Air-water partition coefficients for a suite of polycyclic aromatic and other C10 through C20 unsaturated hydrocarbons

The air-water partition coefficients (K-aw) for 86 large polycyclic aromatic hydrocarbons and their unsaturated relatives were estimated using high-level G4(MP2) gas and aqueous phase calculations with the SMD, IEFPCM-UFF, and CPCM solvation models. An extensive method validation effort was undertaken which involved confirming that, via comparisons to experimental enthalpies of formation, gas-phase energies at the G4(MP2) level for the compounds of interest were at or near thermochemical accuracy. Investigations of the three solvation models using a range of neutral and ionic compounds suggested that while no clear preferential solvation model could be chosen in advance for accurate K-aw estimates of the target compounds, the employment of increasingly higher levels of theory would result in lower K-aw errors. Subsequent calculations on the polycyclic aromatic and unsaturated hydrocarbons at the G4(MP2) level revealed excellent agreement for the IEFPCM-UFF and CPCM models against limited available experimental data. The IEFPCM-UFF-G4(MP2) and CPCM-G4(MP2) solvation energy calculation approaches are anticipated to give K-aw estimates within typical experimental ranges, each having general K-aw errors of less than 0.5 log(10) units. When applied to other large organic compounds, the method should allow development of a broad and reliable K-aw database for multimedia environmental modeling efforts on various contaminants.