negative charge on the F atom and an increase in the positive charge on the H or Cl atom, which is consistent with a shift in electron density from the Cl atom or Kr-Cl/Kr-H bond towards the F atom due to the electric field arising from the molecule Y. There is also some electron density transferred from the lone pairs on Y mainly into the antibonding σ* orbitals of either F-Cl, Kr-Cl or Kr-H. These changes in the atomic charges generally increase with the basicity of Y. However, the shift in electron density away from the Cl atom of FCl or FKrCl by the electric field of the strongly basic NH3
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molecule is counterbalanced by the relatively large electron density transferred (0.16e) from NH3 to FCl/FKrCl. The polarizing effect of Z on the halogen-bonded FKrCl…Y subunit in the trimer is also manifest in the substantially elevated atomic charges shown in Table 9. The F atom of FKrCl becomes more negative, the Kr atom more positive and the Cl atom more positive due to the new interaction. The interaction with Z also results in charge being transferred from the F atom of FKrCl to the Z molecule. The relatively large electron density transferred to the H2Be molecule (into the Be atom’s vacant p orbitals, as previously mentioned) is evident in Table 4 and is consistent with the large perturbations in the FKrCl molecule due to the Be-bonded interaction. To sum up, we have performed energetic calculations for the FKrCl molecule at various levels of theory which suggest that this molecule is metastable and may be amenable to experimental synthesis under cryogenic conditions. Furthermore, we have shown that it forms weak halogenbonded complexes with small molecules like FH and H2O and we compared its computed properties with analogous complexes of its precursor FCl and its FKrH counterpart. The cooperative effect of additional non-covalent interactions introduced at the F end of the FKrCl…Y dimer showed a general strengthening of the intermolecular interactions in the order halogen-bond < hydrogen-bond < beryllium-bond < lithium-bond.
ACKNOWLEDGEMENT The authors wish to acknowledge the support of the School for Graduate Studies at the Cave Hill Campus of the University of the West Indies, for the purchase of the computers used in this work.
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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13.
Khriachtchev, L.; Pettersson, M.; Runeberg, N.; Lundell, J.; Räsänen, M., A Stable Argon Compound. Nature 2000, 406, 874-876. Pettersson, M.; Lundell, J.; Räsänen, M., Neutral Rare-Gas Containing Charge-Transfer Molecules in Solid Matrices .1. HXeCl, HXeBr, HXeI, and HKrCl in Kr and Xe. J. Chem. Phys. 1995, 102, 6423-6431. Khriachtchev, L.; Pettersson, M.; Lignell, A.; Räsänen, M., A More Stable Configuration of HArF in Solid Argon. J. Am. Chem. Soc. 2001, 123, 8610-8611. Khriachtchev, L.; Raesaenen, M.; Gerber, R. B., Noble-Gas Hydrides: New Chemistry at Low Temperatures. Acc. Chem. Res. 2009, 42, 183-191. Gerber, R. B., Formation of Novel Rare-Gas Molecules in Low-Temperature Matrices. Annu. Rev. Phys. Chem. 2004, 55, 55-78. McDowell, S. A. C., Studies of Neutral Rare-Gas Compounds and Their Non-Covalent Interactions with Other Molecules. Curr. Org. Chem. 2006, 10, 791-803. Grochala, W., Atypical Compounds of Gases, Which Have Been Called 'Noble'. Chem. Soc. Rev. 2007, 36, 1632-1655. Hobza, P.; Havlas, Z., Blue-Shifting Hydrogen Bonds. Chem. Rev. 2000, 100, 4253-4264. Scheiner, S., Hydrogen Bonding: A Theoretical Perspective. Oxford University Press: New York, 1997. Lignell, A.; Khriachtchev, L.; Pettersson, M.; Räsänen, M., Interaction of Rare-Gas-Containing Molecules with Nitrogen: Matrix-Isolation and Ab Initio Study of HArF⋯N2, HKrF⋯N2, and HKrCl⋯N2 Complexes. J. Chem. Phys. 2003, 118, 11120-11128. McDowell, S. A. C., Blue Shifting and Red Shifting Hydrogen Bonds: A Study of the HArFN2 and HArFP2 Complexes. Phys. Chem. Chem. Phys. 2003, 5, 808-811. Lignell, A.; Khriachtchev, L., Intermolecular Interactions Involving Noble-Gas Hydrides: Where the Blue Shift of Vibrational Frequency Is a Normal Effect. J. Mol. Struct. 2008, 889, 1-11. Corani, A.; Domanskaya, A.; Khriachtchev, L.; Räsänen, M.; Lignell, A., Matrix-Isolation and Ab Initio Study of the HKrCl···HCl Complex. J. Phys. Chem. A 2009, 113, 10687-10692.
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31.
