Formic and acetic acids in a nitrogen matrix: Enhanced stability of the higher-energy conformer

Abstract

Formic acid ͑HCOOH, FA͒ and acetic acid ͑CH 3 COOH, AA͒ are studied in a nitrogen matrix. The infrared ͑IR͒ spectra of cis and trans conformers of these carboxylic acids ͑and also of the HCOOD isotopologue of FA͒ are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the OH and COH modes and the relative intensities of the OH and C v O bands.

Dimers of the Higher-Energy Conformer of Formic Acid: Experimental Observation

“…11,12 The higher-energy conformer decays back to the ground-state form via tunneling of the −OH hydrogen atom. 12,13 The lifetime of the higher-energy cis conformer strongly depends on the surrounding, changing from 6.25 h in a nitrogen matrix 14 to 75 min in a xenon matrix 13 and to 5 s in a neon matrix. 15 The higher-energy conformer of deuterated FA (HCOOD) is very stable compared to cis-HCOOH.…”

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

“…On the other hand, the νC=O stretching mode is observed at higher frequency in TCA (main bands at 1796.5 and 1793.7 cm -1 ) than in acetic acid (1779.0 and 1775.7 cm -1 ). [14] This result correlates with the stronger C=O bond in TCA compared with acetic acid (120.00 vs. 120.87 pm; B3LYP/cc-PVDZ calculated data). The two modes predominantly related with the νC-O and δCOH coordinates are observed in cis-TCA at 1350.1 / 1344.8 cm -1 and in the 1162-1151 cm -1 range, the lower wavenumber mode having a larger contribution from the νC-O coordinate (see Table 1).…”

“…The multiplet observed in the νOH stretching region (centered at ~3 528 cm -1 ) is a good example of a band exhibiting extensive matrix site-splitting. As it could be expected, the average wavenumber of the νOH stretching vibration in cis-TCA is considerably lower than that of the same vibration in cis-acetic acid (cis-AA) isolated in N2 (~3549 cm -1 ), [14] due to the considerably stronger acid character of TCA. On the other hand, the νC=O stretching mode is observed at higher frequency in TCA (main bands at 1796.5 and 1793.7 cm -1 ) than in acetic acid (1779.0 and 1775.7 cm -1 ).…”

“…[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. [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 stro...…”

“…Subsequently, the spontaneous trans-TCA → cis-TCA decay was investigated and compared with data previously obtained for acetic and formic acids. [2,7,14] The experimental results were enlightened by the comprehensive characterization of the potential energy surface of the molecule and the detailed assignment of the infrared spectra of the two conformers undertaken by quantum chemistry calculations. This is the first reported doubtless experimental observation of the trans-TCA conformer hitherto.…”