title: Adiabatic sweep cross-polarization magic-angle-spinning NMR of half-integer quadrupolar spins.
abstract: The use of frequency-swept radiofrequency (rf) pulses for enhancing signals in the magic-angle spinning (MAS) spectra of half-integer quadrupolar nuclides was explored. The broadband adiabatic inversion cross-polarization magic-angle spinning (BRAIN-CPMAS) method, involving an adiabatic inversion pulse on the S-channel and a simultaneous rectangular spin-lock pulse on the I-channel (1H), was applied to I(1/2)→S(3/2) systems. Optimal BRAIN-CPMAS matching conditions were found to involve low rf pulse strengths for both the I- and S-spin channels. At these low and easily attainable rf field strengths, level-crossing events among the energy levels |3/2〉,|1/2〉,|-1/2〉,|-3/2〉 that are known to complicate the CPMAS of quadrupolar nuclei, are mostly avoided. Zero- and double-quantum polarization transfer modes, akin to those we have observed for I(1/2)→S(1/2) polarization transfers, were evidenced by these analyses even in the presence of the quadrupolar interaction.1H-23Na and1H-11B BRAIN-CPMAS conditions were experimentally explored on model compounds by optimizing the width of the adiabatic sweep, as well as the rf pulse powers of the1H and23Na/11B channels, for different MAS rates. The experimental data obtained on model compounds containing spin-3/2 nuclides, matched well predictions from numerical simulations and from an average Hamiltonian theory model. Extensions to half-integer spin nuclides with higher spins and potential applications of this BRAIN-CPMAS approach are discussed.
authors.affiliation: National High Magnetic Field Laboratory, Tallahassee, Florida 32304, USA. Electronic address: sungsool@magnet.fsu.edu.
authors.forename: Sungsool
authors.initials: S
authors.lastname: Wi
authors.affiliation: Department of Chemistry, Hannam University, Taejeon, 305811, South Korea.
authors.forename: Chul
authors.initials: C
authors.lastname: Kim
authors.affiliation: Department of Chemistry and Biochemistry, University of Windsor, 401 Sunset Avenue, Windsor N9B 3P4, Ontario, Canada.
authors.forename: Robert
authors.initials: R
authors.lastname: Schurko
authors.affiliation: National High Magnetic Field Laboratory, Tallahassee, Florida 32304, USA; Department of Chemical Physics, Weizmann Institute of Sciences, Rehovot 76100, Israel.
authors.forename: Lucio
authors.initials: L
authors.lastname: Frydman
keywords: Adiabatic inversion; BRAIN-CPMAS; Frequency-sweep; Half-integer quadrupolar nuclei; WURST