Sierra Rayne’s research while affiliated with Saskatchewan Polytechnic and other places

The photoisomerization equilibrium between the model tannins (-)-catechin and (-)-epicatechin in aqueous solution was investigated at the density functional level of theory to gain insights into the action of these compounds as natural sunscreens in aquatic systems. Increasing water temperature, as might be expected on seasonal and diurnal bases, is predicted to shift the equilibrium further in favor of catechin. The isomerization energy between catechin and epicatechin was also considered in a range of polar protic, polar aprotic, apolar protic, and apolar aprotic solvents using the solvation model based on density (SMD) and integral equation formalism polarizable continuum model (IEFPCM). The IEFPCM yielded a modest range in isomerization energies depending on solvent polarity or proticity, whereas a substantial variation was observed with the SMD. The SMD results suggest that the solvation environment around catechin and epicatechin will play a major role on the photoisomerization equilibrium between these two compounds. As the freely dissolved monomer in aquatic systems, the catechin–epicatechin photoisomerization equilibrium will be in the range of 11:1 to 14:1. In the less polar environments of associations with dissolved organic matter or within a larger tannin structural framework, the theoretical modeling efforts indicate that the catechin–epicatechin photoisomerization equilibrium could be as low as 3:1.

Aroma compounds in the Flavornet database were screened for potentially hydrolysable carboxylic acid ester functionalities. Of 738 aroma compounds listed in this database, 140 molecules contain carboxylic acid ester groups that may be amenable to hydrolysis in various food and beverage products. Acid- (kA) and base- (kB) catalysed and neutral (kN) hydrolysis rate constants in pure water at 25°C were calculated for these aroma compounds. Where available, good agreement between theoretical and experimental hydrolytic half-lives was obtained at various pH values. Wide ranges and broad frequency distributions for kA, kB, and kN are expected among the various hydrolyzable aroma compounds, with calculated kA ranging from 3.7×10-8 to 4.7×10-4 M-1 s-1, calculated kB ranging from 4.3×10-4 to 43M-1 s-1, and calculated kN ranging from 4.2×10-17 to 7.6×10-9 M-1 s-1. The resulting hydrolytic half-lives also range widely, from 10days to 370years at pH2.8, 18days to 4,900years at pH4.0, 1.8days to 470years at pH7.0, and 26minutes to 5.1years at pH9.0. The findings illustrate the importance of considering abiotic hydrolysis and matrix pH when modelling the evolution of sensory characteristics for foods and beverages with carboxylic acid ester based aroma compounds.

In order to estimate isomer-specific acidity constants (pKa) for the perfluorinated sulfonic acid (PFSA) environmental contaminants, the parameterization method 6 (PM6) pKa prediction method was extensively validated against a wide range of carbon oxyacids and related sulfonic/sulfinic acids. Excellent pKa prediction performance was observed for the carbon oxyacids using the PM6 method, but this approach was found to have a severe positive bias for sulfonic/sulfinic acids. To overcome this obstacle, a correlation was developed between non-adjusted PM6 pKa values and the corresponding experimentally obtained/estimated acidity constants for a range of representative alkyl, aryl and halogen-substituted sulfonic acids. Application of this correction to the PM6 values allows for extension of this computational method to a new acid functional group. When used to estimate isomer-specific pKa values for the C1 through C8 PFSAs, the modified PM6 approach suggests an adjusted pKa range from -5.3 to -9.0, indicating that all members of this class of well-known environmental contaminants will be effectively completely dissociated in aquatic systems.

The air-water partition coefficients (Kaw) 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 Kaw estimates of the target compounds, the employment of increasingly higher levels of theory would result in lower Kaw 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 Kaw estimates within typical experimental ranges, each having general Kaw errors of less than 0.5 log10 units. When applied to other large organic compounds, the method should allow development of a broad and reliable Kaw database for multimedia environmental modeling efforts on various contaminants.

The isomerization energy (ΔisomE) performance of a wide range of density functionals was tested against the set of 24 individual intramolecular reactions collectively comprising the ISOL24/11 benchmark database. Among 50 non-dispersion corrected functionals examined, the M06-2X single-component density functional offered the best estimation capacity, yielding a mean unsigned error (MUE) of 2.4 kcal/mol. The addition of molecular mechanics dispersion-like terms to the M06-2X functional offers minimal improvement in its capacity to reliably estimate isomerization energies, whereas the choice of basis set played a significant role. Triple zeta Dunning, Ahlrichs, and Pople basis sets offer effectively equivalent ΔisomE performance, modestly better than double zeta equivalent basis sets, with ΔisomE performance subsequently rapidly declining at basis set sizes below this level. Among a reduced set of dispersion corrected functionals considered, the APFD functional displayed ΔisomE performance (MUE=1.8 kcal/mol) superior to both the non-dispersion corrected M06-2X functional and the wB97X-D and PBE0-D3 dispersion corrected functionals (1.9 kcal/mol). At this accuracy, the APFD functional is competitive with computationally expensive high-accuracy methods such as MC3MPWB and MC3MPW and nearing that of composite methods and dispersion corrected double-hybrid density functionals.

