Using a dissymmetrically-perturbed particle-in-a-box model, we demonstrate that the induced optical activity of chiral monolayer protected clusters, such as Whetten's Au28(SG)16 glutathione-passivated gold nanoclusters (J. Phys. Chem. B, 2000, 104, 2630-2641), could arise from symmetric metal cores perturbed by a dissymmetric or chiral field originating from the adsorbates. This finding implies that the electronic states of the nanocluster core are chiral, yet the lattice geometries of these cores need not be geometrically distorted by the chiral adsorbates. Based on simple chiral monolayer protected cluster models, we rationalize how the adsorption pattern of the tethering sulfur atoms has a substantial effect on the induced CD in the NIR spectral region, and we show how the chiral image charge produced in the core provides a convenient means of visualizing dissymmetric perturbations to the achiral gold nanocluster core.
United States Environmental Protection Agency (USEPA) researchers are developing a strategy for high-throughput (HT) exposure-based prioritization of chemicals under the ExpoCast program. These novel modeling approaches for evaluating chemicals based on their potential for biologically relevant human exposures will inform toxicity testing and prioritization for chemical risk assessment. Based on probabilistic methods and algorithms developed for The Stochastic Human Exposure and Dose Simulation Model for Multimedia, Multipathway Chemicals (SHEDS-MM), a new mechanistic modeling approach has been developed to accommodate high-throughput (HT) assessment of exposure potential. In this SHEDS-HT model, the residential and dietary modules of SHEDS-MM have been operationally modified to reduce the user burden, input data demands, and run times of the higher-tier model, while maintaining critical features and inputs that influence exposure. The model has been implemented in R; the modeling framework links chemicals to consumer product categories or food groups (and thus exposure scenarios) to predict HT exposures and intake doses. Initially, SHEDS-HT has been applied to 2507 organic chemicals associated with consumer products and agricultural pesticides. These evaluations employ data from recent USEPA efforts to characterize usage (prevalence, frequency, and magnitude), chemical composition, and exposure scenarios for a wide range of consumer products. In modeling indirect exposures from near-field sources, SHEDS-HT employs a fugacity-based module to estimate concentrations in indoor environmental media. The concentration estimates, along with relevant exposure factors and human activity data, are then used by the model to rapidly generate probabilistic population distributions of near-field indirect exposures via dermal, nondietary ingestion, and inhalation pathways. Pathway-specific estimates of near-field direct exposures from consumer products are also modeled. Population dietary exposures for a variety of chemicals found in foods are combined with the corresponding chemical-specific near-field exposure predictions to produce aggregate population exposure estimates. The estimated intake dose rates (mg/kg/day) for the 2507 chemical case-study spanned 13 orders of magnitude. SHEDS-HT successfully reproduced the pathway-specific exposure results of the higher-tier SHEDS-MM for a case-study pesticide and produced median intake doses significantly correlated (p<0.0001, R2=0.39) with medians inferred using biomonitoring data for 39 chemicals from the National Health and Nutrition Examination Survey (NHANES). Based on the favorable performance of SHEDS-HT with respect to these initial evaluations, we believe this new tool will be useful for HT prediction of chemical exposure potential.
A long-standing challenge in molecular stereochemistry is to assess the contributions of the solvent surrounding chiral organic molecules. The observed specific rotation angles and optical rotatory dispersion (ORD) spectra are strongly influenced by solvent-solute interactions. For example, (S)-methyloxirane has a positive optical rotation (OR) in water and a negative OR in benzene.[1] The vapor phase and condensed phase optical rotations of chiral molecules are also known to differ in large and nonintuitive ways.[2a] Access to experimental gas phase optical rotation data [2] and to modern linear-response OR calculations [3] provides the tools needed to analyze and predict the contributions of solute-solvent interactions to OR. Wilson et al. recently reported the OR for a series of organic molecules in the vapor phase using cavity ring-down polarimetry.[2b] These authors compared the solution and vapor phase OR to assess the influence of the condensed phase environment on ORD. They concluded: 1) for flexible solute molecules, solvent stabilization of individual conformers can influence the optical rotation strongly; 2) solvation of rigid molecules like methyloxirane causes changes in the static and dynamic distribution of molecular electron densities; and 3) surprisingly, OR under solvent-free conditions can resemble OR measured in a highly polar solvent. We find that, for methyloxirane in high-polarity solvents such as water, it is essential to include explicit solutesolvent interactions in the theoretical analysis to describe the observed ORD spectrum.Recently, coupled-cluster (CC) and time dependent density functional theory (TD-DFT) methods have been used to predict OR.[3l] However, the substantially greater computational efficiency of TD-DFT methods has led to their favored use in current OR calculations. The solvent dependence of OR for chiral molecules has been computed using TD-DFT and an implicit continuum solvent model. Kongsted et al. recently used CC methods combined with a continuum solvent description to calculate the influence of solvent on the OR of (S)-methyloxirane.[3j,k] The authors concluded that continuum models do not reproduce the experimentally observed solvent shifts in the ORD spectra as a function of solvent. Moreover, implicit solvent models do not predict the sign change in the ORD spectrum of methyloxirane in aqueous solution at wavelengths longer than 400 nm. Accordingly, the inclusion of explicit solvent effects appears to be essential for further progress. Here, we show that ORD calculations using explicit solvent models, combined with continuum models, such as the conductor-like screening model (COSMO), [4] can capture the sign and relative magnitude of the ORD spectrum of methyloxirane in water. More importantly, explicit solvent studies allow elucidation of the solvent contributions to the observed OR spectrum. Comparison of the total computed OR (solute + solvent) with the OR computed for the solvent shell (solvent with the chiral solute removed from the electronic structur...
HighlightsFunctional role of thousands of chemicals is analyzed.These data are combined with chemical weight fractions in personal care products.Empirical compositions for products are developed based on function.Classifier models for function and weight fraction are built.These methods can fill data gaps for consumer product exposure models.
Dilute solutions of (R)-(-)-pantolactone in CCl4 were studied by polarimetry in conjunction with theoretical calculations of [alpha]D. Our data demonstrate that the self-association of a chiral solute results in a change in [alpha]D that can be accounted for by the presence of hydrogen-bonded dimeric species. The theoretical analysis predicts a concentration-dependent specific rotation in good agreement with experiment. Further exploration of monomer and dimer [alpha]D differences, through atomic map analysis, reveals large contributions to [alpha]D from the hydrogen-bonded hydroxyl groups in the tightly-coupled dimer. This study extends the computation of chiroptical properties to the accurate concentration-dependent prediction of [alpha]D for noncovalently interacting self-associating species.
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