Much of the atmospheric chemistry in the stratosphere is governed by radical reactions. For instance, ozone depletion is catalyzed by the presence of chlorine radicals in the upper atmosphere. Experimental gas phase methods for probing the reaction kinetics of radicals with neutral reagents are few. One that has shown significant success is the combination of a flow reactor for studying the radical-neutral chemistry with detection of the products by chemical ionization mass spectrometry. This instrument is a hybrid of a conventional flowing afterglow (FA) apparatus which has been remarkably successful in studying the reactions of organic ions (positive and negative) with neutral reagents.

Variable temperature flowing afterglow - selected ion flow tube - neutral flow reactor.

We are studying the reactions of radicals with neutral reagents of relevance to atmospheric and combustion chemistry. We can generate organic radicals and other transient species in our gas phase instrument and obtain kinetic data for hydrogen atom abstraction reactions, alkene and alkyne addition reactions, radical cyclizations and so on. We will also obtain activation barriers by studying these reactions at different temperatures.

Experiment will provide us with a wealth of data, but mechanistic details may still be unclear. Therefore, we use computational chemistry and molecular modelling (ab initio molecular orbital theoretical methods) to assist our experiments. Besides thermodynamic information, we probe mechanistic details by calculating transition states and kinetic isotope effects. Our experimental data will also aid in calibrating the theoretical calculations in situations where theory has never explored.

In addition, we are examining the reactions of ions with neutral reagents in the gas phase. Many reactions in organic synthesis utilize ionic reagents in solution and aggregates are usually important. We are exploring these solvent effects, experimentally and theoretically.

We are particularly interested in reactions of carbonyl groups and especially their second row analogs, for example thioesters and phosphamides. We have explored the reactions of esters, amides and acid chlorides with a variety of nucleophiles in the gas phase. In particular, we are interested in the nucleophilic acyl substitution pathway that is dominant in aqueous solution but plays a lesser role in the gas phase. We are currently exploring micro-solvation effects on these reactions via mass-selected solvated clusters X-(H2O)n for different values of n.

We are interested in examining the chemistry of radical cations which are known to be formed from X-ray irradiation of biological tissue. These studies will occur in the gas phase and in the condensed phase under cryogenic matrix isolation methods. We are also interested in obtaining the absorption spectra of organic radicals and radical ions under matrix and gas phase conditions.