Research Interests...

• Computational Neuroscience
  
  Recent experimental work has demonstrated the ability of small chondroitin sulfate (CS) molecules to promote neuronal growth. This activity is sensitive to both the presence and location of sulfate groups on CS. The current research project, which is being carried out under the joint supervision of the Neuroscience and Chemistry Departments, involves using recently developed continuum solvation models in conjunction with density functional theory (DFT) to calculate gas and liquid-phase properties of CS molecules. Using calculated energetic and structural data for small CS molecules, possible correlations between the sulfation pattern of these molecules and their ability to moderate neural growth will be investigated. More info...

• Continuum Solvation
  
  Theoretical models for the liquid phase that treat the solvent as a homogenous dielectric continuum are often an attractive alternative to explicit solvent approaches. An advantage of continuum solvation models is that they are able to account for many bulk solvent effects, such as solute and solvent polarization, in an economical way. Under the supervision of Chris Cramer and Don Truhlar, I recently developed a new continuum solvation model called Solvation Model 6 (SM6). This model combines a dielectric continuum (to account for bulk solvent effects) with atomic surface tensions (to account for first shell solvent effects, such as cavitation, dispersion, and hydrogen-bonding) in order to calculate free energies of solvation. Solvation free energies are directly related to partition coefficients and can be combined with other experimental and/or calculated data to determine properties such as vapor pressure, solubility, and acid dissociation constants. Thus, the ability to calculate solvation free energies accurately is important in many areas of scientific research. For more information on the high accuracy of SM6 for calculating aqueous solvation free eneriges, click here.

• High-Energy Materials
  
  I have also been involved in the testing and development of theoretical methods for high-energy materials. The goal of this research is to gain a better understanding of the analytical techniques, such as extraction with supercritical fluids, that are in present use for the safe recycling/disposal of many types of explosive compounds. A theoretical method has been developed for accurately assigning partial atomic charges to molecules containing nitramine functionality, such as the explosives RDX and HNIW (CL-20). Besides giving accurate dipole moments for these types of compounds, the partial atomic charges obtained from this model are relatively invariant to the level of treatment of electron correlation, minor conformational changes, and the presence of buried atoms in the molecule. These qualities make the present charge model potentially useful for use in future continuum solvation calculations and solid-state molecular simulations of high-energy molecules.

• Transport of Pesticides in the Environment
  
  Many pesticides, despite being only sparingly soluble in water, are able to move through the environment by adsorbing on to the surface of fog droplets. One of my earlier research projects involved developing a theoretical model for predicting equilibrium constants for the adsorption of organic molecules at air-water interfaces. The resulting model, which is called SM5.0R-Surf, separates the observable free energy of adsorption into a coupling part, which depends on interactions between the adsorbing solute and solvent molecules at the air-water interface, and a dimensionality change part, which depends only on the mass of the solute. In this model the coupling contribution to the free energy is calculated using the exposed surface areas of atoms in the solute, while the dimensionality change free energy is computed using statistical thermodynamics. Unlike previous models that are based on empirical correlations between other experimental data and air-water interface adsortion coefficients, SM5.0R-Surf requires only the three-dimensional structure of the solute, making it especially useful for molecules for which limited experimental data exist. Application of this model to several pesticides revealed that their air-water interface adsorption coefficients are much greater than those for typical organic compounds, suggesting that adsorption at the air-water interface of fog droplets is indeed an effective mechanism for the transport of pesticides in the environment.

• Software Development
  
  All of the theoretical models that have been developed as part of the research projects described above have been implemented into software programs that are available free of charge to the scientific community. For more information on obtaining this software, click here.