Spectroscopic constants of the X̃(1A1), ã(3B1), and Ã(1B1) states of CF2, CCl2, and CBr2 and heats of formation of selected halocarbenes: An ab initio quantum chemical study

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Abstract
The geometries,rotational constants, harmonic force constants and frequencies, dissociation and term energies of CF 2 , CCl 2 , and CBr 2 in their respective X̃( 1 A 1 ), ã( 3 B 1 ) and Ã( 1 B 1 ) states, computed by complete active space self-consistent field (CASSCF), complete active space second-order purturbation (CASPT2), and coupled-cluster with single, double and perturbative triple excitations [CCSD(T)] methods and cc-pVTZ basis sets, are reported. For CCl 2 and CBr 2 the barriers to linearity are also characterized. The computed spectroscopic constants are in good agreement with the available experimental data. The atomization energies and hence heats of formation at 0 and 298 K of these molecules as well as of CHF, CHCl, and CFCl, all in their lowest singlet ground states were 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 the energies to a complete basis set (CBS) limit and the inclusion of core–valence correlation (with the exception of CBr 2 ). Scalar relativistic corrections, computed by CASPT2, were also taken into account. The final CBS results are Δ f H 298 0 ( CHF )=147±4 kJ mol −1 , Δ f H 298 0 ( CHCl )=320±4 kJ mol −1 , Δ f H 298 0 ( CFCl )=29±4 kJ mol −1 , Δ f H 298 0 ( CCl 2 )=229±4 kJ mol −1 , and Δ f H 298 0 ( CBr 2 )=337±8 kJ mol −1 . The GAUSSIAN-3 (G3) heats of formation of these molecules are within 2 kJ mol −1 of the CBS values, representing significant improvement over the GAUSSIAN-2 predictions which differ from the CBS results by up to ∼10 kJ mol −1 . Similarly, the G3 triplet/singlet energy separations 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.

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