Product branching ratios in simple gas phase reactions
It has long been known that the product branching ratio between different products in multichannel reactions is as important as the overall rate of reaction, both in terms of practical applications (e.g. models of combustion or atmospheric chemistry), and in understanding the fundamental mechanisms of such chemical reactions. However, it is only in the last 10–15 years that the experimental and theoretical tools have become available to quantitatively address the issue of product branching ratios under relevant conditions. This article includes a review of relevant developments in experimental and theoretical techniques and then considers examples of multichannel reactions. Firstly we examine the case of site specific reactions taking place via the same basic mechanism, e.g. abstraction or addition and then move on to review the class of multichannel reactions occurring with the formation of a bound complex with subsequent unimolecular decomposition to multiple sets of products. The final class of reactions considered is where there is a competition between two different mechanisms e.g. abstraction vs. addition. The review is concluded with a brief outlook on to possible developments in the field.