Isomerization and fragmentation products of CH2Cl2 and other dihalomethanes in rare-gas matrices: An electron bombardment matrix-isolation FTIR spectroscopic study

Isodihalomethanes have been isolated by electron bombardment of CH2Cl2, CD2 Cl2, CH2Br2, or CH2ClBr in argon, krypton, or xenon followed by condensatio n on a 15 K matrix-isolation window. Numerous neutral and ionized decomposi tion products of dihalomethane ionization were also observed. Irradiation w ith visible or UV light, isotopic substitution, and previous literature ass ignments 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 invol ving substitution of argon with krypton or xenon gas, mixtures of CH2Cl2 an d CH2Br2. rare-gas resonant emission irradiation, and thermodynamic conside rations support the proposed mechanism for isomerization of the dihalometha ne 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 throu gh electron capture. An upper limit to the barrier for CH2Cl2.+ to CH2ClCl. + isomerization of 43 kJ mol(-1) is deduced following observation of the is odichloromethane product after EBMI of xenon/dichloromethane mixtures. Two isomers of the molecular cation, one resembling the distonic isomer of CH2C l2.+ (HClC.+-ClH) and the other a complex between CH2Cl+ and a chlorine ato m [(CH2Cl+)Cl-.] have been distinguished based on their stability with resp ect 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 x enon matrices are reported for the first time. We propose reasoning for the general observation that ions that have an electron affinity (EA) greater than similar to 10.8 eV (the "5 eV rule") are not observed in argon matrice s, but those with EAs less than 10.8 eV are observed.