Radical thermochemistry and organic reactions*

Most free radicals are transient species and it is therefore difficult to measure their thermochemical properties. We have used two new techniques that address this problem. Photoacoustic calorimetry was used to measureheats of formation of free radicals and thus homolytic bond strengths, while photomodulated voltammetry was used to measure their oxidation and reduction potentials. The data offer fresh insights into the thermochemistry of free radical processes and can therefore be used to design new reactions. INTRODUCTION Almost all of the free radicals that participate in important chemical processes are short lived and it has therefore been particularly difficult to quantify their thermodynamic properties. Nevertheless, these quantities are of critical importance in the understanding of atmospheric and combustion chemistry and in the design of new free radical reactions. Most of the currently available measurements on radical heats of formation and bond strengths have been made using gas kinetic techniques (ref. 1 & 2). This has meant that they are restricted to volatile and hence low molecular weight materials. In addition, the experiments have generally been carried out at high temperatures and often flawed by wall reactions, by assumptions about elements of the reaction kinetics and by the difficulties associated with extrapolation of the results to 300 K. The first part of this paper describes a new technique, photoacoustic calorimetry, which overcomes many of these deficiencies and that can be used to produce reliable heats of formation and bond strengths for a wide variety of free radicals including high molecular weight and organometallic species. While heats of formation of free radicals and homolytic bond strengths are crucial to a thorough understanding of many free radical reactions, it is being increasingly recognized that one electron transfer reactions involving radicals play a vital role in biological chemistry and in some industrially important processes such as photoimaging. In this context, gas phase measurements of the related thermodynamic quantities, i.e. ionization potentials and electron affinities are of little value since they cannot readily be extended to the condensed phases where these reactions take place. In fact, electrochemical oxidation and reduction potentials of free radicals are the parameters that are needed. While some have been measured by pulse radiolysis techniques the amount of available data is extremely limited and certainly does not satisfy the needs of chemists interested in biology or synthesis. The second part of this paper deals with the technique of photomodulated voltammetry which we have developed to measure oxidation and reduction potentials for transient free radicals. As we will see, these can be combined with heats of formation and bond strengths to provide a unified description of the thermodynamic properties of free radicals and their related ions. *Issued as NRCC publication no. 29376