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

Abstract

The geometries, rotational constants, harmonic force constants and frequencies, dissociation and term energies of CF2, CCl2, and CBr2 in their respective X̃(1A1), ã(3B1) and Ã(1B1) 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 CCl2 and CBr2 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 CBr2). Scalar relativistic corrections, computed by CASPT2, were also taken into account. The final CBS results are ΔfH2980(CHF)=147±4 kJ mol−1, ΔfH2980(CHCl)=320±4 kJ mol−1, ΔfH2980(CFCl)=29±4 kJ mol−1, ΔfH2980(CCl2)=229±4 kJ mol−1, and ΔfH2980(CBr2)=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.