The necessity for synchronization in DANTE experiments with rotating samples. Reclamation of the asynchronous DANTE pulse train by combination with the TOSS sequence

Abstract

In recent NMR spectroscopic studies, frequency-selective excitation techniques have become increasingly important. One of their roles is the simplification of spectra by eliminating unwanted resonance lines (1, 2). Another is extracting specific slices of full multidimensional spectra such as two-dimensional exchange/spin diffusion spectra (3), 2D separation spectra (4), and imaging (5). Among such techniques, the DANTE pulse train (6) prevails widely because one can compose the pulse sequence with hard pulses which are more stable in amplitude, and more easily available, than soft pulses, and, moreover, one can readily adjust the effective field strength by altering the pulse spacing. In solid-state NMR, the DANTE pulse train is usually applied to powdered samples under magic-angle spinning (MAS) as well as to single crystals, so that it can operate on specific well-resolved lines ( I). In the MAS experiment, each pulse of the DANTE train should be irradiated synchronously with the spinning period: all the transverse magnetizations due to crystallites are individually modulated by MAS and only become in-phase after every spinning period (7). However, not only is this synchronization difficult in practice but also there are some disadvantages which ensue following a long pulse interval fixed at the spinning period. typically 250400 ps: such a long interval often renders the selective band narrower than the linewidth. Also, the magnetizations decay, in compounds with short T2 values, during the pulse train. Sample heating may occur in the cases where a heteronuclear decoupling field is applied simultaneously. Caravatti et al. ( I ) showed experimentally that synchronization is not necessary for selective saturation with the DANTE-90” pulse train. In addition, Bork and Schaefer (8) recently pointed out that the asynchronous DANTE-180” pulse train inverts an entire spinning sideband family and attempted to explain this observation qualitatively. In fact, we can confirm that the asynchronous DANTE pulse train works properly as long as the spinning frequency, UR, is large compared with chemical-shift anisotropy, AU. Figures 1 a1 c show the ‘%I spectra for polycrystalline glycine observed at vR = 4.0 kHz using the CP (cross-polarization)MAS-DANTE saturation/inversion sequence which consists of the pulse train CP-90”-DANTE-delay-90°-acquisition. ln Figs. 1 a1 C, one can see that the asynchronous pulse train almost completely saturates