On the heats of formation of methylketene, dimethylketene and related cations

Ab initio MO and DFT calculations up to the CCSD(T)/6-311+ + G(3df, 2p) lev el have been applied to the disagreement between theory and experiment conc erning the standard enthalpies of formation of methylketene and dimethylket ene. Our results confirm previous theoretical (G2) values, as well as the e xperimental proton affinities and ionization energies of both alkylketenes. A set of consistent values for Delta H(f, 298)degrees are estimated as fol low (kJ mol(-1)): CH3CH = C = O: -68; CH3CH = C = O.+: 795; CH3CH2C = O+: 6 18; (CH3)(2)C = C = O: -92; (CH3)(2)C = C = O.+: 723 and (CH3)(2)CHC = O+: 583. The proton affinities (kJ mol(-1)) are: PA(methylketene) = 842 and PA( dimethylketene) = 855. The persistent disagreement between the theoretical and experimental values appears to arise from an overestimation of the expe rimental heats of formation of CH3CH2C = O+, CH3CH = C = O.+ and (CH3)(2)C = C = O.+. (C) 1999 Elsevier Science B.V, All rights reserved.