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Politzer, P.; Riley, K. E.; Bulat, F. A.; Murray, J. S., Perspectives on Halogen Bonding and Other SigmaHole Interactions: Lex Parsimoniae (Occam's Razor). Computational and Theoretical Chemistry 2012, 998, 2-8. Politzer, P.; Murray, J. S., Halogen Bonding: An Interim Discussion. Chemphyschem 2013, 14, 278-294. Politzer, P.; Murray, J. S.; Clark, T., Halogen Bonding and Other Sigma-Hole Interactions: A Perspective. Phys. Chem. Chem. Phys. 2013, 15, 11178-11189. Hoppe, R.; Dähne, W.; Mattauch, H.; Rödder, K., Fluorination of Xenon. Angewandte Chemie International Edition in English 1962, 1, 599-599. Turner, J.; Pimentel, G. C., Krypton Fluoride: Preparation by the Matrix Isolation Technique. Science 1963, 140, 974-975. Howard Jr, W. F.; Andrews, L., Synthesis of Noble-Gas Dihalides by Laser Photolysis of Matrix-Isolated Halogens. J. Am. Chem. Soc. 1974, 96, 7864-7868. Khriachtchev, L.; Domanskaya, A.; Lundell, J.; Akimov, A.; Räsänen, M.; Misochko, E., Matrix-Isolation and Ab Initio Study of HNgCCF and HCCNgF Molecules (Ng= Ar, Kr, and Xe). The Journal of Physical Chemistry A 2010, 114, 4181-4187. Lignell, A.; Lundell, J.; Khriachtchev, L.; Räsänen, M., Experimental and Computational Study of HXeY···HX Complexes (X, Y = Cl and Br): An Example of Exceptionally Large Complexation Effect. J. Phys. Chem. A 2008, 112, 5486-5494. Tsuge, M.; Berski, S.; Räsänen, M.; Latajka, Z.; Khriachtchev, L., Experimental and Computational Study of the HXeI⋯HY Complexes (Y = Br and I). J. Chem. Phys. 2013, 138, 104314- 104314. Tsuge, M.; Berski, S.; Räsänen, M.; Latajka, Z.; Khriachtchev, L., Matrix-Isolation and Computational Study of the HXeY⋯H2O Complexes (Y = Cl, Br, and I). J. Chem. Phys. 2014, 140, 044323- 044323. Khriachtchev, L.; Tapio, S.; Räsänen, M.; Domanskaya, A.; Lignell, A., HY⋯N2 and HXeY⋯N2 Complexes in Solid Xenon (Y=Cl and Br): Unexpected Suppression of the Complex Formation for Deposition at Higher Temperature. J. Chem. Phys. 2010, 133, 084309-084309. Arppe, T.; Khriachtchev, L.; Lignell, A.; Domanskaya, A. V.; Räsänen, M., Halogenated Xenon Cyanides ClXeCN, ClXeNC, and BrXeCN. Inorg. Chem. 2012, 51, 4398-4402. Esrafili, M. D.; Juyban, P., CNXeCl and CNXeBr Species as Halogen Bond Donors: A Quantum Chemical Study on the Structure, Properties, and Nature of Halogen···Nitrogen Interactions. J. Mol. Model. 2014, 20, 1-9. Esrafili, M. D.; Ahmadi, B., A Theoretical Investigation on the Nature of Cl⋯N and Br⋯N Halogen bonds in F-Ar-X⋯NCY Complexes (X = Cl, Br and Y = H, F, Cl, Br, OH, NH2, CH3 and CN). Computational and Theoretical Chemistry 2012, 997, 77-82. Bader, R. F. W., Atoms in Molecules — a Quantum Theory. Oxford Univ. Press: Oxford 1990. Reed, A. E.; Curtiss, L. A.; Weinhold, F., Intermolecular Interactions from a Natural Bond Orbital, DonorAcceptor Viewpoint. Chem. Rev. 1988, 88, 899-926. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A.; Vreven, T.; Kudin, K. N.; Burant, J. C., et al. Gaussian 03, Revision B.05, Gaussian, Inc.: Pittsburgh PA, 2003. Bulat, F. A.; Toro-Labbe, A.; Brinck, T.; Murray, J. S.; Politzer, P., Quantitative Analysis of Molecular Surfaces: Areas, Volumes, Electrostatic Potentials and Average Local Ionization Energies. J. Mol. Model. 2010, 16, 1679-1691. Bader, R. F. W.; Carroll, M. T.; Cheeseman, J. R.; Chang, C., Properties of Atoms in Molecules: Atomic Volumes. J. Am. Chem. Soc. 1987, 109, 7968-7979.
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The Journal of Physical Chemistry 19 33. 34. 35.
Keith, T. A. Aimall (Version 12.11.09), TK Gristmill Software: Overland Park KS, USA, 2012. Stone, A. J., Are Halogen Bonded Structures Electrostatically Driven? J. Am. Chem. Soc. 2013, 135, 70057009. Duarte, D. J. R.; Sosa, G. L.; Peruchena, N. M., Nature of Halogen Bonding. A Study Based on the Topological Analysis of the Laplacian of the Electron Charge Density and an Energy Decomposition Analysis. J. Mol. Model. 2013, 19, 2035-2041
Table 1: Molecular properties of F-Cl, F-Kr-Cl and F-Kr-H computed at the MP2 /6-311++G(d,p) level
Properties
F-Cl
F-Kr-Cl
F-Kr-H
r(F-Cl) or r(Kr-X) /Å r(F-Kr) /Å µ /D ∂µ/∂rFCl or ∂µ/∂rKrX α┴ /a.u α║ /a.u ω(F-Cl) or ω(Kr-X) /cm-1 I(F-Cl) or I(Kr-X) / E /a.u
1.673
2.395 2.026 1.032 -3.89 19.121 92.195 240 11 -3311.23480123
1.480 2.074 6.723 -4.52 12.865 57.578 2112 890 -2852.17506514
1.345 2.22 8.675 19.011 733 20 -559.22593226
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Table 2: Two-body dissociation energy for F-Kr-Cl (to Kr…Cl-F) and atomization energy of F-Kr-Cl computed at various levels of theory. The kinetic barrier for the two-body decomposition is given in parentheses. All energy calculations were performed at the MP2/6311++G(d,p) optimized geometry and are in kJ mol-1.
