Structural and Vibrational Assignment of p-Methoxyphenethylamine Conformers

Abstract

A combined wide range of laser spectroscopic techniques and computational quantum chemistry are used to investigate conformer landscapes of large substituted aromatic molecules. The spectroscopic techniques include laser-induced fluorescence (LIF), high-resolution LIF, resonance-enhanced multiphoton ionization (REMPI) with mass detection, two-color resonance ionization with mass detection (R2PI), laser dispersed emission (DE), and ion-dip hole burning (HB). These techniques are balanced with extensive ab initio DFT calculations at the B3LYP/6-31+G* and B3LYP/6-311+G* levels. These methods have been applied to investigate the nine possible conformations of the p−methoxyphenethylamine (MPEA) neurotransmitter molecule. Even when all the spectroscopic techniques play important roles, two color ionization energy (IE) threshold determinations have proven to be especially useful in the study. The gauche and anti conformers exhibit different ionization energy values, and even the amino hydrogen orientation influences their energies. The results allow the unequivocal identification of seven conformer structures which are further confirmed by the assignment of their vibrational spectra. A discussion on the intramolecular interactions, including the N−H···π hydrogen bonding correlation with the molecular stability in the light of the theoretical and experimental results, is also presented.