Molecular Potential Energy Surfaces

               

Principal Investigator

Michael Collins

Research School of Chemistry

Co-Investigators

Ryan Bettens

Simon Petrie

Andrew Chalk

Leo Radom

Research School of Chemistry

Projects

s01 - VPP, PC
This project develops methods for constructing, from
first principles, the molecular potential energy
surface (MPES) for molecules undergoing chemical reactions. This allows us to calculate how the atoms move during the break-up and formation of new molecules. In this way we can understand the mechanisms and rates of important chemical reactions that take place in the atmosphere, in combustion processes and in inter-stellar space, for example.
   
             

     
             
               

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

This year has seen rapid progress made in both methodology development and in application of the methods to important chemical reactions.

We have completed MPES for the reaction of carbon atoms with H3+ molecules. This process initiates the formation of hydrocarbon molecules in space. A new mechanism which leads to the formation of CH2+ was discovered.

We have completed MPES for the reaction of nitrogen atoms with H3+ molecules. This was thought to be an initiator of the formation of ammonia in space, but we have proven that the reported experiment was incorrect.

We have completed MPES for the reactions of BeH with H2 and BeH2 with H. This surface is intended for use in the development of exact quantum chemical reaction dynamics. In collaboration with Dr Donghui Zhang of the National University of Singapore, we have demonstrated that our MPES can be combined with state-of-the-art dynamics methods to yield ab initio quantum rate coefficients. This work is nearing completion.

We have made substantial progress towards constructing a MPES for the reactions of OH with H2 and H2O with H. These reactions are important in combustion processes, and have played a

               
- Appendix A

 
               

       

major role in the development of quantum reaction dynamics methods. This system requires very high level ab initio quantum chemistry calculations.

Substantial progress has been made in constructing MPES for a series of ion-molecule reactions involving Ne + COH+, Ar + COH+, Kr + COH+, and FH + COH+, together with related collisions such as FH2+ + CO. These are important test cases for examining the competition between proton transfer reactions and isomerisation processes. The general aim is to increase our understanding of the competition between energy activation and entropic effects in determining the mechanisms of reactions.

Finally, with Ryan Bettens, we have made important improvements in the interpolation procedure used to construct MPES. This has lead to a significant reduction in the errors associated with interpolation and consequently, to significant increase in the efficiency with which ab initio calculations are used.

What computational techniques are used?

This project employs ab initio quantum chemistry programs together with classical trajectory simulations of chemical reactions and a variety of Fortran routines for the manipulation and transformation of data.

Publications

R. P. A. Bettens and M. A. Collins, Potential energy surfaces and dynamics for the reactions between C (3P) and H3+ (1A1'), Journal of Chemical Physics, 108, 1998, 2424-2433.

R. P. A. Bettens and M. A. Collins, Interpolated potential energy surface and dynamics for the reactions between N (4S) and H3+ (1A1'), Journal of Chemical Physics, 109, 1998, 9728-9736.

M. A. Collins and R. P. A. Bettens, Potential energy surface for the reactions BeH2 + H BeH + H2, Physical Chemistry Chemical Physics, 1999, in press.

       
Appendix A -