Quantitative applications of ¹H and ³¹P chemically induced dynamic nuclear polarization

Abstract

Chemically Induced Dynamic Nuclear Polarization (CIDNP) spectroscopy has been widely used for many years for study of mechanisms of reactions that involve free radical pairs. Unlike Electron Spin Resonance (ESR) spectroscopy CIDNP provides an information on radicals that have been already reacted to paramagnetic species bringing the power of NMR into determination and identification of reaction pathways, yields and environmental effects. Since been discovered in year 1969 CIDNP was mostly applied in purely qualitative mechanistic studies where the simple rules developed by Kaptein allowed to deduce number of important reaction parameters. The quantitative application of CIDNP was rather difficult since there were sometimes no other experimental evidence to be compared especially within the cases where ESR failed. With the fast development of different theoretical calculation procedures which can provide rather precise knowledge on geometry, electron spin distribution and various magnetic properties of free radicals(g-factors, hyperfine coupling constants), CIDNP finds a new broad field of use especially where other physico-chemical methods are not successful by different reasons. Being a variety of Nuclear Magnetic Resonance (NMR) spectroscopy CIDNP is limited by the same drawbacks - bad time resolution (usually microseconds) and poor sensitivity. Despite that it can follow the radical processes that are by far too fast to be observed ESR spectroscopy - the most straightforward method to observe free radicals. The rate of fastest radical reactions that can be observed by CIDNP is determined by the rate of intersystem crossing which is about 10−10 −10−9s. The information on free radical properties and properties of the paramagnetic products of the reaction is then stored within the longitudinal relaxation time of the particular nuclei which is typically 1 - 10s. That very important information includes spin distribution, radical concentration, rotation diffusion and magnetic properties of radicals and rates of reactions. Within the certain conditions it can be extracted by the analysis of CIDNP spectra. The two different novel quantitative and semiquantitative applications of Chemically Induced Dynamic Nuclear Polarization combined with density functional quantum mechanical calculations and NMR studies are the subject of the present thesis. First application is of almost pure fundamental interest whereas the second finds its use in industry.