Near-infrared laser induced conformational change of alanine in low-temperature matrixes and the tunneling lifetime of its conformer VI.

The near- and mid-IR spectra of α-alanine isolated in low-temperature Ar, Kr, and N2 matrixes were measured. Production of the short-lived conformer VI at the expense of the predominant conformer I was observed upon short irradiation with NIR laser light at the first O-H stretching overtone band of conformer I. Conformer VI decays by H-atom tunneling at 12 K with half-lives of 5.7 ± 1 s, 2.8 ± 1 s in Ar (two different sites), 7.0 ± 1 s in Kr, and 2.8 × 10(3) ± 1.2 × 10(3) s in N2. Upon prolonged irradiation, conformer I slowly transformed into conformer IIa. On the basis of these irradiation experiments, the unambiguous vibrational assignments of conformers I, IIa, and VI are given. In contrast to similar experiments for glycine, the irradiation experiments did not lead to the formation of conformer IIIb. This is explained by a very low IIIb → I barrier height computed for alanine, which results in a very fast depletion of conformer IIIb even in low-temperature matrixes.