Principal Investigator Leo Radom Projects k29, p02,p03,

Research School of Chemistry p57, q07, q08, r54, r55, r56, s07, s08

Machine VP

Co-Investigators A Croft, A L L East, J W Gauld, H Heuts, D C Marsden, M P McGrath, A Nicolaides, D R Rasmussen, A P Scott, D M Smith, Research School of Chemistry, M N Glukhovtsev, University of Sydney; C Y Mok, National University of Singapore; R H Nobes, ANUSF; A Pross, University of Sydney; B J Smith, Biomolecular Research Institute, Melbourne; J B Westmore, University of Manitoba and M W Wong, University of Queensland

Computational Quantum Chemistry

Chemistry is traditionally an experimental science. However, recent advances in computational techniques and the development of highly efficient computer algorithms allows the computer to provide a powerful complement to experiment in the study of the chemistry of small molecules. Theory may particularly be used to advantage for reactive or unstable species for which appropriate laboratory experiments are difficult to perform, and these are therefore the systems on which we focus attention. The principal properties of molecules which we examine are their structures, i.e. their shape in terms of bond lengths and bond angles, and their stabilities, i.e. an assessment of whether the molecules are likely to be long-lived or whether they are likely to decompose easily. We also calculate spectroscopic properties. These are particularly useful as fingerprints for eventual experimental identification (e.g. in the laboratory or in interstellar space) of molecules which have hitherto not been observed.

What are the basic questions addressed?

What are the structures of a variety of molecules of current interest and what are the mechanisms of reactions which they undergo? We use ab initio molecular orbital theory to study such problems.

What are the results to date and the future of the work?

One of the highlights for 1994 follows on from our 1992 ab initio characterisation of a new class of neutral saturated hydrocarbons, the alkaplanes, containing a potentially planar tetracoordinate carbon atom. We predict that hemispirooctaplane (pictured below) is extremely basic, with a proton affinity greater than any previously recorded value.

We will continue to use theory to examine reactive species for which appropriate experimental measurements are difficult. We hope that our predictions will continue to stimulate relevant experiments. In the longer term, the computer calculations may prove useful in the design of new materials and pharmaceuticals.

What computational techniques are used and why is a supercomputer required?

The ab initio molecular orbital calculations required to provide reliable information on molecular structures and reaction mechanisms are extremely computationally intensive.

Publications

A reappraisal of the structures and stabilities of prototype distonic radical cations and their conventional isomers, J W Gauld and L Radom, Journal of Physical Chemistry, 98, 777-784 (1994).

Comparison of the addition of CH3*, CH2OH* and CH2CN* radicals to alkenes: a theoretical study of the reaction mechanism, M W Wong, A Pross and L Radom, Journal of the American Chemical Society, 116, 6284-6292 (1994).

A simple failing of G2 theory: heats of combustion, A Nicolaides and L Radom, Journal of Physical Chemistry, 98, 3092-3093 (1994).

Oxirene: to be or not to be? G Vacek, J M Galbraith, Y Yamaguchi, H F Schaefer, R H Nobes, A P Scott and L Radom, Journal of Physical Chemistry, 98, 8660-8665 (1994).

Octaplane: a saturated hydrocarbon with a remarkably low ionization energy leading to a cation with a planar tetracoordinate carbon atom, J E Lyons, D R Rasmussen, M P McGrath, R H Nobes and L Radom, Angewandte Chemie International Edition in English, 33, 1667-1668 (1994).

Is SN2 substitution with inversion of configuration at vinylic carbon feasible? M N Glukhovtsev, A Pross and L Radom, Journal of the American Chemical Society, 116, 5961-5962 (1994).

The Wolff rearrangement: the relevant portion of the oxirene-ketene potential energy hypersurface, A P Scott, R H Nobes, H F Schaefer and L Radom, Journal of the American Chemical Society, 116, 10159-10164 (1994).

Addition of t-butyl radical to alkenes: a theoretical study of the reaction mechanism, M W Wong, A Pross and L Radom, Journal of the American Chemical Society, 116, 11938-11944 (1994).

Seven-membered ring or phenyl substituted cation? Relative stabilities of the tropylium and benzyl cations and their silicon analogues, A Nicolaides and L Radom, Journal of the American Chemical Society, 116, 9769-9770 (1994).

An evaluation of the performance of density functional theory, MP2, MP4, F4, G2(MP2) and G2 procedures in predicting gas-phase proton affinities, B J Smith and L Radom, Chemical Physics Letters, in press.

New theoretical and experimental proton affinities for methyl halides and diazomethane: a revision of the methyl cation affinity scale, M N Glukhovtsev, J E Szulejko, T B McMahon, J W Gauld, A P Scott, B J Smith, A Pross and L Radom, Journal of Physical Chemistry, 98, 13099-13101 (1994).

Chemistry by computer: a theoretical approach to structure and mechanism, L Radom, in "Organic Reactivity: Physical and Biological Aspects", B T Golding, R J Griffin, and H Maskill, Eds., Royal Society of Chemistry, Cambridge, in press.

Ab initio calculations of the gas-phase acidities of diazirine and diazomethane, A J Russell, A P Scott and L Radom, Australian Journal of Chemistry, in press.

Gas-phase identity SN2 reactions of halide anions with methyl halides: a high level computational study, M N Glukhovtsev, A Pross and L Radom, Journal of the American Chemical Society, submitted.

Calculation of proton affinities using the G2(MP2,SVP) procedure, B J Smith and L Radom, Journal of Physical Chemistry, submitted.

Extension of Gaussian-2 (G2) theory to bromine- and iodine-containing molecules: use of effective core potentials, M N Glukhovtsev, A Pross, M P McGrath and L Radom, Journal of Chemical Physics, submitted.

Radical addition to alkenes: an assessment of theoretical procedures, M W Wong and L Radom, Journal of Physical Chemistry, submitted.

Determination of the chirality of enantiomeric [16O, 17O, 18O] sulfate esters by infrared spectroscopy: an ab initio evaluation, A Nicolaides and L Radom, Journal of the Chemical Society, Chemical Communications, submitted.