Broadband excitation and indirect detection of nitrogen-14 in rotating solids using Delays Alternating with Nutation (DANTE)

A train of short rotor-synchronized pulses in the manner of Delays Alternating with Nutations for Tailored Excitation (DANTE) applied to nitrogen-14 nuclei (I = 1) in samples spinning at the magic angle at high frequencies (typically nu(rot) = 62.5 kHz so that tau(rot) = 16 mu s) allows one to achieve uniform excitation of a great number of spinning sidebands that arise from large first-order quadrupole interactions, as occur for aromatic nitrogen-14 nuclei in histidine. With routine rf amplitudes omega(1)(N-14)/(2 pi) = 60 kHz and very short pulses of a typical duration 0.5 < tau(p) < 2 mu s, efficient excitation can be achieved with 13 rotor-synchronized pulses in 13 tau(rot) = 208 mu s. Alternatively, with 'overtone' DANTE sequences using 2, 4, or 8 pulses per rotor period one can achieve efficient broadband excitation in fewer rotor periods, typically 2-4 tau(rot). These principles can be combined with the indirect detection of N-14 nuclei via spy nuclei with S = 1/2 such as H-1 or C-13 in the manner of Dipolar Heteronuclear Multiple-Quantum Correlation (D-HMQC). (C) 2011 Elsevier Inc. All rights reserved.