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James Harrison

Photo of James Harrison


Quantum Chemistry

Advances in fundamental theory and computer technology enable us to construct wavefunctions for atoms and molecules which are of unprecedented ac­curacy. We use these functions to assist in the interpretation of spectroscopic experiments and to develop and refine the qualitative notions of chemical bonding. Our current focus is to understand:

Electron density map of the <sup>1</sup>&sum;<sup>+</sup> state of HCl calculated using a CASSCF wavefunction and the vqz basis set.
Electron density map of the 1+ state of HCl calculated using a CASSCF wavefunction and the vqz basis set.

The electronic structure of the ground and low-lying states of small molecules containing a transition metal atom. Diatomics of interest include MX where M is a transition metal (Sc to Zn) and X is a main group element (H to Cl). These molecules are of great interest as models for the nature of the transition meta-main group element chemical bond. Triatomics include the metal hydroxides MOH & HMO and the carbynes MCH, the understanding of which is fundamental to the reactions of transition metals with hydrocarbons. We are also interested in the structure of the mono- and dipositive ions of these systems.

Molecular quadrupole moments as a function of bond length.
Molecular quadrupole moments as a function of bond length.

The nature of molecular multipole moments and the information contained in these moments about the chemical bond. While we all have an instinctive feeling about the meaning of a molecular dipole moment and how it reflects the charge distribution in a molecule the same instincts often fail when considering for example, the quadrupole moment. Some of this problem is that the quadrupole moment is a second rank tensor while the dipole moment is a tensor of the first rank. However even for homonuclear diatomics where the quadrupole tensor has only one unique component the relationship between this component and the molecular charge density is not well understood. We have recently shown that the molecular quadrupole moment can be written as the sum of the quadrupole moments of the constituent atoms plus a term that depends on the shift in the electron density upon bond formation. In the course of this work we have defined the quadrupole moment density that shows where in the molecule the molecular contribution to the quadrupole moment comes from. We are extending these ideas to other one electron properties like the electric field gradient at a particular nucleus and the dipole moment (still more to learn!).

The spatial distribution of electron spin in open shell molecules. We have recently shown in the open shell nitrogen halides, NF, NCl and NBr that α and β spins flow in opposite directions as the chemical bond forms. We are exploring this observation and the role that electronegativity (chemical potential) plays.