Two-dimensional MAS NMR correlation protocols involving double-quantum filtering of quadrupolar spin-pairs Mattias Edén Physical Chemistry Division, Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91 Stockholm, Sweden article info Article history: Received 22 November 2009 Revised 7 February 2010 Available online 12 February 2010 Keywords: Dipolar recoupling Double-quantum coherence Homonuclear correlation spectroscopy Multiple-quantum filtration Internuclear proximities Quadrupolar tensor orientations Half-integer spin quadrupolar nuclei Sillimanite abstract Three two-dimensional (2D) NMR homonuclear correlation techniques invoking double-quantum (2Q) filtration of the central transitions of half-integer spins are evaluated numerically and experimentally. They correlate directly detected single-quantum (1Q) coherences in the t 2 domain with either of 1Q, two-spin 2Q or single-spin multiple-quantum coherence-evolutions in the indirect ðt 1 Þ dimension. We employ experimental 23 Na and 27 Al NMR on sodium sulfite and the natural mineral sillimanite ðSiAl 2 O 5 Þ, in conjunction with simulated 2D spectra from pairs of dipolar-recoupled spins-3/2 and 5/2 at different external magnetic fields, to compare the correlation strategies from the viewpoints of 2D spectral resolution, signal sensitivity, implementational aspects and their relative merits for establishing internuclear proximities and quadrupolar tensor orientations. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction 1.1. Background There is a growing interest in designing and using techniques to correlate NMR interaction parameters under magic-angle spinning (MAS) conditions, where at least one of the participating nuclei possesses half-integer spin [1,2]. A subset of this wide family of solid state NMR methodologies targets the exploration of internu- clear proximities/connectivities between quadrupolar nuclei of the same species in inorganic materials: the homonuclear correlations are established via spin–spin interactions mediated either through space or chemical bonds. Recent reviews are given in [1,3]. Fig. 1 depicts three distinct 2D NMR protocols for correlating the signals from quadrupolar nuclei in close spatial proximity. They incorporate a pulse-block for exciting two-spin double- quantum coherences (2QC) between the central transitions (CTs) in a pair of dipolar-(re)coupled half-integer spins; such coher- ences are henceforth abbreviated ‘‘2Q CT ”. The MAS-diminished dipolar interactions may partially be restored by using rf pulse sequences conforming to the R-symmetry class [4–7], as described in detail in our previous work on 2Q-recoupling of quadrupolar nuclei [8–10]. In the numerical and experimental demonstrations herein, we employ either of the R2 1 2 R2 1 2 [8] or R2 1 4 R2 1 4 [9] recou- pling schemes. In both cases, the spin-S CT nutation frequency x CT nut ¼ðS þ 1=2Þj c S j B 1 obeys x CT nut ¼ x r =2 during recoupling, where c S is the spin magnetogyric ratio, B 1 the rf amplitude, x r ¼ 2p=s r the angular rotation frequency of the sample and s r its rotational period. When sandwiched between two CT-selective p=2-pulses, the recoupling pulse sequences generate approxi- mately an effective 2Q dipolar Hamiltonian involving CT spin-oper- ators S  j S  k multiplied by a scaled through-space dipolar coupling constant b jk between the homonuclear spins j and k [8–10]. 1.2. 2Q–1Q correlation scheme A double-quantum filtering (2QF) procedure involves the exci- tation of 2Q CT from z-magnetization for an interval s exc [11,12]. The reverse process, labeled by ‘‘2Q CT ! Z” in Fig. 1, is performed by repeating the same pulse train for s rec , but phase-shifting all rf pulses (including the bracketing pulses) by p=2. The 2QC excita- tion and reconversion intervals need not necessarily be equal [5,13]. The NMR signals are detected after a subsequent Z-filter interval ðs Z Þ and a p=2 read pulse. As distinct types of MQC may exist in half-integer spin systems, either involving a single spin or a pair of spins, we denote the 2QF process of the CTs of spin-pairs by ‘‘2QF CT ” (see Fig. 1). The fraction of initial longitudinal CT mag- netization recovered after the 2QF CT event is denoted f 2QF Z!Z and re- ferred to as the 2QF efficiency. The 2QF scheme used herein only differs from the traditional spin-1/2 procedure [11,12] in two re- spects: (i) a CT-selective p-pulse is inserted between the 2Q CT exci- tation and reconversion events [14]. It is phase-cycled to retain 1090-7807/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jmr.2010.02.007 E-mail address: mattias.eden@mmk.su.se Journal of Magnetic Resonance 204 (2010) 99–110 Contents lists available at ScienceDirect Journal of Magnetic Resonance journal homepage: www.elsevier.com/locate/jmr