Comparative semiempirical, ab initio, and density functional theory study on the thermodynamic properties of linear and branched perfluoroalkyl sulfonic acids/sulfonyl fluorides, perfluoroalkyl carboxylic acid/acyl fluorides, and perhydroalkyl sulfonic acids, alkanes, and alcohols

Abstract

A systematic and comprehensive semiempirical, Hartree–Fock (HF) ab initio, and B3LYP density functional theory (DFT) study was conducted on the relative thermodynamic properties of various linear and branched perfluorinated and perhydrogenated alkyl compounds. The semiempirical AM1, PM3, and PM6 methods all consistently and accurately predict that branched alkyl compounds will generally be more thermodynamically stable than their linear counterparts. In contrast, HF and B3LYP calculations with the 6-31G(d,p), 6-31++G(d,p), and 6-311++G(d,p) basis sets predict that linear isomers will be more stable than branched analogs. These different linear versus branched perfluoroalkyl/perhydroalkyl thermodynamic property trends between semiempirical and ab initio/DFT methods were evident in both gas and aqueous phase calculations. Comparison of experimentally determined thermodynamic properties for several classes of linear and branched alkanes and alcohols with values calculated at the PM6 and B3LYP/6-311++G(d,p) levels of theory supported the well known findings that such DFT and HF approaches incorrectly predict branched alkyl compounds will be less thermodynamically stable than linear isomers. Calculations at the MP2/6-311++G(d,p)//B3LYP/6-311++G(d,p) and M05-2X/6-311++G(d,p) levels of theory on a representative subset of the linear and branched perfluorinated compounds supported the thermodynamic conclusions from the PM6 method. Strong agreement between PM6 estimated thermodynamic properties and available experimental data supports use of this computational method for accurately calculating the well established higher thermodynamic stability of branched alkyl compounds. Branched perfluoroalkyl compounds are thus expected to be more thermodynamically stable than their linear analogs.