Multiple-Quantum Cross Polarization in Quadrupolar Spin Systems during Magic-Angle Spinning 1 David Rovnyak, Marc Baldus, and Robert G. Griffin MIT/Harvard Center for Magnetic Resonance, Francis Bitter Magnet Laboratory and Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received June 3, 1999; revised August 18, 1999 We describe the concept of multiple-quantum cross polarization (CP) between an I  3 2 and an I  1 2 spin during magic-angle spinning. Experimental and theoretical results for 23 Na– 1 H pairs are presented that elucidate the transfer mechanism and the ben- eficial effect of adiabatic amplitude modulations of the CP field. The multiple-quantum CP approach is shown to be beneficial for improving the sensitivity of CP-MQMAS experiments and for detecting dipolar correlations. © 2000 Academic Press Key Words: solid-state NMR; multiple-quantum MAS (MQMAS); rotation-induced adiabatic coherence transfer (RI- ACT); dipolar coupling; radiofrequency amplitude modulation. INTRODUCTION During the past few years there has been enormous progress in addressing one of the outstanding problems of solid-state NMR, namely, the observation of high-resolution spectra of half-integer quadrupolar nuclei. Spectroscopy of quadrupolar systems is im- portant since they represent 70% of the periodic table and are intimately involved in many chemical, physical, and biological processes (1). Until recently, these spin species have been largely NMR silent, but the introduction of multiple-quantum magic- angle spinning (MQMAS) has dramatically altered this landscape (2). Specifically, by correlating the evolutions of multiple-quan- tum and central-transition coherences, it is possible to extract isotropic spectra in either a one- or two-dimensional experiment (3–5). With this methodology, high-resolution spectra of S  3 2 species with  Q  600 kHz (3, 6 –11) can be recorded, where  Q = (e 2 qQ/h)/[2S(2S - 1)]. Despite our ability to observe high-resolution spectra of quadrupolar spins in a number of cases, there remain many problems to address. For example, the multiple-quantum exci- tation efficiency in all versions of MQMAS is low and signif- icantly limits the applicability of the technique. Further, in many cases the quadrupole T 1 values are long, exacerbating the already acute signal-to-noise problem. In situations like these involving I = 1 2 systems, it is customary to turn to one of the many variants of cross polarization (12, 13) to both enhance signal intensities and shorten the effective spin–lattice relax- ation times. For example, for proton-coupled I = 1 2 systems, the CP transfer alone can lead to enhancements of the magne- tization of  S /  I , where the  i are the gyromagnetic ratios of the spins involved. In I = S = 1 2 spin systems, the spin dynamics for cross polarization are well understood (14 –17), but for quadrupolar spin systems, this is not the case. In particular, in either static (18, 19) or rotating samples (20, 21), the spectroscopy and spin dynamics are dominated by the quadrupole interaction (22– 25). For this reason, cross-polarization experiments to date have focused on polarizing the central transition by employing a small B 1 for spin locking (20, 21, 26 –30). In the context of high-resolution multiple-quantum MAS (MQMAS) experi- ments (2, 3), the CP step is followed by excitation (29) and reconversion (30) of multiple-quantum coherence for observa- tion. Since low RF power is employed in this approach, it is common to observe poor frequency offset performance and low spin-locking efficiencies for the CP process. In this contribution, we investigate the CP dynamics of an I = 3 2 nucleus coupled to an S = 1 2 nucleus during MAS. We show that a triple-quantum CP (TQCP) process during MAS permits the use of high-power spin-locking fields and improves the sensitivity in CP-MQMAS spectra. We also demonstrate the improved polarization transfer characteristics of an ampli- tude-modulated TQCP transfer and the ability to perform spec- tral filtering. THEORY In spin- 1 2 applications, a theoretical analysis of the CP dy- namics is often performed by considering an isolated I = 1 2 , S = 1 2 spin pair (31). In this case, strong radiofrequency (RF) fields dominate all other interactions in the system Hamiltonian and the spin-lock field can be described in a tilted rotating frame in which the eigenvectors are related to the high-field eigenstates (where k , l = 1 2 represent the magnetic quantum numbers) by a simple rotation given by exp[ i ( I y + S y )( /2)]. In this frame, the initial conditions I x , S x are diagonal, which justifies the concept of polarization transfer under strong RF 1 Presented in part at the 39th ENC Conference, Asilomar, CA, 1998. Journal of Magnetic Resonance 142, 145–152 (2000) Article ID jmre.1999.1922, available online at http://www.idealibrary.com on 145 1090-7807/00 $35.00 Copyright © 2000 by Academic Press All rights of reproduction in any form reserved.