The Polanyi Lecture. Radical–radical reactions: kinetics, dynamics and mechanisms

Abstract

Three facets of radical-radical reactions are examined: their kinetics, i.e. the rates at which such reactions proceed ; their dynamics, i.e. the detailed nature of reactive collisions between radicals; and their mechanisms, i.e. the products which are given when more than one reactive channel is energetically accessible. The goal is to show how information about each of these attributes of radical-radical reactions helps to illuminate the fundamental character of radical-radical collisions. The connecting thread, in the spirit of Michael Polanyi's pioneering work, is the form of the potential-energy surface or surfaces that control the collisions. The general themes are illustrated by reference to studies of particular systems, especially those which have been, or are being, investigated in the author's laboratory. Every effort is made to assess the current level of understanding, to indicate areas where that understanding is deficient, and to suggest how o u r knowledge of radical-radical reactions might be improved.

Theoretical study on the reaction mechanism of the methyl radical with nitrogen oxides

“…But intermediate i 3 crresponds to the twisted configuration. The conversion from i 1 3 i 4 and i 2 3 i 3 can be realized via COO bond rotation transition states TSi 1 i 4 and TSi 2 i 3 with small internal rotation barriers.…”

“…It should be noted that the attempt to locate the transition state from R to a at the B3LYP/6-311G(d,p) level failed. Because both of the C-terminate of CH 3 and N-terminate of NO 2 have a single electron, we expect that middle-N attack is a static attracting process with no barrier. To further confirm our anticipation, we calculate the relaxed potential energy curve for the formative process of H 3 CNO 2 at the B3LYP/ 6-311G(d,p) level of theory.…”

“…Similarly, the isomer b 2 (cis-CH 3 ONO) can alternatively undergo a 1,4-H-shift and NOO bond cleavage to form product P 3 (CH 2 O ϩ HON) via TSb 2 P 3 . These processes can simply be written as: Finally, we turn to the other isomerization and dissociation channels of the isomers a (CH 3 NO 2 ) and b 2 (cis-CH 3 ONO).…”

Reactions between neutral species at low temperatures: Laboratory results and astrophysical modelling

“…Fig. 3 shows the rate coefficients obtained (Smith & Stewart 1994) for the reactions Such systems are characterised by the existence of multiple potential energy surfaces arising from the degenerate and near-degenerate fine structure states in the radical reagents (Smith 1991). As a result, part of the temperature dependence of the rate coefficients shown in Fig.…”

“…When both reagents are radicals (i.e., have one or more unpaired electrons), there will generally be more than one potential energy surface which correlates with the separated reagents (Smith 1991) and one, or possibly more, of these surfaces will be monotonically attractive as the reagents approach. If there are products of lower potential energy than the reagents which can be reached by breaking one bond in the adduct, as in, for example, CN + 0 2 -> (NCOO) -> NCO + Ο and Ο + OH -> (0 2 H) -* 0 2 + H, then the reaction will be rapid and the rate coefficient will almost certainly increase as the temperature is lowered.…”