Thermochemical properties of halogen-substituted methanes, methyl radicals, and carbenes in the gas phase

The thermochemical properties of a series of chlorine- and bromine-substituted methanes, methyl radicals, and carbenes have been (re)examined on the basis of ion-molecule reactions as performed with the use of Fourier transform ion cyclotron resonance mass spectrometry. In particular, the occurrence of reversible proton transfer in the reactions of the CHCLBr-, CHBr2-, and CBr3- anions with selected reference acids has been used to obtain the following gas-phase acidities: Delta H(acid)degrees(CH2ClBr) = 1560 +/- 8 kJ mol(-1), Delta H(acid)degrees(CH2Br2) = 1544 +/- 8 U mol(-1), and Delta H(acid)degrees(CHBr3) = 1463 +/- 8 kJ mol(-1). In addition, the gas-phase acidities of the (CHCl2)-C-., . CHClBr, and . CHBr2 radicals have been placed at 1524 +/- 10, similar to 1509, and similar to 1494 kJ mol(-1), respectively. The values for the (CHClBr)-C-. and (CHBr2)-C-. radicals have been derived on the basis of the occurrence/nonoccurrence of proton transfer in the reactions of the CClBr-. and CBr2-. carbene radical anions with various reference acids in combination with the trend in the gas-phase acidities of bromine containing methanes and methyl radicals. The present gas-phase acidities together with experimentally determined electron affinities of the dihalogen-substituted carbenes have been used to estimate the enthalpies of formation of the CCl2, CClBr, and CBr2 carbenes [Delta H(f)degrees(CCl2) = 226 +/- 25 kJ mol, Delta H(f)degrees(CClBr) similar to 267 kJ mol, and Delta H(f)degrees(CBr2) similar to 324 kJ mol(-1)). (Int J Mass Spectrom 194 (2000) 103-113) (C) 2000 Elsevier Science B.V.