The Bristol group has successfully employed velocity map imaging (VMI) methods to study reactions of polyatomic molecules with large numbers of degrees of freedom. With VMI techniques, quantum-state resolved differential cross sections have been determined for a number of reactions such as Cl + C(CH3)4 → HCl + (CH3)3CCH2 and CH3 + HCl → CH4 + Cl, and the experiments have also proved powerful for characterizing non-adiabatic scattering in such systems. The non-adiabatic pathways involve dynamics on coupled potential energy surfaces (PESs), and therefore require breakdown of the Born-Oppenheimer approximation which neglects terms that couple different electronic PESs through the motions of the nuclei. For a reaction such as CH3 + HCl at high collision energies, our experiments demonstrate ~15% branching via a non-adiabatic path and indicate the regions of the reaction coordinate at which the non-adiabatic couplings become significant.
The experimental studies are complemented by trajectory calculations, performed in collaboration with Prof J.N. Harvey (Bristol) and Prof D. Troya (Virginia Tech), that incorporate all degrees of freedom of the polyatomic systems and provide detailed mechanistic insights.
In 2010, we published a review in PCCP of the use of velocity map imaging in the study of chemical reaction dynamics.
We have also examined the role of shape and structure of polyatomic molecules on the scattering dynamics during a chemical reaction. Much of the theory developed to describe rates of chemical reactions focuses on the transition state, which is a key configuration between reactants and products. Our experiments with polyatomic molecules showed how the rotational excitation of reaction products could be used to explore dynamics in the region of a reaction pathway that lies after the transition state. Extensive studies of the reactions of Cl atoms with organic molecules such as methanol and methyl halides compared the outcomes of molecular beam and laser based experiments with direct dynamics trajectory calculations, and showed that the degree of rotational excitation of the HCl product correlated with the dipole moment of the organic radical co-product of the reaction. The correlation derives from weak electrostatic interactions (e.g. dipole-dipole and dipole-quadrupole) as the products separate (illustrated in the energy diagram below for reaction of Cl with methanol) and the importance of reorientation dynamics on an anisotropic PES was experimentally identified.
This research has benefited greatly from consistent support from EPSRC (e.g. via Programme Grant EP/G00224X/1). Our VMI work has further benefited from collaborations with the groups of Kitsopoulos (FORTH, Heraklion), Parker (Nijmegen), Whitaker (Leeds), and Brouard and Vallance (Oxford) via the successive EU networks IMAGINE, PICNIC and ICONIC.
Direct and indirect hydrogen abstraction in Cl + alkene reactions, T.J. Preston, G.T. Dunning, A.J. Orr-Ewing and S. Vázquez, J. Phys. Chem. A 118, 5595-5607(2014).
Reduced dimensionality spin-orbit dynamics of CH3 + HCl → CH4 + Cl on ab initio surfaces, S.M. Remmert, S.T. Banks, J.N. Harvey, A.J. Orr-Ewing and D.C. Clary, J. Chem. Phys. 134, 204311 (15 pages) (2011).
Velocity map imaging of the dynamics of reactions of Cl atoms with neopentane and tetramethyl silane, R.A. Rose, S.J. Greaves and A.J. Orr-Ewing, J. Chem. Phys.132, 244312 (13 pages) (2010).
Velocity map imaging of the dynamics of bimolecular reactions, S.J. Greaves, R.A. Rose and A.J. Orr-Ewing, PCCP perspective article 12, 9129 – 9143 (2010).
The dynamics of chlorine atom reactions with polyatomic organic molecules, C. Murray and A.J. Orr-Ewing, Int. Rev. Phys. Chem. 23, 435 –482 (2004).
Stereodynamics of chlorine atom reactions with organic molecules, C. Murray, J.K. Pearce, S. Rudić, B. Retail and A.J. Orr-Ewing, J. Phys. Chem. A 109, 11093 – 11102 (2005).