The gas phase standard state (298.15 K, 1 atm) isomerization enthalpy (ΔisomH°(g)) prediction performance of the major semiempirical, ab initio, and density functional levels of theory for environmentally relevant transformations was investigated using the linear to branched heptanes as a representative case study. The M062X density functional, MP2 (and higher) levels of Moller-Plesset perturbation theory, and the CBS and Gaussian-n composite methods are well suited for investigating the thermodynamic properties of environmentally interesting isomerizations, although the M062X functional may not be appropriate for all systems. Where large molecular systems prohibit the use of higher levels of theory, the PM6 and PDDG semiempirical methods may offer an appropriate computational cost-accuracy compromise.

Singlet–triplet adiabatic excitation energies (AES−T) of the parent and variously substituted phenyl cations, as well as the parent benzannelated derivatives up to anthracenyl, were calculated at the G4(MP2) and G4 levels of theory. The G4(MP2)/G4 AES−T estimates range up to 40 kJ/mol higher than prior density functional theory (DFT)-based predictions for these cations and suggest that AES−T and ground state multiplicity structure–property trends for phenyl cations previously proposed in the literature need to be re-assessed at higher levels of theory. In general, Hartree–Fock, DFT, and semiempirical methods do a poor job describing the singlet–triplet excitation energetics of these systems. Only modest effects of different solvation models (SMD, IEF-PCM, and C-PCM) and different polar protic through apolar aprotic solvents are evident on the calculated AES−T of the phenyl cation. Electron-donating substituents on the phenyl cation substantially lower the AES−T to an extent where some functional groups (–NH2, N(CH3)2, OCH3, and SCH3) can result in triplet ground states depending on their position relative to the cation. In contrast to the phenyl and 1- and 2-naphthyl cations, which are predicted to be ground state singlets, the three parent anthracenyl cations will be ground state triplets.

The SPARC software program was used to estimate the acid-catalyzed, neutral, and base-catalyzed hydrolysis rate constants for the polymeric brominated flame retardants BC-58 and FR-1025. Relatively rapid hydrolysis of BC-58, producing 2,4,6-tribromophenol-and ultimately tetrabromobisphenol A-as the hydrolytically stable end products from all potential hydrolysis reactions, is expected in both environmental and biological systems with starting material hydrolytic half-lives (t1/2,hydr) ranging from less than 1 h in marine systems, several hours in cellular environments, and up to several weeks in slightly acid fresh waters. Hydrolysis of FR-1025 to give 2,3,4,5,6-pentabromobenzyl alcohol is expected to be slower (t1/2,hydr less than 0.5 years in marine systems up to several years in fresh waters) than BC-58, but is also expected to occur at rates that will contribute significantly to environmental and in vivo loadings of this compound.

Gas phase standard state (298.15 K, 1 atm) enthalpies of formation (ΔfH°(g)) were calculated at the G4 composite method level of theory for the parent and binary mixed carbon, silicon, nitrogen, and phosphorus cubane derivatives. Increasing nitrogen content increases the ΔfH°(g) for the carbon-nitrogen, nitrogen-phosphorus, and silicon-nitrogen binary cubanes, with the opposite ΔfH°(g) trend for increasing phosphorus content within the carbon-phosphorus, nitrogen-phosphorus, and silicon-phosphorus derivatives. Varying carbon/silicon content in the carbon-silicon cubanes results in no general trends for ΔfH°(g). Analogous ΔfH°(g) structure-property trends were found among the parent and binary mixed carbon, silicon, nitrogen, and phosphorus tetrahedrane derivatives. Cubanoid isomerization enthalpies within the homolog groups having more than one isomer vary widely with atomic composition and substitution patterns. Mass normalized enthalpies of combustion (ΔcombH°(g)) of the binary mixed cubanoids are superior to that of common explosives. The binary mixed carbon, silicon, nitrogen, and phosphorus cubane derivatives are predicted to display potentially tunable thermodynamic properties for high energy materials depending on the atom identities and relative positions.