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

Rotational isomerization of acetic acid (CH3COOH) is studied in Ar, Kr, and Xe matrices. The light-induced trans→cis reaction is promoted using resonant excitation of a number of modes in the 3500–7000 cm−1 region, and the quantum yields for this process are measured for various acetic acid isotopologues and matrix materials. For excitation of acetic acid at energies above the predicted isomerization energy barrier (⩾4400 cm−1), the measured quantum yields are in average 2%–3%, and this is one order of magnitude smaller than the corresponding values known for formic acid (HCOOH). This difference is interpreted in terms of the presence of the methyl group in acetic acid, which enhances energy relaxation channels competing with the rotational isomerization. This picture is supported by the observed large effect of deuteration of the methyl group on the photoisomerization quantum yield. The trans→cis reaction quantum yields are found to be similar for Ar, Kr, and Xe matrices, suggesting similar energy relaxation processes for this molecule in the various matrices. The IR-induced cis→trans process, studied for acetic acid deuterated in the hydroxyl group, shows reliably larger quantum yields as compared with the trans→cis process. For pumping of acetic acid at energies below the predicted isomerization barrier, the trans→cis reaction quantum yields decrease strongly when the photon energy decreases, and tunneling is the most probable mechanism for this process. For the cis→trans dark reaction, the observed temperature and medium effects indicate the participation of the lattice phonons in the tunneling-induced process

Photochemical reactivity of matrix-isolated monomeric carboxylic acids

When used together with conventional spectroscopic methods (e.g. infrared spectroscopy), matrix isolation constitutes a very convenient technique to undertake photochemical studies on single molecules that are not easily accessible to experiment under other sampling conditions, such as carboxylic acid monomers, which show a strong tendency to aggregate. Once a matrix of a given substance has been prepared, in situ irradiation of the target matrix-isolated molecule at a proper wavelength enables the promotion of different types of photochemical processes which, most of times, can also be easily probed spectroscopically. These include conformational interconversions, tautomerization reactions and fragmentation processes (including photodegradation reactions). In this paper, results of both electronic and vibrational photochemical studies carried out on a series of different matrix-isolated carboxylic acid monomers are presented. Besides more conventional techniques, an approach based on irradiation at a frequency matching that of the first overtone of a particular vibrational mode is described.http://www.sciencedirect.com/science/article/B6TGS-435M5SD-5/1/81330f5693ded5ed02059babc57fe62

Photochemistry and Vibrational Spectroscopy of the Trans and Cis Conformers of Acetic Acid in Solid Argon

Acetic acid monomer has two stable geometries, the cis and trans conformers. The high-energy cis conformer has been recently detected experimentally for the first time [Maçôas et al. J. Am. Chem. Soc. 2003, 125, 16188]. The cis conformer can be produced in low-temperature rare-gas matrixes upon vibrational excitation of the ground-state trans conformer. Fast tunneling from cis- to trans-acetic acid takes place even at the lowest working temperatures (8 K), limiting the time available to study the high-energy form. Deuteration of the hydroxyl group reduces the tunneling rate by approximately 4 orders of magnitude, increasing accordingly the lifetime of the unstable conformer and its available concentration. In this work, we present a detailed analysis of the vibrational spectra of the cis form of four acetic acid isotopologues (CH3COOH, CH3COOD, CD3COOH and CD3COOD). Photolysis (193 nm) of the trans and cis forms of the perdeuterated compound was performed to evaluate the possible conformational dependence of photodissociation of acetic acid. However, no evidence of conformer specific photodissociation was found. The UV photolysis of the matrix-isolated acetic acid reveals very different products from the gas phase. Methanol complexed with carbon monoxide is the major product of photolysis of acetic acid isolated in Ar matrixes whereas it has never been observed as a photolysis product in the gas phase

Reactive vibrational excitation spectroscopy of formic acid in solid argon: Quantum yield for infrared induced trans→cis isomerization and solid state effects on the vibrational spectrum.

Formic acid molecules are trapped in two predominant local environments (sites) when isolated in an argon matrix at 8 K. Using narrowband tunable infrared (IR) radiation, we performed site-selective excitation of various vibrational modes of the lower-energy trans conformer. For all excited modes, ranging from 7000 to 2950 cm-1, we detected site-selective isomerization to the higher-energy cis form. By measuring the IR absorption of a selected band of the cis conformer as a function of the excitation frequency, the reactive vibrational excitation (RVE) spectra were obtained. The trans→cis isomerization quantum yields for the excited modes were determined. Remarkably, very high absolute values were obtained for the quantum yield (up to 40%) at excitation energies above the reaction barrier. The efficiency of the photoinduced isomerization is essentially independent of the excited vibrational mode in a broad energy interval. Even when the excitation energy was below the reaction barrier, IR-induced rotational isomerization was observed, which indicates tunneling from the vibrationally excited trans conformer to the cis form. Using the RVE spectra, phonon sidebands were detected on the high-frequency side of the zero-phonon-line of the OH stretching mode of trans-formic acid. These weak and broad bands were not observed in the absorption spectra. Additionally, a relatively narrow band blueshifted by 6 cm-1 from the OH stretching fundamental mode was assigned to a librational satellite based on simulations using the hindered rotation model for an asymmetric top trapped in an octahedral crystal field. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR] Copyright of Journal of Chemical Physics is the property of American Institute of Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.

Cis→trans conversion of formic acid by dissipative tunneling in solid rare gases: Influence of environment on the tunneling rate.

The relaxation of the higher-energy cis conformer of formic acid to the lower-energy trans form by a tunneling mechanism has been investigated in low-temperature rare gas matrices. In the temperature range 8–60 K, the tunneling takes place dominantly from the vibrational ground state of the cis form and the temperature dependence of the tunneling rate constant is influenced by the interactions with the environment. The temperature-dependent tunneling rates for HCOOH and DCOOH in solid Ar, Kr, and Xe are measured including data for molecules in different local environments within each host. It was found that the medium and the local environment has a significant influence on the tunnelin

Spectroscopic and theoretical investigation of the conformational space of a pyrazolo-thiazole precursor of extended dipole diazafulvenium methide intermediates

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Conformational Switching Induced by Near-Infrared Laser Irradiation

This work describes a molecular system where it is possible to effectively control the interconversion between two different stable conformational states, using selective narrow-band near-infrared irradiation. Monomers of hydroxyacetone (HA) isolated in low temperature argon matrices at 15 K exist exclusively in the conformational ground state Cc. Narrow-band near-infrared vibrational excitation of the first OH stretching overtone of the Cc form resulted in conformational isomerization. The photoproduct was identified as the higher energy Tt conformer of HA, and it was found to be stable at 15 K. The reverse isomerization, converting Tt into Cc, could also be induced by laser excitation of the first OH stretching overtone of the Tt form. The interpretation of the experimental observations was supported by anharmonic and harmonic calculations of the vibrational spectra and barriers to intramolecular rotation