Method MP2/6-311++G(d,p) MP2/aug-cc-pVTZ B3LYP/aug-cc-pVTZ B3LYP/aug-cc-pVQZ
F-Kr-Cl→ Kr…Cl-F -229.8 (33.1) -207.1 (107.8) -198.4 (191.6) -200.0 (193.5)
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F-Kr-Cl→ F + Kr + Cl -16.8 65.2 49.9 52.1
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Table 3: Computed MP2/6-311++G(d,p) values for Vs,max and Vs,min (in kJ mol-1) of the electrostatic potential for F-Cl and F-Kr-X (X = Cl, H) and Y (Y = FH, OH2, NH3). Species
Vs,max
Species
Vs,min
F-Cl
51.1 (F-Cl)
FH
-108.8 (F)
F-Kr-Cl
17.7 (Kr-Cl)
OH2
-167.8 (O)
F-Kr-H
62.2 (Kr-H)
NH3
-178.7 (N)
F-Kr-Cl
-39.0 (F)
F-Kr-H
-74.8 (F)
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Table 4: Interaction energies (∆E), bond angles (
Article
A Comparative Computational Study of Model Halogen-bonded Complexes of FKrCl Jerelle A. Joseph, and Sean A. C. McDowell J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/jp5091262 • Publication Date (Web): 15 Oct 2014 Downloaded from http://pubs.acs.org on October 19, 2014
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A Comparative Computational Study of Model Halogen-bonded Complexes of FKrCl
Jerelle A. Joseph Department of Biological and Chemical Sciences, The University of the West Indies, Cave Hill Campus, Barbados
Sean A. C. McDowell * Department of Biological and Chemical Sciences, The University of the West Indies, Cave Hill Campus, Barbados email: [email protected]* Telephone: 246-417-4352*
Abstract Quantum chemical calculations for the FKrCl molecule at various levels of theory were performed and suggest that this molecule is metastable and may be amenable to experimental synthesis under cryogenic conditions. The FKrCl molecule forms weak halogen-bonded complexes FKrCl…Y with small molecules like FH and H2O and its computed properties were compared with analogous complexes of its precursor, FCl, and its rare gas hydride counterpart, FKrH. The cooperative effect of additional non-covalent interactions introduced at the F atom in the FKrCl…Y dimer (to give Z…FKrCl…Y trimers) showed a general strengthening of the intermolecular interactions in the order halogen-bond < hydrogen-bond < beryllium-bond < lithium-bond.
Keywords: rare gas, hydrogen-bond, sigma-hole, lithium-bond, beryllium-bond, Krypton
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INTRODUCTION
The chemistry of rare gas compounds was rejuvenated by the discovery in 2000 of the only known neutral chemical compound of Ar, FArH, by Räsänen and co-workers.1 Their seminal paper spawned a number of experimental and theoretical studies of a general class of XRgH compounds (Rg = rare gas, X = electron-withdrawing atom or fragment). Although metastable, these compounds were prepared under cryogenic conditions in rare gas matrices by UV photolysis of an XH precursor, followed by thermal mobilization of the H atoms, leading to the insertion of the Rg atom between the X and H to form XRgH.1-4 The stability of FArH derives from the strong F- [Ar-H]+ ion-pair character of this species, the Rg-H bond being mainly covalent in nature, whereas the F…Ar bond is predominantly ionic. Several reviews of this topic are to be found in refs.4-7
These XRgH compounds are capable of forming hydrogen-bonded complexes, XRgH…Y, with small molecules like N2 which exhibit the unconventional blue-shifting behaviour, a topic of much interest since it was first reviewed by Hobza and Havlas in 2000.8 Hydrogen bonding normally causes the covalent bond containing the H atom (in a proton donor molecule) to vibrate at a lower vibrational frequency (a red shift) than when it is uncomplexed.9 However, it was found that the characteristically intense Rg-H vibrational stretching frequency, which allows for easy identification by IR spectroscopy, is shifted to higher frequencies due to the interaction with Y.10-13 This blue-shifting effect has been rationalized by invoking enhanced F-[Rg-H]+ ion-pair charge separation when the complex with Y forms.12
Another non-covalent interaction that has received considerable attention over the last ten years or so has been halogen bonding, represented symbolically by R-X…Y (R = electronwithdrawing atom or group; X = halogen atom; Y contains a source of electron density). The key feature of the halogen bond is the attractive and highly directional interaction between a positive region of locally depleted electron density along the extension of the R-X covalent bond, called the σ-hole, and a negative site on the partner molecule Y (e.g. lone pairs or π-electrons).14-16
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Several halogenated rare gas species have been prepared and characterized over the years.17-20 There are also experimental and computational studies on hydrogen-bonded compounds [HRgY…HX/H2O13,
21-23
(Rg = Ar, Kr, Xe; X, Y = Cl, Br, I) and YRgH…N224] containing
halogenated rare gas subunits. The possibility of halogen-bonded complexes of rare gas compounds has been raised by a recent study of halogenated xenon cyanides which reported the synthesis and characterisation of ClXeCN, ClXeNC and BrXeCN by photolysis and thermal annealing of ClCN and BrCN in a low temperature Xe matrix.25 The experimental results for these compounds were in agreement with quantum chemical calculations and this work further suggested the possibility of synthesising molecules of general formula X-Rg-X’, where X and X’ are electronegative fragments like halogens.25
A recent theoretical study of the halogenated xenon isocyanides, ClXeNC and BrXeNC, complexed with several nitrogen-containing molecules via a halogen…nitrogen bond indicates that these weakly-bound complexes may be stable, with computed interaction energies at MP2/aug-cc-pVTZ ranging between -1.0 and -3.1 kcal mol-1 being reported.26 Another high-level theoretical study of F-Ar-X…NCY (X = Cl, Br; Y = H, F, Cl, Br, OH, NH2, CH3 and CN) also predicted stable halogen-bonded complexes, with computed CCSD(T)/aug-cc-pVTZ interaction energies ranging between -0.88 and -3.20 kcal mol-1.27 These two theoretical studies seem to be the only work reported on halogen bonding involving rare gas compounds.
