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

Abstract

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...

Internal Rotation in Propionic Acid: Near-Infrared-Induced Isomerization in Solid Argon

“…The fast dark decay of both C t and C g ( conformers is due to a phononassisted tunneling process, which has been shown to be common for internal rotation around the C-O bond in simple carboxylic acids such as formic and acetic acids. 3,7 For the deuterated isotopologue (PA-OD), the C t and T g ( conformers produced by excitation of the T t form are quite stable at 8 K. In fact, when compared to PA-OH, the C t to T t conversion by tunneling slows down by ca. 4 orders of magnitude.…”

“…The proposed stepwise mechanism is consistent with the model for the dark C f T tunneling decay in formic and acetic acids from the torsional ground state of the cis conformer to a vibrationally excited state of the trans form. 3,7 …”

“…[1][2][3][4][5][6][7] These studies have been mainly motivated by our interest in the conformational dynamics of the carboxylic group as a way to understand the process of intramolecular vibrational energy relaxation, which plays an important role in molecular reactivity. Interestingly, the photolysis of formic acid was shown to be conformationally dependent, being acknowledged as one of the first clear cases of optical control of chemical reactivity.…”

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.…”

“…15 The higher-energy conformer of deuterated FA (HCOOD) is very stable compared to cis-HCOOH. 13,16 The FA molecules can form a number of dimers stabilized by intermolecular hydrogen bonding. The trans−trans dimers have been extensively studied theoretically.…”