Steven Gwaltney

Quantum chemistry has defined much of the way people think about the field of chemistry.  From the molecular orbital to the potential energy surface, many of our basic chemical concepts derive from quantum chemistry.  In addition, in some cases it is now possible computationally to exceed the accuracy of experiments for ground-state properties of small molecules in the gas phase.  However, if we look at an excited state or if we move to a liquid, the quality of the calculated results drops dramatically.  This is true primarily because excited states are more complicated than ground states and because modeling the liquid phase is much more complicated than modeling the gas phase.  Unfortunately, we are left with the situation where we are unable to describe gas- and solution-phase spectroscopy and gas- and solution-phase photochemistry with the level of rigor which has become routine for gas-phase ground states.  Therefore, one focus of my group’s research is to develop new methods for calculating electronic excited states of molecules within a coupled-cluster framework.  These methods will then be combined with good implicit solvent models to get close to chemical accuracy for both ground-state and excited-state reactions in solutions.

Electrophilic aromatic substitutions, in particular electrophilic aromatic nitrations, are considered classic organic chemistry reactions.  And although the basic idea was worked out in the 1950's, the details of the mechanism still remain under debate.  It is now generally accepted that the mechanism of aromatic nitration involves the initial formation of a pi complex, followed by the rearrangement into the sigma intermediate, and ultimately the production of products.  Much of the debate has centered on the nature of the initial pi complex.  Recently, we have used CCSD(T) and DFT calculations to reexamine the potential energy surface for the reaction of the nitronium ion with benzenes.
This work has lead to a consistent picture of how the nitration reaction proceeds in benzene through pi and sigma intermediates.  We are currently studying substituted benzenes to see if the same basic mechanism can be used to explain the known regioselectivity of nitration reactions.

The effects of organophosphate (OP) insecticide exposure on human health are currently of significant concern.  Organophosphates make up the largest by volume class of insecticides in use today.  At high enough doses, acute exposure to OP agents can lead to vomiting, muscle twitches, convulsions, and even death.  The primary route of organophosphate toxicity is phosphorylation of the enzyme acetylcholinesterase (AChE).  The role of AChE is to degrade the neurotransmitter acetylcholine, which is involved in signaling between nerve cells and from nerves to muscles. We have been using molecular dynamics simulations to study how a representative OP, paraoxon, binds with AChE.  Contrary to what has always been assumed, paraoxon seems to initially sit in the entrance channel, rather than binding to the active site.  Further simulations will be run to study how this binding occurs and to determine if this is general to all OP agents.