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

Abstract

Abstract:The monomeric form of trichloroacetic acid (CCl3COOH; TCA) was isolated in a cryogenic nitrogen matrix (15 K) and the higher energy trans conformer (O=C-O-H dihedral: 180°) was generated in situ by narrowband near-infrared selective excitation the 1 st OH stretching overtone of the low-energy cis conformer (O=C-O-H dihedral: 0°). The spontaneous decay, by tunneling, of the generated high-energy conformer into the cis form was then evaluated and compared with those observed previously for the trans conformers of acetic and formic acids in identical experimental conditions. The much faster decay of the high-energy conformer of TCA compared to both formic and acetic acids (by ~3 5 and ca. 25 times, respectively) was found to correlate well with the lower energy barrier for the trans → cis isomerization in the studied compound. The experimental studies received support from quantum chemistry calculations undertaken at the DFT(B3LYP)/cc-pVDZ level of approximation, which allowed a detailed characterization of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers.Keywords: trichloroacetic acid, NIR-induced in situ generation of the higher energy trans-TCA conformer, spontaneous decay by tunneling, matrix-isolation infrared spectroscopy, quantum chemical calculations.

IR-induced and tunneling reactions in cryogenic matrices: the (incomplete) story of a successful endeavor

“…2 Â 10 À3 s À1 , which corresponds to a half-live of the E form of a few minutes. 5 The nature of the mechanism involved in these reactions was since the very beginning proved unequivocally through experiments with isotopically labelled formic and acetic acids, 5,6,50 and this has been confirmed later for other types of molecules containing the carboxylic group (e.g., amino acids and halogenated carboxylic acids [16][17][18][19][35][36][37][38][39][40][41] ). The relevance of spontaneous QMT decay of higher-energy conformers produced in situ in matrices (using either vibrational or electronic excitation of suitable precursors) has also been demonstrated for other types of conformational isomerizations involving movement of hydrogen atoms, for example in derivatives of nucleic acid bases and phenols.…”

“…The studies were promptly expanded to other simple carboxylic and dicarboxylic acids, like acetic, oxalic, fumaric, maleic and malonic acids. 7,11–15 From our fruitful collaboration with the Helsinki group (and also as a result of the work of other groups participating in this adventure) some general conclusions emerged, which became the basis for the subsequent more elaborated studies on more complex closed-shell systems, like amino acids, nucleic acid bases, halogenated carboxylic acids, α-hydroxyl, α-keto and α,β-unsaturated carboxylic acids, aromatic and other cyclic carboxylic acids, and phenol derivatives, 16–47 among others, and also on open-shell species, like the HOCO radical, as more recently reported by Ryazantsev and coworkers. 48…”

Selective conformational control by excitation of NH imino vibrational antennas

“…4 Near-IR irradiation-induced cis-trans conformational changes in hydroxyl-containing compounds isolated in inert matrices is an extensively studied topic. Carboxylic acids [42][43][44] such as formic, [45][46][47][48][49] acetic, [50][51][52] propionic, 53 and pyruvic, 54 for example, were studied successfully using this approach, alongside some dicarboxylic acids, [55][56][57][58] and derivatives containing heteroatoms such as nitrogen (oxamic acid), 59 or halogen atoms (e.g., trifluoroacetic, 60 trichloroacetic, 61 tribromoacetic, 62 and 2-chloropropionic acid). 63 Conformational changes involving -OH groups of cyclic and heterocyclic organic acid compounds have also been investigated using the same experimental approach, [64][65][66][67][68] as well as those occurring in carbonic acid, 69 amino acids, [70][71][72][73][74] and nucleobases or nucleobase derivatives.…”