The paucity of reported studies on halogen-bonded rare gas complexes prompted the present work and this contribution should complement the two previously mentioned theoretical studies. In our study, we chose to explore the stability of a model halogenated Kr compound, F-Kr-Cl, the results of which should indicate whether it is feasible to synthesise these compounds in a low temperature Kr matrix. The stability and properties of model halogen-bonded dimers with general
formula
F-Kr-Cl…Y (Y = FH, OH2, NH3) were also investigated and this allows for the comparison of the halogen-bonded complexes with their hydrogen-bonded F-Kr-H…Y counterparts.
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Furthermore, the effect of the insertion of the Kr atom between the F and Cl atoms of FCl in halogen-bonded FCl…Y complexes can also be assessed by comparison with FKrCl…Y.
An additional halogen-, hydrogen-, lithium and beryllium-bond was introduced into the initial dimer structure by attaching FCl, FH, FLi and the Be atom of BeH2, respectively, to the negative F atom of F-Kr-Cl…Y, followed by subsequent optimization to locate stable structures. This allowed us to investigate the cooperative effect of the additional non-covalent interaction on the model F-Kr-Cl…Y dimers. The changes in the topological properties computed by the Quantum Theory of Atoms in Molecules (QTAIM)28 procedure and the changes in the atomic charges computed by the Natural Bond Order (NBO) procedure29 were useful in elucidating the bonding characteristics of the monomeric, dimeric and trimeric species.
COMPUTATIONAL METHODOLOGY
Computations were performed using the Gaussian 03W suite of programs.30 These computations were mainly performed at the MP2/6-311++G(d,p) level of theory and include optimized geometries and harmonic vibrational frequencies for the monomers, dimers and trimers in our study. The rare gas hydrides FRgH are known to be metastable with respect to fragmentation via the two-body FH + Rg pathway. However, these species are kinetically stable since sufficiently high energetic barriers exist to ensure that these species have a finite lifetime at cryogenic temperatures.1-7 The inherent stability of these rare gas compounds is usually dependent on the energetic barrier to the three-body X + Rg + H fragmentation.1-7
The monomers, FCl, FKrCl and FKrH, were all optimized at the MP2/6-311++G(d,p) level. Several molecular properties of interest were obtained from this computation, including bond lengths, dipole moments, vibrational frequencies and energies. The feasibility of synthesising FKrCl may be gauged from computation of the dissociation energies of FKrCl relative to the two-body FCl + Kr and the three-body F + Kr + Cl decay channels. A transition state for the
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two-body fragmentation was located at MP2/6-311++G(d,p), from which the barrier height for this process was determined. The dissociation energies and the barrier heights were also computed at higher levels of theory using the MP2/-311++G(d,p) optimized geometries. These results were useful in establishing the metastability of FKrCl and suggest the possibility of its experimental preparation.
Complexes of these monomers may be formed by interaction of the positive regions on the Cl atom of FCl/FKrCl (or the proton of FKrH) with negative sites on Y (Y = FH, OH2, NH3). Molecular electrostatic potential plots are useful in mapping the regions of maximum charge depletion (Vs,max) and accumulation (Vs,min) on molecular surfaces. The Wave Function Analysis - Surface Analysis Suite (WFA-SAS)31 was used to compute these properties for the preceding molecules by mapping the MP2/6-311++G(d,p) electrostatic potential onto a 0.001 a.u molecular density surface.32
The FCl/FKrCl/FKrH…Y dimers were then optimized at the MP2/6-311++G(d,p) level. The frequency calculations for these systems confirm that they are true minima on their respective potential energy surfaces. Their interaction energies were computed as the difference between the energy of the optimized dimer and the isolated monomers. Other properties, such as vibrational frequency shifts were also computed relative to the unperturbed monomers.
A
theoretical
study
of
interaction
energy
and
intermolecular
separation
for
the
FCl/FKrCl/FKrH…FH dimers was carried out at MP2/6-311++G(d,p), MP2/aug-cc-pVTZ and B3LYP/aug-cc-pVTZ. The dimers were re-optimized using the latter two methods and minimum energy structures were located. The results were then compared to those previously obtained using the MP2/6-311++G(d,p) procedure, in order to assess the reliability of this method.
