Spectroscopic constants of the (X)over-tilde((1)A(1)), (a)over-tilde(B-3(1)), and (A)over-tilde(B-1(1)) states of CF2, CCl2, and CBr2 and heats of formation of selected halocarbenes: An ab initio quantum chemical study

The geometries, rotational constants, harmonic force constants and frequenc ies, dissociation and term energies of CF2, CCl2, and CBr2 in their respect ive (X) over tilde((1)A(1)), (a) over tilde(B-3(1)) and (A) over tilde(B-1( 1)) states, computed by complete active space self-consistent field (CASSCF ), complete active space second-order purturbation (CASPT2), and coupled-cl uster with single, double and perturbative triple excitations [CCSD(T)] met hods and cc-pVTZ basis sets, are reported. For CCl2 and CBr2 the barriers t o linearity are also characterized. The computed spectroscopic constants ar e in good agreement with the available experimental data. The atomization e nergies and hence heats of formation at 0 and 298 K of these molecules as w ell as of CHF, CHCl, and CFCl, all in their lowest singlet ground states we re also computed by the CCSD(T) method utilizing basis sets ranging from cc -pVDZ to aug-cc-pVQZ, cc-pCVQZ and G3large, enabling the extrapolation of t he energies to a complete basis set (CBS) limit and the inclusion of core-v alence correlation (with the exception of CBr2). Scalar relativistic correc tions, computed by CASPT2, were also taken into account. The final CBS resu lts are Delta(f)H(298)(0)(CHF)=147 +/- 4 kJ mol(-1), Delta(f)H(298)(0)(CHCl )=320 +/- 4 kJ mol(-1), Delta(f)H(298)(0)(CFCl)=29 +/- 4 kJ mol(-1), Delta( f)H(298)(0)(CCl2)=229 +/- 4 kJ mol(-1), and Delta(f)H(298)(0)(CBr2)=337 +/- 8 kJ mol(-1). The GAUSSIAN-3 (G3) heats of formation of these molecules ar e within 2 kJ mol(-1) of the CBS values, representing significant improveme nt over the GAUSSIAN-2 predictions which differ from the CBS results by up to similar to 10 kJ mol(-1). Similarly, the G3 triplet/singlet energy separ ations are found to be in good agreement with those obtained at the CCSD(T) /cc-pVQZ level of theory including core-valence correlation corrections, as well as experiment. (C) 2000 American Institute of Physics. [S0021-9606(00 )30704-8].