J Mol Model (2014) 20:2102 DOI 10.1007/s00894-014-2102-2

ORIGINAL PAPER

Characterization of halogen···halogen interactions in crystalline dihalomethane compounds (CH2Cl2, CH2Br2 and CH2I2): a theoretical study Mehdi D. Esrafili & Mahshad Vakili & Mohammad Solimannejad

Received: 23 October 2013 / Accepted: 30 November 2013 / Published online: 11 February 2014 # Springer-Verlag Berlin Heidelberg 2014

Abstract A theoretical study was performed to examine halogen···halogen (X···X) bonds properties in crystalline dihalomethane CH2X2 compounds (X=Cl, Br and I). MP2/ aug-cc-pVTZ calculations reveal that the interaction energies for CH 2X 2 dimers lie in the range between −3.7 and −9.9 kJ mol−1. One of the most important results of this study is that, according to symmetry-adapted perturbation theory, the X···X interactions are largely dependent on dispersion interactions. The contribution of electrostatic energy in the X···X interaction increases in the order Cl < Br < I, which is consistent with the greater amount of positive electrostatic potential on the halogen atom. Keywords Dispersion . Electrostatic potential . Halogen bond . QTAIM . SAPT

Introduction Noncovalent interactions are of primary significance in various fields of chemistry and biochemistry. They are responsible for stabilizing many important molecules, for example, DNA and proteins [1–6]. The most common of these is hydrogen bonding interaction, frequently defined as a Z−H···Y contact, where Z and Y are electronegative elements and Y possesses Electronic supplementary material The online version of this article (doi:10.1007/s00894-014-2102-2) contains supplementary material, which is available to authorized users. M. D. Esrafili (*) : M. Vakili Laboratory of Theoretical Chemistry, Department of Chemistry, University of Maragheh, P.O. Box: 5513864596, Maragheh, Iran e-mail: [email protected] M. Solimannejad Quantum Chemistry Group, Department of Chemistry, Faculty of Sciences, Arak University, Arak 38156-8-8349, Iran

one or more lone electron pairs. A halogen bond can be described in general by R−X···B interaction, in which X is an electrophilic halogen atom (typically chlorine, bromine, or iodine) and B is any negative site, including Lewis bases, π regions of aromatics or molecules containing double bonds, and anions [7–10]. The R−X···B angle is typically close to 180°, which suggests that the halogen bond is a highly directional interaction. The directionality of this interaction is in general greater than that of the hydrogen bonding [11]. Although the halogen atom X as well as halogen bond electron donor B involved in a halogen bond may have a net negative charge, the stability of the halogen bond is explained by the existence of an electropositive crown at the top of the halogen atom directed toward the electron donor. This positive potential that results from polarization of the halogen along its covalent bond extension, is referred to as a “σ-hole” [12–16]. Extensive theoretical calculations [17–20] and experimental results [21–23] consistently reveal that the greater the polarizability and the lower the electronegativity of a halogen atom, the more positive its σ-hole and the stronger the halogen bond to which it gives rise. On the other hand, the positive region is frequently surrounded by a belt of negative electrostatic potential, which accounts for the possibility of interacting laterally with a positive site. Recently, several groups have reported weak closed-shell bonding interactions between halogens on the basis of the structural and energetics properties [24–28]. Extensive studies show that there are two distinct types of angular preferences for halogen···halogen interactions [23]. The first possibility occurs when θ1=θ2, where θ1 and θ2 are the C–X···X and X···X–C angles, respectively. The second geometry arises when θ1 =180° and θ2 =90°; the perpendicular arrangement; this is simply halogen bonding interaction where the negative site is the lateral side of a halogen (Scheme 1): It has been frequently demonstrated that for the X···X interactions of type I and II geometries, the former is believed

2102, Page 2 of 6

Scheme 1 Classification of halogen-halogen interactions

to be a result of close packing, whereas the latter is ascribed to the polarization of adjacent halogen atoms [29, 30]. It should be noted that most of the studies on X···X interactions in the literature are restricted to the heavier halogens, since they have higher polarizabilities [31]. As a result of a combination of extreme electronegativity and limited polarizability, the F atom is frequently deemed to not participate in X···X bonding. However, it has recently been shown that the fluorine atom has the capability of forming noncovalent F···F bonds and can also affect recognition and self assembly processes, but only under specific circumstances [32, 33]. The potential of halogen···halogen bonding shown by useful applications in the fields of synthetic chemistry, material science, and bioorganic chemistry was reported recently by Awwadi and co-workers [34]. According to our knowledge, the X···X interactions have received less attention especially from the theoretical point of view. In this paper we report the results of quantum chemical calculations of the strength and characteristics of the intermolecular interactions in the crystalline dihalomethane CH2X2 compounds (X=Cl, Br and I). The nature of X···X interaction is analyzed using parameters derived from topological analysis of its electron density and an energy decomposition analysis. Such a theoretical study may provide some useful information of the nature and strength of X···X interactions, which would be very important for the design and synthesis of new materials and effective drugs containing halogenated compounds.

