Far-Infrared Investigation of the Benzene-Water Complex: The Identification of Large-Amplitude Motion and Tunneling Pathways.

The far-infrared spectrum of the weakly OH···π hydrogen-bonded benzene-water complex has been studied in neon and argon matrices, below 30 K. The in-plane water libration has been observed in both neon and argon for H2O and D2O complexed with C6H6 and C6D6 but not for the corresponding complexes involving HDO. Both H2O and D2O can tunnel between the two possible hydrogen bonds. This is not possible for HDO. The reported far-infrared observations have implications for the interpretation of the previously obtained molecular beam microwave spectrum of the benzene-water complex.

Communication: THz absorption spectrum of the CO2-H2O complex: observation and assignment of intermolecular van der Waals vibrations.

Terahertz absorption spectra have been recorded for the weakly bound CO2-H2O complex embedded in cryogenic neon matrices at 2.8 K. The three high-frequency van der Waals vibrational transitions associated with out-of-plane wagging, in-plane rocking, and torsional motion of the isotopic H2O subunit have been assigned and provide crucial observables for benchmark theoretical descriptions of this systems' flat intermolecular potential energy surface. A (semi)-empirical value for the zero-point energy of 273 ± 15 cm(-1) from the class of intermolecular van der Waals vibrations is proposed and the combination with high-level quantum chemical calculations provides a value of 726 ± 15 cm(-1) for the dissociation energy D0.

Hydrogen-bonded OH stretching modes of methanol clusters: a combined IR and Raman isotopomer study.

A comprehensive study of the OH and OD stretching fundamentals in clusters of methanol and its isotopomers CH(3)OD, CD(3)OH, and CD(3)OD provides detailed insights into the hydrogen-bond mediated coupling as a function of cluster size. The combination of infrared and Raman supersonic jet spectroscopy enables the observation and assignment of all hydrogen-bonded OH stretching modes of isolated methanol trimer and methanol tetramer. A consistent explanation for the spectral complexity observed more than a decade ago in methanol trimer in terms of low-frequency methyl umbrella motions is provided. Previous explanations based on cluster isomerism or anharmonic resonances are ruled out by dedicated jet experiments. The first experimental lower bound for concerted quadruple proton transfer in S(4) symmetric methanol tetramer is derived and compared with theoretical predictions. The observed isotope effects offer insights into the anharmonicity of the localized OH bond. The performance of harmonic B3LYP and MP2 calculations in predicting hydrogen-bond-induced spectral shifts and couplings is investigated.

Observation and rovibrational analysis of the intermolecular HCl libration band upsilon6(1) of HCN-HCl, DCN-HCl and H13CN-HCL.

The high-resolution far-infrared absorption spectrum of the intermolecular HC1 libration band upsilon6(1) (upsilonB) of the gaseous molecular complex H12CN-HCl and the two isotopically substituted species H13CN-HCl and D12CN-HCl is recorded by means of static gas-phase Fourier transform far-infrared spectroscopy at 205 K using an electron storage ring source. The rotational structure of the upsilon6(1) band has the typical appearance of a perpendicular type band of a linear polyatomic molecule. The structure is analyzed using a standard semi-rigid linear molecule model including l-type doubling to yield the band origin upsilon0, together with values for the upper state rotational constant B', the upper state quartic centrifugal distortion constant D'j and the value for the l-type doubling constant q6. The values for the ground-state spectroscopic constants B" and D"j for D12CN-H35Cl and H13CN-H35Cl are determined for the first time by ground state combination difference analyses. A number of upsilon6(1) + upsilon7(1) - upsilon7(1) and upsilon6(1) + 2upsilon7(2) - 2upsilon7(2) hot bands are observed in the spectra and the sum of the anharmonicity constants X(6,7) + g(6,7) is estimated. The observed decrease of the rotational constant B together with the simultaneous increase of the quartic centrifugal distortion constant Dj upon excitation of the HCl libration mode indicate that the hydrogen bond in the molecular complex is significantly destabilized upon intermolecular vibrational excitation. The calculated harmonic force constants for the intermolecular hydrogen bond stretching vibration upsilon(sigma) for the ground state and the excited HCl libration state indicate that the excitation of the HCl libration mode destabilizes the intermolecular interaction between HCN and HCl by almost 20%. The hydrogen bond is elongated by 0.030 A upon excitation of the upsilon6(1) mode.