THE JOURNAL OF CHEMICAL PHYSICS 139, 154304 (2013)
High resolution spectroscopy of HCl–water clusters: IR bands of undissociated and dissociated clusters revisited Melanie Letzner,1 Sarah Gruen,1 Daniel Habig,1 Kenny Hanke,1 Torsten Endres,1 Pablo Nieto,1 Gerhard Schwaab,1 Łukasz Walewski,2 Miriam Wollenhaupt,1,2 Harald Forbert,2 Dominik Marx,2 and Martina Havenith1,a) 1 2
Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany Lehrstuhl für Theoretical Chemistry, Ruhr-Universität Bochum, 44780 Bochum, Germany
(Received 21 July 2013; accepted 26 September 2013; published online 15 October 2013) We report a detailed study on the IR spectroscopy of HCl-water complexes in superfluid helium nanodroplets in the frequency range from 2660 to 2675 cm−1 . We have recorded spectra of HCl − H16 2 O as well as of HCl − H18 O complexes and compared these results with theoretical predictions. In ad2 dition, we have carried out mass-selective intensity measurements as a function of partial pressure of HCl as well as of H18 2 O (pick-up curves). The results support a scenario where the IR-absorption in this part of the spectrum contains contributions from undissociated as well as from dissociated clusters with Cl− (H2 O)3 (H3 O)+ being the smallest dissociated complex. These findings are corroborated by additional electric field measurements yielding the orientation of the vibrational transition moment with respect to the permanent dipole moment. As a result we are able to assign a broad absorption band starting at 2675 cm−1 to dissociated HCl-water clusters (HCl)1 (H2 O)n with n ≥ 4. The two narrow absorption lines at 2667.9 cm−1 and 2670 cm−1 are assigned to an undissociated cluster, in agreement with previous studies. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824858] I. INTRODUCTION
Brønsted acid/base dissociation, leading to proton transfer and solvation1, 2 of the charged fragments in nanoenvironments,3–13 is one of the most fundamental chemical processes at the root of a myriad of subsequent reactions. The process plays a central role not only in bulk aqueous chemistry but also in heterogeneous reactions in polar stratospheric clouds,14, 15 the spontaneous synthesis of molecules in interstellar space at ultracold temperatures,16 and dissociative adsorption and ionization at surfaces.17–22 A further focus has been on the atmospherically relevant dissociation of HCl at low temperatures, e.g., on amorphous ice surfaces and on ice nanocrystals.15, 18, 19, 23 Even though these studies led to a progress on the fundamental steps of HCl dissociation on ice,18, 23 the processes at ultracold temperatures remained poorly understood. Laser spectroscopy in combination with molecular beam techniques allows to unravel such solvation processes relevant for acid dissociation on a molecular scale. The first infrared (IR) spectra of HCl-water complexes were reported only a decade ago by Suhm and coworkers.24, 25 They were able to observe several IR bands in an HCl/H2 O/He molecular beam expansion and could assign two IR bands between 2000 and 3000 cm−1 to undissociated HCl(H2 O)n with n = 1 and 2. A prominent and unusually broad absorption band centered at ≈2570 cm−1 was observed. In their paper they stated that “the background probably contains important contributions from complexed H3 O+ Cl− hydronium stretch vibrations.” An IR spectroscopic study of Vilesov and co-workers in the OH stretch region lead to the a) Electronic mail: [email protected]
0021-9606/2013/139(15)/154304/11/$30.00
