The Low-temperature Quantum-mechanical Limit for the Rates of Chemical Reactions

The influence of quantum-mechanical effects on one of the fundamental laws of chemical kinetics – the Arrhenius law – is considered. Criteria characterising the limits of the low-temperature region where the extent of quantum-mechanical tunnelling transitions exceeds exponentially the transitions over the barrier are quoted. Studies of the low-temperature tunnelling of electrons and hydrogen atoms are briefly mentioned and the history of research on low-temperature radiation-induced solid-phase polymerisation, the development of which led to the discovery of the phenomenon of the low-temperature quantum-mechanical limit for the rates of chemical reactions in relation to the formaldehyde polymerisation reaction, is briefly considered.
The results of experiments using low-inertia calorimeters, whereby it is possible to determine directly the average time (τ₀) required to add one new link to the polymer chain of formaldehyde during its polymerisation by radiation and during postpolymerisation and to establish that below 80 K the increase of τ₀ slows down and that at T≈10–4 K the time τ₀ reaches a plateau (τ₀≈0.01 s), are described. Possible explanations of the observed low-temperature limit for the rate of a chemical reaction are critically examined and a semiquantitative explanation is given for this phenomenon, which may be particularly common in combined electronic-confirmational transitions in complex biological molecules and may play a definite role in chemical and biological evolution (cold prehistory of life?).
The bibliography includes 56 references.