Sensitivity enhancement and spectral editing of quadrupolar nuclei in solid state NMR.

摘要

The problem of poor resolution in Solid State NMR has been greatly improved by first- and second-order line-narrowing techniques, such as MAS (1958), DOR (1988), DAS (1988), MQ-MAS (1995), and ST-MAS (2000). The problem of poor sensitivity in Solid State NMR, however, continues to be a subject of ongoing research. A new approach for enhancing NMR sensitivity of the central transition of half-integer quadrupolar nuclei is presented in this thesis. This new scheme is called Rotor Assisted Population Transfer (RAPT), and enhances the central transition by a factor of I + 1/2 as a result of a selective saturation of the satellite transitions during magic-angle spinning (MAS). Most importantly, we find no significant anisotropic lineshape distortion using RAPT, as a saturation of all crystalline in polycrystalline sample during MAS has been achieved.; The original RAPT pulse sequence has been improved by replacing the on-resonance square pulse train with alternating off-resonance Gaussian pulse train. This improvement overcomes a number of shortcomings of the on-resonance square pulse sequence. Off-resonance excitation surmounts the need for fast phase shifting and higher rf field strengths. It also allows us to explore a wider range of frequency offsets. The use of Gaussian pulses instead of square pulses also improves the selectivity of satellite excitation. The new approach has been investigated for several quadrupolar nuclei, including 87Rb in polycrystalline RbClO₄, 27Al in VPI-5, and 93Nb in polycrystalline NaNbO₃ with enhancement factors of 2.0, 2.6, and 3.0, respectively.; When applied to higher spin nuclei the RAPT sensitivity enhancement are less than the ideal value, I + 1/2. The origins of the RAPT experimental shortcomings are discussed and new scheme that improves the RAPT efficiencies for higher spin nuclei is proposed. This new approach employs the fact that there is more central transition sensitivity enhancement that can be obtained from the outer satellite even after perfect RAPT. Thus, it is possible to get a greater sensitivity gain by performing RAPT-enhanced experiments multiple times before the recycle delay. A factor of 2.73 and 4.94 sensitivity enhancements has been achieved for 27Al in polycrystalline Albite (NaSi₃AlO₈) and 93Nb in polycrystalline NaNbO₃ by using Multi-RAPT.; In the last section of the thesis, a simple and fast method of measuring quadrupolar coupling constants of half-integer quadrupolar nuclei by using RAPT is discussed. With this approach, we obtain the bandwidth and sensitivity advantages of a second-order method while retaining the C_q measurement precision of a first-order method. Additionally, two approaches using RAPT for selective suppression and discrimination of Solid State NMR of half-integer quadrupolar nuclei based on their quadrupolar coupling constants are proposed. Two 87Rb sites in polycrystalline Rb₂SO ₄, three 87Rb sites in the mixture of Rb₂SO ₄ and RbClO₄, and three 27Al sites in VPI-5 have been used to demonstrate these new approaches.