Cross polarization in solid state NMR spectroscopy. Efficient polarization transfer via the non-Zeeman spin reservoir

Abstract

A novel and highly efficient method of polarization transfer between nuclear spin ensembles is described and demonstrated with proton enhanced solid state 13C NMR spectra of methanol and hexamethylbenzene. Concepts known from dynamic nuclear polarization of paramagnetic systems are invoked and used for polarization enhancement via a mechanism different from that of conventional cross polarization (CP); in particular, no energy matching by fulfillment of the Hartmann-Hahn condition is necessary, thereby making requirements for the applied rf pulses less stringent. Since the non-Zeeman energy reservoir plays a central role in the polarization transfer pathway, we use the term non-Zeeman polarization transfer (NZPT) for out method. The NZPT approach has especially clear advantages for nuclei with small gyromagnetic ratio, e.g. 15N, or for spin I> 1 2, e.g. 2H. We show for the test systems that NZPT transfer efficiency compares well with the CP technique, while the adjustment of experimental parameters is easier. © 1993.