Isomer structures and vibrational assignment of the methyl-p-aminobenzoate(H2O)1 complex

Methyl-p-aminobenzoate$({\mathrm{H}}_{\mathrm{2}}\mathrm{O}{)}_1$ complex, henceforth ${\mathrm{MAB(H}}_{\mathrm{2}}\mathrm{O}{)}_1,$ prepared by pulsed supersonic expansion, has been examined by a broad range of laser based spectroscopic, mass and isomer selective techniques and density functional theory (DFT) calculations, in order to identify its isomer structures, ionization energies, and vibrational frequencies. The experimental techniques used include laser induced fluorescence (LIF), mass resolved excitation spectroscopy (MRES) either with one (REMPI) or two laser colors (R2PI), laser excited dispersed emission (DE), high resolution MRES, pressure controlled R2PI, hole burning (HB) spectroscopy, and photoion fragmentation threshold (PIFT). Experimental results have been interpreted, rationalized and extended with density functional theory (DFT) computations at the B3LYP/6-31G and B3LYP/6-31+G^* levels. Although bare MAB molecule have four possible solvation sites, prone to yielding hydrogen bonds with the water molecule, LIF, R2PI, and HB spectroscopy of ${\mathrm{MAB(H}}_{\mathrm{2}}\mathrm{O}{)}_1$ only pick out the presence of three blue shifted isomers, each accompanied by a number of vibrational features extending to ≈500 cm⁻¹. The high intensity bands have been demonstrated to originate in three isomers and their ionization energies, dispersed emission, vibrational spectra, and photoion fragmentation threshold have been measured and characterized. Isomer shifts and structures are discussed in the light of experimental and theoretical results.