assignment of the OH stretching bands for the undissociated clusters HCl(H2 O)n with n = 1-3 and (HCl)2 H2 O.26 In theoretical studies27–31 it was postulated that four H2 O molecules might provide the smallest possible hydrogen-bonded water network, in which the most stable structure incorporating HCl is the (charge-separated) solvent-shared ion pair (SIP) H3 O+ (H2 O)3 Cl− ; note that this species is also called a solvent-separated ion pair in the literature. In a recent study using high-resolution mass-selective infrared laser spectroscopy we were able to record the IR spectrum of microsolvated HCl in helium nanodroplets.32 Based upon the experimental data we proposed that successive aggregation of the acid HCl with water molecules, HCl(H2 O)n , results in the formation of hydronium, H3 O+ , for n = 4. Accompanying ab initio simulations32, 33 of the very aggregation process showed that undissociated clusters can assemble by stepwise addition of water molecules in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ring-like undissociated HCl(H2 O)3 complex then spontaneously yields the compact dissociated H3 O+ (H2 O)3 Cl− ion pair even at temperatures of 0.37 K. This new dissociation mechanism, called “aggregation-induced dissociation,” thus bypasses deep local energy minima on the n = 4 potential energy surface by using energy released during a step-by-step aggregation and offers a general paradigm for reactivity at ultracold temperatures. Experimentally, we observed two strong IR absorption bands centered at 2669.854(1) and 2667.804(2) cm−1 on top of a background upon sequential pick-up of HCl and H2 O.32 These signals disappeared when pure HCl or H2 O was added alone to the He droplets. In order to assign the observed bands to a specific cluster, we measured the dependence of the
139, 154304-1
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depletion signal on the partial pick-up pressure of both dopants. The variation with pressure supported an assignment to a cluster aggregated from one HCl and four H2 O molecules. In a subsequent infrared spectroscopic study, Flynn et al.34 reassigned these bands to a cyclic hydrogen-bonded form of the (HCl)2 (H2 O)2 cluster. Their assignment was based on measurements of the pick-up pressure dependence of the spectra, the electric dipole moment, and the vibrational transition moment angle (VTMA) associated with the transition. In all IR studies24, 25, 32, 34, 35 a spectrally broad background signal was observed, which rises with increasing partial water pressure. Flynn et al.34 stated that their results “do not exclude the existence of the charge dissociated forms of (HCl)m (H2 O)n ” and speculated that these “may contribute to the broad background.” In a further paper by Douberly and co-workers, they investigated the pressure dependence and suggested that the broad background “originates at least partially from mixed (HCl)m (H2 O)n clusters with m > 3 and n > 2.”35 Here, we report new IR studies of microsolvated HCl. While an unambiguous assignment of the previous optically mass selective measurements32 was hampered due to the fact that protonated HCl and (H2 O)2 are both detected at the same mass channel, we have now used instead H2 18 O as a solvent. Thus, in the present study we are able to distinguish between HCl, H2 O, and mixed cluster fragments in our mass spectrometer. In addition, we have rebuilt our molecular beam machine to carry out electric field measurements. Based on these measurements, the results of all previous and newly reported measurements can be explained by proposing an overlap of several distinct absorption features: We give now direct experimental evidence that the broad background contains IR absorption bands of the dissociated clusters including those of (H3 O)+ (H2 O)3 Cl− . The two sharp peaks are due to an undissociated cluster, in agreement with the assignment of Flynn et al.34
II. METHODS A. Experimental
