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Timescales of chemical reactions

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Chemical reactions occur on timescales from femtoseconds to many years. Some reactions occur before the energy can be randomly distributed in the reacting molecule; the reaction between cyclopentyne and ethene, for example, requires a dynamical description in order to understand the mechanism. For many other reactions, occurring on longer timescales, energy is randomly distributed in the reacting molecule, but not equilibrated with the bath gas, and it is necessary to consider the interaction between collisional energy transfer and reaction, using a master equation approach. Such an approach may be necessary even in solution phase reactions, despite the high collisional frequency, as illustrated by the reaction between propene and BH3 .

Gas phase reactions following the association of two molecules can lead, on a single chemical timescale, to the association product. In other cases, such as the reaction between an organic radical and O2, the reaction may involve several isomers, and can lead to dissociation to form bimolecular products. There are now several chemical timescales, which depend on the details of the isomerisation, dissociation and energy transfer processes occurring in the reacting system. Examples will be drawn from combustion and atmospheric chemistry. Solution of the master equation is the key to understanding these timescales and interpreting experimental results to obtain the underlying chemical kinetics and mechanism.

Timescales of complex sequences of chemical reactions can be interpreted in a similar way, but now using the Jacobian of the system. There is a distinction between the chemical lifetime – the reciprocal of the reactant’s overall pseudo first-order rate constant for loss – and the system timescales, as has been appreciated for many years for methane in the atmosphere. Other examples will be briefly discussed.

This talk is part of the Physical Chemistry series.

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