Laser mass-resolved spectroscopy and theoretical study of methyl-p-aminobenzoate(H2O)n (n=2,3,4) complexes

A combined computational and experimental study of the methyl-p-aminobenzoate(H2O)(n), (n=2,3,4) complexes [MAB(H2O)(n)] is reported. Complexes potential energy surfaces were explored by ab initio density functional theory (DFT) methods, at the B3LYP/6-31G level, and the stable isomer structures and vibrational modes further computed at the B3LYP/6-31+G* level. A set of self-contained experimental techniques, including laser induced fluorescence (LIF), resonance enhanced multiphoton ionization mass-resolved spectroscopy (REMPI), two-color resonance enhanced multiphoton ionization mass-resolved spectroscopy (R2PI), "hole burning" spectroscopy (HB), and two-color ionization thresholds were used to study the spectra and other physical features of the complexes. Of the three title complexes only MAB(H2O)(4) has been observed with our experimental methods, while the MAB(H2O)(3) was formed by evaporation and MAB(H2O)(2) was not detected at all. It has been shown that the observed MAB(H2O)(4) complex has only one isomer with a hydrogen bonded water ring structure attached to the amino hydrogens and its low vibrational modes (up to 165 cm(-1)) have been assigned. A discussion of the results, including structures of stable isomers, isomer energies, ionization thresholds, and the difficulties in observing some solvated complexes is presented. (C) 2000 American Institute of Physics. [S0021-9606(00)00333-0].