THE SHAPES OF NEUROTRANSMITTERS BY MILLIMETER-WAVE SPECTROSCOPY - 2-PHENYLETHYLAMINE

We have detected two conformers, (x) and (y), of 2-phenylethylamine and determined their shapes using microwave spectroscopy. By comparing observed and ab initio predicted spectroscopic rotational constants, dipole moments, amino-deuteration isotopic substitution coordinates, and the N-14 quadrupole coupling hyperfine patterns, species (x) has been identified with conformer III and species (y) with conformer II (see Figure 2). Both conformers are gauche with III being present in greater concentration. No other conformers occur in significant concentration in the vapor. Molecular orbital calculations correctly predict that III and II are the two most stable conformers. However at the RHF level the ab initio calculations predict that other conformers are of comparable stability to II, indicating that predictions of the relative energies of the conformers are not entirely reliable. When electron correlation is included at the MP2 level, the predicted energies of III and II are sufficiently lower than those of an the other conformers that their mole fractions in the prejet vapor at 120 degrees C would be too low for detection of the species in the jet spectrometer. A limited ab initio study of some of the barriers to conformer relaxation indicates that they are too high (> 1000 cm(-1)) for relaxation of the less stable conformers to III or II, except in the case of I where the low barrier to relaxation to III (about 270 cm(-1)) implies that it would rapidly relax to III in the jet.