Structures and Anharmonic Analyses of the O–H Stretching Vibrations of Jet-Cooled Benzoic Acid (BA), (BA)(H₂O)_n, and (BA)₂(H₂O)_n (n = 1, 2) Clusters, and Their Ring-Deuterated Isotopologues Measured with IR-VUV Spectroscopy─Unraveling the Complex Anharmonic Couplings in the Cyclic Structures

Abstract

The IR spectra of benzoic acid (BA), (BA)­(H2O) n and (BA)2(H2O) 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)­(H2O) and (BA)­(H2O)2, only a single isomer was observed for each species, whereas for (BA)2(H2O) and (BA)2(H2O)2, more than one isomers were present. The observed IR spectra were very complex and showed similar structures between (BA) m (H2O) n and their ring-deuterated isotopologues (BA-d 5) m (H2O) 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 H2O 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.

Aspartame and Its Microhydrated Aggregates Revealed by Laser Spectroscopy: Water–Sweetener Interactions in the Gas Phase

“…The band at 3718 cm –1 corresponds to the stretching vibration of the free OH, whereas the one at 3524 cm –1 is assigned to the stretch of the water’s OH that participates in the hydrogen bond with the carboxylic acid. A similar displacement to lower frequencies of the “bridge” hydrogen was also observed in the aggregates of benzoic acid and 3-indole propionic acid with one water molecule.…”

“…Nonetheless, due to an intense cooperative and anharmonic effect, the acid OH vibration is strongly shifted to lower frequencies, and it is predicted to appear as a very broad absorption around 2950 cm –1 . A similar situation is observed for the broad band around 3172 cm –1 , since this one is predicted to be the OH stretch of the water in bridged position and its combination bands, as observed in benzoic acid·water 2 and 3-indole propionic acid·water 2 aggregates. , …”

“…Comparison with the computational predictions led us to assign the most energetic absorption to the O–H stretching mode (3587 and 3588 cm –1 in conformer A and B, respectively), in good agreement with previously reported benzoic acid and tyrosine experimental spectra, under similar experimental conditions. The band corresponding to the antisymmetric stretching mode of the –NH 2 group lies above 3425 cm –1 .…”