Through-space NMR correlations between two different half-integer quadrupolar nuclei using T-HMQC sequences

Proximities between spin-1/2, e.g. ¹H and ¹³C, and quadrupolar nuclei can be analyzed using HMQC (Heteronuclear Multiple-Quantum Correlation) experiments, in which two continuous-wave irradiations similar to those used in TRAPDOR (TRAnsfer of Population in DOuble-Resonance) experiments are applied on the indirectly detected quadrupolar isotope during the defocusing and refocusing delays. Here, we demonstrate that this sequence, called T-HMQC (T stands for TRAPDOR), can be applied to probe proximities between distinct half-integer spin quadrupolar isotopes. We introduce two novel variants of this sequence to reduce the number of resonances along the indirect dimension. These selective variants employ either (i) an echo-antiecho quadrature detection to only retain the single-quantum (1Q) coherences or (ii) two π-pulses selective of the central-transition (CT) to observe only the 1Q-CT coherences. We analyze how the effects of various experimental parameters, including the synchronization of the TRAPDOR recoupling pulses with the sample rotation, and their radio-frequency (rf) field amplitude and frequency offset, affect the efficiency of ¹¹B-²⁷Al T-HMQC experiments on a magnesium aluminoborate glass. The performances of these T-HMQC sequences are compared to those of the D-HMQC scheme employing the SPI-R³ (Synchronous Phase-Inversion Rotary-Resonance-Recoupling) or REDOR (Rotational-Echo DOuble-Resonance) symmetry-based heteronuclear dipolar recouplings built from CT-selective pulses. We demonstrate that the two TRAPDOR pulses in the T-HMQC sequence must be separated by an integer number of rotor periods and must employ the maximum rf field strength compatible with the probe specifications. Furthermore, as the TRAPDOR pulses distribute the populations equally to all possible coherences, the sensitivity of the T-HMQC selective variants is lower than that of the D-HMQC techniques. To limit this sensitivity decrease and the number of cross-peaks, it is preferable to detect indirectly the quadrupolar nucleus I with the lowest spin number, and in the case of I = 3/2, the resolution along the indirect dimension can be enhanced with respect to a MAS spectrum (for instance, by a factor of 27/7, without taking into account the quadrupolar-induced shift (QIS), through the sole indirect detection of triple-quantum (3Q) coherences). Moreover, owing to the use of a high-power TRAPDOR recoupling, the T-HMQC technique benefits from a wider excitation bandwidth than the D-HMQC methods, which is advantageous for broad NMR spectra, especially at high magnetic fields.