Temperature dependence of cryochemical H-tunneling reactions

Ivanov, G. K.; Kozhushner, M. A.; Трахтенберг, Л. И.

doi:10.1063/1.482005

Cited by 40 publications

(42 citation statements)

References 29 publications

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“…1 In a condensed environment, phonons participate in a tunneling reaction and the environment should have some effect on tunneling reactions. 2,3 However, in several previous experiments it was found that the tunneling rate constant was unaffected by the change of solvent. 1,4 -6 In this work, we have studied the conversion of cis formic acid ͑HCOOH͒ to trans formic acid in solid rare gases ͑Ar, Kr, Xe͒.…”

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confidence: 97%

“…18͒ and the origin of the effect was considered theoretically. 3,19 The present observations should be valuable in testing theories for the temperature dependence of dissipative tunneling. In addition to the fundamental value the observed phenomenon can be used for solid-phase stabilization of desired species affected by tunneling decay by choosing a suitable environment.…”

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confidence: 99%

“…The temperature dependencies of the rate constants are roughly described as k(T)ϰT 3 or T 4 . Previously, roughly T 4 dependence was observed for translational tunneling of the acidic protons in benzoic acid dimers.…”

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

Pettersson

Maçôas

Khriachtchev

et al. 2002

The Journal of Chemical Physics

172

195

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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 tunneling rate. In reaction kinetics, tunneling of atoms is often negligible compared with over-barrier transitions. At very low temperatures, however, the population of energy states above the barrier becomes exceedingly small and tunneling becomes comparatively more important.1 In a condensed environment, phonons participate in a tunneling reaction and the environment should have some effect on tunneling reactions.2,3 However, in several previous experiments it was found that the tunneling rate constant was unaffected by the change of solvent. 1,4 -6 In this work, we have studied the conversion of cis formic acid ͑HCOOH͒ to trans formic acid in solid rare gases ͑Ar, Kr, Xe͒. This reaction is dominated by tunneling from the vibrational ground state at temperatures below 60 K. The results show that the tunneling rate depends strongly on the environment.The samples were made by mixing vapors of formic acid ͑FA͒ ͑KEBO lab, Ͼ99%͒ or its isotopomers ͑IT Isotope 95%-98% deuteration͒ with rare gases ͑Rg͒ Ar ͑AGA, 99.9999%͒, Kr ͑Air Liquid, 99.95%͒, Xe ͑AGA, 99.997%͒ in the gas phase in a proportion FA/RgϷ1/1000. The gas mixture was deposited on a CsI substrate at 15 K ͑Ar͒, 25 K ͑Kr͒ or 35 K ͑Xe͒ yielding highly monomeric matrices with respect to FA. Thickness of the sample was typically about 100 m. After deposition, the samples were cooled to ϳ8 K which was the lower limit for the cryostat ͑APD DE 202 A͒. The spectra were measured with a FTIR spectrometer ͑Nico-let 60 SX͒ with a resolution of 1 or 0.25 cm Ϫ1 . FA has energy minima in two planar forms differing by orientation of the hydroxyl group as shown in Fig. 1. The interconversion of the conformers involves mainly the torsional motion of the hydroxyl group. In the gas phase, cis-FA is 1365Ϯ30 cm Ϫ1 higher in energy than trans-FA. 7 The barrier from trans to cis has been calculated to be ϳ4200 cm Ϫ1. 8 In this work, cis-FA was prepared by exciting the vibrational transitions of trans-FA in rare-gas matrices with narrowband infrared radiation of an optical parametric oscillator ͑Sunlite, Continuum, FWHM ϳ0.1 cm Ϫ1 ͒. The excitation energy flows into the torsional coordinate inducing the conformer conversion. 9 The IR spectra of cis and trans FA differ significantly from each other making it possible to distinguish them easily in rare-gas matrices.9 FA is trapped in several sites correspo...

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

Pettersson

Maçôas

Khriachtchev

et al. 2002

The Journal of Chemical Physics

172

195

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“…An increase in temper ature causes the rate constant for tunneling (k) to abrupt ly increase and, after passing through a region with a linear temperature dependence, the parameter lnk obeys the Arrhenius law. 13,14 This type of the temperature de pendence of the rate constants for chemical reactions involving tunneling of an H atom was first explained in More recently, these studies were continued by other authors (see references cited in Refs 13 and 22).…”

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confidence: 93%

The role of intermolecular vibrations and reorganization of a reaction system in tunneling reactions with H atom transfer. A Debye model for the medium

2008

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Mechanisms of temperature dependence of the rate constants for two types of solid state tunneling chemical reactions, namely, transfer of an H atom between two molecules and intramolec ular transfer, are analyzed. To this end, an analytical expression for the rate constant for tunneling atom transfer in solids is derived in the framework of a modified theory of nonradiative transitions. The mechanisms of the temperature dependence of the rate constant considered in this work include oscillations of the potential barrier to chemical reaction in intermolecular fluctuations and reorgani zation of the medium. The effect of pressure on the distance between reactants and on the frequency of intermolecular vibrations is taken into account. The theory developed is used to interpret experimental data on tunneling transfer of an H atom in two reactions: а) intramolecular hydrogen transfer in a matrix isolated formic acid molecule entrapped in an argon crystal and b) H atom transfer from a fluorene molecule to an excited acridine molecule in a fluorene crystal.

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“…Additionally, isomerization studies performed on hydroquinone and on a halogenated derivative of dihydroxy-benzoquinone isolated in argon matrices [21,22] seem to indicate that the participation of tunneling in isomerization processes involving mainly motion of hydrogen atoms is quite general. Solid-state medium is expected to influence the tunneling rate [23][24][25], which depends also on the temperature due to the participation of the lattice phonons in a dissipative tunneling mechanism [26][27][28].…”

Section: Introductionmentioning

confidence: 99%

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

Maçôas

Khriachtchev

Pettersson

et al. 2004

Vibrational Spectroscopy

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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. #

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Temperature dependence of cryochemical H-tunneling reactions

Cited by 40 publications

References 29 publications

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

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

The role of intermolecular vibrations and reorganization of a reaction system in tunneling reactions with H atom transfer. A Debye model for the medium

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

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