Unimolecular React ions

BY H. M. FREY AND R. WALSH

1 Introduction

In the first volume of this series Robinson presented a comprehensive account of the subject which together with his book brought together in tabular form most of the unimolecular reactions that had been reported and for which there were accurate Arrhenius parameters. Quack and Troe in Volume 2 did not attempt a complete compilation of experimental data but concentrated on progress in theory together with selected .experiments related to fundamental aspects of unimolecular reactions. The high standard set by these previous Reports has been hard to follow and the volume of work which has since been published difficult to digest. This Report is intended to be 'consumer oriented' in that coverage of theory, in the main, is limited to those theories which can be readily applied to practical calculation. In selecting topics for coverage we have if anything reverted to the pattern of Robinson's article although inevitably with some modifications. For example, multiphoton i.r. photochemistry, a rapidly expanding area, is covered for the first time (although relatively briefly).

Unfortunately we have found it too difficult to attempt a comprehensive tabulation of high-pressure rate constants since the Robinson Report (Quack and Troe covered a good deal although by no means all of these). In this area we have tried to cover most of what appeared in 1976 and 1977 and additionally we have included the relevant area of radical recombinations (the reverse of unimolecular dissociation). Diatomic molecules (and atom recombination) present special problems and have, therefore, been omitted. There has been a tendency in the past, which we have not been able to avoid, of giving undue emphasis to presenting data where Arrhenius parameters have been reported without a critical analysis of the likely errors in the quoted values. Such a critical evaluation (which would be extremely time-consuming) is now overdue; the work reported since Benson and O'Neal's, 'Kinetic Data on Gas-phase Unimolecular Reactions' has probably equalled in volume all that available prior to 1969. We would like to reiterate a plea we have made previously, that all rate constants should be reported as well as the Arrhenius parameters on which they are based, and again that values of the pre-exponential factor and the energy of activation should always be given even if values of ΔS* and ΔH* are presented.

The number of recent reviews that have been published relevant to unimolecular reactions is not large. Troe's chapter in 'Chemical Kinetics' and Tardy and Rabinovitch's review on intermolecular vibrational energy transfer in unimolecular systems are required reading. However, there have been numerous reviews which have dealt with some aspects of particular series of unimolecular reactions which contain useful compilations of data, e.g. on the vinylcyclopropane rearrangements and azo decomposition. Mechanisms of some specific cyclopropane rearrangements have been dealt with in great detail. Other reviews have dealt with some thermal sigmatropic rearrangements and the decomposition of a number of heterocycles. A comprehensive survey on strained organic molecules contains much useful information on data and theories and especially energetic. Other reviews on aspects of Transition State Theory (TST) and use of local modes in the description of highly vibrationally excited molecules raise interesting ideas in relation to unimolecular reactions. Lee's review on laser photochemistry of selected states outlines an area that may well become of great importance for testing theory in the future.

After much debate it was decided not to devote a major section of this Report to the reactions of ions. We judged the time not quite ripe but it was a marginal decision. The area is growing very rapidly and as well as now covering a large number of species of a great range of complexity the precision of the quantitative information is approaching that of conventional thermal studies. Williams has discussed how the unimolecular decompositions (and isomerization) of ions in the field free region of a conventional (magnetic sector) mass spectrometer can be studied to give detailed information on potential energy surfaces. It is often necessary for the construction of 'unimolecular potential surfaces' to use information obtained from ion cyclotron resonance experiments on bimolecular reactions. Indeed RRKM calculations have been helpful in the latter area. Field isomerization techniques allow one to look at ion isomerizations occurring in the pico second time scale and the use of photoelectron-photoion coincidence spectrometry can give very precise values of the translational energy ielease during ion fragmentation reactions at specific energizations and time delays. Some of these techniques have yielded results which have provided searching tests of RRKM theory and later we mention phase-space theory.

2 General Theoretical Considerations

Despite many independent developments and much criticism it is still clear that RRKM theory occupies the centre of the stage of unimolecular reaction theory. This is partially because workers in this field have become familiar with its use and there are excellent textbooks describing it. But it is also because alternative and more fundamental theories, making fewer (or different) assumptions have not yet been fully developed to a point where they can be easily and practically applied. It is, however, true that the underlying assumptions of the RRKM theory have come increasingly under fire. The assumption of strong collisions is no longer regarded as integral to the theory, and indeed has been largely abandoned. RRKM calculations are routinely carried out incorporating specific models of weak collisional behaviour.

The related random lifetime and free intramolecular exchange assumptions, while still seemingly reliable under a variety of experimental conditions, are however being probed with increasing vigour. There has been considerable discussion in recent years of the application to unimolecular reactions of the ergodic hypothesis (which states that the time average of a system property is the same as the average of the property over all parts of the system at a single time instant). In this context, of course, ergodic behaviour may be identified with free and complete energy randomization, implying that a specifically energized molecule decays with a single rate constant irrespective of its initial mode of excitation. The limitations of this are being increasingly explored (see Section 3). However, it should be noted that ergodic behaviour is not necessarily synonymous with rapid intramolecular...