Robert Ditchfield
Professor of Chemistry
Professor Ditchfield received his B.Sc. degree with special honors in Chemistry from Sheffield University in 1965. He then undertook graduate work in theoretical chemistry under the direction of Professor J. N. Murrell at the University of Sussex, obtaining his D.Phil. degree in 1968. From Sussex, he moved to Mellon Institute in Pittsburgh, where his postdoctoral studies with Professor J.A. Pople involved the development of practical ab-initio methods for calculating molecular electronic structure. After two years (1970-72) at Bell Telephone Laboratories, he joined the faculty of Dartmouth College where he is now Professor.
Research Interests
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.