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Physical and Theoretical Chemistry
Theoretical Chemical Physics
Professor Michael Collins
http://rsc.anu.edu.au/research/collins.php
The group's main interest is in
understanding the mechanisms of chemical reactions. This pursuit
involves the development of methods for constructing potential energy
surfaces for chemical reactions and the reaction dynamics on these
surfaces.
Ab
initio quantum chemistry provides accurate information about the
energetics of chemical reactions. The potential energy surfaces
(PESs) are constructed as an interpolation of this ab initio
data evaluated at a relatively small number of ab initio
calculations to estimate the molecular energy at any relevant
molecular geometry. Significant progress has now been achieved for
moderate sized molecules, so that many different chemical reactions
have been investigated. Most of these reactions involve competing
mechanisms or reaction pathways and could not be treated using
simpler approximate methods. The end result of this work should be a
much clearer understanding of the mechanisms of reaction at the
molecular level. We are currently pursuing a number of objectives,
including improved accuracy of PES for larger molecular systems,
highly accurate PES for quantum reaction dynamics studies of four and
five atom systems, many-body expansions for the weak interactions
involved in collisional energy transfer and molecular clusters, and
coupled surfaces for nonadiabatic reactions.
The
group's work has been enhanced through collaborations with overseas
scientists including Dr Mark Brouard at Oxford University
and the dynamics group of Professor Aoiz and
Dr Jesus F. Castillo, Universidad Complutense de
Madrid, Associate Professor Dong Hui Zhang at the National
University of Singapore, Professor David Yarkony, Johns Hopkins
University (nonadiabatic dynamics), and Dr C. Crespos and
Professor G. J. Kroes, University of Leiden (reaction at
surfaces).
Reaction Dynamics for H+H2O <->
OH+H2 and H+D2O <-> HOD+D
Recent
experiments by Mark Brouard at Oxford have extended the available
data on these seminal reactions to high translation energy in the
reactants. We are pursuing a combined classical and quantum study of
the reaction dynamics. (with M. Brouard [Oxford U.],
J. Castillo [U. Complutense de Madrid], D.H. Zhang
[National U. Singapore])
Classical Reaction Dynamics for H+N2O <->
OH+N2
We
have completed a preliminary study of this reaction using classical
dynamics on a PES constructed with density functional theory. The
theoretical dynamics confirm the presence of two competing reaction
mechanisms which was inferred from experimental observations by the
Brouard group. A more accurate PES, evaluated with high level ab
initio data, is currently under construction. (with
J. Castillo and colleagues [U. Complutense de Madrid])
Radical-radical and Combustion Reactions
We
have developed ab initio PES for the reaction H + HCO
(important in hydrocarbon combustion). This reaction is a prototype
example of the competition between direct abstraction and
addition-elimination mechanisms. Analogous reactions are currently
under investigation. (with M.H. Smith)
Hydrogen Abstraction in H + CH4
The
abstraction reactions, H + RH ® H2
+ R-, have been observed to yield an
unusual distribution of rotation-vibration states in the H2
product. To investigate the mechanism of this class of important
combustion reactions, we have constructed a very accurate PES for the
simplest example, H + CH4. Further development of this
surface to high accuracy is underway in order to provide a basis for
very high dimensional quantum scattering calculations. (with
D.H. Zhang [National U. Singapore])
Interpolation for High Dimensional Surfaces
Quantum
scattering calculations for systems of four or more atoms require
evaluation of the molecular potential energy over a high dimensional
grid comprised of 107 to 109 or more vertices.
A new algorithm has been developed to minimise the computer time
involved.
Reactions on Surfaces
The
method for constructing PES for gas phase chemical reactions has been
extended to evaluate the PES for a chemical reaction on a crystalline
surface. Initially, we have demonstrated the accuracy of this
approach for a model potential which describes the dissociation of H2
on a metal surface. (with C. Crespos, G.-J. Kroes [U. Leiden])
Nonadiabatic Chemical Reactions
Many
reactions, particularly in combustion and atmospheric chemistry, take
place in more than one electronic state. The PES for these
electronic states can intersect, and new methods are being developed
to describe all the energy surfaces involved and their
"interactions". Significant progress on code development
for this very difficult and important project has been accomplished
this year. (with C. Evenhuis, and D. Yarkony [Johns
Hopkins U., USA])
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