K. Schmidt-Rohr

NMR-STUDY OF XENON DYNAMICS AND ENERGETICS IN NA-A ZEOLITE

Abstract For xenon atoms adsorbed in Na—A zeolite, electronic interactions cause shifts in NMR frequencies, resulting in a spectrum with discrete peaks from xenon atoms in cages with different xenon occupancies. Using two-dimensional exchange NMR, it is possible to determine the microscopic rates of intercage motion and to relate them to the adsorption and activation energies of the xenon atoms. The dependence of the adsorption energies on xenon cage occupancy reflects the importance of the intracage interactions and is directly related to the cage occupancy distribution. Variable temperature measurements yield an activation energy of about 60 kJ/mol for the transfer of a xenon from one cage to another.

Study of phospholipid structure by 1H, 13C, and 31P dipolar couplings from two-dimensional NMR.

Various motionally averaged 31P-1H, 13C-1H, 1H-1H, and 31P-13C dipolar couplings were measured for natural-abundance and unoriented phosphocholine in the L alpha phase. The couplings were obtained and assigned by a variety of advanced and partly novel two-dimensional solid-state NMR experiments. Whereas 31P-1H and 31P-13C dipolar couplings provide long-range structural constraints, geminal 1H-1H couplings and the signs of 13C-1H couplings are important new elements in a segmental order-tensor analysis of the lipid headgroup and glycerol backbone. The implications of these measured dipolar couplings for the conformational exchange of the lipid headgroup and the bending of the headgroup from the glycerol backbone are discussed. These dipolar couplings are also analyzed semiquantitatively in terms of the segmental order tensor.

ISOTROPIC SECOND-ORDER DIPOLAR SHIFTS IN THE ROTATING FRAME

An experiment is described that utilizes the truncation of the Hamiltonian in the rotating frame by a radio‐frequency field designed to yield an isotropic shift for the dipolar coupling. This approach allows the measurement of a normally orientation‐dependent coupling constant by a single isotropic value. The dipolar isotropic shift is closely related to the field‐dependent chemical shift in solids due to the second‐order dipolar perturbation observed in magic‐angle spinning experiments. In the rotating frame, larger shifts of up to 1000 Hz can be observed for the case of a one‐bond C–H coupling compared to a shift of a few Hertz in the laboratory‐frame experiment. In addition to the isotropic shift, a line broadening due to the P4(cos β) terms is observed when the experiment is carried out under magic‐angle sample spinning (MAS) conditions, leading to the requirement of higher‐order averaging such as double rotation (DOR) for obtaining narrow lines. As an application of this new experiment the separation...

Rotational diffusion measurements of suspended colloidal particles using two-dimensional exchange nuclear magnetic resonance

We present here an experimental and theoretical study of the application of two‐dimensional exchange nuclear magnetic resonance spectroscopy (NMR) to the investigation of the rotational diffusion of colloidal particles. The theoretical discussion includes the nature of the NMR frequency time‐correlation function where the NMR interaction is represented by the chemical shift anisotropy (CSA). Time‐correlation functions for the isotropic rotational diffusion of a suspension of colloidal particles containing single and multiple sites are derived in addition to time‐correlation functions for the rotational diffusion of a suspension of symmetric top particles containing an isotropic distribution of a single CSA interaction. Simulations of two‐dimensional exchange spectra for particles undergoing isotropic rotational diffusion are presented. We performed two‐dimensional exchange NMR experiments on a colloidal suspension of spherical poly(methyl methacrylate) (PMMA) particles which were synthesized with a 20% en...

Two-Dimensional 29Xe Exchange NMR Measurements of Xenon Dynamics in Na-A Zeolite

Two-dimensional (2D) exchange NMR is a powerful tool for measuring the dynamics and energetics of adsorbed xenon atoms undergoing slow exchange between the alpha-cages of Na-A zeolite. In this proceedings, we present recently published results, as well as additional discussion and data obtained, using 2D Xe-129 NMR to determine rate coefficients for intercage xenon hopping and to correlate them with adsorption and activation energies. Variable-temperature experiments establish the activation energy for hopping between alpha-cages to be 60+/-10 kJ/mol. The dependence of these kinetic and thermodynamic quantities on xenon occupancy of the alpha-cages reflects the importance of intracage interactions on the behavior of the adsorbed guest molecules.