EPSRC Research Grant Final Report Grant no. GR/L31340

ELECTRONIC STRUCTURE AND CHEMICAL PROCESSES IN MOLECULES AND SOLIDS

N.L. Allan (and J. Gerratt) (Bristol)

NLA: We have developed a major new computer code, SHELL, available via the WWW (http://dougal.chm.bris.ac.uk/programs/shell), which incorporates significant general advances in the methodology for the direct minimisation of the free energies of periodic solids and surfaces with possibly very large unit cells. This is achieved using a combination of lattice statics and dynamics together with analytic derivatives of the vibrational frequencies with respect to all external and internal variables. We have used this new code to calculate for the first time defect free energies and surface free energies as a function of temperature, based on the full minimisation. With the aid of a CASE student, equations of state calculated using SHELL have been implemented in a hydrodynamics code used in the explosives industry.

A range of powerful new methods have also been developed for grossly disordered systems and solid solutions. These include novel exchange and hybrid Monte Carlo techniques and a new configurationally averaged lattice dynamics approach. Applications have included olivines, high-temperature superconducting cuprates and the phase diagrams of ceramics and minerals at high temperatures and/or high pressures.

A series of ab initio studies using periodic Hartree-Fock theory has examined the electronic and magnetic structure of sulphides, oxide films and high-T_C superconducting oxyhalide cuprates. We have published the first comparative ab initio study of competing mechanisms of phase transitions in ionic solids and our results for the structures adopted by MnO and NiO at high pressure have prompted new experimental studies.

JG: Spin-coupled projects concentrated on the development of an alternative strategy to optimise a fully-variational spin-coupled (SC) wavefunction, where a set of doubly occupied orbitals are determined simultaneously with a group of valence singly occupied orbitals. Applications of SC theory included the role of transition metal ions in the removal of furan and thiophene from crude petroleum, and modern-VB descriptions of the bonding in cyclooctatetraene, borabenzene, benzocylobutadiene, thiophene oligomers, bicyclic 1,6-methano[10]annulene, and various strained three- and four-membered ring systems. A particularly unusual bonding arrangement was found for the N₂S₂ ring system and other N-S heterocycles. We settled a dispute regarding the relative energies of different spin states of cyclobutadiene chains: it is now clear that these systems feature a pattern of antiferromagnetic spin coupling. SC theory was also applied to the reaction mechanism of the ring opening of cyclobutene to cis-butadiene. The transition structure is neither aromatic nor antiaromatic. The discriminating factor between the conrotatory and disrotatory pathways stems from the recoupling of the electron spins, which occurs earlier along the conrotatory reaction coordinate. Such results give new insights into mechanistic organic chemistry since, for the first time, it is possible to 'see' how bonds break and form in pericyclic reactions.

Publications: To date approximately 40 papers have appeared or are in press. Key papers are:

Conrotatory and disrotatory pathways in the electrocyclic isomerization of cyclobutene to cis-butadiene: The spin-coupled viewpoint, J.M. Oliva, J. Gerratt, P.B. Karadakov and D.L. Cooper, J. Chem. Phys. 107, 8917-8926 (1997).

Modern valence bond theory, J. Gerratt, D.L. Cooper, P.B. Karadakov and M. Raimondi, Chem. Soc. Rev., 26, 87-100 (1997).

Free energy derivatives and structure optimisation within quasiharmonic lattice dynamics, M.B. Taylor, G.D. Barrera, N.L. Allan and T.H.K. Barron, Phys. Rev. B56, 14380-14390 (1997).

Monte Carlo and hybrid Monte Carlo/molecular dynamics approaches to order-disorder in alloys, oxides and silicates, J.A. Purton, G.D. Barrera, N.L. Allan and J.D. Blundy, J. Phys. Chem. B102, 5202-5207 (1998).

Hybrid Monte Carlo and lattice dynamics simulations: the enthalpy of mixing of binary oxides, J.A. Purton, J.D. Blundy, M.B. Taylor, G.D. Barrera and N.L. Allan, Chem. Commun. 627-628 (1998).

Ionic solids at high temperatures and pressures: ab initio, lattice dynamics and Monte Carlo studies, N.L. Allan, G.D. Barrera, C.E. Sims and M.B. Taylor, Phys. Chem., Chem. Phys 2, 1099-1111 (2000)

Contact: Dr. Neil L. Allan, School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS.

Email: n.l.allan@bris.ac.uk