Recent Progress in Homonuclear Correlation Spectroscopy of Quadrupolar Nuclei

Improved Magnetization Transfers among Quadrupolar Nuclei in Two-Dimensional Homonuclear Correlation NMR Experiments Applied to Inorganic Network Structures

“…The family of SR2 1 2 p sequences with p = {1, 2, 3, ...} is generated recursively by combining cyclic pulse-permutations and phase-inversions, as described in detail in refs. [9,57,58]. For increasing p, the compensation to resonance offsets and rf-amplitude errors enhances for the SR2 1 2 p sequence [9,57,58], as demonstrated further in Section 4.1.…”

“…The engineering of rf-pulse sequences to achieve homonuclear dipolar recoupling among spins-1/2 (e.g., 1 H, 13 C, and 31 P) is well-developed [3][4][5][6][7], whereas it is exceedingly difficult to devise efficient and robust dipolar recoupling methods for half-integer spin quadrupolar nuclei [8,9], such as the 11 B (spin S = 3/2) and 27 Al (S = 5/2) nuclides considered herein. Underlying these problems is the necessity to solely observe and control the central transition (CT) of the quadrupolar nucleus [10], thereby requiring the lowest possible rf-field amplitudes ("rf power") to avoid NMR-signal leakages out to the satellite transitions (STs) during the dipolar recoupling, while it must also cope with large second-order broadenings (and chemical-shift dispersion for structurally disordered materials [2,10]).…”

“…These pulse sequences were introduced for two-spin 2QC excitation among the CTs of half-integer spins [9,38,49]. The recoupling schemes have been exploited for probing proximities among homonuclear 11 B, 23 Na, and 27 Al sites in a multitude of structurally well-ordered, as well as disordered, network-based materials [8,9,38,41,[49][50][51][52][53][54][55][56]. Moreover, they have also been employed in several structural studies of both organic and inorganic systems where spin-1/2 nuclei were used as probes, e.g., 13 C [57-60], 1 H [61,62], and 31 P [63][64][65].…”

“…Then in the absence of resonance-broadenings from second-order quadrupolar interactions and (potential) chemical-shift dispersions from structural disorder, the 1Q-1Q correlation NMR spectrum manifests two narrow cross peaks from the j-k spin-pair. In practice, however, distributions of resonances around each ν j and ν k frequency value produce broad "ridge-like" cross peaks in the 2D NMR spectrum [8,9].…”

“…Such "windowed" SR4 1 4 schemes were originally developed for dipolar recoupling of spins-1/2 by Ishii [74], who called the pulse-sequences "finite-pulse RFDR" (whereas the original RFDR scheme involves strong pulses [75]). As will be shown herein and discussed further in an upcoming paper, a significantly improved recoupling performance is accomplished by instead utilizing two-fold symmetry schemes, SR2 1 2 p [9,38,49,57,58] as the basic pulse-train S in the MQ phase cycle (Equation (1)). The SR2 1 2 p sequences were initially designed for usage in 2Q-1Q correlation NMR protocols [38,41,49].…”