The Z…FKrCl…Y (Z = FCl, FH, H2Be and FLi ;Y = FH, OH2) trimers were optimized in order to investigate the cooperative effects due to the new non-covalent interaction introduced by the
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molecule Z. The pair-wise non-additive contribution (∆Enon-add) to the interaction energy (∆E) is a measure of the cooperativity of the interactions in the trimer and this was computed as: ∆Enon-add = ∆E – (∆Eab + ∆Ebc + ∆Eac)
(1)
where, ∆Eab, ∆Ebc and ∆Eac are pair energies – each calculated at the geometry that the pair adopts in the optimized trimer.
The electron density (ρ) and negative Laplacian of electron density (L) at the bond critical points (bcp) were computed with the QTAIM method as implemented in the AIMAll software.33 The partial atomic charges were computed using the NBO procedure as implemented in Gaussian03W.29, 30
DISCUSSION Table 1 The molecular properties in Table 1 illustrate the effect of the insertion of the Kr atom between the F and Cl atoms of FCl. There is a modest decrease in the dipole moment but the dipole moment derivative ∂µ/∂rFCl is positive, whereas ∂µ/∂rKrCl is negative. However, the most significant change is in the dipole polarizabilities, especially the parallel polarizability α║, which is substantially larger for FKrCl due to the large Kr atom. Comparing the FKrCl with FKrH, it can be seen that the latter species has a substantially larger dipole moment (which is typical for the rare gas hydrides). However, the dipole derivatives have the same (negative) sign and are similar in magnitude. The α║ is larger for FKrCl (relative to FKrH) since Cl is more polarizable than H. It is notable that the IR intensity of the Kr-H stretch is orders of magnitude larger than that IR intensity of the Kr-Cl stretch and suggests larger perturbations in the electron density in the FKrH relative to FKrCl (since IR intensity is proportional to |∂µ/∂r|2). Table 2
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The metastability of F-Kr-Cl may be assessed by computing the barrier height for the two-body F-Kr-Cl → FCl + Kr dissociation pathway, which is highly exothermic – having a ∆E of -229.8 kJ/mol at MP2/6-311++G(d,p), Table 2. A transition state for this pathway was located at MP2 /6-311++G(d,p) and is shown in Figure 1 (figure 1(a) gives the reactant, 1(b) the transition state and 1(c) the product) . The barrier height is computed to be 33.1, 107.8, 191.6 and 193.5 kJ/mol at MP2/6-311++G(d,p), MP2/aug-cc-pVTZ, B3LYP/aug-cc-pVTZ and B3LYP/aug-cc-pVQZ, respectively. It is clear that the barrier height is sensitive to the level of theory. Though the 6311++G(d,p) basis set gives a substantially smaller value for the barrier height than for the larger basis sets, all methods predict FKrCl to be kinetically stable with respect to the two-body dissociation pathway. For the three-body atomization, FKrCl is predicted to be energetically unstable with respect to the atoms at MP2/6-311++G(d,p). However, with the larger basis sets FKrCl was found to be energetically more stable and the values for the atomization energies are in good agreement. The larger basis sets should produce more reliable results and therefore we can conclude that FKrCl is more energetically stable than its separated atoms. The FKrCl molecule was also fully optimized at MP2 and B3LYP using the aug-cc-pVTZ basis set and the atomization energies were found to be 70.6 and 54.0 kJ/mol respectively. These values are in good agreement with the corresponding values of 65.2 and 49.9 kJ/mol listed in Table 2, which were computed at the MP2/6-311++G(d,p) optimized geometry. The substantial barrier height for the two-body F-Kr-Cl → Kr + Cl-F dissociation and the computed three-body F-Kr-Cl → F + Kr + Cl atomization energies, computed at both MP2 and B3LYP, indicate that F-Kr-Cl may be stable enough for synthesis at cryogenic temperatures – possibly under rare-gas matrix isolation conditions. Table 3 Table 3 summarizes the molecular surface electrostatic potentials for the monomers in this study. The maximum positive surface potential Vs,max along the extension of the F-Cl or Kr-X (X = H, Cl) bond is in the order F-Kr-H > F-Cl > F-Kr-Cl. We would, therefore, expect that the strength of the F-Cl/F-Kr-X…Y interaction would vary in this order as well. Similarly, the Vs,min for Y
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correlates with its basicity. We expect that the binding strength of the complexes would also increase in the order of the Vs,min for the atoms of Y directly bonded to F-Cl/F-Kr-X; i.e., FH < OH2 < NH3.
Table 4 The dimer results in Table 4 allow us to compare the effect of insertion of the Kr atom into the F-Cl molecule (i.e. F-Cl…Y vs F-Kr-Cl…Y). It also allows comparison of the hydrogen-bonded rare gas compound (F-Kr-H…Y) with the halogen-bonded rare gas compound (F-Kr-Cl…Y, Figure 2; figure 2(a), 2(b), 2(c) showing the optimized structures for Y = FH, OH2 and NH3 respectively). As far as we are aware, this is the first time that a prototypical halogen-bonded rare gas molecule has been compared with its hydrogen-bonded rare gas counterpart. A number of pertinent observations can be made. (a) The binding energies for the dimers are in the order F-Kr-H…Y > F-Cl…Y > F-KrCl…Y. This is also the order of the FKrX/FCl dipole moments, which indicates that the electrostatic and polarization components dominate the attraction; for the Kr-containing species, the dispersion energy can be expected to make a larger contribution to the attraction than in the F-Cl complexes. (b) The binding strength also generally increases with the basicity of Y; i.e., NH3 > OH2 > FH. (c) All dimers are nearly linear, though F-Kr-Cl…FH deviates by 13° from linearity. (d) Comparing F-Kr-Cl…Y with F-Cl…Y (Y = FH, OH2, NH3): i.
