Relaxivity in paramagnetic systems: Theory and mechanisms

Fundamental understanding of nuclear relaxation in the presence of ions with unpaired electrons exists in the field of relaxation enhancement by paramagnetic molecules. Computer programs based on a stochastic Liouville approach, or on spin dynamics, allow the calculation of intra- and intermolecular relaxivities for very general cases. Both techniques are computationally very demanding and can in general not be used for data fitting. Recently, computationally much more efficient analytical approaches have been developed that are valid under conditions such as high or low magnetic field, and fast or slow rotation of the paramagnetic species. Even if the theoretical understanding of relaxivity in paramagnetic systems has made considerable progress there still items to be considered. These items include theoretical models, theoretical calculation of the lineshape of EPR spectra, and nuclear relaxivity by intermolecular relaxation.