Theoretical and experimental studies of water complexes of p- and o-aminobenzoic acid

We report studies of supersonically cooled water complexes of p- and o-aminobenzoic acid with one or two water molecules using two-color resonantly enhanced multiphoton ionization (REMPI) spectroscopy. Density functional theory calculations are carried out to identify structural minima of water complexes in the ground state. According to the calculation, water molecules are bound to both the C=O and -OH groups to form a cyclic hydrogen-bond network in the most stable isomer. Vibrational frequency calculations for the first electronically excited state of the most stable isomer agree well with the experimental observation. On the basis of this agreement, we believe that only one isomer exists in our molecular beam. The frequency shifts of a few normal modes caused by the water molecules further confirm the site of water addition. A surprising observation is that, for OABA(H2O)n complexes, abundant intermolecular vibrational modes are clearly observable in the REMPI spectra, while for PABA(H2O), complexes, these modes are conspicuously missing. A red shift in the transition energy is observed for OABA(H2O)(1), while blue shifts are observed for the rest of the complexes. This difference alludes to the relative stabilities of the water complexes of the two aminobenzoic acids in both the ground and excited electronic states. These observations will be discussed in comparison with those from the meta isomer.