Insertion of Kr into the F-Cl bond leads to less strongly-bound F-Kr-Cl…Y complexes (relative to F-Cl…Y) presumably because of the smaller dipole of FKr-Cl and the reduced Vs,max on Cl, which results from the insertion.
ii.
Consistent with the smaller ∆E, there is a greater Cl…Y separation in the F-Kr-Cl complexes than in the F-Cl complexes.
iii.
For F-Cl…Y, the F-Cl bond extends, accompanied by a red shift of the F-Cl stretch, both of which increase as the base Y becomes stronger.
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iv.
By contrast, for F-Kr-Cl…Y, there is a small Kr-Cl bond contraction (by 0.001 Å) and negligible blue shift for the most weakly bound complex (Y = FH). However, as we go to Y = OH2, we obtain a small Kr-Cl bond extension and a small red shift. For the most strongly-bound F-Kr-Cl..NH3 complex, a large Kr-Cl extension (0.133 Å) and sizeable red shift (-55 cm-1) are obtained. For both F-Cl…Y and F-Kr-Cl…Y, the electrostatic dipole-dipole attraction and the attraction between the σ-hole from FCl/FKrCl and the lone pairs on Y, are enhanced by the polarization of the large Cl atom by the electric field arising from the dipole of Y. This explains the general tendency for FCl/KrCl bond extension and red shift observed for these complexes.
v.
Interestingly, the Kr-Cl extension in F-Kr-Cl…NH3 is substantially larger than the F-Cl extension in F-Cl…NH3, whereas the Kr-Cl red shift is much smaller than the F-Cl red shift. This may be a consequence of the much larger Kr-Cl separation (2.395 Å) compared with F-Cl (1.673 Å); i.e. it is much easier to displace the longer Kr-Cl bond than the F-Cl bond. However, the stronger non-covalent FCl bond in the F-Cl complex leads to a larger red shift of the F-Cl stretch compared with the Kr-Cl stretch in F-Kr-Cl.
(e) Comparing F-Kr-Cl…Y with F-Kr-H…Y, it is evident that: i.
The hydrogen-bonded dimers are much more strongly-bound than the halogenbonded dimers, since µ (F-Kr-H) >> µ (F-Kr-Cl), as is Vs,max (Kr-H) compared to Vs,max (Kr-Cl). Also the repulsion between the Cl lone pairs and the lone pairs on Y would tend to diminish the attraction between F-Kr-Cl and Y; note the substantially larger X…Y distances in the F-Kr-Cl dimers compared with the F-Kr-H dimers.
ii.
As expected, ∆E increases with increasing basicity of Y. However, the F-Kr-H…NH3 dimer does not optimize to a minimum, probably because the lone pair of NH3 so strongly attracts the proton on F-Kr-H that the linear orientation leads to a destabilized complex; i.e., the NH3 pulls the H away from F-Kr-H to the extent that the entire complex falls apart.
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iii.
For the least-strongly bound FH complexes, the Kr-H bond contracts and a large blue shift of the Kr-H stretch is predicted. For F-Rg-H molecules, hydrogen bonding usually leads to a blue-shifted Rg-H stretch (with concomitant Rg-H bond contraction) [6]. These molecules usually have negative ∂µ/∂rRgH values (i.e. their permanent dipole moments increase when the Rg-H bond is compressed). Table 1 shows that F-Kr-H has a negative dipole derivative, whereas FCl has a positive dipole derivative. The F-Rg-H…Y interaction is consequently stabilized by the increased dipole-dipole attraction that results from bond compression, while the F-Cl…Y interaction is enhanced by bond extension (Table 4). However, the bond contraction (in F-Kr-H…Y) is counterbalanced by the attraction of the proton to the electron density (lone pairs) on Y. The increased basicity of H2O (relative to FH) leads to the latter effect dominating so that for FKr-H…OH2, an extension of the Kr-H bond (by 0.003 Å) and a large red shift (of -108 cm-1) are obtained. The F-Kr-Cl molecule also has a negative dipole derivative (similar in magnitude to the F-Kr-H value), but its substantially larger polarizability (Table 1) leads to larger induction and dispersion energy contributions to the attractive potential in
FKr-Cl…Y, which in turn yields larger bond extensions compared to the F-KrH…Y dimers (Table 4). iv.
