@article {335, title = {Diamagnetic clusters of paramagnetic endometallofullerenes: A solid-state MAS NMR study}, journal = {Journal of Physical Chemistry B}, volume = {108}, year = {2004}, note = {J Phys Chem B846BKTimes Cited:10Cited References Count:39}, month = {Aug 19}, pages = {12450-12455}, abstract = {

Solid powder samples of complexes of the endometallofullerenes (EMF) La@C-82 and Y@C-82 with hexamethylphosphoramide (HMPA) were studied by magic-angle spinning (MAS) NMR. We have obtained well-resolved P-31 NMR spectra and C-13 NMR spectra for both La-EMF/HMPA and Y-EMF/HMPA and La-139 spectra for the La-EMF/HMPA. The (31)p measurements on La-EMF/HMPA and Y-EMF/HMPA have revealed considerable chemical shifts of (31)p signals relative to pure HMPA. Two-dimensional exchange P-31 experiments revealed that HMPA molecules at different sites in the EMF/HMPA complex do not change positions at a time scale of up to 1 s. Both EMF samples demonstrate a vast chemical shift range for 31p of the bound HMPA molecules. In addition, the La-EMF/HMPA exhibits the enormous spreading of the chemical shifts for La-139. The experimental results suggest that paramagnetic La@C-82 and Y@C-92 in the solid state form clusters (nanoparticles) in which the exchange coupling of the EMF takes place with quenching of the most electron spins.

}, keywords = {complexes}, isbn = {1520-6106}, doi = {Doi 10.1021/Jp048610z}, url = {://WOS:000223289500027}, author = {Koltover, V. K. and Logan, J. W. and Heise, H. and Bubnov, V. P. and Estrin, Y. I. and Kareev, I. E. and Lodygina, V. P. and Pines, A.} } @article {337, title = {Rational reduction of periodic propagators for off-period observations}, journal = {Journal of Magnetic Resonance}, volume = {166}, year = {2004}, note = {J Magn Reson767TUTimes Cited:2Cited References Count:19}, month = {Feb}, pages = {174-181}, abstract = {

Many common solid-state nuclear magnetic resonance problems take advantage of the periodicity of the underlying Hamiltonian to simplify the computation of an observation. Most of the time-domain methods used, however, require the time step between observations to be some integer or reciprocal-integer multiple of the period, thereby restricting the observation bandwidth. Calculations of off-period observations are usually reduced to brute force direct methods resulting in many demanding matrix multiplications. For large spin systems, the matrix multiplication becomes the limiting step. A simple method that can dramatically reduce the number of matrix multiplications required to calculate the time evolution when the observation time step is some rational fraction of the period of the Hamiltonian is presented. The algorithm implements two different optimization routines. One uses pattern matching and additional memory storage, while the other recursively generates the propagators via time shifting. The net result is a significant speed improvement for some types of time-domain calculations. (C) 2003 Published by Elsevier Inc.

}, keywords = {simulation}, isbn = {1090-7807}, doi = {Doi 10.1016/J.Jmr.2003.10.002}, url = {://WOS:000188469400005}, author = {Blanton, W. B. and Logan, J. W. and Pines, A.} } @article {359, title = {Investigations of low-amplitude radio frequency pulses at and away from rotary resonance conditions for I=5/2 Nuclei}, journal = {Solid State Nuclear Magnetic Resonance}, volume = {22}, year = {2002}, note = {Solid State Nucl Mag613WLTimes Cited:8Cited References Count:40}, month = {Sep-Nov}, pages = {97-109}, abstract = {

Additional experimental evidence of rotary resonance effects for multiple-quantum coherence conversion in a spin-5/2 system is presented. Two-dimensional plots of the relative efficiency of MQ excitation and conversion are given as a function of radio frequency (rf) amplitude and pulse width. Data are presented for the excitation of five-quantum coherence (5QC), as well as for 5QC to three-quantum coherence (3QC) conversion, 5QC to 1QC (the central transition coherence) conversion, and 3QC to 1 QC conversion. A two-fold increase in the signal-to-noise ratio is achieved by substituting low amplitude rf pulses in place of hard rf pulses for 5QC excitation and 5QC to 3QC conversion in a mixed multiple-quantum magic angle spinning (MAS) (MMQMAS) experiment. The anisotropic line shape for the low-amplitude rf pulse version of the MMQMAS experiment was observed to be distorted from the MAS line shape. The cause and implications of the distortion are discussed. (C) 2002 Elsevier Science (USA).

