Conformers of beta-aminoisobutyric acid probed by jet-cooled microwave and matrix isolation infrared spectroscopic techniques

Abstract

beta-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N-2 matrix. A solid sample of the compound was vaporized by laser ablation and investigated through their rotational spectra in a supersonic expansion using two different spectroscopic techniques: broadband chirped pulse Fourier transform microwave spectroscopy and conventional molecular beam Fourier transform microwave spectroscopy. Four conformers with structures of two types could be successfully identified by comparison of the experimental rotational and N-14 nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH center dot center dot center dot N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral similar to 180 degrees), and type B, having an NH center dot center dot center dot O bond and the cis arrangement of the carboxylic group. These two types of conformers could also be trapped from the gas phase into a cryogenic N-2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N-2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (similar to 6930 cm(-1)) of these forms allowed to selectively convert them into type A conformers and into a new type of conformers of higher energy (type D) bearing an NH center dot center dot O=C bond and a O-H "free" trans carboxylic group