We have recorded the IR-absorption bands of (HCl)m (H2 O)n aggregates using the Bochum helium nanodroplet apparatus with an infrared optical parametric oscillator (IR-OPO) as a radiation source. Further details on the experimental set-up are given elsewhere.36 Briefly, the helium nanodroplets were formed by expanding ultra-pure helium gas through a 5 μm diameter cold nozzle into vacuum. The temperature of the nozzle was kept at 16 K by a continuous flow liquid helium dewar. At a stagnation pressure of 50 bars, droplets with an average cluster size of 9000 atoms/droplet were formed. In the pick-up chamber the partial pressures of HCl and H2 O were adjusted by computer controlled leak valves (Leybold MOVE1250). The purity and partial pressures of the dopants were controlled by use of a rare gas analyser (Scientific Instruments SRS RGA 200). The IR laser beam was amplitude-modulated by a chopper at 28 Hz and coupled with the helium droplet beam either (a) in antiparallel configuration to achieve high sig-
J. Chem. Phys. 139, 154304 (2013)
nal to noise ratio or (b) in a perpendicular configuration in case of Stark field measurements by means of parallel goldcoated mirrors in a multipass arrangement. Beam depletion by evaporative cooling was detected using a Pfeiffer QMS 422 quadrupole mass spectrometer, which was either operated in high pass mode with m/z ≥ 8 or set to a single mass channel. The signal from the mass spectrometer was demodulated using a lock-in amplifier to deduce the light-induced beam depletion. As a radiation source we used a homebuilt continuous wave optical parametric oscillator based on a MgO doped periodically poled lithium niobate (PPLN) crystal, which is described in detail by our group.37 The OPO was pumped by a Nd:YAG master oscillator (LightwaveElectronics Model No. 126-1064-500) power amplifier system (Lightwave-Electronics Model No. 126-MOPA), providing an output power of typically 14 W at 1064 nm. The PPLN crystal was kept in an oven (30–200 ◦ C), where the temperature is kept constant within 0.01 ◦ C, to maintain quasi-phase matching conditions. In the crystal, the pump wave is split into a signal and an idler wave. The idler wave has a maximum output power of 1.7 W and a linewidth of 0.0001 cm−1 . The laser is tunable in the region of 2600–3300 cm−1 . The OPO can be continuously tuned over a range of 1.5 cm−1 before a 6.78 cm−1 free spectral range intracavity etalon has to be tilted for frequency adjustment. The idler wavelength is monitored with a Bristol Instruments wavemeter (Model No. 621A-IR). The absolute frequency calibration has an accuracy of 0.1 cm−1 , while the relative frequency calibration is much more precise and only limited by the resolution of the pump laser (0.0001 cm−1 ). In this study we report measurements with isotopically pure H2 18 O (Euriso-Top, 97% purity) and H35 Cl. The H35 Cl synthesis was performed under an atmosphere of dry, oxygen free nitrogen using standard Schlenk techniques. An excess of 10 ml of sulfuric acid (1.1 ml, 20.5 mmol) (SigmaAldrich) was added drop-wise to isotopically pure sodium chloride (1.08 g, 18.6 mmol) (Euriso-Top) i.vac. Byproducts were separated in two cooling traps with iso-propanol/dry ice. The H35 Cl gas was frozen out by means of a liquid nitrogen trap. As a result we obtained a yield of 200 mg (30%, MS/EI: m/z = 36[M]+ ). For Stark spectroscopy, a DC electric field of up to 55 kV/cm was applied between two electrodes positioned perpendicular to the multipass cell and the helium cluster expansion. The laser electric field was aligned either parallel or perpendicular with respect to the Stark field, denoted as Epara or Eperp , respectively. The ratios Apara or Aperp describe the intensity ratio with the electric field on or off for a laser polarization which is either parallel or perpendicular to the applied electric field.
