Simulation Studies of Collisional Deactivation in Combustion-Model and Atmospheric-Model Systems


Principal Investigator

Kieran F. Lim

School of Biological and Chemical Sciences

Deakin University


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Collisional processes are an important part of the
overall chemistry occurring in combustion and
atmospheric systems. We have investigated the collisional dynamics of ethane with monatomic bath gases and will continue to investigate larger hydrocarbon molecules and halogenated molecules.

In addition to the collisional processes, the competition between bond-fission ("loose" transition-state) and elimination-type ("tight" transition-state) reactions controls whether a combustion system will explode or merely burn.

In recent years, organometallic compounds have been increasingly used as catalysts for various industrial processes. We have found that in the gas-phase, organo-tin cations undergo reactions which are analogous to the combustion chemistry of the hydrocarbon molecules.



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

Ethane is the simplest model hydrocarbon fuel molecule that incorporates an internal rotation. This methyl torsional motion behaves like a vibration at low vibrational energies with a transition to a near-free rotor at high vibrational energies. Since vibration-to-rotation (VÆR) energy transfer is more efficient than vibration-to-translation (VÆT) energy transfer, our studies have shown that torsional motion acts as a gateway for energy transfer (VÆtorsionÆT) and that any simulation of collisional energy transfer must include the correct torsional dynamics. This "gateway" behaviour is the most probable explanation for previous experimental observations that toluene transfers much more energy per collision than does benzene.

The computational study of organotin dissociation complements our experimental observations of gas-phase organotin dissociation pathways which consist of both beta-elimination ("tight" transition-state) and radial loss ("loose" transition-state) reactions. Calculations on the C2H5SnH2+ and the C3H7SnH2+ potential energy surfaces have been completed and reveal that where alkene loss is possible, bHelimination is energetically favorable to b-methyl-elimination which is in turn favorable to simple Sn-C bond cleavage. These results are in complete agreement with our experimental observations and will be submitted for publication shortly. The extension of this work to the other organo-Group 14 cations is in progress. To date, we have successfully located first-order saddle points corresponding to b-H-transfer and b-methyl-transfer on the C3H7MH2+ potential energy surfaces (M = Si, Ge) along with other relevant features of the potential surfaces.

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Future calculations will concentrate on two areas:

· the completion of the investigation of the C3H7MH2+ potential energy surface (with possible extension to the corresponding organolead species). Time permitting, we will then commence an investigation on the dissociation pathways of the corresponding organo-Group 14 radicals.

· the collisional dynamics of larger hydrocarbon fuels and halogenated molecules.

What computational techniques are used?

Molecular dynamics calculations used a customised version of VENUS96. This program has been customised for collisional-energy-transfer calculations, with vectorisation of the partial-derivatives subroutines which account for most of the computing time.

Ab initio quantum calculations use GAUSSIAN. All structures were initially optimised using the double-zeta pseudopotential basis sets of Hay and Wadt supplemented with d functions for Sn at the HF level of theory. Frequency calculations were then performed to ascertain the nature of the stationary points. Further geometry optimisations were then performed using MP2 theory and QCISD single point energies were obtained for the MP2 structures.


A.E.K. Lim, D. Dakternieks and K.F. Lim, Gas-phase dissociation of triorganostannyl cations, paper presented at 2nd Australian Conference on Physical Chemistry, Brisbane, Australia, July, 1998.

A.E.K. Lim, D. Dakternieks and K.F. Lim, Gas-phase dissociation of triorganostannyl cations, paper presented at IXth International Conference on the Coordination and Organometallic Chemistry of Germanium, Tin and Lead, Melbourne, Australia, September, 1998.

D. Dakternieks, A.E.K. Lim, and K.F. Lim, The gas-phase fragmentation of trineopentylstannyl cation: a rare example of beta-methyl migration within a main group organometallic compound, submitted to the Journal of the American Chemical Society.

Appendix B -