Chemical Reactions at Very Low Temperatures

INTRODUCTION The Arrhenius law, k = Aexp[ -ElkB T], predicts that the rate of a chemical reaction vanishes in the limit T -+ O. Here, k is the rate constant, E the activation energy, and kB the Boltzmann constant. However, as early as 1903 Moissan & Dewar (1) observed rather fast fluorination of hydrocarbons at low temperatures. The next thirty years were marked by observations of efficient oxidation of nitric oxide (2), chlorination of ammonia (3), and NO-NCI3 interaction (4), as well as of hctcrogencous hydrogcn ortho-para convcrsion (5) and hydrogcn-dcutcrium exchange (6) at low temperatures, up to the boiling point of nitrogen (77°K). The interest in chemical reactions at low temperatures increased in the 1950s, when new experimental methods for studying them emerged, e.g. tracer methods, radiospectroscopy, etc. Attention was drawn to high free-radical concentrations in solid matrices and to various solid-phase radiation reactions involving electrons and ions, radicals and molecules. Gradually it became clear that most low-temperature reactions actually proceed faster than would be predicted on the basis of an Arrhenius-type extrapolation. There exist two main reasons for such deviations from the Arrhenius law at low temperatures. First, if the conversion of a certain species A can proceed via several parallel channels-I. A -+ B, 2. A -+ C, 3. A -+ D, etc-each with its own activation energy E" at low temperatures the channels with higher activation energies are suppressed and only one single channel of the least (Ei)min magnitude remains. For example, since ion-molecule reactions are activationless, the ionic mechanism of the polymer chain growth often prevails over the radical one at low temperaturcs, and this can even alter the polymer structure. Second, even for single-channel systems quantum-mechanical tunneling can result in very strong deviations from the Arrhenius law and an apparent decrease of activation energy with the decreasing temperature, and in high observable reaction rates at very low temperatures. The definitions of low and very low temperatures are quite conditional and their sense in physics, chemistry, and biology differs. Our paper is devoted to "very low"