Figure 2.

(a) The exponential dependence of reaction rates upon absolute temperature . Deviations from linearity at low temperatures can be observed in the plot as showing a ‘concave’ curvature, i.e. higher than expected rates as temperature decreases. (b) The sub-Arrhenius behaviour is accentuaded in an Arrhenius plot view, where ln is reported against . In elementary reactions at temperatures where quantum mechanical tunnelling is operative, there is a relation between the deformation parameter and features of (height and width) of the barrier (see 4b(i)). Panel (c) shows that deviations from constancy of the apparent activation energy, expected from the Arrhenius Law, for a sub-Arrhenius behaviour manifests as a decrease with decreasing temperature. Panel (d) exhibits the linear relationship of the transitivity (equation (2.4)) with inverse temperature, basic to our derivation of the -Arrhenius formula. Sub-Arrhenius behaviour, discussed as a manifestation of quantum mechanical tunnelling through a potential energy barrier as exemplified in the panel. (Online version in colour.)