Comment on “Structural and Vibrational Assignment of p-Methoxyphenethylamine Conformers”

Assignment of molecular conformers is difficult when the evidence is incomplete. In the past this has led to mistaken assignments, which have subsequently been corrected in the light of additional, firmer evidence: See, for example, 3-(4-hydroxyphenyl)propionic acid,1 2-phenylethylamine (PEA),2 2-phenylethanol,3 and benzyl alcohol.4 Unfortunately, a recent study of p-methoxyphenylethylamine (MPEA)5 has provided another example. The implicit starting point for these assignments was the early work of Martinez et al.,6 in which origin bands in the LIF spectrum of MPEA (and tyramine) were assigned to gauche or anti conformers on the assumption that pairs of gauche conformers which vary only in the orientation of the methoxy (or hydroxy) group on the ring have their band origins split by a constant amount. Data obtained subsequently, and compiled in Table 1, show this assumption to be false: the splitting of gauche conformer band origins varies considerably with the group terminating the side chain.