DENSITY-FUNCTIONAL STUDY OF ISOMERIZATION OF FLUOROFORMALDEHYDE AND CHLOROFORMALDEHYDE RADICAL CATIONS

Ab initio and density functional (DFT) calculations were performed on radical cations with the formula HXCO(+.) (X = H, F, and Cl) and their isomers XCOH(+.). Hartree-Fock, Moller-Plesset at second order (MP2), and quadratic configuration interaction including singles and doubles (QCISD) methods were employed for geometry optimizations at the ab in itio level. Becke's 1988 and three-parameter exchange potentials, toge ther with Vosko-Wilk-Nusair (VWN) and Lee-Yang-Paar (LYP) correlation potentials (BVWN, BLYP, and B3LYP) were used for the DFT calculations. HF and MPn isomerization energies are severely in error, mostly due t o a bad description of the XHCO(+.) cation radicals at the Hartree-Foc k level, leading to oscillatory behavior of the perturbation series. Q CISD methods, at least, are needed to obtain accurate results at the c onventional ab initio level, even using large extended basis sets. B3L YP results are most similar to QCISD results for the isomerization ene rgies of the three cation radicals. Parent neutral species are also de scribed to a high degree of accuracy. B3LYP methods are faster than QC ISD (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. Th e best theoretical results obtained indicate that fluoro- and chlorofo rmaldehyde cation radicals are less stable than their hydroxyfluoro- a nd 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), +3 7 +/- 2 kJ/mol (X = F), and +28 +/- 2 kJ/mol (X = Cl). Ionization ener gies of 10.9, 12.3, and 11.4 eV are predicted for the XHCO species (X = H, F, CI). (C) 1996 by John Wiley & Sons, Inc.