Molecular, spectroscopic and thermochemical characterization of C₂Cl₃, C₂F₃ and C₂Br₃ radicals and related species

This work reports a detailed theoretical study of the molecular parameters, harmonic vibrational frequencies, UV absorption spectra and standard enthalpies of formation for the radicals C₂X₃ (with X = F, Cl and Br) and a comparison with the corresponding determinations for the rest of the members of the family C₂X _n (with n = 2-4). Molecular properties were calculated using different levels of theory: density functional theory employing the B3LYP, X3LYP, BMK, M06-2X and M08-HX functionals combined with the basis sets 6-311++G(3df,3pd) and aug-cc-pVTZ, and the ab initio composite models G3B3 and G4. Structural and spectroscopic characterization of the C₂F₃, C₂Cl₃ and C₂Br₃ radicals, along with the estimation of the enthalpies of formation of C₂F₃ and C₂Cl₃, were derived here for the first time, to our knowledge. In particular, values of -220.9 ± 2.9, 230.8 ± 3.8 and 375.4 ± 5.9 kJ mol^-1 were computed for enthalpies of formation of C₂F₃, C₂Cl₃ and C₂Br₃, respectively. Additionally, enthalpies of formation for related closed-shell molecules were obtained with less uncertainty compared to those found in the literature. The recommended values of -669.6 ± 3.8, -23.0 ± 4.6 and 155.3 ± 5.0 kJ mol^-1 were derived for C₂F₄, C₂Cl₄ and C₂Br₄, while corresponding values of 0.6 ± 6.3, 228.1 ± 2.1 and 319.6 ± 5.4 kJ mol^-1 were estimated for C₂F₂, C₂Cl₂ and C₂Br₂, respectively.