Vibrational dynamics of the hydrogen bond in H 2 S–HF: Fourier-transform-infrared spectra and ab initio theory Pierre Asselin, Pascale Soulard, Bruno Madebe`ne, M. Esmail Alikhani and Marius Lewerenz* Received 16th December 2005, Accepted 14th February 2006 First published as an Advance Article on the web 8th March 2006 DOI: 10.1039/b517814b The rotationally resolved infrared spectrum of the hydrogen bonded complex H 2 S–HF and of its isotopomer D 2 S–DF in the HF/DF stretching range have been observed in a supersonic jet Fourier-transform infrared (FTIR) experiment and indicate a predissociation lifetime of 130 ps for H 2 S–HF. Complementary spectra taken at a temperature of 190 K in a cell without resolved rotational structure indicate the presence of strong anharmonic couplings between low frequency intermolecular modes and the HF donor stretch mode previously observed in other complexes with heavier acceptor molecules without rotational fine structure. The anharmonic analysis of the hot band progressions and of the rotational data confirm the coupling mechanism. The coupling constants and the absolute frequency of the hydrogen bonded stretch mode are in excellent agreement with theoretical predictions based on adiabatic variational calculations on potential surfaces computed at MP2 and CCSD(T) level. Complementary calculations with a perturbational approach further confirm the coupling model. 1. Introduction Hydrogen bonded dimers like the HF dimer, 1 HCl dimer 2–4 and the water dimer 5–7 have been and still are the subject of extensive experimental and theoretical studies of very high quality. These studies have provided very important insight into the dynamics of the hydrogen bond. Dimers formed from two different small monomers, however, have received a lot less attention, in part due to experimental difficulties, but are probably more relevant to understanding solvation in polar liquids and proton transfer reactions. 8,9 While microwave data on complexes like HF–H 2 O, 10–12 HCl–H 2 O, 13 HF–H 2 S, 14,15 and HCl–H 2 S 16 have provided reliable structural information and some estimates of low frequency intermolecular modes, vibrational spectra of the predissociating stretching states are scarce and remain limited to water complexes 8,9,17–20 except for a matrix isolation study of H 2 SHF in solid argon. 21 The authors observed a strong n s HF stretching mode at 3652 cm 1 and two modes assigned as librational n l motions (hindered rotations) of HF relative to H 2 S. They concluded that the complex has a pyramidal structure. In addition, a weak stable complex HFHSH where HF acts as an acceptor was characterized by a much less red-shifted single n(H–F) absorption at 3799 cm 1 , and the presence of a H 2 S–(HF) 2 complex was revealed by n(H b –F) and n(H a –F) stretching modes above and below n s . The large red shift of n s (H–F) relative to the HF monomer frequency of 3961 cm 1 , Dn s = 267 cm 1 , indicates a relatively strong intermolecular bond. The formation of a sufficient dimer density in simple thermodynamic equilibrium under properly chosen conditions should therefore be possible, allowing the use of ordinary gas phase spectroscopic techniques. Previous Fourier transform infrared (FTIR) studies of medium strength hydrogen bonded one to one complexes between thiirane and oxirane and deuterium fluoride and hydrogen fluoride carried out in our group, 22–24 have shown the presence of significant anharmonic couplings between intra- and intermolecular vibrational modes, consistent with a stiffening of hydrogen bond modes upon excitation of the donor stretch mode. The deconvolution of the observed bands resulted in homogeneous widths providing a lower bound for the predissociation lifetime but probably containing significant IVR contributions due to the high density of states. A natural extension of this work is the present combined experimental and theoretical analysis of the smaller but essentially analo- gous systems H 2 S–HF and D 2 S–DF which are attractive for several reasons: (1) the small size of the system reduces spectral congestion through IVR related processes and the elevated rotational constants allow the observation of resolved rotational struc- ture thereby providing a direct pathway to lifetime estimates for donor stretch excitation without deconvolutions and further confirmation for vibrational coupling through the contraction of the complex upon HF excitation; (2) the inhomogeneous hot band progressions can be com- pletely separated at cell temperatures around 190 K; (3) high level ab initio explorations of the potential surface are possible for this complex. The analysis of this surface allows a very detailed comparison with experiments and the evaluation of several strategies to account for anharmonicity and multidimensional vibrational coupling. Universite ´ Pierre et Marie Curie-Paris 6, CNRS, Laboratoire Dynamique, Interactions et Re´activite´, UMR 7075, Case 49, Place Jussieu, 75252 Cedex Paris, France. E-mail: lewerenz@spmol.jussieu.fr This journal is  c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 1785–1793 | 1785 PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics