Acidic protons before take-off: A comparative jet Fourier transform infrared study of small HCl– and HBr–solvent complexes

Abstract

A comprehensive study of the proton vibrational dynamics in isolated (HX)m⋅(M)n (X=Cl, Br, and M=H2O, D2O, CH3OH) clusters is presented. Mid-IR (1900–3900 cm−1) spectra are measured in pulsed, seeded slit nozzle expansions. A close similarity among the HCl and HBr systems is revealed and exploited for cluster assignment. Distinct absorptions by small HX and mixed HX–M clusters are superimposed on broadbands, partly due to electrolytically dissociated HX in larger mixed clusters. Assignments of bands corresponding to mixed dimers and trimers based on their X–H and O–H stretching fundamentals are provided. The broad Cl–H stretching band profile of the 1:2 complex is indicative of sub-picosecond intracluster vibrational energy redistribution. The assignments are compared to matrix isolation experiments, when available. Systematic quantum chemistry calculations are performed for HCl–water complexes with up to three molecules and their isotopomers. Hydrogen bond induced frequency shifts of Cl–H stretching vibrations are best described at the coupled-cluster [CCSD(T)] level of theory. Lower level correlated quantum-chemical calculations clearly overestimate Cl–H frequency shifts. The low abundance of mixed clusters in the size range close to electrolytic dissociation (n=3–5) is discussed.

Perturbation Approach for Computing Infrared Spectra of the Local Mode of Probe Molecules

“…Interestingly, the composite result is in better accord with experimental results than CCSD(T)-F12b, perhaps due to fortuitous error cancellations. Furthermore, the computed fwhm from the composite strategy is 21 cm −1 , also in good agreement with experimental results (18 cm −1 ), 16,17 and consistent with pure CCSD(T)-F12b and M06-2X calculations. The agreement between theory and experimental results both in peak position and in line shape suggests that the QVP1 composite approach can be used for studying IR spectra of probe molecules in the condensed-phase and in biomolecular systems.…”

“…Figure 5 depicts the line shape of the main peak, which is in good agreement with the experimental peak. 16,17 The peak position is centered at 2634 cm −1 , which is 54 cm −1 greater than the experimental value. The calculated line shape is similar to that from experimental results, indicated by the blue arrow of the spectrum in red.…”

“…Figure 3b shows that the maximum peak position from QVP1 using CCSD(T)-F12b is only 14 cm −1 greater than the experimental value (2723.5 cm −1 ). 16,17 The M06-2X frequency in the HCl(H 2 O) complex is blue-shifted by 84.6 cm −1 from the coupled-cluster value, and this is mainly due to the difference in the reference state between the two methods: 80.6 cm −1 from 2650.5 cm −1 (CCSD(T)-F12b) to 2569.9 cm −1 (M06-2X). Thus, the solvatochromic shifts are not very sensitive to the quantum chemical models used, here, differing only by 4 cm −1 (84.6 vs 80.6 cm −1 ).…”

“…Experimentally, Suhm and co-workers reported the first IR spectrum of HCl–water clusters 16,17 and found that a minimum of four water molecules is required for HCl dissociation. Recently, Havenith and co-workers confirmed that the broad band at 2675 cm −1 is due to the dissociated cluster, H 3 O + (H 2 O) 3 Cl − .…”

Communication: A new ab initio potential energy surface for HCl–H2O, diffusion Monte Carlo calculations of D and a delocalized zero-point wavefunction

“…4,23 A building block to understanding the clusters at the four-water limit and beyond is a detailed knowledge of even smaller HCl-H 2 O clusters, the first of which is the HCl-H 2 O dimer. The dimer, which is of great interest in its own right, has been firmly identified using a variety of techniques including velocity-map imaging with resonanceenhanced multiphoton ionization, 1, 2 infrared studies in liquid helium, 3,4,24,25 microwave spectroscopy, 26,27 ragout-jet FTIR 28,29 cavity ringdown spectroscopy, 30 and infrareda) Email: jmbowma@emory.edu matrix isolation spectroscopy. [31][32][33][34][35] Of these studies Refs.…”