The conformers of phenylglycine

We report the first vapor phase investigation of the unnatural aminoacid phenylglycine. The rotational spectra of two conformers have been assigned by using a laser ablation system in combination with Fourier transform microwave spectroscopy in a supersonic jet. Rotational, centrifugal distortion and quadrupole coupling constants have been determined, the latter playing a decisive role to assign the observed conformers beyond any doubt. Ab initio calculations at the MP2/6-311++G(d,p) level of theory have been performed to guide the spectral search. The conformers detected are stabilized by internal hydrogen bonds in the side chain, in one case by a N-H···O=C and an N-H···p interaction with the closest double bond in the aromatic ring, and in the other one by a O-H···N interaction. The conformer bearing the N-H···O=C bond is the global minimum.