Journal of Molecular Spectroscopy 205, 269–279 (2001) doi:10.1006/jmsp.2000.8266, available online at http://www.idealibrary.com on Infrared and EPR Spectroscopic Studies of 2-C2H2F and 1-C2H2F Radicals Isolated in Solid Argon I. U. Goldschleger,* A. V. Akimov,* E. Ya. Misochko,* and C. A. Wight† *Institute of Problems of Chemical Physics of the Russian Academy of Sciences, 142432, Chernogolovka, Moscow region, Russian Federation; and †Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 E-mail: [email protected], [email protected] Received August 17, 2000; in revised form October 12, 2000 2-fluorovinyl radicals were generated in solid argon by solid-state chemical reactions of mobile F atoms with acetylene and its deuterated analogues. Highly resolved EPR spectra of the stabilized radicals CHFA•CH, CDFA•CD, CHFA•CD, and CDFA•CH were obtained for the first time. The observed spectra were assigned to cis-2-fluorovinyl radical based on excellent agreement between the measured (a F ϭ 6.50, a ␤H ϭ 3.86, a ␣H ϭ 0.25 mT) hyperfine constants and those calculated using density functional (B3LYP) theory. Analogous experiments carried out using infrared spectroscopy yielded a complete assignment of the vibrational frequencies. An unusual reversible photochemical conversion is observed in which cis-2- fluorovinyl radicals can be partially converted to 1-fluorovinyl radicals by pulsed laser photolysis at 532 nm. Photolysis at 355 nm converts 1-fluorovinyl back to cis-2-fluorovinyl. High-resolution EPR and infrared spectra of 1-fluorovinyl were obtained for the first time. The measured hyperfine constants (a F ϭ 13.71, a H1 ϭ 4.21, a H2 ϭ 1.16 mT) are in good agreement with calculated values. © 2001 Academic Press Key Words: cryochemistry; EPR; infrared spectroscopy; fluorine; acetylene; 2-fluorovinyl radical; 1-fluorovinyl radical. INTRODUCTION in the matrix. The most challenging aspect of this method is that reactants are normally closed-shell species; therefore, most Free radicals play an important role in many chemical and of the likely reaction products will also be closed-shell. How- biological processes. Therefore considerable attention has been ever, we have recently explored a modification of the method given to study of their properties (1). The matrix isolation that is based on the high mobility of fluorine atoms in rare gas technique has been a highly successful method for spectro- matrices and on the combination of two complementary spec- scopic investigations of radical products of chemical reactions. troscopic techniques, IR and EPR (3, 4). The ability of fluorine A common variant of this method is to trap gas-phase reaction atoms (e.g., generated by photolysis of F2) to migrate in solid products and freeze them in a matrix consisting of a large argon over long distances (5, 6) allows them to react with excess of argon or neon. The relatively weak guest–host in- closed-shell species to form free radicals at sufficiently high teractions allow spectroscopic characterization of the matrix- concentrations for spectroscopic study. The advantages of us- isolated species under conditions that are similar to the gas ing of two complementary spectroscopic techniques (EPR for phase but at much higher density than can ordinarily be sensitive detection of radicals and IR for characterization of achieved in the gas phase. closed-shell products) were demonstrated earlier for reactions Infrared absorption and EPR spectra of many radicals were of fluorine atoms with methane (3), ethylene (7), nitrogen obtained using this technique and received general acceptance oxide (8), and ammonia (4). (2). However, in some cases, radicals were misidentified or Using this approach, we have shown that complete sets of poorly characterized. In particular, difficulties in their identi- isotropic hyperfine constants (hf) can be determined from fication by IR spectroscopy included (a) the presence of sec- high-resolution EPR spectra of radicals in solid argon. The- ondary gas-phase reaction products; (b) stabilization of prod- oretically calculated isotropic hf constants of radicals and ucts in different structural traps of the solid matrix results in radical-molecular complexes are generally in good agree- multiple infrared bands, or “site splittings”; (c) registration of ment with those measured. The isotropic hf constants low-frequency IR bands ␯ Յ 300 cmϪ1, which are required for change drastically upon even minor distortion from the reliable identification of radical-molecular complexes, struc- equilibrium geometry, whereas vibrational frequencies gen- tural isomers, or conformational isomers, is experimentally erally do not (3). Therefore EPR technique is a more reliable challenging under ordinary conditions. method for identification and clarification of the structure Most of these difficulties can be overcome if the radicals are and geometry of stabilized radicals than the infrared tech- generated by atom–molecular reactions that take place directly nique. The main advantage of using IR is that most of the 269 0022-2852/01 $35.00 Copyright © 2001 by Academic Press All rights of reproduction in any form reserved. 