Isomerization and Fragmentation Products of CH 2 Cl 2 and Other Dihalomethanes in Rare-Gas Matrices: An Electron Bombardment Matrix-Isolation FTIR Spectroscopic Study ² Travis D. Fridgen, § Xiaokui K. Zhang, | J. Mark Parnis,* and Raymond E. March Department of Chemistry, Trent UniVersity, Peterborough, Ontario, Canada K9J 7B8, and Department of Chemistry, Queen’s UniVersity, Kingston, Ontario, Canada K7L 3N6 ReceiVed: September 7, 1999; In Final Form: NoVember 17, 1999 Isodihalomethanes have been isolated by electron bombardment of CH 2 Cl 2 , CD 2 Cl 2 , CH 2 Br 2 , or CH 2 ClBr in argon, krypton, or xenon followed by condensation on a 15 K matrix-isolation window. Numerous neutral and ionized decomposition products of dihalomethane ionization were also observed. Irradiation with visible or UV light, isotopic substitution, and previous literature assignments of the matrix-isolated products allow definitive identification of most of the observed product bands in the infrared spectra recorded after electron bombardment matrix-isolation experiments (EBMI). Experiments involving substitution of argon with krypton or xenon gas, mixtures of CH 2 Cl 2 and CH 2 Br 2 , rare-gas resonant emission irradiation, and thermodynamic considerations support the proposed mechanism for isomerization of the dihalomethane radical cation in the gas phase. This mechanism involves charge-exchange ionization of dihalomethane followed by gas-phase isomerization, isolation, and stabilization in the solid matrix and subsequent neutralization through electron capture. An upper limit to the barrier for CH 2 Cl 2 •+ to CH 2 ClCl •+ isomerization of 43 kJ mol -1 is deduced following observation of the isodichloromethane product after EBMI of xenon/dichloromethane mixtures. Two isomers of the molecular cation, one resembling the distonic isomer of CH 2 Cl 2 •+ (HClC •+ -ClH) and the other a complex between CH 2 Cl + and a chlorine atom [(CH 2 Cl + )Cl • ] have been distinguished based on their stability with respect to UV-visible light irradiation, their infrared spectra, and published ab initio calculations. Vibrational wavenumbers for isodichloromethane and various other products of dichloromethane EBMI experiments in krypton and xenon matrices are reported for the first time. We propose reasoning for the general observation that ions that have an electron affinity (EA) greater than ∼10.8 eV (the “5 eV rule”) are not observed in argon matrices, but those with EAs less than 10.8 eV are observed. 1. Introduction The fate of molecular ions following their formation has been one of the central foci of gas-phase ion chemistry and mass spectrometry. Recently, we have investigated fragmentation and isomerization processes that follow formation of molecular ions using a combination of gas-phase electron bombardment and matrix-isolation infrared spectroscopy. Application of this technique to the formation and characterization of 1-propen-2- ol, 1 to the mixed rare-gas cations 2 (RgHRg′) + (Rg ) rare gas and Rg * Rg′), and to the elucidation of some gas-phase ion processes pertaining to the oxalyl chloride radical cation 3 has recently been reported. Physical, electronic, and spectroscopic properties of halo- methane systems have been investigated extensively 4-14 through a variety of methods in both the gas and the condensed phases. The neutral and ionic decomposition products of photoionization and radiolysis of halogenated methanes were first investigated by Andrews et al. 9 using matrix-isolation infrared and ultraviolet spectroscopy. Other techniques, such as excimer-laser irradia- tion, 10,14 electron impact 12 (EI), and chemical ionization 15 have since been employed to generate and/or to characterize spec- troscopically various chlorinated species. Maier et al. 10 have reported IR and UV-visible spectroscopic studies of the photoisomerization of dihalomethanes (CH 2 XY, X/Y ) I/I, Br/I, Cl/I, F/I, Br/Br, Cl/Br, Cl/Cl) isolated in argon matrices. UV irradiation of dihalomethanes trapped in argon matrices at 12 K generated visible-light-absorbing species that decomposed on subsequent irradiation. These species were proposed to be isomers of dichloromethane having an unusual structure involving a CH 2 X-Y linkage that was supported by both spectroscopic evidence and ab initio calculations. More recently, Lugez et al. 16 have observed the neutral tetrachlo- romethane isomer, Cl 2 CCl-Cl, in a neon matrix by co-depositing Ne/CCl 4 mixtures with neon atoms that have been excited in a microwave discharge. In the present work, the relatively new technique of electron bombardment and subsequent matrix isolation was applied to Rg/dihalomethane mixtures (Rg ) Ar, Kr, or Xe). Subsequently-recorded infrared spectra revealed that isodihalomethanes are produced in abundance. To our knowledge, isodichloromethane and many of the other fragmentation products of ionized dichloromethane have not been observed previously in krypton or xenon matrices. Wave- number positions of these species in both krypton and xenon are reported here, and spectroscopic assignments are supported by isotopic substitution and photochemical behavior. In addition, the mechanism for the formation of isodichloromethane via the ² Part of the special issue “Marilyn Jacox Festschrift”. * To whom all correspondence should be addressed at Trent University. § Present address: Department of Chemistry, University of Waterloo, Waterloo, ON, Canada N2L 3G1. E-mail: tdfridge@sciborg.uwaterloo.ca. | Present address: Genzyme Corporation, P.O. Box 9322, Framingham, MA, 01701. 3487 J. Phys. Chem. A 2000, 104, 3487-3497 10.1021/jp993162u CCC: $19.00 © 2000 American Chemical Society Published on Web 02/18/2000