J Mol Model (2014) 20:2102

calculation of the X···X bond energies. The quantum theory of atoms in molecules (QTAIM) [42] was performed with the help of AIM 2000 software [43] using the wave functions generated at the M06-2X/aug-cc-pVTZ(-PP) level. Surface electrostatic potentials were calculated at the same level of theory using WFA code [44]. In order to analyze halogen···halogen bonds properties in terms of meaningful physical components, interaction energies were decomposed using symmetry-adapted perturbation theory (SAPT) [45] by means of SAPT2008 program [46]. In this study, we have employed the so-called SAPT2 approximation to the intermolecular interaction energies: ð10Þ

ð10Þ

ð20Þ

ð20Þ

ð20Þ

ESAPT2 ¼ E elst þ E exch þ E ind;resp þ E exch−ind;resp þ Edisp þ int ð20Þ

ð12Þ

E exch−disp þ E elst;resp þ t Eð22Þ þ t E ð22Þ þ E11 exch þ ðHF Þ

E 12 exch þ δE int;resp ;

ð1Þ

where ð10Þ

ð10Þ

ð20Þ

ð20Þ

HF δEHF int;resp ¼ E int −E elst −E exch −E ind;resp −E exch−ind;resp :

The right side of Eq. (1) is a sum of perturbative energy correction terms that are the consequences of various physical interaction forces. These energy correction terms, except for the very last one, were collected into four fundamental physical components: electrostatic (Eelst), exchange (Eexch), induction (Eind), and dispersion (Edisp) as follows: ð10Þ

ð12Þ

Eelst ¼ E elst þ E elst;resp ð10Þ

ð11Þ

ð3Þ ð12Þ

Eexch ¼ E exch þ Eexch þ E exch ð20Þ

ð4Þ ð20Þ

Eind ¼ Eind;resp þ t Eð22Þ þ E exch−ind;resp þ t Eð22Þ ð20Þ

ð20Þ

Edisp ¼ E disp þ E exch−disp :

ð5Þ ð6Þ

Computational details All quantum chemical calculations were carried out with the GAMESS suite of programs [35]. The crystalline structures of CH2X2 were adopted from X-ray diffraction data [36, 37]. Since the positions of hydrogen atoms are not located accurately by X-ray diffraction, a geometry optimization of just the hydrogen atoms in the structure was needed. Partial geometry optimizations were carried out using M06-2X and B97D density functionals, which have been widely used to study noncovalent interactions [38, 39]. In this study, the standard aug-cc-pVTZ basis set was used for hydrogen, carbon, chlorine, and bromine atoms. For iodine atom, the aug-cc-pVTZPP [40] basis set was used. The counterpoise method [41] was used to correct the basis set superposition error in the

The above grouping scheme in SAPT2 analysis is used because it was indicated that such a scheme is more appropriate and easier to interpret than other possible schemes [47].

Results and discussion To give a comprehensive description of the intermolecular interactions involved in the crystalline CH2X2 compounds, we first discuss the electrostatic potentials and geometries of the three compounds characterized by X-ray diffraction. We then use quantum chemical calculations to gain a detailed insight into the strength and origin of the interactions. Unless

J Mol Model (2014) 20:2102

otherwise noted, the following results refer to the M06-2X/ aug-pVTZ(-PP) level of theory. Surface electrostatic potentials and geometries Figure 1 presents the molecular electrostatic potentials V(r) on the surface of the isolated CH2Cl2, CH2Br2, and CH2I2 molecules, which are computed on the 0.001 au contour of the molecule’s electronic density. Table 1 lists the most positive (VS,max) and the most negative (VS,min) electrostatic potentials on X atom of the CH2X2 molecules. The most striking feature of Fig. 1 is a positive electrostatic potential cap at the end region of the X atom along the C−X bond vector, which is surrounded by an electroneutral area and, next, a large electronegative domain. Such halogen positive region is referred as the “σ-holes”, because it is centered on the C–X axis and is surrounded by a negative electrostatic potential. This positive region can interact with any negative site, thereby giving rise to a highly directional interaction. From Table 1, the σ-hole of atom X becomes more positive as follows: I > Br > Cl, which is the reverse of the electronegativity sequence Cl > Br > I. On the other hand, VS,min of X becomes more negative along the sequence X=Cl > Br > I. Therefore, it may be concluded that the weakness of the X···X interactions in solid CH2X2 can be rationalized by looking at the general range of VS,min values on the lateral sides of the X (−22.6 to −26.4 kJ mol−1) compared to, for example, the negative potential of the ammonia (−165.3 kJ mol−1). The graphical illustration of the crystalline CH2X2 under consideration is depicted in Fig. 2. The X···X interaction is between the positive σ-hole on one X atom and the negative lateral side of another, illustrating the point discussed above. It is also seen from Fig. 2 that the bond distances of the X···X interactions increase in the order Cl < Br < I. The distances are slightly shorter than the sum of vdW radii of the respective atoms [49], which are consistent with these being weak noncovalent interactions. In all the cases, the X···X contacts belong to the type II interaction since the two XXC angles are approximately linear (169–179°). An interesting aspect of the solid-state structure of CH2Cl2 is that the self-assembled CH2Cl2 dimers are interlinked to each other through two unconventional Cl···H hydrogen bonding interactions. The Cl···H bonds distances are slightly greater than the sum of