The bonding in the F-Kr portion of F-Kr-X is best described as being ionic; i.e. F-Kr+. The F-Kr-H…Y interaction usually causes a shift of electron density out of the covalent Kr-H σ orbital (leading to bond shortening) and into the σ* orbital of F-Kr (leading to bond elongation) [6]. This is consistent with the F-Kr bond extension and red shift in F-Kr-H…FH and F-Kr-H…OH2. However, the net electron density shift in the F-Kr-Cl…Y analogues appears to be smaller, as indicated by smaller F-Kr bond extensions (0.008 Å H2Be > FH > FCl; i.e. the lithium-bond yields the most strongly-bound, while the halogenbond yields the most weakly-bound trimers. The stronger base OH2 produces more stronglybound trimers than FH. Interestingly, the Kr-Cl…Y orientation becomes more linear as ∆E increases; i.e. the stronger the perturbation due to Z, the closer to 180° is the Kr-Cl…Y angle. For example, the Kr-Cl…F angle in Z…FKrCl…FH is 168°, 172°, 176° and 179° for Z = FCl, FH, H2Be and FLi, respectively. The molecule Z polarizes FKrCl by shifting electron density from the Kr-Cl bond towards the F atom. This results in the observed Kr-Cl bond contraction and associated blue shift of the Kr-Cl stretch. The F-Kr bond elongation and associated red shift of the F-Kr stretch are also consistent with this electron density displacement. For FCl…FKrCl…Y, there is a small change in the F-Kr
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bond length and blue shifts of the F-Kr vibration are obtained – presumably the repulsion between the Cl lone pairs on FCl and the F lone pairs on FKrCl restricts the F-Kr bond extension and increases the F-Kr vibration. The polarization of the dimer subunit by the third molecule in Z…FKrCl…FH/OH2 leads to strengthening of the Cl…F/O halogen bond as reflected in the decrease in r(Cl…F/O), relative to the FKrCl…FH/OH dimers. The Cl…Y separation in the trimers decreases as the strength of the additional non-covalent interaction increases (i.e. FCl > FH > H2Be), but there is a slight increase in going from H2Be to the most strongly-bound FLi species. The extent of polarization of the FKrCl…Y dimer, as well as the electrostatic dipole-dipole contribution to the binding strength in the trimer, follows the order of increasing FX dipole moment (FLi > FH > FCl) and is reflected in the relative interaction energies of the trimers. On the other hand, the H2Be molecule polarizes FKrCl by shifting electron density from the F atom towards the vacant 2p orbitals of the Be atom, facilitated by the short Be…F separation shown in Table 6. The separation of Z from the F atom of FKrCl…Y in the trimers is another measure of the strength of the interaction (or polarization) introduced by the molecule Z – the Z…F distance decreases from 2.582 Å (for the halogen bond) to 1.645 Å (for the beryllium bond) in Z…FKrCl…FH, However, there is an increase of the Z…F distance to 1.833 Å, going to the Libonded interaction, even though this is the strongest non-covalent bond – it is likely that the repulsion between the 1s electrons of Li and the lone pairs of the F atom of FKrCl restricts how closely FLi can approach FKrCl in the trimer. Furthermore, it should be noted that although the Li-bonded interaction produces the most strongly-bound trimer, the Be-bonded interaction yields the largest FKr and KrCl bond length changes and frequency shifts, as well as the shortest Z…F and Cl…Y separations. Table 7
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The cooperative effect of the new non-covalent interaction due to Z and the Cl…Y halogen bond in Z…FKrCl…Y trimers is evident from the results in Table 7. The pair interaction (ac) involving the terminal molecules Z and Y are negligible, except for the Be-bonded interaction, where it makes a substantial repulsive contribution. The largest non-additive contribution to ∆E (70 and 84% of ∆E for Z = FH, OH2) occurs for the Be-bonded trimer, which again is an indication of the large perturbations of the FKrCl…Y dimer caused by this interaction. The percentage non-additive contributions for the other non-covalent interactions is in the order of their relative strengths. Hence, the order of the percentage contribution of the non-additive energy to ∆E is in the order H2Be > FLi > FH > FCl. For all trimers, the Z…FKrCl pair is the dominant two-body interaction, and as expected, follows the order of the total interaction energy ∆E. Particularly, for the FLi…FKrCl…Y complexes ∆Eab is atleast 20 kJmol-1 higher than for the corresponding value for the H2Be species. For FLi…FKrCl…FH, a relatively short F…Kr separation of 2.9 Å was computed and is consistent with the structure shown in Figure 4d, which suggests an additional interaction between the F of FLi and the Kr atom. Similar results were obtained for the FLi…FKrCl…OH2 trimer. This additional interaction is probably the reason why the Li species are more strongly bound than the Be species, despite the latter having larger non-additive energy contribution. Table 8 The values for the electron density ρ and the negative Laplacian of the electron density L displayed in Table 8 indicate the mainly covalent character of the FCl bond (L > 0), while the ion-pair character of FKrH and FKrCl is illustrated by L < 0 for the F…Kr bcp. The covalency of Kr-H (L > 0) is evident, but the Kr-Cl bond appears to be a weak non-covalent interaction (L < 0 and relatively small). The covalency of the FCl bond appears to be reduced by the formation of the FCl…Y dimers (L < 0). For the Kr-containing dimers, the ρ and L values are similar to the corresponding values for the uncomplexed Kr species, though there is a significant increase in the small negative L values for the Kr-Cl bond, which probably indicates stabilization of this particular bond in the respective dimers. The L values for the Cl…Y or H…Y interactions in the dimers are negative
