The missing conformers of glycine and alanine: relaxation in seeded supersonic jets

Abstract

Certain conformers of glycine and alanine that are predicted by ab initio calculations to be among those of the lowest energy have not been detected in jet spectroscopy studies of these amino acids, although others of similar or higher predicted energy are observed. Selective conformational relaxation of the missing conformers to lower energy species during the free jet expansion, broadly consistent with the criteria established by Ruoff et al. and by Felder and Günthard, has been found to provide a satisfactory explanation. Relevant regions of the molecular potential energy hypersurface have been modelled via ab initio calculations [ MP2 6–31 G(d,p) ] and these show that glycine 4 (see Fig. 3) and glycine 7 can relax to glycine 1, leading to the conclusion, in accordance with observation, that only the latter and glycine 2 should be detected in the jet. In the case of alanine, the ab initio study of the molecular potential energy hypersurface has yielded revised data on predicted spectroscopic constants for some conformers. Because of the combined effects of relaxation and conformational coalescence through large-amplitude vibrational motion, the revised data do not change the interpretation of the previous spectroscopic study of alanine. The barrier separating conformers 2 and 3 (see Fig. 8) is predicted to be no more than 129 cm −1, so that in addition to alanine 1 only a single additional coalesced conformer { 2, 3}, undergoing a large-amplitude motion in a double-minimum potential, is expected to appear in the jet spectrum rather than two separate species. The ab initio calculations on alanine indicate that conformers 4 and 5 are separated by a low energy barrier and a similarly low barrier separates 4 from the much lower energy conformer 1. The resulting coalesced { 4, 5} is effectively a “ledge” on one wall of the potential well containing 1, and is occupied via an excited vibrational state of 1. In this light, the conformer { 4, 5} is not expected to be observed in the jet, as the vibrational relaxation of low-frequency vibrational modes is known to be very efficient in free jet expansions. Overall, no conflicts are found to remain between the experimental observations of jet spectra and the ab initio calculations for glycine and alanine, provided that the scope of such calculations is broadened from a narrower focus on equilibrium structures to consideration of the potential hypersurface. This approach should be important in the correct interpretation of the spectra of other multi-conformational species in free jet expansions.