ABSOLUTE HEATS OF FORMATION OF PHENYLCARBENE AND VINYLCARBENE

The method of collision-induced dissociation threshold analysis for de termining carbene thermochemistry is applied to the ground-state tripl et carbenes CH2 (methylene, 1), CH2=CHCH (vinylcarbene, 2), and PhCH ( phenylcarbene, 3). The chief aims of this study are to evaluate the en ergetic and dynamical consequences of the obligatory curve-crossing th at characterizes dissociation of a halide ion from an alpha-halocarban ion to form a carbene with a triplet ground state, and to determine ac curate heats of formation for 2 and 3, Threshold collision energies fo r loss of halide from CH2X- (X = Cl, Br), CH2CHCHX- (X = Cl, Br, I), a nd PhCHX- (X = Cl, Br, I) are determined with use of a flowing aftergl ow-triple quadrupole apparatus. The dissociation energies are combined with the measured gas-phase acidities of the corresponding methyl, al lyl, and benzyl halides in simple thermochemical cycles in order to de rive the absolute heats of formation for the carbenes. The value of De lta H-f,H-298(1) derived from the results for the two different methyl halides (92.2 +/- 3.7 kcal/mol) is in excellent agreement with the we ll-established literature value for the triplet ground state of methyl ene: Delta H-f,H-298[(X) over tilde B-3(1) CH2] = 92.9 +/- 0.6 kcal/mo l. The goad agreement indicates that dissociation of the halomethyl an ions occurs adiabatically to produce the triplet state of the product without any significant reverse activation energy or dynamical constra ints. The measured dissociation energies for 1-chloro-, 1-bromo-, and 1-iodoallyl anions are combined with the bracketed acidities of allyl chloride, bromide, and iodide to yield three independently determined but closely matched values for Delta H-f,H-298(2): 92.3 +/- 2.6, 93.9 +/- 3.4, and 93.2 +/- 3.1 kcal/mol, respectively. The average value fr om the three determinations, 93.3 +/- 3.4 kcal/mol, is in fair agreeme nt with the estimated heat of formation for the triplet ground state o f 2 obtained from various MCSCF and density functional calculations (9 0 kcal/mol), but much lower than the predicted heat of formation for t he lowest singlet state of 2 (100 kcal/mol). As with the halomethyl io ns, efficient adiabatic dissociation of the haloallyl anions at the th ermodynamic limit is indicated by these results. The apparent heats of formation for 3 derived from the measured dissociation energies for P hCHCl-, PhCHBr- and PhCHI-, and the bracketed acidities of the corresp onding benzyl halides show a somewhat larger (non-systematic) variatio n, but are all within the assigned uncertainties. The derived values f or Delta H-f,H-298(3) are 103.2 +/- 3.2, 105.5 +/- 2.7, and 100.9 +/- 2.8 kcal/mol for the benzyl chloride. bromide, and iodide systems, res pectively, giving an average value of 102.8 +/- 3.5 kcal/mol. The meas ured heats of formation for 2 and 3 are compared with the predictions obtained from various levels of ab initio theory. Density functional c alculations with the BVWN5 and B3LYP functionals in conjunction with p olarized, triple-zeta basis sets are found to perform best with respec t to the singlet-triplet splittings and absolute heats of formation, w hile MCSCF and CISD methods lead to S-T gaps and heats of formation th at are too high, The experimental thermochemistry is used to derive va lues for the alpha-CH bond strengths in allyl radical and benzyl radic al: DH298[CH2=CHCH-H] = 104.0 +/- 3.4 kcal/mol and DH298[PhCH-H] = 105 .2 +/- 3.5 kcal/mol.