B. Quantum chemical calculations
Accompanying calculations have been carried out using the RI-MP2 method with an aug-cc-pVTZ basis set as implemented in the TURBOMOLE38, 39 program package. The harmonic frequencies and their isotope shifts were obtained by diagonalizing the numerical Hessian matrix weighted with
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J. Chem. Phys. 139, 154304 (2013)
the corresponding masses and are reported without any scaling. It is noted that the harmonic approximation can only provide rough guidelines for comparing to the measured frequencies and isotope shifts, which are known to be subject to anharmonicities and couplings for such rather floppy, hydrogen-bonded complexes. Transition dipole angles are obtained from the normal modes and the dipole derivatives. For the (HCl)2 (H2 O)2 case an extensive search produced 12 local minima of the potential energy surface (see the Appendix for all these structures and the relevant properties). Note that structures I, II, and III reported in the Appendix (corresponding to 1, 2, and 3 in the main text) are not only close in energy (within about 0.5 kJ/mol ∼0.1 kcal/mol relative to the global energy minimum), but also well separated in energy from all other structures (being higher in energy by one order of magnitude, i.e., by about 5 kJ/mol, relative to structure III/3) found in our broad search. We thus consider only these structures in our analysis of the experimental data but include also structure V (corresponding to structure 4 in the text) since that has also been considered earlier34 in the present context. Furthermore, inspection of the relevant properties compiled in Table III shows that none of the other structures comes close to providing a set of frequencies, IR activities, dipole moments, VTMAs, and isotope shifts that would be in line with the known experimental data, see the discussion below. The investigated structures are displayed in Fig. 1 and the relevant properties are compiled in Table I. III. EXPERIMENTAL RESULTS
In helium nanodroplets, aggregation occurs after subsequent pick-up of water and HCl molecules. The pick-up probabilities can be described by Poisson statistics.40 In consequence, even when optimizing pressures for the specific species, e.g., for (HCl)1 (H2 O)4 , it is inevitable that additional
1
4
2
3
5
TABLE I. Computed center frequencies ν, permanent dipole moments μ, vibrational transition moment angles VTMA, and 16 O → 18 O isotopic frequency shifts |ν|oxygen for selected undissociated clusters containing two H2 O and two HCl molecules and dissociated clusters with 4, 5, and 6 water molecules containing a single HCl; the corresponding structures with labels are depicted in Fig. 1. Values in parentheses are taken from Ref. 34.
Structure 1 2 3 4 5 6 7 8
ν (cm−1 )
μ (D)
VTMA (deg)
|ν|oxygen (cm−1 )
2426.9 (2463.2) 2755.1 (2785.0) 2432.6 (2471.8) 2885.5 (2919.4) 2913.7 (2921.5) 2902.7 2907.6 2877.6
0.0 (0.0) 2.80 (2.76) 2.39 (2.37) 2.73 (2.82) 4.16 (4.22) 3.57 3.92 3.43
N/A (N/A) 60.0 (58.4) 90.0 (90.0) 72.9 (72.5) 39.6 (36.6) 0.0 65.2 83.5
High resolution spectroscopy of HCl–water clusters: IR bands of undissociated and dissociated clusters revisited Melanie Letzner,1 Sarah Gruen,1 Daniel Habig,1 Kenny Hanke,1 Torsten Endres,1 Pablo Nieto,1 Gerhard Schwaab,1 Łukasz Walewski,2 Miriam Wollenhaupt,1,2 Harald Forbert,2 Dominik Marx,2 and Martina Havenith1,a) 1 2
Lehrstuhl für Physikalische Chemie II, Ruhr-Universität Bochum, 44780 Bochum, Germany Lehrstuhl für Theoretical Chemistry, Ruhr-Universität Bochum, 44780 Bochum, Germany
(Received 21 July 2013; accepted 26 September 2013; published online 15 October 2013) We report a detailed study on the IR spectroscopy of HCl-water complexes in superfluid helium nanodroplets in the frequency range from 2660 to 2675 cm−1 . We have recorded spectra of HCl − H16 2 O as well as of HCl − H18 O complexes and compared these results with theoretical predictions. In ad2 dition, we have carried out mass-selective intensity measurements as a function of partial pressure of HCl as well as of H18 2 O (pick-up curves). The results support a scenario where the IR-absorption in this part of the spectrum contains contributions from undissociated as well as from dissociated clusters with Cl− (H2 O)3 (H3 O)+ being the smallest dissociated complex. These findings are corroborated by additional electric field measurements yielding the orientation of the vibrational transition moment with respect to the permanent dipole moment. As a result we are able to assign a broad absorption band starting at 2675 cm−1 to dissociated HCl-water clusters (HCl)1 (H2 O)n with n ≥ 4. The two narrow absorption lines at 2667.9 cm−1 and 2670 cm−1 are assigned to an undissociated cluster, in agreement with previous studies. © 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4824858] I. INTRODUCTION
