@article {379, title = {NMR study of InP quantum dots: Surface structure and size effects}, journal = {Journal of Chemical Physics}, volume = {110}, year = {1999}, note = {J Chem Phys192HUTimes Cited:30Cited References Count:28}, month = {May 8}, pages = {8861-8864}, abstract = {

We report the results of P-31 NMR measurements on trioctylphosphine oxide (TOPO) passivated InP quantum dots. The spectra show distinct surface-capping sites, implying a manifold of crystal-ligand bonding configurations. Two In P-31 surface components are resolved and related to different electronic surroundings. With decreasing particle size the In P-31 core resonance reveals an increasing upfield chemical shift related to the overall size dependence of the InP electronic structure. (C) 1999 American Institute of Physics. [S0021-9606(99)70718-X].

}, keywords = {clusters}, isbn = {0021-9606}, doi = {Doi 10.1063/1.478858}, url = {://WOS:000080073200002}, author = {Tomaselli, M. and Yarger, J. L. and Bruchez, M. and Havlin, R. H. and deGraw, D. and Pines, A. and Alivisatos, A. P.} } @article {400, title = {In situ NMR analysis of fluids contained in sedimentary rock}, journal = {Journal of Magnetic Resonance}, volume = {133}, year = {1998}, note = {J Magn Reson116WNTimes Cited:23Cited References Count:23}, month = {Aug}, pages = {385-387}, abstract = {

Limitations of resolution and absorption in standard chemical spectroscopic techniques have made it difficult to study fluids in sedimentary rocks. In this paper, we show that a chemical characterization of pore fluids may be obtained in situ by magic angle spinning (MAS) nuclear magnetic resonance (NMR), which is normally used for solid samples. H-1 MAS-NMR spectra of water and crude oil in Berea sandstone show sufficient chemical shift resolution for a straightforward determination of the oil/water ratio, (C) 1998 Academic Press.

}, keywords = {oil}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.1998.1459}, url = {://WOS:000075748200020}, author = {de Swiet, T. M. and Tomaselli, M. and Hurlimann, M. D. and Pines, A.} } @article {405, title = {Low field magnetic resonance images of polarized noble gases obtained with a dc superconducting quantum interference device}, journal = {Applied Physics Letters}, volume = {72}, year = {1998}, note = {Appl Phys LettZg930Times Cited:37Cited References Count:24}, month = {Apr 13}, pages = {1908-1910}, abstract = {

Using a low transition temperature superconducting quantum interference device as a detector, we have obtained magnetic resonance images of laser-polarized He-3 gas and solid Xe-129 at 4.2 K in magnetic fields as low as 0.54 mT (He-3) and 1 mT (Xe-129), corresponding to Larmor frequencies of 17.6 and 11.8 kHz, respectively. The experimental resolution of the images is similar to 500 mu m for He-3 ill the gas phase and similar to 950 mu m for Xe-129 in the solid state. (C) 1998 American Institute of Physics.

}, keywords = {mri}, isbn = {0003-6951}, doi = {Doi 10.1063/1.121223}, url = {://WOS:000073054300038}, author = {Augustine, M. P. and Wong-Foy, A. and Yarger, J. L. and Tomaselli, M. and Pines, A. and TonThat, D. M. and Clarke, J.} } @article {399, title = {NMR spectra with peaks at the principal values of the chemical shielding tensor}, journal = {Chemical Physics Letters}, volume = {285}, year = {1998}, note = {Chem Phys LettZf354Times Cited:10Cited References Count:25}, month = {Mar 13}, pages = {59-63}, abstract = {

The NMR chemical shielding in a solid powder sample produces featured, but broad, lineshapes, or powder patterns, because the shielding experienced by a nuclear spin depends on the spatial orientation of its local molecular frame with respect to the external magnetic field. The Lineshape, however, is fully determined by the three principal values of the shielding tenser. We present a simple approach that uses sample spinning NMR to extract peaks at the principal-value frequencies from chemical shielding powder patterns. Such techniques may simplify spectra with overlapping powder patterns without the information loss inherent in fast magic angle spinning. Experimental data and numerical simulations are presented for two P-31 model compounds. (C) 1998 Elsevier Science B.V.

}, keywords = {SPECTROSCOPY}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(97)01478-4}, url = {://WOS:000072889100010}, author = {de Swiet, T. M. and Tomaselli, M. and Pines, A.} } @article {401, title = {Reversal of radio-frequency-driven spin diffusion by reorientation of the sample spinning axis}, journal = {Journal of Chemical Physics}, volume = {108}, year = {1998}, note = {J Chem PhysYr359Times Cited:9Cited References Count:39}, month = {Jan 15}, pages = {826-829}, abstract = {

The dipolar Hamiltonian in a rapidly rotating sample is scaled by the second Legendre polynomial of the cosine of the angle between the rotation axis and the static magnetic field. It is, therefore, possible to refocus the spatial polarization-transfer process, often termed spin diffusion, in extended spin systems by reorienting the rotor axis such that the dipolar interaction Hamiltonian changes sign. We present experimental results which demonstrate that a rapid mechanical sample reorientation leads to a time reversal of the \"radio-frequency-driven\" spin diffusion among C-13 spins. (C) 1998 American Institute of Physics. [S0021-9606(98)03503-X].

