Indirect Detection via Spin-1/2 Nuclei in Solid State NMR Spectroscopy: Application to theObservation of Proximities between Protons and Quadrupolar Nuclei

摘要

We present a comprehensive comparison of through-space heteronuclear correlation techniques for solid stateNMR, combining indirect detection and single-channel recoupling method. These techniques, named D-HMQCand D-HSQC, do not suffer from dipolar truncation and can be employed to correlate quadrupolar nucleiwith spin-1/2 nuclei. The heteronuclear dipolar couplings are restored under magic-angle spinning by applyingsupercycled symmetry-based pulse sequences (SR4_1~2) or simultaneous frequency and amplitude modulation(SFAM). The average Hamiltonian theory (AHT) of these recoupling methods is developed. These resultsare applied to analyze the performances of D-HMQC and D-HSQC sequences. It is shown that, whatever themagnitude of spin interations, D-HMQC experiment offers larger efficiency and higher robustness thanD-HSQC. Furthermore, the spectral resolution in both dimensions of proton detected two-dimensional D-HMQCand D-HSQC spectra can be enhanced by applying recently introduced symmetry-based homonuclear dipolardecoupling schemes that cause a z-rotation of the spins. This is demonstrated by ~1H-~(13)C and ~1H-~(23)Nacorrelation experiments on L-histidine and NaH_2PO_4, respectively. The two-dimensional heteronuclear ~1H-~(23)Nacorrelation spectrum yields the assignment of ~(23)Na resonances of NaH_2PO_4. This assignment is corroboratedby first-principles calculations.