Strange vibrations

Exciting a vibration between the two atoms in a chemical bond should increase the likelihood of the bond breaking during a chemical reaction or, at the very least, have no impact on the reaction, according to conventional wisdom. Now, researchers in Taiwan have shown that precisely the opposite can happen - that in a relatively simple molecular system the induced vibrations can inhibit the breaking of the bond and slow the reaction down. The finding could have key implications for modelling reaction dynamics and could lead to new approaches to control molecular reactions by stimulating particular chemical bonds.

Weiqing Zhang, Hiroshi Kawamata and Kopin Lui, of the Institute of Atomic and Molecular Sciences in Taipei, studied the reaction between atomic fluorine and a methane molecule in which hydrogen was substituted by deuterium in three of the four positions - CHD₃. This is a so-called ’early-barrier’ reaction, in which the transition state resembles the reactants more than the products (as opposed to a late-barrier reaction, for which the reverse applies).   Using an infra-red laser pulse tuned to the frequency of the C-H vibration, the researchers excited the C-H bond exclusively in the methane reactant. 

According to a set of chemical rules of thumb called the Polanyi rules, vibration of a molecular bond in an early-barrier reaction is unlikely to make a significant contribution to the reaction rate.   However, when the Taiwan researchers reacted fluorine with CHD₃   they found that excitation of the C-H bond actually prevented the H atom from participating in the reaction - only D atoms were removed from the methane, not H atoms. Furthermore, the rate of the D atom reaction was slowed down.   The researchers describe the result as ’counterintuitive’ and state that ’clearly a conceptual framework of vibrational effects on chemical reactivity is far from complete’.

Claire Vallance, who researches chemical reaction dynamics at the University of Oxford in the UK, points out that Polanyi’s rules are a simplified model based upon a system involving a single atom reacting with a diatomic molecule, so the observation that the more complicated, six-atom system investigated by the Taiwan team does not obey these rules is not in itself especially surprising given the huge complexity of the ’potential energy surface’ of the reaction system. However, the fact that exciting one bond in one of the reactants almost completely excludes that bond from participating in the reaction is intriguing, Vallance says. 

’This is probably the best example I have seen of using light to control and steer a chemical reaction, and is unique in its simplicity - they have succeeded where vastly more complex approaches have failed.   This type of approach may potentially be a new way in to the type of control that synthetic chemists, for example, are very interested in.’

Simon Hadlington