Brønsted acid/base dissociation, leading to proton transfer and solvation1, 2 of the charged fragments in nanoenvironments,3–13 is one of the most fundamental chemical processes at the root of a myriad of subsequent reactions. The process plays a central role not only in bulk aqueous chemistry but also in heterogeneous reactions in polar stratospheric clouds,14, 15 the spontaneous synthesis of molecules in interstellar space at ultracold temperatures,16 and dissociative adsorption and ionization at surfaces.17–22 A further focus has been on the atmospherically relevant dissociation of HCl at low temperatures, e.g., on amorphous ice surfaces and on ice nanocrystals.15, 18, 19, 23 Even though these studies led to a progress on the fundamental steps of HCl dissociation on ice,18, 23 the processes at ultracold temperatures remained poorly understood. Laser spectroscopy in combination with molecular beam techniques allows to unravel such solvation processes relevant for acid dissociation on a molecular scale. The first infrared (IR) spectra of HCl-water complexes were reported only a decade ago by Suhm and coworkers.24, 25 They were able to observe several IR bands in an HCl/H2 O/He molecular beam expansion and could assign two IR bands between 2000 and 3000 cm−1 to undissociated HCl(H2 O)n with n = 1 and 2. A prominent and unusually broad absorption band centered at ≈2570 cm−1 was observed. In their paper they stated that “the background probably contains important contributions from complexed H3 O+ Cl− hydronium stretch vibrations.” An IR spectroscopic study of Vilesov and co-workers in the OH stretch region lead to the a) Electronic mail: [email protected]
0021-9606/2013/139(15)/154304/11/$30.00
assignment of the OH stretching bands for the undissociated clusters HCl(H2 O)n with n = 1-3 and (HCl)2 H2 O.26 In theoretical studies27–31 it was postulated that four H2 O molecules might provide the smallest possible hydrogen-bonded water network, in which the most stable structure incorporating HCl is the (charge-separated) solvent-shared ion pair (SIP) H3 O+ (H2 O)3 Cl− ; note that this species is also called a solvent-separated ion pair in the literature. In a recent study using high-resolution mass-selective infrared laser spectroscopy we were able to record the IR spectrum of microsolvated HCl in helium nanodroplets.32 Based upon the experimental data we proposed that successive aggregation of the acid HCl with water molecules, HCl(H2 O)n , results in the formation of hydronium, H3 O+ , for n = 4. Accompanying ab initio simulations32, 33 of the very aggregation process showed that undissociated clusters can assemble by stepwise addition of water molecules in electrostatic steering arrangements up to n = 3. Adding a fourth water molecule to the ring-like undissociated HCl(H2 O)3 complex then spontaneously yields the compact dissociated H3 O+ (H2 O)3 Cl− ion pair even at temperatures of 0.37 K. This new dissociation mechanism, called “aggregation-induced dissociation,” thus bypasses deep local energy minima on the n = 4 potential energy surface by using energy released during a step-by-step aggregation and offers a general paradigm for reactivity at ultracold temperatures. Experimentally, we observed two strong IR absorption bands centered at 2669.854(1) and 2667.804(2) cm−1 on top of a background upon sequential pick-up of HCl and H2 O.32 These signals disappeared when pure HCl or H2 O was added alone to the He droplets. In order to assign the observed bands to a specific cluster, we measured the dependence of the
139, 154304-1
© 2013 AIP Publishing LLC
154304-2
Letzner et al.
