title: Second-order quadrupolar line shapes under molecular dynamics: An additional transition in the extremely fast regime.
abstract: NMR spectroscopy is a powerful tool for probing molecular dynamics. For the classic case of two-site exchange, NMR spectra go through the transition from exchange broadening through coalescence and then motional narrowing as the exchange rate increases passing through the difference between the resonance frequencies of the two sites. For central-transition spectra of half-integer quadrupolar nuclei in solids, line shape change due to molecular dynamics occurs in two stages. The first stage occurs when the exchange rate is comparable to the second-order quadrupolar interaction. The second spectral transition comes at a faster exchange rate which approaches the Larmor frequency and generally reduces the isotropic quadrupolar shift. Such a two-stage transition phenomenon is unique to half-integer quadrupolar nuclei. A quantum mechanical formalism in full Liouville space is presented to explain the physical origin of the two-stage phenomenon and for use in spectral simulations. Variable-temperature17O NMR of solid NaNO3in which the NO3-ion undergoes 3-fold jumps confirms the two-stage transition process. The spectra of NaNO3acquired in the temperature range of 173-413K agree well with simulations using the quantum mechanical formalism. The rate constants for the 3-fold NO3-ion jumps span eight orders of magnitude (102-1010s-1) covering both transitions of the dynamic17O line shape.
authors.affiliation: National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USA.
authors.forename: Ivan
authors.initials: I
authors.lastname: Hung
authors.affiliation: Department of Chemistry, Queen's University, 90 Bader Lane, Kingston, Ontario, Canada, K7L 3N6.
authors.forename: Gang
authors.initials: G
authors.lastname: Wu
authors.affiliation: National High Magnetic Field Laboratory, 1800 East Paul Dirac Drive, Tallahassee, FL 32310, USA. Electronic address: gan@magnet.fsu.edu.
authors.forename: Zhehong
authors.initials: Z
authors.lastname: Gan