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

Abstract

β-aminoisobutyric acid (BAIBA) has been studied in isolation conditions: in the gas phase and trapped into a cryogenic N2 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 (14)N nuclear quadruple coupling constants with those predicted theoretically: type A, bearing an OH⋯N intramolecular hydrogen bond and its carboxylic group in the trans geometry (H-O-C=O dihedral ∼180°), and type B, having an NH⋯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 N2 matrix and probed by Fourier transform infrared (IR) spectroscopy. In situ irradiation of BAIBA isolated in N2 matrix of type B conformers using near-IR radiation tuned at the frequency of the O-H stretching 1st overtone (∼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⋯O=C bond and a O-H "free" trans carboxylic group.

Narrowband NIR-Induced In Situ Generation of the High-Energy Trans Conformer of Trichloroacetic Acid Isolated in Solid Nitrogen and its Spontaneous Decay by Tunneling to the Low-Energy Cis Conformer

“…Alternatively, energy is introduced in the molecule by excitation of a different higher-energy vibration with appreciable absorption cross section. [2][3][4][5][6][7][8][9][10][11][12] This energy is then partially transferred to the reaction coordinate through a cascade relaxation mechanism involving participation of matrix phonons.…”

“…[2][3][4]7,9,[13][14][15][16] Together with additional studies on other molecules, these investigations allowed to reach some general conclusions: i) in general, once formed, the highenergy conformer of the carboxylic acids convert back spontaneously to the more stable form by tunnelling, even at cryogenic temperatures, in time scales that vary from a few seconds to several hours; [7,8,[13][14][15][16] ii) the stability of the higher-energy forms depends upon several factors, the dominant one appearing to be the height/wideness of the energy barrier separating the higher energy form from the lower energy one; [3,7,14] iii) specific interactions with the matrix material affect also the stability of the higher-energy form, which has been found to be greater in solid N2 and CO2 than in rare gases; [7,8,14,15] iv) differences in the matrix microenvironments also influence the life-time of the higher-energy conformer, even if they appear spectroscopically indistinguishable, leading to decay rates that very often obey to a dispersive-type kinetics instead of following a single exponential behaviour. [5][6][7][8][9][10][11][12] The present study focuses on trichoroacetic acid (Cl3CCOOH; TCA), which differs from acetic acid by replacement of the methyl group by the strongly electron-attractor trichloromethyl moiety. Such structural modification can be expected to lead to a reduction of the double bond character of the carboxylic C-O bond, implying a lower energy barrier for the trans → cis conformational isomerization in TCA, and reducing the stability of the higher-energy trans conformer (O=C-O-H dihedral angle: 180°; see Figure 1) once it is generated in a suitable matrix media using the strategy presented above.…”

“…[2][3][4] It is now possible to choose the vibrational mode used to excite the targeted precursor conformer in a very selective way, [2][3][4][5][6][7][8] and since then many successful examples of application of the technique have been reported. [2][3][4][5][6][7][8][9][10][11][12] In several cases, the application of the method aims to generate high-energy rare conformers otherwise inaccessible to experiment. [2][3][4][5][6][8][9][10][11][12] A conformational isomerisation occurs when enough energy is deposited into the reaction coordinate, so that the system can overcome the energy barrier and pass into the products valley.…”

“…[2][3][4][5][6][7][8][9][10][11][12] In several cases, the application of the method aims to generate high-energy rare conformers otherwise inaccessible to experiment. [2][3][4][5][6][8][9][10][11][12] A conformational isomerisation occurs when enough energy is deposited into the reaction coordinate, so that the system can overcome the energy barrier and pass into the products valley. The reaction coordinate for internal rotation around a single bond connecting two conformers is essentially described by the torsion about that bond.…”

“…[2][3][4][7][8][9][10][11] Formic and acetic acids have been studied in detail in various matrix-media. [2][3][4]7,9,[13][14][15][16] Together with additional studies on other molecules, these investigations allowed to reach some general conclusions: i) in general, once formed, the highenergy conformer of the carboxylic acids convert back spontaneously to the more stable form by tunnelling, even at cryogenic temperatures, in time scales that vary from a few seconds to several hours; [7,8,[13][14][15][16] ii) the stability of the higher-energy forms depends upon several factors, the dominant one appearing to be the height/wideness of the energy barrier separating the higher energy form from the lower energy one; [3,7,14] iii) specific interactions with the matrix material affect also the stability of the higher-energy form, which has been found to be greater in solid N2 and CO2 than in rare gases; [7,8,14,15] iv) differences in the matrix microenvironments also influence the life-time of the higher-energy conformer, even if they appear spectroscopically indistinguishable, leading to decay rates that very often obey to a dispersive-type kinetics instead of following a single exponential behaviour.…”