Isomerization and Fragmentation Products of CH2Cl2 and Other Dihalomethanes in Rare-Gas Matrices:  An Electron Bombardment Matrix-Isolation FTIR Spectroscopic Study

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2000-04, Vol.104 (16), p.3487-3497
• Main Authors: Fridgen, Travis D, Zhang, Xiaokui K, Parnis, J. Mark, March, Raymond E
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp993162u

Isodihalomethanes have been isolated by electron bombardment of CH2Cl2, CD2Cl2, CH2Br2, or CH2ClBr 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 CH2Cl2 and CH2Br2, 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 CH2Cl2 •+ to CH2ClCl•+ 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 CH2Cl2 •+ (HClC•+−ClH) and the other a complex between CH2Cl+ and a chlorine atom [(CH2Cl+)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.