Field-cycling NMR investigations of 13C-1H cross-relaxation and cross-polarisation: The nuclear solid effect and dynamic nuclear polarisation

A field-cycling NMR investigation of 1H-13C polarisation transfer using cross-relaxation and the nuclear solid effect (NSE) is described. Dynamic nuclear polarisation (DNP) of the 13C spins is observed when forbidden transitions are driven by r.f. irradiation at the sum and difference Larmor frequencies of the two nuclei. When the 1H spins are pre-polarised, a significant transfer of polarisation to the 13C nuclei is achieved in a time short compared with the spin-lattice relaxation time of 13C. The cross-polarisation arising from the NSE is studied as a function of B-field and time. These results are compared with the solutions of the differential equations that govern the coupled system of 1H-13C spins. The effects of cross-relaxation are incorporated into the model for the first time and good agreement between theory and experiment is obtained. The experiments have been conducted at 20 K on a 13C-enriched sample of benzoic acid.