Fig. 1 Surface electrostatic potential maps for isolated CH2X2 molecules

Page 3 of 6, 2102 Table 1 Calculated electrostatic potentials (in kJ mol−1) for isolated CH2X2 monomers

Molecule

VS,max

VS,min

CH2Cl2 CH2Br2 CH2I2

37.2 59.4 89.1

−26.4 −24.3 −22.6

the vdW radii of the Cl and H atoms (2.95 Å) [48], hence, they do not contribute significantly to the stabilization of the crystal packing. However, the two hydrogen bonds cause the Cl···Cl interactions to deviate slightly from linearity. The structure of CH2Br2 presents two different Br···Br interactions with distances of 3.62 and 3.49 Å. These are, respectively, 2 and 6 % shorter than the sum of the vdW for two Br atom (3.70 Å). In the crystalline structure, the two CH2I2 molecules are linked

Fig. 2 Crystalline structure of dihalomethanes : a CH2Cl2, b CH2Br2 and c CH2I2. Bond distances in Å

2102, Page 4 of 6

J Mol Model (2014) 20:2102

Table 2 QTAIM results (in au) for crystalline CH2X2 compoundsa Interaction CH2Cl2 Cl···Cl H···Cl CH2Br2a Br···Br(4) Br···Br(5) CH2I2 I···I a

ρBCP

∇ ρBCP GBCP 2

VBCP

HBCP

−GBCP/ VBCP

0.009 0.034 0.004 0.013

0.007 −0.005 0.002 1.355 0.003 −0.002 0.001 1.319

0.010 0.031 0.008 0.025

0.007 −0.005 0.001 1.220 0.005 −0.004 0.001 1.280

0.009 0.023

0.005 −0.004 0.001 1.165

The value within the parenthesis refers to molecule number in Fig. 2

by a I···I interaction (3.81 Å), forming a dimeric unit. This unit is further connected with other CH2I2 molecules, expanding the structure along crystallographic directions. QTAIM analysis According to the QTAIM [42], properties of bond critical points (BCPs) are widely used to characterize covalent and noncovalent interactions, for instance via correlations between the electron density at the BCP and the strength of interaction. The electron density ρBCP at the X···X critical points, the Laplacian of the electron density ∇2ρBCP, the total electron energy density HBCP, and the absolute ratio of kinetic and potential energies densities −GBCP/ VBCP for the title complexes are presented in Table 2. Figure S1 (Supporting information) clearly shows the existence of a BCP for each X···X interaction, accompanied by a bond path between the two corresponding atoms. It can be seen that the values of ρBCP are calculated to be in a range of 0.008–0.010 au, whereas the values of ∇2ρBCP are all positive, ranging from 0.025 to 0.034 au. This indicates very little sharing between the two atomic basins, which leads one to anticipate small delocalization between the basins of the two corresponding atoms. We noted, however, the ρBCP value is slightly higher for I···I and Br···Br bonds compared to the Cl···Cl interactions. This confirms the idea that the strength of the X···X bond is connected with the anisotropy of the

electron charge distribution of X atom (as a Lewis acid). The electron density at the Cl···Cl critical points compare well to those reported in the literature, for example, dichloroacetic acid [26]. For the Cl···H interactions of CH2Cl2, it can be seen that the values of ρBCP are calculated to be 0.004 au, whereas the values of ∇2ρBCP are positive (0.013 au). These values are within the commonly accepted values for hydrogen bonding interactions [49], thus indicating the closed-shell interaction in the solid CH2Cl2. Arnold and Oldfied [50] pointed out that electronic energy density (H BCP ) is an appropriate index to understand noncovalent interactions, and its sign at the BCP determines whether the interaction is electrostatic dominant (HBCP >0) or covalent dominant (HBCP