Rare spin polarization by the nuclear solid effect

Abstract

The development of the cross-polarization method has led to numerous applications of high-resolution solid state NMR spectroscopy by making it possible to obtain the spectra of magnetically dilute-“rare spin” -nuclei (I). Using the CP scheme, the polarization of a rare spin can be enhanced by as much as the abundant to rare spin gyromagnetic ratio (e.g., 4 for H-C experiments) (2). This maximum enhancement is rarely achieved in practice, and a rapid decay of the spin-locked proton magnetization (short proton T,,) is often the cause of this problem. The CP scheme also requires that the strength of the proton (and, because of matching requirements, the carbon) spin-lock field be greater than the width of the proton line, a criterion that can be difficult to satisfy. In this communication, we discuss a method for polarizing rare spins (e.g., carbon13) which neither requires a long Tr, for the abundant spin (e.g., proton) nor very strong matching rf fields at both Larmor frequencies. We also report results which illustrate characteristics of the method that are complementary, and under certain conditions superior, to the cross-polarization technique. The nuclear solid effect (NSE) is analogous to the solid effect associated with dynamic nuclear polarization (DNP) (3). In DNP, the nuclei are polarized by saturation of a forbidden transition involving simultaneous flips of an electron and nuclear spin. This transition is at the sum or difference of the electron and nuclear Larmor frequencies. In the nuclear solid effect, however, the abundant nuclear spins assume the role of the: electrons, so that the forbidden transition is at the sum or difference of the Larmor frequencies of the abundant and rare nuclei (4). For example, this would be either 45 or 75 MHz for a proton-carbon system in a field of 1.4 T. ‘The polarization step merely requires irradiation of the forbidden transition. The time dependence of the carbon polarization during this irradiation is given by