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

Abstract

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 ν rot = 62.5 kHz so that τ rot = 16 μs) 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 ω 1( 14N)/(2 π) = 60 kHz and very short pulses of a typical duration 0.5 < τ p < 2 μs, efficient excitation can be achieved with 13 rotor-synchronized pulses in 13 τ rot = 208 μs. 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 τ rot. These principles can be combined with the indirect detection of 14N nuclei via spy nuclei with S = ½ such as 1H or 13C in the manner of Dipolar Heteronuclear Multiple-Quantum Correlation (D-HMQC).