The gas-phase structure of alanine.

The jet-cooled rotational spectrum of neutral alanine has been studied using laser-ablation molecular-beam Fourier transform microwave spectroscopy (LA-MB-FTMW). The spectra of the two most stable forms were observed in the frequency range 6-18 GHz for the parent, (15)N alanine, three single (13)C species, and four single D species. The (14)N nuclear quadrupole coupling hyperfine structures have been resolved, and their comparison with those calculated theoretically confirms unambiguously the conformer assignments. The independent structures of both conformers have been determined experimentally for the first time using r(s) and r(0) procedures. In both cases, the amino acid backbone is nonplanar. For the most stable conformer I, the COOH group adopts a cis configuration and an asymmetric bifurcated hydrogen bond is formed between the amino group and carbonyl oxygen (r(N-H(a)...O=C) = 2.70(2) A and r(N-H(b)...O=C) = 2.88(2) A). For conformer IIa, the COOH group adopts a trans configuration and is stabilized by a O-H...N hydrogen bond (r(O-H...N) = 1.96(2) A). The relative conformer abundances in the supersonic expansion have also been investigated.