Structures and Anharmonic Analyses of the O-H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H 2 O) n , and (BA) 2 (H 2 O) n ( n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures.

The IR spectra of benzoic acid (BA), (BA)(H 2 O) n and (BA) 2 (H 2 O) n ( n = 1, 2) clusters, and their ring-deuterated isotopologues in the 2800-3750 cm -1 region were measured with IR-vacuum ultraviolet spectroscopy under the jet-cooled condition. For (BA)(H 2 O) and (BA)(H 2 O) 2 , only a single isomer was observed for each species, whereas for (BA) 2 (H 2 O) and (BA) 2 (H 2 O) 2 , more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA) m (H 2 O) n and their ring-deuterated isotopologues (BA- d 5 ) m (H 2 O) n for specific values of m and n . The anharmonic analysis based on the vibrational second-order perturbation theory indicated that the complexity of the IR spectra in these clusters was due to the appearance of many bands of (i) the overtone and combination modes involving the O-H bend of H 2 O and the in-plane C-O-H bends and the C═O stretch of BA, and (ii) the combination modes involving the hydrogen-bonded O-H stretch and low-frequency intermolecular vibrations, with considerable intensities.