Computational Studies of Chemical Reaction Dynamics


The alkane molecules, and their halogenated analogs, are important fuel and atmospheric-chemistry molecules. Yet there is little reliable experimental data on collisional energy transfer involving these molecules, because of a lack of suitable spectroscopic probes. For example, the shape of the colliders is one of many factors in determining whether liquified petroleum gas molecules gain mainly vibrational or rotational energy in a collision. The partitioning of the energy, in turn, determines the competition between different reactions and controls whether a combustion system will explode or merely burn. This project investigates the collisional energy transfer process between hydrocarbon fuels (methane, ethane, propane, butane) and monatomic colliders, the reactions of gas-phase organo-tin cations (which undergo reactions similar to hydrocarbon combustion), and the development of better computer algorithms for collisional energy transfer.


Principal Investigator

Kieran Fergus Lim

Centre for Chiral and Molecular Technologies
School of Biological and Chemical Sciences
Deakin University

Project

g72, t52

Facilities Used

PC, SC

Co-Investigators

Allan E.K. Lim
Apichart Linhananta

Centre for Chiral and Molecular Technologies
School of Biological and Chemical Sciences
Deakin University

RFCD Codes

250601, 250603, 259901

Significant Achievements, Anticipated Outcomes and Future Work

Ethane, propane (liquified petroleum gas) and butane (lighter fluid) are the simplest model hydrocarbon fuel molecules that incorporate an internal rotation. The methyl torsional motion behaves like a vibration at low vibrational energies with a transition to a near-free rotor at high vibrational energies. Our simulations of collisions between these fuel molecules and rare gas atoms have shown that torsional motion acts as a gateway for energy transfer. The implication, for any future studies of larger fuel molecules (e.g. octane), is that simulations of collisional energy transfer must include the correct torsional dynamics.

As part of these collisional energy transfer studies, we have developed a "multiple direct-encounter hard-sphere model", which enables qualitative prediction of collisional energy transfer trends and behaviours, but at less computational cost than a full quantitative prediction. This will be be useful for future investigations of large molecules, for which full simulation studies are impractical.

In collaboration with Dr Collins (Australian National University), we have shown that his interpolated potential energy surface method is suitable for collisional energy transfer studies with quantitative prediction. This is a generalisation of the GROW methodology to weakly interacting molecules. We are currently using this for methane + helium collisional energy transfer studies, for which there are no reliable experimental data.

In other work, we have investigated the structure and energetics of molecular metallasiloxanes, which are model compounds for structurally modified silica surfaces and minerals. These metallasiloxanes have interesting physical properties, and are potential precursors for new inorganic materials. We have compared the ring flexibilities of cyclo-tetrasiloxanes and cyclo-stannasiloxanes in order to determine the effects of replacing silicon in eight-membered cyclo-tetrasiloxanes with tin. Ab initio and DFT calculations indicate that the energetic preference to adopt puckered structures increases and the ring flexibility decreases with an increasing number of tin atoms in the ring. The rich diversity of puckered conformations is attributed to the steric demand of the different organic substituents.

We have also studied gas-phase organotin dissociation pathways, which are analogous to some reactions in combustion systems. We are currently comparing the organotin results with those of other organo-Group-14 molecules. These studies are highlighting similarities, differences and trends in chemical reaction dynamics in the transition from organic molecules to organometallic molecules moving down Group-14 (M=C, Si, Ge, Sn, Pb).

 

Computational Techniques Used

Molecular dynamics calculations on ethane+Rg, propane+Rg and butane+Rg systems (Rg=rare gas) used the program MARINER, a customised version of VENUS96 [QCPE Program 671: Quantum Chem. Program Exchange Bull., 16(4), 1996, 43]. MARINER 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 were performed using GAUSSIAN. All organo-Group-14 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.

Molecular dynamics calculations on methane+He systems were performed using the GROW suite of programs and scripts written by M.A. Collins and coworkers (Research School of Chemistry, Australian National University, http://www.rsc.anu.edu.au/RSC/ChemResearch/Groups/DD-home.html). GROW calls on GAUSSIAN to calculate ab initio points from which a potential energy surface is interpolated. The interpolated surface is then used for molecular dynamics calculations.

 

Publications, Awards and External Funding

ARC Large Grant A29701382 (1997-1999): "Theoretical studies of collisional energy transfer in combustion-model and atmospheric-model systems at high vibrational excitation".

A. Linhananta and K. F. Lim, "Quasiclassical trajectory calculations of collisional energy transfer: The methyl internal rotor in ethane", Physical Chemistry Chemical Physics, 1999, 1 (15), 3467-3472

A. Linhananta and K. F. Lim, "Quasiclassical trajectory calculations of collisional energy transfer in propane systems", Physical Chemistry Chemical Physics, 2000, 2 (7), 1385-1392

J. Beckmann, K. Jurkschat, M. Schürmann, D. Dakternieks, A. E. K. Lim and K. F. Lim, "Comparison of the flexibility of eight-membered tetrasiloxane and stannasiloxane rings: A crystallographic and computational study", Organometallics, 2001, 20 (24), 5125-5133.

A. Linhananta and K. F. Lim, "Quasiclassical trajectory calculations of collisional energy transfer in propane systems: Multiple direct-encounter hard-sphere model", Physical Chemistry Chemical Physics, 2002, 4 (4), 577-585
http://pubs.rsc.org/ej/CP/2002/b109074g.pdf