In 2014, the U.S. National Response Center recorded more than 30000 incidents of oil spills, chemical releases, or maritime security issues, including crude (4-methylcyclohexyl)­methanol (MCHM) that contaminated river and drinking water in West Virginia. This research yielded physicochemical partitioning data for the two major compounds released in West Virginia, cis- and trans-(4-methylcyclohexyl)­methanol. Octanol–water partition coefficients (K OW) were 225 for cis-4-MCHM and 291 for trans-4-MCHM. The aqueous solubility for total 4-MCHM was 2250 mg/L at 23 °C; solubilities of individual isomers were dependent on their mole fractions. The cis isomer was more soluble and less well sorbed to activated carbon than the trans isomer, consistent with its lower K OW. The partition behavior is supported by a larger computed solvated dipole moment for the cis form than for the trans form at the MP2 aug-cc-pwCVDZ SMD level of theory. Different partition properties would result in the differential fate and transport of cis- and trans-4-MCHM in aqueous environments.

Accurate computational estimates of fundamental physical properties can be used as inputs in the myriad of extant models employed to predict toxicity, transport, and fate of contaminants. However, as molecular complexity of contaminants increases, it becomes increasingly difficult to determine the magnitude of the errors introduced by ignoring the 3D conformational space averaging within group-additivity and semi-empirical approaches. The importance of considering 3D molecular structure is exemplified for the dipole moments of cis and trans isomers of (4-methylcyclohexyl)methanol (4-MCHM). When 10 000 gallons of 4-MCHM was spilled into the Elk River in January 2014, a lack of toxicological data and environmental partitioning coefficients hindered the immediate protection of human health and the local water supply in West Virginia, USA. Post-spill analysis of the contaminants suggested that the cis and trans isomers had observably different partitioning coefficients and solubility, and thus differing environmental fates. Obtaining high-quality dipole moments using ab initio quantum chemical methods for the isomeric pair was crucial in validating their experimental differences in solubility [Environ. Sci. Technol. Lett., 2015, 2, 127]. The use of first principles electronic structure theory is further explored here to obtain accurate conformer relative energies and dipole moments of cis- and trans-4-MCHM. Overall, the MP2 aug-cc-pVDZ level of theory affords the best balance between accuracy and computational cost.

We report the intermolecular binding energies (IBEs) between UF6 and over 50 different functionalized small organic molecules as predicted by electronic structure calculations. Optimized geometries of UF6–molecule dimers were found at the MP2/aug-cc-pwCVDZ (non-U), cc-pVDZ-PP (U) level. IBEs were calculated using MP2 and dispersion-corrected DFT theory. We characterize the various functional groups based on the inclusion of specific heteroatoms. Those functional groups containing “nitrogen only” heteroatoms result in larger IBEs than groups containing both nitrogen and oxygen or oxygen alone. Halogen-containing and regular hydrocarbon molecules show the lowest IBEs with UF6. Nonorganic phosphoryl species are also shown to display large IBEs with UF6. These interactions are characterized in part by how much the impinging functionalized molecule distorts the UF6 from its optimal octahedral geometry. Of all the investigated groups, the amine group displayed the largest IBE values (IBE ∼ >12–14 kcal/mol for methyl amine), while hydrocarbons and perfluorocarbons both showed the weakest interactions (IBE ∼ 0.5–1.5 and 0.1–0.8 kcal/mol for methane and perfluoromethane, respectively). The study examines how the strength of the IBE is contingent on a combination of conformational deformation, stabilizing nonbonding interactions, and sterics.