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With theoretical calculations it is possible to investigate details of chemical reactions and molecular properties that are often difficult to study experimentally. Molecular orbital computations can be used to explore potential energy surfaces for reactions, determine equilibrium geometries, locate transition states, follow reaction paths and choose between different proposed reaction mechanisms. Heats of formation, NMR spectra, vibrational spectra and a variety of molecular properties can also be calculated reliably by ab initio molecular orbital methods. These calculations are particularly useful for highly reactive molecules and unstable intermediates that are problematic to observe experimentally.

Our lab is involved in both the development and the application of new methods in ab initio molecular orbital (MO) methods. The development efforts are centered around analytical energy derivatives and the use of these derivatives to explore potential energy surfaces. Over the past 25 years, our group and others have developed efficient computer programs to calculate energy derivatives for a variety of levels of ab initio MO theory. Our current efforts include the development of new algorithms using derivatives for geometry optimization, searching for transition states, and following reaction paths. We have also developed efficient code to compute classical trajectories for molecular dynamics directly from the MO calculations. To aid in the study of radicals, we have formulated spin projection methods to obtain more accurate energetics for open shell systems.

The remarkable advances in quantum chemical software and the rapid increase in the speed of computers have opened new realms of chemistry for investigation by ab initio molecular orbital methods. Many of our applications of quantum chemical calculations are in direct collaboration with experimental groups in order to maximize the benefits of our studies. In physical chemistry, we are studying reaction path branching and molecular dynamics by ab initio trajectory methods, and looking at excited states of large conjugated systems. Our new efforts in strong field chemistry are directed toward studying the behavior of conjugated molecules in short, intense laser pulses. In materials, we have a long standing interest in chemical vapor deposition (CVD). We have studied the thermochemistry and reactivity of a variety of systems pertaining to silicon, zinc oxide and titanium-nitride CVD. In the area of organic chemistry, we have examined reactions involving the oxidation of guanine, reactivity and rearrangements of organic radicals, and the behavior of nitric oxide releasing agents. In applications to biochemistry, we are using electronic structure methods to examine reactivity and inhibition in the active sites of enzymes.

References

a. Hratchian, H. P.; Schlegel, H. B.; Finding Minima, Transition States, and Following Reaction Pathways on Ab Initio Potential Energy Surfaces, in Theory and Applications of Computational Chemistry: The First 40 Years, Dykstra, C.E.; Kim, K. S.; Frenking, G.; Scuseria, G. E. (eds.), Elsevier, 2005, pg 195 - 259.

b. Zhou, J.; Schlegel, H. B.; Dissociation of acetone radical cation (CH₃COCH₃^{+ ∙} ® CH₃CO⁺ + CH₃^∙): An ab initio direct classical trajectory study of the energy dependence of the branching ratio. J. Phys. Chem. A 2008, ASAP (10.1021/jp8057492)

c. Munk, B. H.; Burrows, C. J.; Schlegel, H. B.; An Exploration of Mechanisms for the Transformation of 8-Oxoguanine to Guanidinohydantoin and Spirodihydantoin by Density Functional Theory. J. Am Chem. Soc. 2008, 130, 5245-5256.

d. Schlegel, H. B.; Smith, S. M.; Li, X.; Electronic Optical Response of Molecules in Intense Fields: Comparison of TD-HF, TD-CIS and TD-CIS(D) Approaches. J. Chem. Phys. 2007, 126, 244110 .

e. Li, J.; Cross, J. B.; Vreven, T.; Meroueh, S. O.; Mobashery, S.; Schlegel, H. B.; A Theoretical Study of Lysine Carboxylation in Proteins: OXA-10 b Lactamase. Proteins 2005, 61, 246-257.