Thermodynamic stability of PFOS: M06-2X and B3LYP comparison

The thermodynamic stability in the gas phase, water and octanol of the neutral and anionic forms of the 89 PFOS isomers is studied at the M06-2X/6-311++G(d,p) level of theory using the SMD solvation model. The structure–stability relationship is explored and compared to previous B3LYP calculations. This study aims to improve the thermodynamic data for these compounds using a functional that is expected to overcome known deficiencies of the B3LYP functional when considering medium-range electron correlation effects. The M06-2X results place emphasis on the substitution patterns at the head and tail ends of the molecule as a factor in stability, while substitution patterns on the middle carbons tend to decrease stability. Analysis of the electron density revealed a subtle balance between weak interactions in these perfluorinated compounds. M06-2X ranks highly-substituted isomers as more stable than B3LYP did, and ranks linear isomers quite low in relative stability compared to B3LYP. It has been shown that the difference in stability for these compounds is mostly enthalpic, with small entropy changes between the two functionals for most compounds.