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Ab initio and density functional (OFT) calculations were performed on radical cations with the formula HXCO+' (X = H, F, and Cl) and their isomers XCOH+·. Hartree-Fock, Meller-Plesset at second order (MP2), and quadratic configuration interaction including singles and doubles (QCISO) methods were employed for geometry optimizations at the ab initio level. Becke's 1988 and three-parameter exchange potentials, together with Vosko-Wilk-Nusair (VWN) and Lee-Yang-Paar (LYP) correlation potentials (BVWN, BLYP, and B3LYP) were used for the OFT calculations. HF and MPn isomerization energies are severely in error, mostly due to a bad description of the XHCO+' cation radicals at the Hartree-Fock level, leading to oscillatory behavior of the perturbation series. QCISO methods, at least, are needed to obtain accurate results at the conventional ab initio level, even using large extended basis sets. B3LYP results are most similar to QCISO results for the isomerization energies of the three cation radicals. Parent neutral species are also described to a high degree of accuracy. B3LYP methods are faster than QCISO (and as exact as them) for all the cases studied here. MP2 methods, although giving incorrect results, are faster than B3LYP up to about 80 basis functions. For larger problems, B3LYP methods are faster. The best theoretical results obtained indicate that fluoroand chloroformaldehyde cation radicals are less stable than their hydroxyfluoroand hydroxychloromethylene isomers, the reverse situation than for the formaldehyde cation radical. The best values found in this article for the isomerization energy of XHCO+' are -26 ± 2 kJ/mol (X = H), +37 ± 2 kJ/mol (X = F), and + 28 ± 2 kJ/mol (X = Cl), Ionization energies of 10.9, 12.3, and 11.4 eV are predicted for the XHCO species (X = H, F, en © 1996 by John Wiley & Sons, Inc. * Author to whom all correspondence should be addressed. E-mail: oscar@bilbo.edu.uy Journal of Computational Chemistry, Vol. 17, No. 11, 1309 -1317 (1996) © 1996 by John Wiley & Sons, Inc. CCC 0192-8651 /96/111309-09 VENTURA, KIENINGER, AND COITINO

Density functional study of isomerization of fluoro- and chloroformaldehyde radical cations