Infrared-induced conformational interconversion in carboxylic acids isolated in low-temperature rare-gas matrices

Abstract

An overview of our recent studies dealing with infrared-induced conformational interconversion of carboxylic acids isolated in rare-gas matrices is presented. Extensive rotational photoisomerization studies have been performed on formic acid, which is the simplest organic acid enabling this kind of processes. Formic acid has two conformers and interconversion between them can be induced by vibrational excitation. As such, it is an ideal model system to study the conformational dynamics of the carboxylic group. Formic acid molecules were found to be isolated in different local environments within the rare-gas matrices, as shown by the site splitting of the vibrational bands. Narrowband tunable infrared (IR) radiation was used to induce site-selective isomerization processes. The induced changes in the IR absorption spectra allowed for a detailed analysis of the vibrational properties of both conformers of formic acid isolated in solid argon. In particular, derived from the intermode coupling constants the local environment was shown to affects the intramolecular potential energy surface. Tunneling is involved in the rotamerization of formic acid, with the tunneling rate being affected by the local environment. Additionally, formic acid exhibits isomer-selective photodissociation where narrowband IR excitation can control the conformer-dependent photodissociation channels. Tunable IR radiation was also used to promote rotamerization in a series of matrix-isolated dicarboxylic acids (ethanedioic, propanedioic, and 2-butenedioic acids) by exciting the first overtone of the O-H stretching mode or a suitable combination mode at similar energies. Efficient isomerization involving rotation around the C-O bond was observed in most cases whereas the internal rotation around the C-C bond was found to be constrained for ethanedioic and (Z)-2-butenedioic acids. #

Rotational isomerism of acetic acid isolated in rare-gas matrices: Effect of medium and isotopic substitution on IR-induced isomerization quantum yield and cis→trans tunneling rate

“…The energy above which the quantum yield reaches a relatively stable value ͑4500 cm Ϫ1 ͒ agrees with the computational barrier height, which supports the theoretical estimation. The lack of evident mode specific effects on the quantum yield was also reported for formic acid in solid Ar 7,19,31 For both molecules, the range of excitation energies probed corresponds to the discrete region of vibrational states and, thus, mode specific effects are generally possible. We can speculate that the observed lack of mode specificity in the isomerization quantum yield is connected with a slow isomerization rate, allowing effective randomization of the excitation energy between various modes.…”

“…3͒. 7,19,31 At energies below ϳ4400 cm Ϫ1 , the quantum yield depends strongly on the excitation energy.…”

Rotational isomerization of small carboxylic acids isolated in argon matrices: Tunnelling and quantum yields for the photoinduced processes

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] This process occurs when the energy deposited into the initially excited vibrational mode is transferred to the reaction coordinate. The mechanism of energy relaxation in the solid was described as a phonon-assisted intramolecular vibrational energy relaxation (IVR) where the lattice modes provide a thermal bath compensating the energy mismatch between the intramolecular vibrational states.…”

UV-light induced conformational changes of 2-pyridinecarboxylic acid in low-temperature argon matrices

“…2 after correction of zero-point vibrational energy, showing that TT is the most stable conformer. This result is in contrast to the fact that general carboxylic acids prefer cis to trans conformation about the OCAOH bond in the COOH group [13][14][15][16][17][18][19], as described in Introduction, implying that TT of 2-PA is strongly stabilized by the IMHB of COOHÁ Á ÁN in spite of the repulsion between lone pairs of the two O atoms in the COOH group. This is supported by comparison of the OAH bond lengths; the r(OAH) of TT is longer than that of other conformers by $0.01 Å as shown in Table 1.…”

“…For example, the less stable conformers, trans, in formic acid [13][14][15], acetic acid [13,16,17], propionic acid [13,18], and benzoic acid [19] are produced from the more stable conformers, cis, by IR radiation and return to cis in darkness through hydrogen-atom tunneling. Another example is reported that the trans conformer of 2-chlorobenzoic acid produced from the cis conformer by UV irradiation is unable to exist stably even in a low-temperature argon matrix in spite of the stabilization due to the IMHB of COOHÁ Á ÁCl [20].…”