@article {362, title = {Two-dimensional high-resolution NMR spectra in matched B-0 and B-1 field gradients}, journal = {Journal of Magnetic Resonance}, volume = {156}, year = {2002}, note = {J Magn Reson568UETimes Cited:15Cited References Count:26}, month = {May}, pages = {146-151}, abstract = {

In a recent publication we presented a method to obtain highly resolved NMR spectra in the presence of an inhomogeneous B-0 field with the help of a matched RF gradient. If RF gradient pulses are combined with \"ideal\" 90degrees pulses to form inhomogeneous z rotation pulses, the line broadening caused by the B-0 gradient can be refocused, while the full chemical shift information is maintained. This approach is of potential use for NMR spectroscopy in an inhomogeneous magnetic field produced by an \"ex-situ\" surface spectrometer. In this contribution, we extend this method toward two-dimensional spectroscopy with high resolution in one or both dimensions. Line narrowing in the indirect dimension can be achieved by two types of nutation echoes, thus leading to depth-sensitive NMR spectra with full chemical shift information. If the nutation echo in the indirect dimension is combined with a stroboscopic acquisition using inhomogeneous z-rotation pulses, highly resolved two-dimensional correlation spectra can be obtained in matched field gradients. Finally, we demonstrate that an INEPT coherence transfer from proton to carbon spins is possible in inhomogeneous B-0 fields. Thus, it is possible to obtain one-dimensional C-13 NMR spectra with increased sensitivity and two-dimensional HETCOR spectra in the presence of B-0 gradients of 0.4 mT/cm. These schemes may be of some value for ex-situ NMR analysis of materials and biological systems. (C) 2002 Elsevier Science (USA).

}, keywords = {echoes}, isbn = {1090-7807}, doi = {Doi 10.1006/Jmre.2002.2545}, url = {://WOS:000176562100016}, author = {Heise, H. and Sakellariou, D. and Meriles, C. A. and Moule, A. and Pines, A.} } @article {353, title = {Variable rotation composite pulses for high resolution nuclear magnetic resonance using inhomogeneous magnetic and radiofrequency fields}, journal = {Chemical Physics Letters}, volume = {363}, year = {2002}, note = {Chem Phys Lett591TPTimes Cited:17Cited References Count:23}, month = {Sep 2}, pages = {25-33}, abstract = {

An approach toward high-resolution NMR spectroscopy on samples located outside the physical confines of a magnet (ex situ NMR), has recently been described [C.A. Meriles, D. Sakellariou, H. Heise, A.J. Moule, A. Pines, Science 293 (2001) 82]. Nutation echoes are generated by a train of z-rotation pulses in the presence of spatially matched static and rf field gradients. These pulses were based on a combination of \&$\#$39;perfect\&$\#$39; pi/2 constant rotation composite pulses and \&$\#$39;imperfect\&$\#$39; ordinary pulses of variable length. Here we introduce a new class of \&$\#$39;self-compensated\&$\#$39; z-rotation composite pulses based only on variable rotation inversion pulses that lead to an rf dependent phase shift. Experiments and simulations show that these new pulses perform well at high B-0 gradients and require less rf power than previous schemes. (C) 2002 Elsevier Science B.V. All rights reserved.

}, keywords = {echoes}, isbn = {0009-2614}, doi = {Doi 10.1016/S0009-2614(02)01116-8}, url = {://WOS:000177896600005}, author = {Sakellariou, D. and Meriles, C. A. and Moule, A. and Pines, A.} } @article {369, title = {Approach to high-resolution ex situ NMR spectroscopy}, journal = {Science}, volume = {293}, year = {2001}, note = {Science451BBTimes Cited:109Cited References Count:20}, month = {Jul 6}, pages = {82-85}, abstract = {

Nuclear magnetic resonance (NMR) experiments are typically performed with samples immersed in a magnet shimmed to high homogeneity. However, there are many circumstances in which it is impractical or undesirable to insert objects or subjects into the bore of a high-field magnet. Here we present a methodology based on an adaptation of nutation echoes that provides resolved spectra in the presence of matched inhomogeneous static and radiofrequency fields, thereby opening the way to high-resolution exsitu NMR, The observation of chemical shifts is regained through the use of multiple-pulse sequences of correlated, composite z-rotation pulses, producing resolved NMR spectra of liquid samples.

}, keywords = {echoes}, isbn = {0036-8075}, doi = {Doi 10.1126/Science.1061498}, url = {://WOS:000169780300032}, author = {Meriles, C. A. and Sakellariou, D. and Heise, H. and Moule, A. J. 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 {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.} }