Absolute Heats of Formation of Phenylcarbene and Vinylcarbene

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Abstract

The method of collision-induced dissociation threshold analysis for determining carbene thermochemistry is applied to the ground-state triplet carbenes CH2 (methylene, 1), CH2CHCH (vinylcarbene, 2), and PhCH (phenylcarbene, 3). The chief aims of this study are to evaluate the energetic and dynamical consequences of the obligatory curve-crossing that characterizes dissociation of a halide ion from an α-halocarbanion to form a carbene with a triplet ground state, and to determine accurate heats of formation for 2 and 3. Threshold collision energies for loss of halide from CH2X- (X = Cl, Br), CH2CHCHX- (X = Cl, Br, I), and PhCHX- (X = Cl, Br, I) are determined with use of a flowing afterglow−triple quadrupole apparatus. The dissociation energies are combined with the measured gas-phase acidities of the corresponding methyl, allyl, and benzyl halides in simple thermochemical cycles in order to derive the absolute heats of formation for the carbenes. The value of ΔHf,298(1) derived from the results for the two different methyl halides (92.2 ± 3.7 kcal/mol) is in excellent agreement with the well-established literature value for the triplet ground state of methylene: ΔHf,298[X̃ 3B1 CH2] = 92.9 ± 0.6 kcal/mol. The good agreement indicates that dissociation of the halomethyl anions occurs adiabatically to produce the triplet state of the product without any significant reverse activation energy or dynamical constraints. 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 ΔHf,298(2): 92.3 ± 2.6, 93.9 ± 3.4, and 93.2 ± 3.1 kcal/mol, respectively. The average value from the three determinations, 93.3 ± 3.4 kcal/mol, is in fair agreement with the estimated heat of formation for the triplet ground state of 2 obtained from various MCSCF and density functional calculations (90 kcal/mol), but much lower than the predicted heat of formation for the lowest singlet state of 2 (100 kcal/mol). As with the halomethyl ions, efficient adiabatic dissociation of the haloallyl anions at the thermodynamic limit is indicated by these results. The apparent heats of formation for 3 derived from the measured dissociation energies for PhCHCl-, PhCHBr- and PhCHI-, and the bracketed acidities of the corresponding benzyl halides show a somewhat larger (non-systematic) variation, but are all within the assigned uncertainties. The derived values for ΔHf,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, respectively, giving an average value of 102.8 ± 3.5 kcal/mol. The measured heats of formation for 2 and 3 are compared with the predictions obtained from various levels of ab initio theory. Density functional calculations with the BVWN5 and B3LYP functionals in conjunction with polarized, triple-ζ basis sets are found to perform best with respect to the singlet−triplet splittings and absolute heats of formation, while MCSCF and CISD methods lead to S−T gaps and heats of formation that are too high. The experimental thermochemistry is used to derive values for the α-CH bond strengths in allyl radical and benzyl radical: DH298[CH2CHCH−H] = 104.0 ± 3.4 kcal/mol and DH298[PhCH−H] = 105.2 ± 3.5 kcal/mol.

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