@article {427, title = {Measurement and assignment of long-range C-H dipolar couplings in liquid crystals by two-dimensional NMR spectroscopy}, journal = {Journal of Physical Chemistry}, volume = {100}, year = {1996}, note = {J Phys Chem-UsVe373Times Cited:41Cited References Count:22}, month = {Aug 29}, pages = {14815-14822}, abstract = {

We describe multidimensional NMR techniques to measure and assign C-13-H-1 dipolar couplings in nematic liquid crystals with high resolution. In particular, dipolar couplings between aromatic and aliphatic sites are extracted, providing valuable information on the structural correlations between these two components of thermotropic liquid crystal molecules. The NMR techniques are demonstrated on 4-pentyl-4\&$\#$39;-biphenylcarbonitrile (5CB), a well-characterized room-temperature nematic liquid crystal. Proton-detected local-field NMR spectroscopy is employed to obtain highly resolved C-H dipolar couplings that are separated according to the chemical shifts of the carbon sites. Each C-13 cross section in the 2D spectra exhibits several doublet splittings, with the largest one resulting from the directly bonded C-H coupling, The smaller splittings originate from the long-range C-H dipolar couplings and can be assigned qualitatively by a chemical shift heteronuclear correlation (HETCOR) experiment. The HETCOR experiment incorporates a mixing period for proton spin diffusion to occur, so that maximal polarization transfer can be achieved between the unbonded C-13 and H-1 nuclei. To assign the long-range C-H couplings quantitatively, we combined these two techniques into a novel reduced-3D experiment, in which the H-1 chemical shift-displaced C-H dipolar couplings are correlated with the C-13 chemical shifts. The time domain of this experiment involves separate but synchronous incrementation of the evolution periods for the C-H dipolar couplings and the H-1 chemical shifts, with a variable ratio of the respective dwell times to optimize the resolution and facilitate resonance assignment in the spectrum.

}, keywords = {shape}, isbn = {0022-3654}, doi = {Doi 10.1021/Jp960972m}, url = {://WOS:A1996VE37300036}, author = {Hong, M. and Pines, A. and Caldarelli, S.} } @article {432, title = {Measurement of carbon-proton dipolar couplings in liquid crystals by local dipolar field NMR spectroscopy}, journal = {Journal of Physical Chemistry}, volume = {100}, year = {1996}, note = {J Phys Chem-UsVv354Times Cited:54Cited References Count:31}, month = {Nov 28}, pages = {18696-18701}, abstract = {

The performance of several different two-dimensional NMR methods for the measurement of carbon-proton dipolar couplings in liquid crystalline phases is analyzed. Proton-detected local field spectroscopy allows the measurements of short range C-H couplings, which correspond to directly bond pairs, by direct inspection of the spectra. Off magic angle (OMAS) spinning techniques can be applied to anisotropic phases that can be oriented mechanically at an angle to the magnetic field, such as nematic phases. The consequent scaling of the chemical shift anisotropy and dipolar couplings can be used to resolve otherwise overlapping resonances, Moreover, an estimate of the accuracy of the technique can be achieved by performing a series of OMAS experiments with different sample orientations.

}, keywords = {solids}, isbn = {0022-3654}, doi = {Doi 10.1021/Jp962023z}, url = {://WOS:A1996VV35400008}, author = {Caldarelli, S. and Hong, M. and Emsley, L. and Pines, A.} } @article {444, title = {Low-Power Decoupling Sequences for High-Resolution Chemical-Shift and Local-Field Nmr-Spectra of Liquid-Crystals}, journal = {Journal of Magnetic Resonance Series A}, volume = {113}, year = {1995}, note = {J Magn Reson Ser AQw158Times Cited:11Cited References Count:29}, month = {Apr}, pages = {169-176}, abstract = {

Various new and well-established multiple-pulse sequences that provide homonuclear dipolar decoupling are compared on a high-resolution spectrometer/probe configuration using low RF-irradiation powers. Proton-proton decoupled carbon-13 spectra of singly C-13-labeled benzene dissolved in a nematic liquid crystal and of the liquid crystal 152 are presented. Two classes of multiple-pulse sequences are studied that produce either broadband decoupled carbon spectra or separated local-held spectra. Several new approaches for the design of windowless sequences are demonstrated to be valuable in these heteronuclear experiments. They include implementations of the iterative MLEV scheme and the use of 270 degrees pulses. Furthermore, noncyclic propagators have been found that exhibit advantages over closely related cyclic analogues in both classes of decoupling sequences. (C) 1995 Academic Press, Inc.

}, keywords = {dipolar}, isbn = {1064-1858}, doi = {Doi 10.1006/Jmra.1995.1077}, url = {://WOS:A1995QW15800005}, author = {Nanz, D. and Ernst, M. and Hong, M. and Ziegeweid, M. A. and Schmidt-Rohr, K. and Pines, A.} } @article {451, title = {Nmr Measurement of Signs and Magnitudes of C-H Dipolar Couplings in Lecithin}, journal = {Journal of the American Chemical Society}, volume = {117}, year = {1995}, note = {J Am Chem SocQn458Times Cited:45Cited References Count:25}, month = {Mar 22}, pages = {3310-3311}, keywords = {orientation}, isbn = {0002-7863}, doi = {Doi 10.1021/Ja00116a051}, url = {://WOS:A1995QN45800051}, author = {Hong, M. and Schmidt-Rohr, K. and Pines, A.} } @article {461, title = {3-Dimensional Variable-Angle Nuclear-Magnetic-Resonance Exchange Spectroscopy without Rotor Axis Hopping}, journal = {Journal of Chemical Physics}, volume = {101}, year = {1994}, note = {J Chem PhysNy002Times Cited:12Cited References Count:48}, month = {Aug 1}, pages = {1852-1864}, abstract = {

Slow, large-amplitude chain motions play an important role in determining the macroscopic mechanical properties of polymers. Although such motions have been studied quantitatively by two-dimensional (2D) nuclear: magnetic resonance (NMR) exchange experiments, overlapping anisotropic patterns hamper spectral analysis, and limit applications. Variable angle correlation spectroscopy (VACSY) has proven useful in resolving such problems for rapidly spinning samples by separating anisotropic spectral patterns according to isotropic chemical shifts. In a previous study [J. Am. Chem. Sec. 115, 4825 (1993)], we described a three-dimensional (3D) NMR experiment that incorporates; the VACSY method and a hop of the rotor axis to correlate the isotropic chemical shifts to 2D anisotropic exchange patterns. The hop of the rotor axis, however, presents experimental difficulties and limits the range of motional rates that may be studied. We present in this paper a new 3D VACSY exchange experiment that obtains the same correlations without the need for the rotor axis hop. A series of 2D exchange spectra are recorded with the sample spinning at different rotation axis angles.\&$\#$39; Then using the scaling of the anisotropic frequency at the different angles, we construct the data onto a 3D matrix so that a Fourier transformation directly yields the desired correlations. The technique is applied to C-13 exchange NMR to study the slow molecular motion of ordered isotactic polypropylene.

}, keywords = {distributions}, isbn = {0021-9606}, doi = {Doi 10.1063/1.467696}, url = {://WOS:A1994NY00200011}, author = {Lee, Y. K. and Emsley, L. and Larsen, R. G. and Schmidt-Rohr, K. and Hong, M. and Frydman, L. and Chingas, G. C. and Pines, A.} }