Free radicals play an important role in many important atmospheric and combustion reactions. Their study however, is often hampered due to their high reactivity, and thus short lifetimes, limiting the number densities which can be generated in the laboratory. While a great deal of progess has been made by studying radicals isolated in the gas-phase, much less is known about the interactions between radicals, or the interaction between a radical and another closed shell species. Clearly of interest is what happens at the transition state of a chemical reaction, the point at which bonds are broken and reformed. The areas of the potential energy surface that lead up to the transition state are the entrance and exit channel valleys, which represent weakly bound clusters between two (or more) reagent or product molecules in a chemical reaction. Due to the fact that at long-range the interaction potential is almost always attractive, minima develop near the base of the transition state which may support bound or quasi-bound states. Given that the barriers to a chemical reaction are typically several orders of magnitude larger than the dispersion forces holding these entrance channel complexes together, their importance to reaction dynamics has largely been neglected. However, there is now compelling evidence that these weak forces may actually control the dynamics in some instances due to the corresponding orientational effects. The work detailed in this thesis is focused on applying super_uid helium droplets to the study of these entrance and exit channel complexes. The extremely cold and gentle nature of the helium matrix allows us to stabilize highly reactive complexes and using high-resolution infrared spectroscopy we are able to probe the structure and dynamics of these systems
