CHEMICAL AND THERMODYNAMIC PROPERTIES OF METHYL-CHLORIDE DIMER RADICAL CATIONS IN THE GAS-PHASE

Dimer radical cations of methyl chloride have been prepared in a Fouri er transform ion cyclotron resonance mass spectrometer using a consecu tive exchange reaction of the xenon dimer radical cation with methyl c hloride. Both theory and experiment have indicated the existence of tw o different stable methyl chloride dimer radical cations, that is a tw o-center three-electron (2c/3e) bonded dimer radical cation, CH3Cl the refore ClCH3.+, with C-2 symmetry and a methyl chloride chloronium yli dion structure, CH3Cl . H . ClCH2.+, with C-s symmetry. The CH3(C)l th erefore ClCH3.+ species has been observed to react both as an electron acceptor and as a proton donor. The CH3Cl . H . ClC(H)3(.+) structure , however, reacts only as an electron acceptor. Calculations of the CH 3Cl therefore ClCH3.+ structure at the MP2/6-311G(d,p) level of theory indicate a bond dissociation energy of 105.4 kJ mol(-1) relative to i ts ionized and neutral monomer products, while experiments show a bond dissociation energy of 109 +/- 5 kJ mol(-1). The bond dissociation en ergy of the CH3Cl . HClCH2.+ species has been determined to be 62.8 kT mol(-1) at the MP2/6-311G(d,p) level of theory, whereas experiments i ndicate 76 kT mol(-1) relative to the radical cation of methyl chlorid e and its neutral counterpart as products. These results indicate that CH3Cl therefore ClCH3.+ is the intermediate complex in the electron t ransfer reaction and CH3Cl . H . ClCH2.+ the intermediate complex in t he proton transfer reaction between the methyl chloride radical cation and neutral methyl chloride.