Hyung-Tae Kwak

Sensitivity enhancement of the central transition NMR signal of quadrupolar nuclei under magic-angle spinning

An approach for enhancing the NMR sensitivity of the central transition of spin-3/2 nuclei is presented. Through selective excitation of the satellite transitions using a fast 180 degrees phase alternating pulse train during magic-angle spinning a selectively excited state is prepared where the populations of all eigenstates textbackslash m > with the same sign of m are equal, resulting in an enhanced central m = -1/2 --> 1/2 transition polarization. Numerical simulations predict enhancements up to a factor of 2 and values of 1.7 and 1.9 have been obtained experimentally for Na-23 in Na2C2O4 and Rb-87 in RbClO4, respectively. We observe no significant anisotropic lineshape distortion. The conditions for optimum enhancement are discussed

Enhancing sensitivity of quadrupolar nuclei in solid-state NMR with multiple rotor assisted population transfers

Rotor-assisted population transfer (RAPT) was developed as a method for enhancing MAS NMR sensitivity of quadrupolar nuclei by transferring polarization associated with satellite transitions to the central m=12-->-12 transition. After a single RAPT transfer, there still remains polarization in the satellite transitions that can be transferred to the central transition. This polarization is available without having to wait for the spin system to return to thermal equilibrium. We describe a new RAPT scheme that uses the remaining polarization of the satellites to obtain a further enhancement of the central transition by performing RAPT-enhanced experiments multiple times before waiting for re-equilibration of the spin system. For 27Al (I=5/2) in albite we obtain a multiple RAPT enhancement of 3.02, a 48% increase over single RAPT. For 93Nb (I=9/2) in NaNbO(3) we obtain a multiple RAPT enhancement of 5.76, an 89% increase over single RAPT. We also describe a data processing procedure for obtaining the maximum possible signal-to-noise ratio.

Double-quantum filtered STMAS

Double-quantum and double-quantum-filtered satellite-transition magic-angle spinning (STMAS) experiments are proposed. The experiments efficiently convert satellite-transition coherence from single- to double-quantum with a central-transition selective pi-pulse. The conversion allows the selection of double-quantum coherence transfer pathways with phase cycling that completely filters out unwanted diagonal and outer satellite-transition peaks. Both experiments are demonstrated with RbNO3 and AlPO4-berlinite as model compounds for obtaining clean STMAS spectra of spins 3/2 and 5/2, respectively.

High-field NMR using resistive and hybrid magnets.

Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution.

Selective suppression and excitation of solid-state NMR resonances based on quadrupole coupling constants.

The dependence of the (Rotor Assisted Population Transfer) RAPT enhancement on offset frequency for nuclei experiencing different quadrupolar couplings has been exploited to design two new spectral editing schemes, pi/2-RAPT and RAPT-pi-RAPT, for the selective excitation or suppression, respectively, of nuclei with large quadrupolar couplings. Both approaches are demonstrated on the 87 Rb spectrum of Rb(2)SO(4), which contains two resonances with C(q) values of 2.6 and 5.3 MHz. The conditions for optimal selectivity are discussed. Combining pi/2-RAPT with the RIACT MQ-MAS experiment it is also demonstrated how a pure absorption mode triple quantum MQ-MAS spectrum devoid of narrow resonances can be obtained.

Satellite transition rotational resonance of homonuclear quadrupolar spins: magic-angle effect on spin-echo decay and inversion recovery

Abstract Satellite transition rotational resonance and magic-angle effects on central-transition spin-echo decay and inversion recovery of half-integer quadrupolar nuclei are presented. Small magic-angle offsets can change the satellite transition rotational resonance condition and consequently lead to dramatic effects on line width, spin-echo decay and inversion recovery. An half degree magic-angle offset lengthens α-Al 2 O 3 spin-echo decay by 50% and inversion-recovery by nearly two orders of magnitude. The theory of satellite transition rotational resonance is presented and the origin of these magic-angle effects is explained.