High-Pressure In Situ NMR Methods for the Study of Reaction Kinetics in Homogeneous Catalysis

High-pressure NMR methods for the study of the kinetics of gas–solution reactions are presented, and the importance/interplay of mass transport and chemical resistances are discussed. For reactions that are slow compared with mass transport, the true reaction kinetics can be obtained and used to confirm that the observed species are relevant to the catalytic reaction. Conversely, the ability to determine the catalyst speciation during the catalytic reaction aids interpretation of the kinetic data. When chemical reaction is fast compared with diffusion across the gas–liquid interface, reaction is shown to occur in the liquid layer; for such a transport-controlled reaction, chemical reaction can increase the concentration gradient across the boundary layer, enhancing diffusion both across the phase boundary and within the liquid layer, resulting in apparent positive orders in catalyst, substrate, or both. These dependencies do not reflect the chemical dependence of the reaction rate on these concentrations.