Effects of Molecular Mobility on Reaction Rates in Liquid Solutions

Abstract

This review concerns molecular motions, which include motions of geminate pairs (cage effects), the formation of encounter complexes, motions of solvent molecules, and conformational changes of the reagent molecules. With rare exceptions, we discuss these motions only to the extent that they have an effect on the rates or kinetically-controlled product distributions of chemical reactions. Because they are covered in separate reviews in this or preceding volumes, we have intentionally neglected picosecond processes, electron tunneling, and reactions in liquid crystals. Although theoretical and experimental tools have been developed to the point where they are useful for studying the effects of molecular motions in liquid-phase reactions, thest: tools are being continually refined and new ones developed. In particular, we noted a continued concern with cooperative effects in diffusion (1-3), effects of local depletion of the reagent (1, 3), possible effects of excess energy in a caged pair (1), and hydrodynamic models (4-8). Physical methods currently in use for probing rotational motions of solutes in liquids include studies of infrared and Raman lineshapes (9), studies of NMR relaxation times (10-13) and picosecond flash experiments (13a). In addition, considerable information about molecular rotation is obtained from studies of caged pairs in which one or both of the partners are chiral. Information about fast conformational changes is obtainable from picosecond flash experiments and from NMR, ultrasonic, and other forms of relaxation spectrometry. Heywood et al (14) have described a useful statistical method of analyzing data from ultrasonic relaxation experiments on molecules undergoing conformational changes, and Strehlow & Frahm (15) have presented a theory for obtaining, from NMR Tl relaxation times, not only rates of conformational inter­ conversion but exchange rates in general.