}, keywords = {solid-state}, isbn = {0926-2040}, doi = {Doi 10.1006/Snmr.2002.0084}, url = {://WOS:000179154400003}, author = {Logan, J. W. and Urban, J. T. and Walls, J. D. and Lim, K. H. and Jerschow, A. and Pines, A.} } @article {347, title = {Rotary resonance recoupling for half-integer quadrupolar nuclei in solid-state nuclear magnetic resonance spectroscopy}, journal = {Journal of Chemical Physics}, volume = {117}, year = {2002}, note = {J Chem Phys599CVTimes Cited:22Cited References Count:53}, month = {Oct 15}, pages = {7024-7033}, abstract = {

Investigations were made of rotary resonance recouplings (R-3) of chemical shift anisotropy (CSA), heteronuclear dipolar (HTD), and homonuclear dipolar (HMD) couplings involving half-integer quadrupolar nuclei under magic-angle sample spinning condition. Under rotary resonance conditions provided by a low amplitude rf field and a high spinning speed, the spectrum of the central transition coherence of half-integer quadrupolar nuclei shows recouplings of CSA, HTD, and HMD interactions that depend on the ratio of the rf field to the spinning speed. These new properties can be used to extract electronic and structural information about the sample that are otherwise difficult to extract in the presence of a dominant quadrupolar interaction. An average Hamiltonian theory is used to explain the recoupling properties of various interactions. Experimental implementations of the R-3 are demonstrated on model compounds with spin-3/2 systems. (C) 2002 American Institute of Physics.

}, keywords = {polarization}, isbn = {0021-9606}, doi = {Doi 10.1063/1.1506907}, url = {://WOS:000178317300016}, author = {Wi, S. and Logan, J. W. and Sakellariou, D. and Walls, J. D. and Pines, A.} } @article {351, title = {Theoretical investigations of I=5/2 quadrupolar spin dynamics in the sudden-passage regime}, journal = {Journal of Chemical Physics}, volume = {117}, year = {2002}, note = {J Chem Phys566JRTimes Cited:3Cited References Count:44}, month = {Jul 8}, pages = {518-532}, abstract = {

The theoretical approach utilizing bimodal Floquet theory in the quadrupolar/central-transition interaction frame, presented in an earlier article [J. D. Walls, K. H. Lim, and A. Pines, J. Chem. Phys. 116, 79 (2002)], is extended to describe the more complicated spin dynamics of I=5/2 spin systems. Rotary resonance effects occur when the strength of the radio-frequency irradiation, omega(1), matches the sample spinning speed, omega(r), at the conditions omega(1) = 2/3nomega(r) (n integral). At these conditions, conversions of both triple-quantum and five-quantum coherences to central-quantum coherence are observed. Between rotary resonance conditions [ 2n/3omega(r)\<ω(1)\<[2(n+1)]/3ω(r)], five-quantum as well as triple-quantum coherences can be created from equilibrium z-magnetization via a nutation mechanism. In addition, effective transfer between five-quantum and triple-quantum coherences also is observed in between rotary resonance conditions. These effects have been investigated theoretically and verified by both numerical calculations and experimental results. (C) 2002 American Institute of Physics.

}, keywords = {SPECTROSCOPY}, isbn = {0021-9606}, doi = {Doi 10.1063/1.1483256}, url = {://WOS:000176424800003}, author = {Walls, J. D. and Lim, K. H. and Logan, J. W. and Urban, J. T. and Jerschow, A. and Pines, A.} } @article {371, title = {Dynamic NMR microscopy of gas phase Poiseuille flow}, journal = {Journal of Magnetic Resonance}, volume = {149}, year = {2001}, note = {J Magn Reson421TCTimes Cited:17Cited References Count:16}, month = {Mar}, pages = {144-148}, abstract = {

Dynamic NMR microscopy has been used to study xenon gas undergoing Poiseuille flow in the regime where deterministic and stochastic motions are the same order of magnitude. For short observation time, the flow profile images are largely influenced by the longitudinal diffusion, manifested by large displacements in both positive and negative directions. For longer observation time, the effect of the mixing between the fast and slow flow components due to transverse diffusion becomes apparent. A spin-echo version of the dynamic NMR experiment yields images exhibiting strong distortions for longer observation time due to fast diffusion under the \"natural\" gradient from magnetic field inhomogeneity (compared to results obtained with a stimulated echo version). This effect is used as an edge-enhancement filter by employing a longer time duration of the imaging gradient in a stimulated echo experiment. (C) 2001 Academic Press.

