Carbyne thermochemistry from energy-resolved collision-induced dissociation. The heats of formation of CH, CF, and CCl

The 298 K heats of formation of the carbynes CH, CF, and CCl have been dete rmined from measurements of the chloride dissociation energies of the haloc arbene anions, CHCl.-, CFCl.-, and CCl2.-. The chloride dissociation enthal pies were measured by energy-resolved collision-induced dissociation in a f lowing afterglow-triple quadrupole instrument to be 37.7 +/- 2.3, 21.9 +/- 1.1, and 33.4 +/- 2.3 kcal/mol, for CHCl.-, CFCl.-, and CCl2.-, respectivel y. The dissociation enthalpies were combined with the known heats of format ion and electron affinities of the corresponding halocarbenes and other the rmochemical data to give: Delta H-f,H-298 (CH) = 142.2 +/- 3.2, Delta H-f,H -298 (CF) = 60.6 +/- 3.4, and Delta H-f,H-298 (CCl) = 105.9 +/- 3.1 kcal/mo l. The experimental results for all three carbynes are in good agreement wi th the predicted heats of formation obtained from G2 calculations. The meas ured values for CH and CF are in excellent agreement with other experimenta l values, whereas results for CCl do not support the heats of formation for this carbyne given in current thermochemical data compilations. The haloge n substituent effects on carbyne stabilities and on the sequential C-H bond energies of CH3F and CH3Cl are shown to be dominated by pi-donor interacti ons. (Int J Mass Spectrom 185/186/187 (1999) 745-757) (C) 1999 Elsevier Sci ence B.V.