}, keywords = {echoes}, isbn = {0021-9606}, doi = {Doi 10.1063/1.475494}, url = {://WOS:000071487500002}, author = {De Paul, S. M. and Tomaselli, M. and Pines, A. and Ernst, M. and Meier, B. H.} } @article {391, title = {Scalar and anisotropic J interactions in undoped InP: A triple-resonance NMR study}, journal = {Physical Review B}, volume = {58}, year = {1998}, note = {Phys Rev B125HUTimes Cited:29Cited References Count:40}, month = {Oct 1}, pages = {8627-8633}, abstract = {

The heteronuclear J-coupling tensor between nearest neighbor P-31 and In-113 spins in undoped InP is investigated by means of In-113--\>P-31 polarization transfer under rapid magic angle spinning (MAS). The scalar contribution can be measured directly and is found to have the value \J(iso)(P-31-In-113,In-115)\ = (225+/-10) Hz. The principal value of the traceless anisotropic J-coupling tensor (pseudodipolar coupling) is determined to be J(aniso)(P-31-In-113,In-115)= 2/3[J(parallel to)(P-31-In-113,In-115)-J(perpendicular to)(P-31-In-113,In-115)] = (813+/-50) or (1733+/-50) Hz, assuming axial symmetry with the principal axis parallel to the In-P bond. Our values deviate from those reported previously [M. Engelsberg and R. E. Norberg, Phys. Rev. B 5, 3395 (1972)] [based on a moment analysis of the P-31 resonance \J(iso)(31P-In-113,In-115)\ = 350 Hz and J(aniso)(P-31-In-113,In-115) = 1273 Hz], but confirm the postulate that the nearest neighbor P-31-In-113,In-115 magnetic dipolar and pseudodipolar interactions are of the same order of magnitude and partially cancel each other.

}, keywords = {solids}, isbn = {0163-1829}, doi = {Doi 10.1103/Physrevb.58.8627}, url = {://WOS:000076232100075}, author = {Tomaselli, M. and deGraw, D. and Yarger, J. L. and Augustine, M. P. and Pines, A.} } @article {398, title = {Time reversal of cross-polarization in solid-state NMR}, journal = {Molecular Physics}, volume = {95}, year = {1998}, note = {Mol Phys146XYTimes Cited:4Cited References Count:21}, month = {Dec 10}, pages = {849-858}, abstract = {

Cross-polarization at the Hartmann-Hahn condition in solid-state NMR frequently is described in terms of thermodynamics. Spin temperatures characterizing the canonical density operator are assigned to the Zeeman reservoirs of the two spins and the cross-polarization process brings about a state of equilibrium of the two reservoirs with a common temperature. In such a model, cross-polarization from an initially polarized spin species (I spins) to another spin species (S spins) is inherently an irreversible process accompanied by an increase in the entropy of the system. However, a cross-polarization echo can be generated whereby the polarization transferred to the S spins returns to the I spins, restoring the initial density operator. Therefore a thermodynamic description should be applied with care even in samples where the build-up and the decay of the magnetization can be approximated well by multiexponential processes. Such cross-polarization echoes are formed by the consecutive application of two pulse trains that produce effective Hamiltonians differing in sign. The \&$\#$39;time reversal\&$\#$39; of cross-polarization is consistent with both the increase in Zeeman entropy during the approach to equilibrium and with the constraint of unitary quantum evolution.

}, keywords = {echoes}, isbn = {0026-8976}, doi = {Doi 10.1080/002689798166477}, url = {://WOS:000077460200014}, author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.} } @article {397, title = {Time-reversal of cross-polarization in nuclear magnetic resonance}, journal = {Journal of Chemical Physics}, volume = {108}, year = {1998}, note = {J Chem Phys108FUTimes Cited:11Cited References Count:17}, month = {Jun 15}, pages = {9611-9613}, abstract = {

It is demonstrated that the time evolution of the heteronuclear polarization-transfer process in a dipolar-coupled nuclear spin system can be reversed, leading to the observation of cross polarization echoes. The cross-polarization echoes are induced by consecutive application of two pulse trains that produce effective Hamiltonians that differ only in sign, Cross-polarization echoes have been recorded for a powder sample of alanine. The time evolution of the spin;system is consistent with both unitary quantum dynamics and with spin thermodynamics of two systems approaching a common spin temperature. (C) 1998 American Institute of Physics.

}, keywords = {echoes}, isbn = {0021-9606}, doi = {Doi 10.1063/1.476435}, url = {://WOS:000075256100001}, author = {Ernst, M. and Meier, B. H. and Tomaselli, M. and Pines, A.} } @article {418, title = {Determination of dihedral angles in peptides through experimental and theoretical studies of alpha-carbon chemical shielding tensors}, journal = {Journal of the American Chemical Society}, volume = {119}, year = {1997}, note = {J Am Chem SocXt037Times Cited:90Cited References Count:57}, month = {Aug 20}, pages = {7827-7831}, abstract = {

A simple method for the determination of backbone dihedral angles in peptides and proteins is presented. The chemical-shift anisotropies (CSA) of the central alanine alpha-carbon in powdered crystalline tripeptides, whose structures have been determined previously by X-ray crystallography, were measured by cross-polarization magic-angle-spinning nuclear magnetic resonance. The experimental CSA values were correlated with ab initio chemical-shielding calculations over Ramanchandran phi/psi space on an N-formyl-L-alanine amide fragment. Using this correlation, phi/psi probability surfaces for one of the tripeptides were calculated based only on the alpha-carbon CSA, allowing a prediction of backbone angles. Dihedral angles predicted by these calculations fall within +/-12 degrees of the values determined by crystallography. This approach should be useful in the determination of solid-slate protein structure.

}, keywords = {crystal-structure}, isbn = {0002-7863}, doi = {Doi 10.1021/Ja970124k}, url = {://WOS:A1997XT03700025}, author = {Heller, J. and Laws, D. D. and Tomaselli, M. and King, D. S. and Wemmer, D. E. and Pines, A. and Havlin, R. H. and Oldfield, E.} }