Uniform broadband excitation of crystallites in rotating solids using interleaved sequences of delays alternating with nutation Veronika Vitzthum a,⇑ , Marc A. Caporini a , Simone Ulzega a,1 , Julien Trébosc b , Olivier Lafon b , Jean-Paul Amoureux b , Geoffrey Bodenhausen a,c,d,e a Institut des Sciences et Ingénierie Chimiques (ISIC), Ecole Polytechnique Fédérale de Lausanne, 1015 Lausanne, Switzerland b Unité de Catalyse et de Chimie du Solide (UCCS), CNRS 8181, University of Lille North of France, 59652 Villeneuve d’Ascq, France c Département de Chimie, Ecole Normale Supérieure, 24 rue Lhomond, 75005 Paris, France d Université Pierre et Marie Curie, Paris, France e CNRS, UMR 7203, Paris, France article info Article history: Received 19 December 2011 Revised 24 April 2012 Available online 13 June 2012 Keywords: Nitrogen-14 NMR Delays Alternating with Nutations for Tailored Excitation (DANTE) Interleaved DANTE Dipolar Heteronuclear Multiple-Quantum Correlation (D-HMQC) Solid-state NMR Magic Angle Spinning (MAS) Broadband excitation Quadrupolar nuclei abstract In solids that are spinning about the magic angle, trains of short pulses in the manner of Delays Alternat- ing with Nutations for Tailored Excitation (DANTE) allow one to improve the efficiency of the excitation of magnetization compared to rectangular pulses. By interleaving N pulse trains with N > 1, one obtains ‘DANTE–N’ sequences comprising N pulses per rotor period that extend over K rotor periods. Optimized interleaved DANTE schemes with N > 1 are shorter than basic DANTE-1 sequences with N = 1. Therefore, they are less affected by coherent or incoherent decays, thus leading to higher signal intensities than can be obtained with basic DANTE-1 or with rectangular pulses. Furthermore, the shorter length of DANTE-N with N > 1 increases the width of the spikelets in the excitation profile, allowing one to cover the range of isotropic chemical shifts and second-order quadrupolar effects typical for side-chain and backbone amide 14 N sites in peptides at B 0 = 18.8 T. In DANTE-N, spinning sidebands only appear at multiples of the spin- ning frequency m rot , as if the samples were rotating at Nm rot . We show applications to direct detection of nitrogen-14 nuclei with spin I = 1 subject to large quadrupole interactions, using fast magic angle spin- ning (typically m rot P 60 kHz), backed up by simulations that provide insight into the properties of basic and interleaved DANTE sequences. When used for indirect detection, we show by numerical simulations that even basic DANTE-1 sequences can lead to a four-fold boost of efficiency compared to standard rect- angular pulses. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction In static solids, nuclear magnetic resonance (NMR) spectra are often very broad, particularly for quadrupolar nuclei with I > ½, and for spins with I = ½ in paramagnetic samples. In the early days of continuous-wave excitation, it was straightforward to sweep the radio-frequency (rf) carrier over large bandwidths, and the main challenge of broad spectra arose from the weakness of signals that were spread over wide ranges of frequencies. Although the advent of pulsed excitation and Fourier transformation (FT) [1] heralded decisive advantages for spectra with bandwidths Dx that are com- parable with typical rf intensities x 1 , pulsed excitation is challeng- ing for spectra with larger bandwidths. Various approaches to broadband excitation have been reviewed recently by Shurko et al. [2] with particular emphasis on 14 N spectra of static solids. Broadband excitation may benefit from pulses with very short durations s p that can cover large bandwidths Dx  2p/s p , although this usually entails small flip angles b =|cB 1 s p | << p/2, and hence poor S/N ratios, since the rf intensities are necessarily limited. Broadband excitation may be improved by recording com- plementary spectra in (possibly overlapping) segments. On the other hand, narrow-band excitation can be achieved either by using weak rf amplitudes, or equivalently by applying long trains of intense short pulses at regular intervals s d . This type of excitation, dubbed Delays Alternating with Nutations for Tai- lored Excitation (DANTE) [3], has become popular for selective excitation, saturation and inversion in solution-state NMR [4] and in magnetic resonance imaging [5]. During a DANTE sequence, the magnetization vectors in the rotating frame undergo nutation and precession in alternation. If the offset X = x 0  x rf between the precession frequency of the spins x 0 and the carrier frequency x rf coincides with a multiple of the pulse repetition rate, i.e., if X = x 0  x rf =2pj/s d , with integer j, the spins precess through an 1090-7807/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jmr.2012.05.024 ⇑ Corresponding author. Tel.: +41 21 6939386; fax: +41 21 6933604. E-mail address: veronika.vitzthum@epfl.ch (V. Vitzthum). 1 Present address: Bruker BiosSpin AG, Industriestrasse 26, 8117 Fällanden, Switzerland. Journal of Magnetic Resonance 223 (2012) 228–236 Contents lists available at SciVerse ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr