Kerwin D. Dobbs

Ab initio prediction of the barrier height for abstraction of H from CH4 by OH

The reaction CH4+OH→CH3+H2O has been investigated by ab initio molecular orbital theory with a variety of basis sets and levels of correlation. At the highest level of basis set (Dunning’s correlation consistent) and highest level of correlation [QCISD(T)], the value of ΔH(0) is −13.44 kcal/mol and the classical barrier height is 6.62 kcal/mol which is reduced to 5.24 kcal/mol if zero point effects are included. Transition state theory is used to predict the rate constants and good agreement with the experimental values is found over a broad temperature range.

On the proposed existence of a ketene derived from carbon monoxide and 1,3-di-1-adamantylimidazol-2-ylidene

Abstract The reaction of carbon monoxide with the stable carbene, 1,3-di-1-adamantylimidazol-2-ylidene, is reinvestigated experimentally. With the aid of high level ab initio density functional theory calculations the predicted stability of the putative ketene product is determined. No evidence is found for the existence of a stable ketene product.

Origin of side bands in the FTIR spectrum of acetone oxime vinyl ether

Abstract The origin of two minor but enigmatic side bands in the Fourier-transform infrared spectrum (FTIR) of acetone oxime vinyl ether is resolved with density functional theory calculations. A comparison of vibrational frequencies as computed with Hartree–Fock self-consistent-field theory and density functional theory is carried out for acetone oxime vinyl ether, and N-(3-nitrobenzylidene)-p-phenylenediamine. Implications for molecular electronics are discussed.

2-Phenylpyridine: To Twist or Not To Twist?

Density functional theory methods were used to investigate the structures associated with 2-phenylpyridine, ppy, and several of its electronic states. The structure of ppy has the aromatic rings twisted with respect to one another by ∼21°, which is about half the value found for biphenyl. In comparison with ppy, both the isoelectronic cation, ppyH(+), and anion, ppy(-), have larger twist angles. The extent of twisting is governed by the delicate balance between π conjugation and repulsive orbital/steric interactions, and the magnitudes of these interactions were investigated by examining the torsional energy barriers for all three molecular species. In contrast, every one of the investigated open-shell structures [Formula: see text] ppy(•)(+), ppy(•)(-), ppy*, ppyH(+)*, and ppy(-)* [Formula: see text] has coplanar aromatic rings, that is, no twist angle. Frontier molecular orbital analyses reveal that the π-type bonding between the bridging carbons becomes dominant over any repulsive orbital and steric interactions, thereby leading to coplanar rings. Also, the energetics associated with ppy and its various electronic states were investigated and reported.

Computational Characterization of Metal Binding Groups for Metalloenzyme Inhibitors.

The mode of action of many pest or disease control agents involves inhibition of some metalloenzyme that is essential for the survival of the target organism. These inhibitors typically consist of a functional group that is capable of a primary binding interaction with the metal and a scaffold that is capable of secondary interactions with the remainder of the enzyme. To characterize the binding ability of various metal binding groups (BGs), we have performed electronic structure calculations on ligand displacement reactions in a model system related to the metalloenzyme, peptide deformylase:  E-M-R + BG → E-M-BG + R. Here E represents a model coordination environment for the metal M, and R is a reference ligand (e.g., water) that may be displaced by a metal binding group. Since the oxidation state of many of the metals considered allows for multiple spin states, we also studied the influence of spin state on the coordination environment. Qualitative considerations of electronic structure inspired by the calculations provide an understanding of binding energy trends across a variety of ligands for a given metal and across a variety of metals for a given ligand.

Computational Thermochemistry and Benchmarking of Reliable Methods

During the first and second years of the Computational Thermochemistry and Benchmarking of Reliable Methods project, we completed several studies using the parallel computing capabilities of the NWChem software and Molecular Science Computing Facility (MSCF), including large-scale density functional theory (DFT), second-order Moeller-Plesset (MP2) perturbation theory, and CCSD(T) calculations. During the third year, we continued to pursue the computational thermodynamic and benchmarking studies outlined in our proposal. With the issues affecting the robustness of the coupled cluster part of NWChem resolved, we pursued studies of the heats-of-formation of compounds containing 5 to 7 first- and/or second-row elements and approximately 10 to 14 hydrogens. The size of these systems, when combined with the large basis sets (cc-pVQZ and aug-cc-pVQZ) that are necessary for extrapolating to the complete basis set limit, creates a formidable computational challenge, for which NWChem on NWMPP1 is well suited.