}, keywords = {transport}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.2000.2283}, url = {://WOS:000168078800021}, author = {Kaiser, L. G. and Logan, J. W. and Meersmann, T. and Pines, A.} } @article {372, title = {High-resolution NMR of quadrupolar nuclei using mixed multiple-quantum coherences}, journal = {Journal of Magnetic Resonance}, volume = {149}, year = {2001}, note = {J Magn Reson431JNTimes Cited:17Cited References Count:16}, month = {Apr}, pages = {268-270}, abstract = {

A multiple-quantum magic angle spinning (MQMAS) NMR experiment of quadrupolar nuclei is demonstrated, which uses two different multiple quantum coherences in tl to refocus the quadrupolar broadening. This experiment has the potential of achieving improved resolution over current techniques. (C) 2001 Academic Press.

}, keywords = {pulses}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.2001.2303}, url = {://WOS:000168628200013}, author = {Jerschow, A. and Logan, J. W. and Pines, A.} } @article {376, title = {Exploring single-file diffusion in one-dimensional nanochannels by laser-polarized Xe-129 NMR spectroscopy (Cover Article)}, journal = {Journal of Physical Chemistry A}, volume = {104}, year = {2000}, note = {J Phys Chem A383EFTimes Cited:75Cited References Count:22}, month = {Dec 21}, pages = {11665-11670}, abstract = {

Single-file diffusion behavior is expected for atoms and molecules in one-dimensional gas phases of nanochannels with transverse dimensions that do not allow for the particles to bypass each other. Although single-file diffusion may play an important role in a wide range of industrial catalytic, geologic, and biological processes, experimental evidence is scarce despite the fact that the dynamics differ substantially from ordinary diffusion. We demonstrate the application of continuous-flow laser-polarized Xe-129 NMR spectroscopy for the study of gas transport into the effectively one-dimensional channels of a microporous material. The novel methodology makes it possible to monitor diffusion over a time scale of tens of seconds, often inaccessible by conventional NMR experiments. The technique can also be applied to systems with very small mobility factors or diffusion constants that are difficult to determine by currently available methods for diffusion measurement. Experiments using xenon in nanochannel systems can distinguish between unidirectional diffusion and single-file diffusion. The experimental observations indicate that single-file behavior for xenon in an organic nanochannel is persistent even at long diffusion times of over tens of seconds. Finally;using continuous flow laser-polarized Xe-129 NMR spectroscopy, we describe an intriguing correlation between the observed NMR line shape of xenon within the nanochannels and the gas transport into these channels.

}, keywords = {model}, isbn = {1089-5639}, doi = {Doi 10.1021/Jp002322v}, url = {://WOS:000165869600001}, author = {Meersmann, T. and Logan, J. W. and Simonutti, R. and Caldarelli, S. and Comotti, A. and Sozzani, P. and Kaiser, L. G. and Pines, A.} } @article {374, title = {A porous crystalline molecular solid explored by hyperpolarized xenon}, journal = {Angewandte Chemie-International Edition}, volume = {39}, year = {2000}, note = {Angew Chem Int Edit343KUTimes Cited:116Cited References Count:38}, pages = {2695-2698}, keywords = {c-13}, isbn = {1433-7851}, doi = {Doi 10.1002/1521-3773(20000804)39:15<2695::Aid-Anie2695>3.0.Co;2-M}, url = {://WOS:000088702200008}, author = {Sozzani, P. and Comotti, A. and Simonutti, R. and Meersmann, T. and Logan, J. W. and Pines, A.} } @article {378, title = {Visualization of gas flow and diffusion in porous media}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {97}, year = {2000}, note = {P Natl Acad Sci USA294ZRTimes Cited:29Cited References Count:18}, month = {Mar 14}, pages = {2414-2418}, abstract = {

The transport of gases in porous materials is a crucial component of many important processes in science and technology. In the present work, we demonstrate how magnetic resonance microscopy with continuous flow laser-polarized noble gases makes it possible to \"light up\" and thereby visualize, with unprecedented sensitivity and resolution, the dynamics of gases in samples of silica aerogels and zeolite molecular sieve particles. The \"polarization-weighted\" images of gas transport in aerogel fragments are correlated to the diffusion coefficient of xenon obtained from NMR pulsed-field gradient experiments. The technique provides a unique means of studying the combined effects of flow and diffusion in systems with macroscopic dimensions and microscopic internal pore structure.

}, keywords = {xenon}, isbn = {0027-8424}, doi = {Doi 10.1073/Pnas.050012497}, url = {://WOS:000085941400005}, author = {Kaiser, L. G. and Meersmann, T. and Logan, J. W. and Pines, A.} }