A Theoretical and Experimental Study of Water Complexes of m-Aminobenzoic Acid MABA·(H₂O)_n (n = 1 and 2)

We report studies of supersonically cooled water complexes of m-aminobenzoic acid MABA.(H(2)O)n (n = 1 and 2) using two-color resonantly enhanced multiphoton ionization (REMPI) and UV-UV hole-burning spectroscopy. Density functional theory calculations are also carried out to identify structural minima of water complexes in the ground state. For the most stable isomers of both complexes, water molecules bind to the pocket of the carboxyl group in a cyclic hydrogen bond network. Vibrational frequency calculations for the first electronically excited state (S(1)) of these isomers agree well with the experimental observation. The addition of water molecules has a major impact on the normal mode that involves local motion of the carboxyl group, while negligible effects are observed for other normal modes. On the basis of the hole-burning experiment, two major isomers for each complex are identified, corresponding to the two conformers of the bare compound. Compared with the other two isomers of aminobenzoic acid, the red shifts of the origin bands due to water complexation in MABA are considerably larger. Similar to p-aminobenzoic acid and different from o-aminobenzoic acid, the existence of the intermolecular stretching mode is ambiguous in the REMPI spectrum of MABA.(H(2)O)n.