The simpler the better? Enthalpies of formation and entropies of short-chain chlorinated paraffins by ab initio, DFT, group additivity and semiempirical approximations

An automated reaction-based approach coupled with DLPNO-CCSD(T)/CBS calculations was used to derive enthalpies of formation (Δ_fH⁰) of 32 short-chain chlorinated paraffins (SCCPs) containing 10–13 C atoms and 3–11 Cl atoms. Absolute entropies (S) were obtained using B3LYP-D3(BJ)/def2-TZVP optimized geometries and vibrational frequencies in the framework of the modified and scaled rigid rotor – harmonic oscillator (msRRHO) approximation. Conformational corrections to entropies were estimated using the CREST program and GFNn-xTB/FF methods. Examination of the performance of fast semiempirical methods reveals them to be suitable for the express evaluation of molecular entropies (mean unsigned deviation MUD_RM1 = 2.3 cal mol⁻¹ K⁻¹), while only PM6 method reproduces reference Δ_fH⁰ with MUD_PM6 = 2.4 kcal mol⁻¹, approximately within the estimated average error limits (2.8 kcal mol⁻¹). In contrast, simple Benson's group additivity method (GA) yields Δ_fH⁰ values with MUD_GA = 0.8 kcal mol⁻¹ but turns out to significantly overestimate entropies with MUD_GA = 20.4 cal mol⁻¹ K⁻¹. The obtained datasets of reliable Δ_fH⁰ and S values can be used for the further development of fast and efficient methods for the gas-phase thermochemistry evaluations of medium-sized molecules with rich conformational landscapes.