Professor of Chemistry
The accuracy of electronic structure methods has now reached a point where such techniques are used rather widely by all kinds of chemists to investigate molecular properties. These methods not only provide information which supplements that obtained from a variety of spectroscopic techniques, but they can also yield important insight about arrangements of nuclei that are difficult to obtain experimentally. My research focusses mainly on developing and applying theoretical techniques that will enable one to understand how various ground and excited state properties depend on molecular electronic structure. Of particular interest are those properties which reflect how the molecular electronic structure responds to the application of external magnetic and/or electric fields. Magnetic shielding tensors have been calculated for many types of nuclei in a wide range of molecular environments.
For hydrogen-bonded systems, we have been able to explain the isotropic deshielding which accompanies hydrogen bond formation in terms of charge polarization effects in the proton donor and magnetic shielding effects of the proton acceptor. These latter contributions are strongly anisotropic and are almost completely responsible for the change in proton shielding anisotropy which occurs on hydrogen bond formation. More recently, we have been using calculated 27Al shielding tensors to provide insight into the electronic structure of aluminium alkoxides. Electronic structure techniques are also being used to study the relative energies of various pi- and sigma-structures produced when a carbocation binds to an aromatic ring. Such complexes are thought to be important in stabilization of certain protein-substrate conjugates, and are, of course, relevant to an understanding of the mechanism of electrophilic aromatic substitution. Computational studies on model isomeric carbocations derived from bicyclic alkene precursors indicate that carbocation-pi interaction preferentially stabilizes those isomers that can achieve a geometry in which the carbocation moiety can be positioned above the periphery of the aromatic ring.
Lancaster, C. R. D., Hunte, C., Kelley, J., Trumpower, B. L., and Ditchfield, R. "A Comparison of Stigmatellin Conformations, Free and Bound to the Photosynthetic Reaction Center and the Cytochrome bc1 Complex." J. Mol. Biol. 368, 197 (2007).
Circumannular long range orbital interactions affect thefree energy of activation for the inversion of groups on nitrogen in3-imino-2,2,4,4-tetramethylcyclobutanones, James J. Worman, Robert Ditchfield,Allen Chong, Grzegorz Mloston, and Martha Wozicka, J. Amer. Chem. Soc., Book of Abstracts, 229thACS National Meeting, San Diego, CA (2005), ORGN-474.
Hydroxycarbocation - π interaction: A computational andexperimental study of protonation of aromatic ketones, Bobbijo van Beusichem,Celeste P. Viscardi, Thomas A. Spencer, Jr., and Robert Ditchfield, Abstr. Pap. J. Amer. Chem. Soc., (2000), ORGN-497.
Benzobicyclo[4.2.1]nonene model system for study ofcarbocation-π interaction, Janeta V. Popovici-Muller, Robert Ditchfield, andThomas A. Spencer, Jr., J. Amer. Chem. Soc., Book of Abstracts, 219th ACS National Meeting, SanFrancisco, CA (2000), ORGN-508.J. Am. Chem. Soc.
Carbocation-π Interaction: Computational Study ofComplexation of Methyl Cation with Benzene and Comparisons with RelatedSystems, P. C. Miklis, R. Ditchfield, and T. A. Spencer. J. Am. Chem. Soc., 120,10482, (1998)
Metal Complexes of Mesitylphosphine: Synthesis, Structure,and Spectroscopy I. V. Kourkine, S. V. Maslennikov, R. Ditchfield, D. S.Glueck, G. P. A. Yap, L. M. Liable-Sands, A. L. Rheingold, InorganicChemistry, 35, 6708 (1996).
Synthesis and Dynamic NMR Studies of h3- Triphenyl - and h3-Trimethylcyclopropenyl Complexes of Ruthenium, [Ru(h5 - C5R5)(h3 - C3R'3)X2] (R= H, Me; R' = Me, Ph; X = Cl, Br, I). Extended Hückel Molecular Orbital Study of Barriers to Rotation of h3 -Cyclopropenyl Ligands in Isoelectronic Ruthenium and Molybdenum Complexes, R.Ditchfield, R. P. Hughes, D. S. Tucker, E. P. Bierwagen, J. Robbins, D. J.Robinson, and J. A. Zakutansky, Organometallics, 12, 2258 (1994)
Ab-Initio Computations of 29SiNuclear Magnetic Resonance Chemical Shifts for a range of representativecompounds, J.R. van Wazer, C.S. Ewig, and R. Ditchfield, J. Phys. Chem., 95,2222 (1990).
Phosphorus Compounds and their 31P Chemical Shifts, J.R. van Wazer and R. Ditchfield, Chapter 1,p.1 in Phosphorus NMR in Biology, (edited by C.T. Burt), (CRC Press,1987).
Proton and Carbon-13 Chemical Shifts: Comparison between Theory andExperiment, C. Rohlfing, L.C. Allen, and R. Ditchfield, Chemical Physics, 87, 9(1985).
Works in Progress
“Carbocation-π Interaction: The 1,1-Dimethylallyl Cation and Benzene,” “A Comparison of Stigmatellin Conformations, Free and Bound to the Photosynthetic Reaction Center and the Cytochrome bc 1 Complex”