Methods for Interpreting the Partitioning and Fate of Petroleum Hydrocarbons in a Sea Ice Environment.

Decreases in Arctic Sea ice extent and thickness have led to more open ice conditions, encouraging both shipping traffic and oil exploration within the northern Arctic. As a result, the increased potential for accidental releases of crude oil or fuel into the Arctic environment threatens the pristine marine environment, its ecosystem, and local inhabitants. Thus, there is a need to develop a better understanding of oil behavior in a sea ice environment on a microscopic level. Computational quantum chemistry was used to simulate the effects of evaporation, dissolution, and partitioning within sea ice. Vapor pressures, solubilities, octanol-water partition coefficients, and molecular volumes were calculated using quantum chemistry and thermodynamics for pure liquid solutes (oil constituents) of interest. These calculations incorporated experimentally measured temperatures and salinities taken throughout an oil-in-ice mesocosm experiment conducted at the University of Manitoba in 2017. Their potential for interpreting the relative movements of oil constituents was assessed. Our results suggest that the relative movement of oil constituents is influenced by differences in physical properties. Lighter molecules showed a greater tendency to be controlled by brine advection processes due to their greater solubility. Molecules which are more hydrophobic were found to concentrate in areas of lower salt concentration.

Conformational preference of fused carbohydrate-templated proline analogues--a computational study.

The structure of fused C-glucosylproline hybrid (GlcProH) has been studied in detail computationally. A systematic molecular mechanics/Monte Carlo search has been performed in order to cover the entire conformational space of GlcProH. This has been followed by density-functional (DFT B3LYP) calculations in the gas phase and in aqueous solution, using the polarizable continuum model (PCM). In the gas phase, a large excess of the cis conformation with respect to the prolyl amide bond is found. This is reversed in aqueous solution where the calculations show 80% trans conformers, which is in accordance with experimental data. Thus, the PCM model is capable of accurately predicting cis-trans ratios. The free energy of solvation is not correlated with the dipole moment. Hence, a model (such as PCM) is required that takes into account the complete charge distribution. The reversal of the cis-trans ratio between gas phase and solution also emphasizes the effects of different free energies of solvation for the distinct conformers. Nevertheless, the energy difference between the cis and trans conformers is very small in solution (0.18 kcal/mol). Intramolecular hydrogen bonding is found to stabilize the cis conformers exclusively, which is a result of the rigid geometry of the fused rings. This can be contrasted to related more flexible molecules that show hydrogen bonding for both cis and trans isomers. The hydrogen bonding is at least partially responsible for the preferential stabilization of the cis conformers in the gas phase and a very small cis-trans energy difference in solution.

Aggregation of alkyllithiums in tetrahydrofuran.

Density functional theory was used to examine the solvation number and aggregation state of several alkyllithium compounds in clusters with tetrahydrofuran molecules coordinated to each lithium atom. We then made the microsolvation approximation and approximated the bulk free energy of solvation by the free energy of clustering with solvent molecules in the gas phase. The trends in the computed results are in reasonable agreement with the available experimental data.

Structures and aggregation states of fluoromethyllithium and chloromethyllithium carbenoids in the gas phase and in ethereal solvent.

Using high-level quantum mechanical calculations and various models to account for solvation effects, monomers and dimers of fluoromethyllithium and chloromethyllithium carbenoids are studied in the gas phase and in dimethyl ether solvent. A combination of explicit microsolvation and a continuum reaction field is required to account fully for the structural and energetic effects of solvation. One important effect of solvent is the stabilization of charge-separated structures in which the lithium-halogen distance is much greater than in the gas-phase structures. At the most complete level of theory the 173 K standard-state free energy of dimerization of fluoromethyllithium in dimethyl ether is predicted to be -0.9 kcal mol(-)(1), while that for chloromethyllithium in the same solvent is predicted to be 3.7 kcal mol(-)(1). This suggests that, under typical experimental conditions, dimers of chloroalkyllithiums will not be observed, while dimers of fluoroalkyllithiums may contribute to the equilibrium population at a detectable level.

An ab Initio Study of Facial Selectivity in the Diels-Alder Reaction.

Facial selectivities in the Diels-Alder reactions of 5-substituted 1,3-cyclopentadienes with a variety of dienophiles are predicted reliably at the ab initio HF/6-31G level. The ranges of activation energies for syn addition are large relative to those for anti addition, which are all similar to the activation energy for cyclopentadiene itself. Partitioning the activation energy into diene deformation, dienophile deformation, and diene-dienophile interaction energies shows that the major factor in determining facial selectivity is in the energy required to deform the diene into its transition state geometry. Deformation of the 5-fluoro-, 5-hydroxy-, and 5-amino-1,3-cyclopentadienes into their syn transition state geometries is predicted to require less energy than deformation of cyclopentadiene itself, which is in accord with experimental observation of syn addition with these dienes. The first definition of an ab initio steric factor is presented which correlates very well with syn activation energies. This indicates that facial selectivity with these dienes is primarily due to steric hindrance between the dienophile and the plane-nonsymmetric groups on the diene. However, we have also identified a significant lone pair-lone pair interaction with the reacting nitrogens when the dienophile is 1,2,4-triazoline-3,5-dione.