title: 1H-2H cross-polarization NMR in fast spinning solids by adiabatic sweeps.
abstract: Cross-polarization (CP) experiments employing frequency-swept radiofrequency (rf) pulses have been successfully used in static spin systems for obtaining broadband signal enhancements. These experiments have been recently extended to heteronuclear I, S = spin-1/2 nuclides under magic-angle spinning (MAS), by applying adiabatic inversion pulses along the S (low-γ) channel while simultaneously applying a conventional spin-locking pulse on the I-channel (1H). This study explores an extension of this adiabatic frequency sweep concept to quadrupolar nuclei, focusing on CP from1H (I = 1/2) to2H spins (S = 1) undergoing fast MAS (νr= 60 kHz). A number of new features emerge, including zero- and double-quantum polarization transfer phenomena that depend on the frequency offsets of the swept pulses, the rf pulse powers, and the MAS spinning rate. An additional mechanism found operational in the1H-2H CP case that was absent in the spin-1/2 counterpart, concerns the onset of a pseudo-static zero-quantum CP mode, driven by a quadrupole-modulated rf/dipolar recoupling term arising under the action of MAS. The best CP conditions found at these fast spinning rates correspond to double-quantum transfers, involving weak2H rf field strengths. At these easily attainable (ca. 10 kHz) rf field conditions, adiabatic level-crossings among the {|1⟩,|0⟩,|-1⟩} mSenergy levels, which are known to complicate the CP MAS of quadrupolar nuclei, are avoided. Moreover, the CP line shapes generated in this manner are very close to the ideal2H MAS spectral line shapes, facilitating the extraction of quadrupolar coupling parameters. All these features were corroborated with experiments on model compounds and justified using numerical simulations and average Hamiltonian theory models. Potential applications of these new phenomena, as well as extensions to higher spins S, are briefly discussed.
authors.affiliation: National High Magnetic Field Laboratory, Tallahassee, Florida 32304, USA.
authors.forename: Sungsool
authors.identifier: 000000030728095X
authors.initials: S
authors.lastname: Wi
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.
authors.forename: Lucio
authors.initials: L
authors.lastname: Frydman