Structural and vibrational assignment of p-methoxyphenethylamine conformers

A combined wide range of laser spectroscopic techniques and computational q uantum chemistry are used to investigate conformer landscapes of large subs tituted aromatic molecules. The spectroscopic techniques include laser-indu ced fluorescence (LIF), high-resolution LIF, resonance-enhanced multiphoton ionization (REMPI) with mass detection, two-color resonance ionization wit h mass detection (R2PI), laser dispersed emission (DE), and ion-dip hole bu rning (HB). These techniques are balanced with extensive ab initio DFT calc ulations 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-metho xyphenethylamine (MPEA) neurotransmitter molecule. Even when all the spectr oscopic techniques play important roles, two color ionization energy (IE) t hreshold determinations have proven to be especially useful in the study. T he gauche and anti conformers exhibit different ionization energy values, a nd even the amino hydrogen orientation influences their energies. The resul ts allow the unequivocal identification of seven conformer structures which are further confirmed by the assignment of their vibrational spectra. A di scussion on the intramolecular interactions, including the N-H ...pi hydrog en bonding correlation with the molecular stability in the light of the the oretical and experimental results, is also presented.