NMR cross polarization in solids using multipulse irradiation

Abstract

A double resonance NMR scheme (MPSLCP) is presented in which a WHH-4 type multipulse irradiation of the abundant spins I is applied during cross polarization. When using the HW-8 sequence, the locking Hamiltonian for the I spins is provided by the average pulse length error Hamiltonian. Double resonance is obtained with a weak pulse irradiation of the rare spin S, whose amplitude matches (Hartmann–Hahn condition) the average pulse error field. Experimental results on 13C–1H in powdered adamantane are presented. Owing to the high symmetry of both the crystal and the molecular reorientations, the 13C multipulse cross-polarization rise is described by considering one carbon coupled to N equivalent protons and applying the calculations of L. Muller and R. R. Ernst [Mol. Phys. 38, 963 (1979)] for an SIN system with, here N=196 (or better N=235). Such a cross-polarization scheme (MPSLCP) is shown to be applicable to rigid organic solids. For S spins with very low gyromagnetic ratio and/or far from I spins, MPSLCP (i) is selective; (ii) increases the rate of polarization (it is suggested that this might help for quantitative analysis purposes); (iii) provides a very important rf power saving (104 here for adamantane) at the S spin frequency.