270 GOLDSCHLEGER ET AL. molecules can be detected, providing a more comprehensive each other, and the measured hf constants (11, 13) are actually probe of reactants and products. close to those calculated for cis-2-C2H2F. The calculated har- This paper is devoted to the study of fluorovinyl radicals monic vibrational frequencies of both isomers are nearly the formed in the reactions of mobile F atoms with C2H2 mole- same (i.e., the differences in the frequencies are comparable cules. In the gas phase, this reaction leads to the formation of with the accuracy of calculations). two types of products: In this paper, we show how it is possible to form relatively high concentrations of isolated fluorovinyl radicals at 24 K by reaction of thermally diffusing F atoms with matrix-isolated [1] acetylene molecules. We make a definitive assignment of the spectroscopic properties of cis-2-C2H2F based on an experi- It was determined that the dominant channel is the formation of mental approach (4, 8) that combines measurements made using infrared and EPR spectroscopies. The correlation of the 2-C2H2F radical (9, 10). There are two isomers of this radical, which differ in the location of the F atom with respect to the results is made possible by the discovery of a reversible pho- orbital of unpaired electron: toconversion between cis-2-C2H2F and 1-C2H2F that is ob- served in both types of experiments. EXPERIMENTAL The experimental techniques are similar to those used in our previous studies (3). Samples were prepared by vacuum co- The first attempt to obtain the EPR spectrum of ␤-C2D2F deposition of gaseous mixtures Ar/C2H2 and Ar/F2 onto a radical by F2 photolysis of solid ternary mixtures Ar/F2/C2D2 at substrate (CsI salt window in IR experiments, or a sapphire rod 4.2 K was made in 1970 by Cochran et al. (11). They observed in the EPR experiments) maintained at 15 K. The composition a poorly resolved anisotropic spectrum, but they were able to of typical samples was Ar:F2:C2H2 ϭ 3000:1:1, though the Ϫ3 determine isotropic hf constants for the F atom (a F ϭ 6.9 mT) relative concentration of reactants were varied from 3 ϫ 10 Ϫ4 and deuterium (a D ϭ 0.6 mT). Based on the known isotropic hf to 2.5 ϫ 10 . The thickness of the samples was approximately constants of both isomers of C2H3 radical (12), they ascribed 100 ␮m. the detected EPR spectrum to trans-␤-C2D2F radical. Better- The EPR experiments were carried out with a homemade resolved spectra of 2-fluorovinyl radical in an SF6 matrix were helium flow cryostat with a movable helium shaft (3). The obtained by Shiotani et al. (13). These authors also assigned temperature stability was ϳ0.1 K over the range 15–40 K. EPR the observed spectra to trans-2-C2H2F radical. Such assign- spectra were recorded using standard 9 GHz spectrometer. ment was based on the results of semiempirical INDO MO Infrared spectra were recorded with a Mattson Model RS/ calculations, which predicted that trans isomer is more stable 10000 FTIR spectrometer (spectral region from 500 to 4000 than the cis isomer. The infrared absorption spectrum of ma- cmϪ1 and spectral resolution 0.5 cmϪ1). The closed-cycle he- trix-isolated fluorovinyl radical was reported by Jacox (14)by lium refrigerator (APD Cryogenics Model DE-202) was used trapping the gas-phase products of the F ϩ C2H2 reaction. to maintain the sample temperature in these experiments. In a Many different species were generated in this experiment, and separate experiment we verified that the light from the ceramic it required exceptional skill and scrupulousness to separate the globar source did not induce any chemical reactions. infrared bands of 2-C2H2F radical from the variety of other Fluorine atoms were generated by F2 photolysis at ␭ ϭ 355 bands. Jacox obtained a reasonable fit of the isotopic data to a nm (third harmonic of Nd:YAG laser in IR experiments) and diagonal valence force field potential using the assumed struc- ␭ ϭ 337 nm (N2 laser in EPR experiments). The average power 2 ture of trans-2-C2H2F radical, and with the support of the did not exceed 10 mW/cm in either type of experiment. To previous EPR study (11), she assigned the observed spectrum distinguish the chemical reactions involving photogenerated F to trans-2-C2H2F. atoms from those of diffusing thermal atoms, photolysis of F2 The first reliable calculations of spectroscopic characteristics molecules was performed at 15 K. Fluorine atoms are able to of fluorovinyl radicals performed with post-Hartree–Fock diffuse in solid argon at temperatures above 20 K. To initiate methods and density functional methods appeared in 1993 reactions of thermal F atoms, we performed annealing of (15).