High-resolution through-space correlations between spin-1/2 and half-integer quadrupolar nuclei using the MQ-D-R-INEPT NMR experiment

The multiple-quantum dipolar refocused-INEPT (MQ-D-R-INEPT) NMR experiment is presented. This new pulse sequence allows the generation of two-dimensional through-space NMR correlation spectra between spin-½ and quadrupolar nuclei under high-resolution. Compared to the existing sequences, the MQ-D-R-INEPT sequence benefits from increased robustness, due to the reduced sensitivity to offsets and radio-frequency field inhomogeneity of the R-INEPT recoupling scheme as compared to the classical cross-polarization transfer involving a quadrupolar nucleus, and applicability to quadrupolar nuclei with any half-integer spin value. Here, the heteronuclear dipolar couplings have been reintroduced using the rotary resonance recoupling (R(3)) sequence, chosen because it is γ-encoded, and thus robust to spinning speed instabilities and to radio-frequency inhomogeneities in the case of nuclei subjected to large chemical shift anisotropy (e.g., (31)P). In the course of the article, a new definition of γ-encoding is introduced, more general and practical than the previous ones, as it does not depend on the implementation of the recoupling sequence, and is independent of the considered frame. The efficiency of the MQ-D-R-INEPT experiment is demonstrated for the observation of (31)P-(27)Al proximities. It is first tested on AlPO4-VPI-5, and then applied to a complex aluminophosphate AlPO4-(Al5P7)-DAE of incompletely established structure. For this latter sample, the high-resolution provided by the MQMAS filter allows the resolution of a large number of (31)P-(27)Al correlations, which represent an essential step towards the determination of its structural model.