Research Guides

Department of Chemistry University of Oxford

Dr Nicholas Green

Director of Studies for the Department of Chemistry

Physical & Theoretical Chemistry

nicholas.green@chem.ox.ac.uk

Telephone: 44 (0) 1865 282 760

 

 

 

 

 

 

 

 

Stochastic processes in chemical kinetics

We are interested in the theory of reaction rates, particularly the way they depend on dynamical random processes, such as the trajectories of diffusing particles in solution, or the energy of a molecule as it changes collision by collision.

The systems we investigate can be categorised according to the nature of the underlying random process:

The motion of particles in solution (Brownian motion)

  • The reactivity of particles in solution (reactivity dynamics)
  • Reactions in solution controlled by spin (diffusion and relaxation)
  • Unimolecular reactions in the gas phase (collisional energy transfer)
  • Stochastic kinetics in systems with small numbers of reactants

The main applications of this research are in radiation chemistry, which is the study of the chemical effects of ionising radiation. Radiation tracks contain microscopic clusters (spurs) containing small numbers of highly reactive particles. Recombination reactions take place on a picosecond timescale and are strongly dependent on the numbers of particles in the clusters, their relative spatial arrangement and the structure of the track. Most of the species are free radicals and the chemistry may also be influenced by spin correlations. Normal theories of geminate recombination or homogeneous reaction are not applicable.

The reactive particles produced by radiation can attack other molecules in the vicinity, leading to biological damage or corrosion of metal surfaces. To understand these effects it is important to understand both the kinetics and the spatial distributions of surviving radicals.

A proper understanding of radiation chemistry requires a good theory for all the following stages:

  • the interaction between the radiation and molecules giving a spectrum of energy losses
  • the fragmentation pathways for excited molecules and negative ion resonances in solution
  • the thermalisation, capture and solvation of the fragments
  • ultrafast proton transfer reactions
  • diffusion controlled recombination of radicals and ions
  • effects of spin correlation between radicals
  • competitive scavenging by other molecules including DNA

We have made contributions in many of these areas, especially in diffusion kinetics, where our simulation method (the IRT method) is now used world-wide.

For unimolecular reactions we are developing fast and simple methods for the global analysis of rate data, using a stochastic description of the energy transfer process, and sometimes of the microcanonical reactivity. We are also investigating the analysis of complex schemes, such as reversible systems and systems with multiple linked minima in the potential energy surface.

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