Accurate ab initio determination of spectroscopic and thermochemical properties of mono- and dichlorocarbenes

Abstract

The best technically feasible values for the triplet–singlet energy gap and the enthalpies of formation of the HCCl and CCl₂ radicals have been determined through the focal-point approach. The electronic structure computations were based on high-level coupled cluster (CC) methods, up to quadruple excitations (CCSDTQ), and large-size correlation-consistent basis sets, ranging from aug-cc-pVDZ to aug-cc-pV6Z, followed by extrapolation to the complete basis set limit. Small corrections due to core correlation, relativistic effects, diagonal Born–Oppenheimer correction, as well as harmonic and anharmonic zero-point vibrational energy corrections have been taken into account. The final estimates for the triplet–singlet energy gap, T₀(ã), are 2170 ± 40 cm⁻¹ for HCCl and 7045 ± 60 cm⁻¹ for CCl₂, favoring the singlet states in both cases. Complete quartic force fields in internal coordinates have been computed for both the and ã states of both radicals at the frozen-core CCSD(T)/aug-cc-pVQZ level. Using these force fields vibrational energy levels of {HCCl, DCCl, CCl₂} up to {6000, 5000, 7000} cm⁻¹ were calculated both by second-order vibrational perturbation theory (VPT2) and variationally. These results, especially the variational ones, show excellent agreement with the experimentally determined energy levels. The enthalpies of formation of HCCl (¹A^′) and CCl₂(¹A₁), at 0 K, are 76.28 ± 0.20 and 54.54 ± 0.20 kcal mol⁻¹, respectively.