depletion signal on the partial pick-up pressure of both dopants. The variation with pressure supported an assignment to a cluster aggregated from one HCl and four H2 O molecules. In a subsequent infrared spectroscopic study, Flynn et al.34 reassigned these bands to a cyclic hydrogen-bonded form of the (HCl)2 (H2 O)2 cluster. Their assignment was based on measurements of the pick-up pressure dependence of the spectra, the electric dipole moment, and the vibrational transition moment angle (VTMA) associated with the transition. In all IR studies24, 25, 32, 34, 35 a spectrally broad background signal was observed, which rises with increasing partial water pressure. Flynn et al.34 stated that their results “do not exclude the existence of the charge dissociated forms of (HCl)m (H2 O)n ” and speculated that these “may contribute to the broad background.” In a further paper by Douberly and co-workers, they investigated the pressure dependence and suggested that the broad background “originates at least partially from mixed (HCl)m (H2 O)n clusters with m > 3 and n > 2.”35 Here, we report new IR studies of microsolvated HCl. While an unambiguous assignment of the previous optically mass selective measurements32 was hampered due to the fact that protonated HCl and (H2 O)2 are both detected at the same mass channel, we have now used instead H2 18 O as a solvent. Thus, in the present study we are able to distinguish between HCl, H2 O, and mixed cluster fragments in our mass spectrometer. In addition, we have rebuilt our molecular beam machine to carry out electric field measurements. Based on these measurements, the results of all previous and newly reported measurements can be explained by proposing an overlap of several distinct absorption features: We give now direct experimental evidence that the broad background contains IR absorption bands of the dissociated clusters including those of (H3 O)+ (H2 O)3 Cl− . The two sharp peaks are due to an undissociated cluster, in agreement with the assignment of Flynn et al.34
II. METHODS A. Experimental
We have recorded the IR-absorption bands of (HCl)m (H2 O)n aggregates using the Bochum helium nanodroplet apparatus with an infrared optical parametric oscillator (IR-OPO) as a radiation source. Further details on the experimental set-up are given elsewhere.36 Briefly, the helium nanodroplets were formed by expanding ultra-pure helium gas through a 5 μm diameter cold nozzle into vacuum. The temperature of the nozzle was kept at 16 K by a continuous flow liquid helium dewar. At a stagnation pressure of 50 bars, droplets with an average cluster size of 9000 atoms/droplet were formed. In the pick-up chamber the partial pressures of HCl and H2 O were adjusted by computer controlled leak valves (Leybold MOVE1250). The purity and partial pressures of the dopants were controlled by use of a rare gas analyser (Scientific Instruments SRS RGA 200). The IR laser beam was amplitude-modulated by a chopper at 28 Hz and coupled with the helium droplet beam either (a) in antiparallel configuration to achieve high sig-
J. Chem. Phys. 139, 154304 (2013)
nal to noise ratio or (b) in a perpendicular configuration in case of Stark field measurements by means of parallel goldcoated mirrors in a multipass arrangement. Beam depletion by evaporative cooling was detected using a Pfeiffer QMS 422 quadrupole mass spectrometer, which was either operated in high pass mode with m/z ≥ 8 or set to a single mass channel. The signal from the mass spectrometer was demodulated using a lock-in amplifier to deduce the light-induced beam depletion. As a radiation source we used a homebuilt continuous wave optical parametric oscillator based on a MgO doped periodically poled lithium niobate (PPLN) crystal, which is described in detail by our group.37 The OPO was pumped by a Nd:YAG master oscillator (LightwaveElectronics Model No. 126-1064-500) power amplifier system (Lightwave-Electronics Model No. 126-MOPA), providing an output power of typically 14 W at 1064 nm. The PPLN crystal was kept in an oven (30–200 ◦ C), where the temperature is kept constant within 0.01 ◦ C, to maintain quasi-phase matching conditions. In the crystal, the pump wave is split into a signal and an idler wave. The idler wave has a maximum output power of 1.7 W and a linewidth of 0.0001 cm−1 . The laser is tunable in the region of 2600–3300 cm−1 . The OPO can be continuously tuned over a range of 1.5 cm−1 before a 6.78 cm−1 free spectral range intracavity etalon has to be tilted for frequency adjustment. The idler wavelength is monitored with a Bristol Instruments wavemeter (Model No. 621A-IR). The absolute frequency calibration has an accuracy of 0.1 cm−1 , while the relative frequency calibration is much more precise and only limited by the resolution of the pump laser (0.0001 cm−1 ). In this study we report measurements with isotopically pure H2 18 O (Euriso-Top, 97% purity) and H35 Cl. The H35 Cl synthesis was performed under an atmosphere of dry, oxygen free nitrogen using standard Schlenk techniques. An excess of 10 ml of sulfuric acid (1.1 ml, 20.5 mmol) (SigmaAldrich) was added drop-wise to isotopically pure sodium chloride (1.08 g, 18.6 mmol) (Euriso-Top) i.vac. Byproducts were separated in two cooling traps with iso-propanol/dry ice. The H35 Cl gas was frozen out by means of a liquid nitrogen trap. As a result we obtained a yield of 200 mg (30%, MS/EI: m/z = 36[M]+ ). For Stark spectroscopy, a DC electric field of up to 55 kV/cm was applied between two electrodes positioned perpendicular to the multipass cell and the helium cluster expansion. The laser electric field was aligned either parallel or perpendicular with respect to the Stark field, denoted as Epara or Eperp , respectively. The ratios Apara or Aperp describe the intensity ratio with the electric field on or off for a laser polarization which is either parallel or perpendicular to the applied electric field.
