Fourier transform infrared spectroscopy and ab initio theory of acid-hydrogen sulfide clusters: H2S-HCl, D2S-DCl and H2S-(HCl)(2).

The rotationally resolved Fourier transform infrared (FTIR) spectrum of the nu(s) HCl and DCl stretching bands for the hydrogen bonded complex H2S-HCl and its isotopomer D2S-DCl have been observed in a supersonic jet at 0.02 cm(-1) resolution. In the same experimental conditions, two additional bands observed without rotational structure in the HCl range of the dimer have been assigned to the cyclic trimer H2S-(HCl)(2). The multidimensional coupling picture involving the donor stretch mode nu(s) and low frequency intermolecular modes already evidenced in several medium strength hydrogen bonded complexes is beautifully confirmed by the observation of completely separated hot band progressions in the 198 K cell spectrum of both dimers. Based on our anharmonic adiabatic approach for the treatment of the coupled vibrations, absolute vibrational frequencies, diagonal and off-diagonal anharmonicities as well as rovibrational coupling constants obtained from analyses of several 2-D subspaces at MP2 and CCSD(T) level are in excellent agreement with spectroscopic results. In the case of small light complexes, the combination of elevated rotational constants and a negligible contribution of intramolecular vibrational redistribution (IVR) improve the reliability of predissociation lifetime measurements, estimated to 180 ps for H2S-HCl and above 200 ps for D2S-DCl.