SPECTROSCOPY OF NEUROTRANSMITTERS AND THEIR CLUSTERS .1. EVIDENCE FOR5 MOLECULAR CONFORMERS OF PHENETHYLAMINE IN A SUPERSONIC JET EXPANSION

The neurotransmitter molecule phenethylamine (PEA) is studied by fluor escence excitation, mass-resolved excitation, hole burning, and disper sed emission spectroscopies. The excitation spectrum of PEA exhibits a group of five transition bands in the S-1 <-- S-0 0(0)(0) region. Thi s group of transitions appears repetitively with the same spectral sep arations and intensity pattern in the higher energy regions up to 1000 cm(-1) above the origin, The hole burning spectra of these five bands show different independent spectral hole depletion features. With the observation of no blue shifted emission bands higher than the excitat ion energy in the dispersed emission spectra, these five transition ba nds can be assigned to five different molecular conformers of PEA. The se molecular conformers are distinguished from each other in their alk ylamine side chain conformations and the amino group spatial orientati ons. With the assistance of ab initio calculations, we are able to ide ntify the conformations of these five isomers. The effects of the alky lamino chain conformation, as well as the amino group orientation, on the relative potential energies of these conformers are discussed. The significance of this work for future studies of PEA/solvent molecule clusters and the other neurotransmitters is demonstrated by the amphet amine (AMP) excitation spectrum, The conformation of the alkylamine si de chain is found to have little impact on in-plane vibrational modes of the phenyl ring such as nu(6b) and nu(12).