B. Quantum chemical calculations
Accompanying calculations have been carried out using the RI-MP2 method with an aug-cc-pVTZ basis set as implemented in the TURBOMOLE38, 39 program package. The harmonic frequencies and their isotope shifts were obtained by diagonalizing the numerical Hessian matrix weighted with
154304-3
Letzner et al.
J. Chem. Phys. 139, 154304 (2013)
the corresponding masses and are reported without any scaling. It is noted that the harmonic approximation can only provide rough guidelines for comparing to the measured frequencies and isotope shifts, which are known to be subject to anharmonicities and couplings for such rather floppy, hydrogen-bonded complexes. Transition dipole angles are obtained from the normal modes and the dipole derivatives. For the (HCl)2 (H2 O)2 case an extensive search produced 12 local minima of the potential energy surface (see the Appendix for all these structures and the relevant properties). Note that structures I, II, and III reported in the Appendix (corresponding to 1, 2, and 3 in the main text) are not only close in energy (within about 0.5 kJ/mol ∼0.1 kcal/mol relative to the global energy minimum), but also well separated in energy from all other structures (being higher in energy by one order of magnitude, i.e., by about 5 kJ/mol, relative to structure III/3) found in our broad search. We thus consider only these structures in our analysis of the experimental data but include also structure V (corresponding to structure 4 in the text) since that has also been considered earlier34 in the present context. Furthermore, inspection of the relevant properties compiled in Table III shows that none of the other structures comes close to providing a set of frequencies, IR activities, dipole moments, VTMAs, and isotope shifts that would be in line with the known experimental data, see the discussion below. The investigated structures are displayed in Fig. 1 and the relevant properties are compiled in Table I. III. EXPERIMENTAL RESULTS
In helium nanodroplets, aggregation occurs after subsequent pick-up of water and HCl molecules. The pick-up probabilities can be described by Poisson statistics.40 In consequence, even when optimizing pressures for the specific species, e.g., for (HCl)1 (H2 O)4 , it is inevitable that additional
1
4
2
3
5
TABLE I. Computed center frequencies ν, permanent dipole moments μ, vibrational transition moment angles VTMA, and 16 O → 18 O isotopic frequency shifts |ν|oxygen for selected undissociated clusters containing two H2 O and two HCl molecules and dissociated clusters with 4, 5, and 6 water molecules containing a single HCl; the corresponding structures with labels are depicted in Fig. 1. Values in parentheses are taken from Ref. 34.
Structure 1 2 3 4 5 6 7 8
ν (cm−1 )
μ (D)
VTMA (deg)
|ν|oxygen (cm−1 )
2426.9 (2463.2) 2755.1 (2785.0) 2432.6 (2471.8) 2885.5 (2919.4) 2913.7 (2921.5) 2902.7 2907.6 2877.6
0.0 (0.0) 2.80 (2.76) 2.39 (2.37) 2.73 (2.82) 4.16 (4.22) 3.57 3.92 3.43
N/A (N/A) 60.0 (58.4) 90.0 (90.0) 72.9 (72.5) 39.6 (36.6) 0.0 65.2 83.5
