Experimental and theoretical study of methyl-p-aminobenzoate/ammonia complexes. II. $MAB(NH3)2–4$ Available to Purchase

A complementary laser spectroscopy and computational study of the $MAB(NH3)2–4$ complexes, hereafter referred to by its stoichiometry, i.e., 1:2, 1:3, and 1:4, prepared in a supersonic expansion, is reported. Experimental evidence shows the existence of abundant fragmentation cascades, the most notorious being the observation of the 1:4 complex spectrum in the 1:3 and to 1:2 mass channels, in fact, the observed spectra of the 1:2 and 1:3 complexes are not genuine but a consequence of fragmentation. The observed 1:4 complex resonance enhanced multiphoton ionization (REMPI) spectrum has a significant redshift of −1160 cm⁻¹ from the bare MAB $000$ transition and appears over a noisy background that decreases, although it does not disappear, in resonance enhanced two-color photo ionization (R2PI) studies. “Hole burning” spectroscopy corroborates the presence of only one 1:4 isomer. Calculations at the $B3LYP/6−31+G*$ level conduct to a number of 1:2, 1:3, and 1:4 stable isomer structures, the most stable being the 1:4 with a four ammonia chain coordinated to the $NH2$ group. The good agreement between calculated and experimental vibrational frequencies confirms the ammonia ring structure and allows us to assign a number of $MAB(NH3)4$ inter- and intramolecular vibrational bands.