The shapes of neurotransmitters by millimetrewave spectroscopy: amphetamine

We have studied the amphetamine molecule both experimentally by rotational spectroscopy and theoretically by ab initio molecular orbital calculations at the MP2/6-31 G(d,p) level. Two species x and y of amphetamine detected by millimetre wave argon free jet expansion spectroscopy are identifiable from values of their rotational constants together with the quadrupole hyperfine patterns of some spectral lines. The more abundant conformer x is amp(l) and the less abundant y is amp(2) corresponding to the theoretical conformers I and II [see Fig. 2]. The identity of conformer x as amp(l) is supported by amino-hydrogen coordinate data obtained from detection of the N-deutero isotopomer of this conformer. Both amp(1) and amp(2) appear to be stabilized by non-classical hydrogen bonds from an amino hydrogen to the aromatic pi-electron cloud, and have the methyl group trans to the phenyl substituent. Calculations of the relative energies of the conformers at the MP2/6-31 G(d,p) level suggest that the corresponding gauche-methyl conformers V and VI may be of energy similar to H. However, calculations of details of the potential energy surface at levels of theory higher than that used in the present work will be needed to clarify this question. By analogy with smaller disubstituted ethanes, we would expect the barriers to rotation about the C-C bond to be too high to enable relaxation of one conformer to another during the jet expansion. (C) 2001 Elsevier Science B.V. All rights reserved.