A REINVESTIGATION OF SINGLET BENZYNE THERMOCHEMISTRY PREDICTED BY CASPT2, COUPLED-CLUSTER AND DENSITY-FUNCTIONAL CALCULATIONS

Recent CASPT2 calculations of the heats of formation of the isomeric b enzynes by R. Lindh and M. Schutz [Chem. Phys. Lett. 258 (1996) 409] a re reexamined. The unrealistically low value reported for p-benzyne (1 32.7 kcal/mol) is shown to be an artifact of the use of incorrect CASS CF and CASPT2 energies for p-benzyne, as well as a flawed isodesmic re action analysis. Use of the correct energies and an appropriate isodes mic reaction leads to excellent agreement between the calculated and m easured heats of formation for p-benzyne. The performance of coupled-c luster methods and density functional theory in predicting benzyne the rmochemistry and singlet-triplet splittings is also evaluated. (C) 199 7 Elsevier Science B.V.