Spectroscopy of Neurotransmitters and Their Clusters. 1. Evidence for Five Molecular Conformers of Phenethylamine in a Supersonic Jet Expansion

The neurotransmitter molecule phenethylamine (PEA) is studied by fluorescence excitation, mass-resolved excitation, hole burning, and dispersed emission spectroscopies. The excitation spectrum of PEA exhibits a group of five transition bands in the S1 r S0 00 0 region. This group of transitions appears repetitively with the same spectral separations 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 excitation energy in the dispersed emission spectra, these five transition bands can be assigned to five different molecular conformers of PEA. These molecular conformers are distinguished from each other in their alkylamine side chain conformations and the amino group spatial orientations. With the assistance of ab initio calculations, we are able to identify the conformations of these five isomers. The effects of the alkylamino 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 amphetamine (AMP) excitation spectrum. The conformation of the alkylamine side chain is found to have little impact on in-plane vibrational modes of the phenyl ring such as ν6b and ν12.