Gerard S. Harbison

Structure and order in partially oriented solids: Characterization by 2D-magic-angle-spinning NMR

Theory and applications of a new 2D‐NMR technique for characterizing the molecular structure and order of partially oriented solids are described. After deriving a general expression for the signal intensities in two dimensions in cases of perfect order, the influence of disorder on the spectra is discussed. It is demonstrated that an expansion of the orientational distribution function in terms of Wigner rotation matrices allows spectra from residues with an arbitrary degree of order and arbitrary chemical shielding tensor orientation to be fully characterized. The technique is illustrated by obtaining the orientational distribution function for the crystalline and amorphous regions in semicrystalline polyethylene terephthalate, showing that the molecular orientation is quite different in the two phases.

Two‐dimensional zero‐field nutation nuclear quadrupole resonance spectroscopy

We introduce a new two‐dimensional nuclear quadrupole resonance experiment, in which the first time period (t1) is the duration of the radiofrequency exciting pulse; and the second (t2) is the normal free precession of a quadrupolar nucleus at zero field. After double Fourier transformation, the result is a 2D spectrum in which the first frequency dimension is the nutation spectrum for the quadrupolar nucleus at zero field. For single crystals, this spectrum contains narrow lines, whose frequency, for axially symmetric tensors, is proportional to sin θ, where θ is the angle between the unique axis of the quadrupolar tensor and that of the transmitter/receiver coil. For polycrystalline samples we obtain powder line shapes which are reminiscent of high‐field nuclear magnetic resonance (NMR) powder patterns, and which allow determination of the asymmetry parameter η, which has previously only been obtainable using Zeeman perturbed nuclear quadrupole resonance (NQR) methods. Both theoretical spectra and sever...

Homonuclear J-couplings and rotationally induced sideband enhancements in NMR spectra of rotating solids

We demonstrate direct detection of 13C-13C J-couplings in magic angle sample spinning NMR spectra despite the presence of the much larger homogeneous broadening caused by the homonuclear dipolar couplings. Carbon-carbon J-couplings were observed in doubly 13C-enriched samples of sodium acetate, glycine and glucose. The resolved J-coupling permits carbon-carbon connectivities to be established with standard two-dimensional techniques. Interesting spectral features are observed when the rotational sidebands of the coupled spins overlap: when a sideband from a dipolar-coupled pair approaches the centerband of its partner, a significant enhancement in sideband intensity is observed as well as small shifts in the resonance frequency.

High-field 2D exchange spectroscopy in rotating solids

Abstract Methods for studying chemical exchange in solid samples are discussed and a new two-dimensional magic angle sampling spinning experiment is introduced. The new experiment is derived from the basic two-dimensional experiment used in solution spectroscopy but it employs chemical-shift scaling during t1 to ensure isotropic evolution. Thus, one dimension contains isotropic shifts while the second contains isotropic plus anisotropic. Chemical exchange between inequivalent spins manifests itself by cross peaks. The new method is used to study slow exchange in three different compounds containing aromatic rings which undergo twofold flips.

Two-Dimensional Dipolar-Chemical Shift NMR in Rotating Solids

An overview of dipolar-chemical shift spectroscopy is presented with emphasis on recent developments to improve sensitivity, enhance information content, and simplify the spectra. Three new experiments are examined. First, a simplified experiment which is not synchronized with the sample rotation is shown to yield the same 2D spectra as previous methods, but with higher signal intensities. Second, an experiment is demonstrated to enhance dipolar sideband intensities, allowing the measurement of weaker couplings at spinning speeds which would otherwise yield vanishingly small sidebands in the dipolar dimension. Lastly, a mechanical technique involving the changing of the spinning speed between the evolution and detection periods of the dipolar chemical shift experiment is shown to yield a dramatic compression of the information in the two-dimensional landscape. The signals obtained are calculated using a versatile formalism which comprises a basis for comparison of the results of different experiments.

Determination of the structure of oriented samples using two-dimensional solid state NMR techniques

Abstract One dimensional and two-dimensional MAS techniques can give detailed information about the structure and dynamics of oriented systems. We describe the application of such techniques to the liquid-crystalline polymer poly ( p -phenyleneterephthalimide) (PPTA), and thence deduce the solid-state structure of the material.

Determination of Chromophore Structure and Photochemistry in Bacteriorhodopsin with Resonance Raman, NMR, and Chemical Analogues

Bacteriorhodopsin (BR) is an intrinsic membrane protein that transduces light-energy into a trans-membrane protonmotive force. To elucidate the mechanism of this important process, it is necessary to determine the structure of the retinal chromophore in BR and its intermediates and to characterize important chromophore-protein interactions. This paper will review recent physical chemical and bio-organic approaches that we have developed to study chromophore structure and environment in bacteriorhodopsin. A model for the molecular mechanism of proton-pumping based on inversion of the unprotonated Schiff base nitrogen is presented.