Conformational Switching in Pyruvic Acid Isolated in Ar and N₂ Matrixes: Spectroscopic Analysis, Anharmonic Simulation, and Tunneling

Abstract

Monomers of pyruvic acid (PA) isolated in cryogenic argon and nitrogen matrixes were characterized by mid- and near-infrared spectroscopy. Interpretation of the experiments was aided by fully anharmonic calculations of the fundamental modes, overtones, and combinations up to two quanta, including their infrared intensities. The initially dominating PA conformer (Tc) has a cis CCOH arrangement and is stabilized by a strong intramolecular H-bond. Selective near-infrared excitation of Tc at the first OH overtone (6630 cm(-1) in Ar, 6643 cm(-1) in N2) induced a large scale conformational conversion to the higher-energy conformer (Tt) with trans CCOH arrangement. Tt was then converted back to Tc by selective NIR irradiation at the first Tt OH overtone (6940 cm(-1) in Ar, 6894 cm(-1) in N2). In N2 matrix, the Tt form was stabilized due to interaction between the OH group and the matrix molecules. This stabilization manifested itself in the absence of Tt → Tc relaxation and in a considerable change of the vibrational Tt signature upon going from argon to nitrogen matrix. In argon, the Tt form spontaneously decayed back to Tc in the dark (characteristic lifetime +16 h). In the presence of broad-band near-infrared light, the Tt → Tc relaxation speed considerably increased. The decay mechanisms are discussed.

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

“…[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.…”

“…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][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.…”

“…[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.…”