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and correlate well with their relative strengths; i.e. the magnitude of the negative L values are generally in the order FKrH > FCl > FKrCl. The relative strengths of the new non-covalent interactions introduced by Z in the Z…FKrCl…Y trimers are also generally reflected by the order of the L values for the Z…F bond, except that the values for the Be-bond are larger than those for the Li-bond; i.e. the negative L values are in the order Z = H2Be > FLi > FH > FCl. This is also the order of the L values for the Cl…Y halogen bond in the trimer complexes. Hence, it would appear that the L values are a good indicator of the relative strength of these intermolecular interactions, as noted in previous work by Duarte et al.35 The ρ and L values for the F-Kr bond in the trimers are similar, but there seems to be an inverse correlation between the smaller negative L values for the Kr-Cl bond and the relative strength of the Z…F interaction; i.e. the weaker Z interactions have larger negative L values, while the stronger interactions have correspondingly smaller values. For example, the L values for the weaker FCl and FH interactions in Z…FKrCl…FH are -0.093 and -0.087 a.u., whereas for the stronger H2Be and FLi interactions, the L values are -0.067 and -0.072 a.u. Table 9 The changes in the atomic charges on the FCl, FKrCl and FKrH molecules, as well as the intermolecular charge transfer, due to dimer and trimer formation can be assessed from the NBO results given in Table 9. The changes in the atomic charges are assumed to be more informative than their absolute values. For all three molecules, dimer formation results in an increase in the negative charge on the F atom and an increase in the positive charge on the H or Cl atom, which is consistent with a shift in electron density from the Cl atom or Kr-Cl/Kr-H bond towards the F atom due to the electric field arising from the molecule Y. There is also some electron density transferred from the lone pairs on Y mainly into the antibonding σ* orbitals of either F-Cl, Kr-Cl or Kr-H. These changes in the atomic charges generally increase with the basicity of Y. However, the shift in electron density away from the Cl atom of FCl or FKrCl by the electric field of the strongly basic NH3
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molecule is counterbalanced by the relatively large electron density transferred (0.16e) from NH3 to FCl/FKrCl. The polarizing effect of Z on the halogen-bonded FKrCl…Y subunit in the trimer is also manifest in the substantially elevated atomic charges shown in Table 9. The F atom of FKrCl becomes more negative, the Kr atom more positive and the Cl atom more positive due to the new interaction. The interaction with Z also results in charge being transferred from the F atom of FKrCl to the Z molecule. The relatively large electron density transferred to the H2Be molecule (into the Be atom’s vacant p orbitals, as previously mentioned) is evident in Table 4 and is consistent with the large perturbations in the FKrCl molecule due to the Be-bonded interaction. To sum up, we have performed energetic calculations for the FKrCl molecule at various levels of theory which suggest that this molecule is metastable and may be amenable to experimental synthesis under cryogenic conditions. Furthermore, we have shown that it forms weak halogenbonded complexes with small molecules like FH and H2O and we compared its computed properties with analogous complexes of its precursor FCl and its FKrH counterpart. The cooperative effect of additional non-covalent interactions introduced at the F end of the FKrCl…Y dimer showed a general strengthening of the intermolecular interactions in the order halogen-bond < hydrogen-bond < beryllium-bond < lithium-bond.
ACKNOWLEDGEMENT The authors wish to acknowledge the support of the School for Graduate Studies at the Cave Hill Campus of the University of the West Indies, for the purchase of the computers used in this work.
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Table 1: Molecular properties of F-Cl, F-Kr-Cl and F-Kr-H computed at the MP2 /6-311++G(d,p) level
Properties
F-Cl
F-Kr-Cl
F-Kr-H
r(F-Cl) or r(Kr-X) /Å r(F-Kr) /Å µ /D ∂µ/∂rFCl or ∂µ/∂rKrX α┴ /a.u α║ /a.u ω(F-Cl) or ω(Kr-X) /cm-1 I(F-Cl) or I(Kr-X) / E /a.u
1.673
2.395 2.026 1.032 -3.89 19.121 92.195 240 11 -3311.23480123
1.480 2.074 6.723 -4.52 12.865 57.578 2112 890 -2852.17506514
1.345 2.22 8.675 19.011 733 20 -559.22593226
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Table 2: Two-body dissociation energy for F-Kr-Cl (to Kr…Cl-F) and atomization energy of F-Kr-Cl computed at various levels of theory. The kinetic barrier for the two-body decomposition is given in parentheses. All energy calculations were performed at the MP2/6311++G(d,p) optimized geometry and are in kJ mol-1.
Method MP2/6-311++G(d,p) MP2/aug-cc-pVTZ B3LYP/aug-cc-pVTZ B3LYP/aug-cc-pVQZ
F-Kr-Cl→ Kr…Cl-F -229.8 (33.1) -207.1 (107.8) -198.4 (191.6) -200.0 (193.5)
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F-Kr-Cl→ F + Kr + Cl -16.8 65.2 49.9 52.1
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Table 3: Computed MP2/6-311++G(d,p) values for Vs,max and Vs,min (in kJ mol-1) of the electrostatic potential for F-Cl and F-Kr-X (X = Cl, H) and Y (Y = FH, OH2, NH3). Species
Vs,max
Species
Vs,min
F-Cl
51.1 (F-Cl)
FH
-108.8 (F)
F-Kr-Cl
17.7 (Kr-Cl)
OH2
-167.8 (O)
F-Kr-H
62.2 (Kr-H)
NH3
-178.7 (N)
F-Kr-Cl
-39.0 (F)
F-Kr-H
-74.8 (F)
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Table 4: Interaction energies (∆E), bond angles (
