Tuesday, September 09, 2008

Journal of Magnetic Resonance - Vol 194 Issue 1

Exploring the limits of broadband excitation and inversion: II. Rf-power optimized pulses
Kyryl Kobzar, Thomas E. Skinner, Navin Khaneja, Steffen J. Glaser and Burkhard Luy
In [K. Kobzar, T.E. Skinner, N. Khaneja, S.J. Glaser, B. Luy, Exploring the limits of broadband excitation and inversion, J. Magn. Reson. 170 (2004) 236–243], optimal control theory was employed in a systematic study to establish physical limits for the minimum rf-amplitudes required in broadband excitation and inversion pulses. In a number of cases, however, experimental schemes are not limited by rf-amplitudes, but by the overall rf-power applied to a sample. We therefore conducted a second systematic study of excitation and inversion pulses of varying pulse durations with respect to bandwidth and rf-tolerances, but this time using a modified algorithm involving restricted rf-power. The resulting pulses display a variety of pulse shapes with highly modulated rf-amplitudes and generally show better performance than corresponding pulses with identical pulse length and rf-power, but limited rf-amplitude. A detailed description of pulse shapes and their performance is given for the so-called power-BEBOP and power-BIBOP pulses.

Improved J-compensated sequences based on short composite pulses
A.M. Torres, W.A. Bubb, D.J. Philp and P.W. Kuchel
Efficient J-compensated sequences that are shorter in duration and use less RF pulses have been created from short but very efficient composite 90° RF pulses. The improved J-compensation transforms in-phase into antiphase magnetization and can be incorporated in any pulse sequence that involves evolution of heteronuclear J-couplings. The compensated sequences were tested and incorporated into an HMBC sequence. J-compensated experiments referred to as HMBC-J45 + 90A and HMBC-J45 + 90B, were found to be effective over a wide range of J values.

Clean HMBC: Suppression of strong-coupling induced artifacts in HMBC spectra
Peter Würtz, Perttu Permi, Niels Chr. Nielsen and Ole W. Sørensen
A new experiment, clean HMBC, is introduced for suppression of strong-coupling induced artifacts in HMBC spectra. The culprits of these artifacts are an inherent shortcoming of low-pass J filters in the presence of strong coupling and the 1H π pulse in the middle of the evolution period aimed at suppressing evolution under heteronuclear J couplings and 1H chemical shifts. A π pulse causes coherence transfer in strongly coupled spin systems and, as is well known in e.g., homonuclear J spectra, this leads to peaks that would not be there in the absence of strong coupling. Similar artifacts occur in HMBC spectra, but they have apparently been overlooked, presumably because they have been assigned to inefficiency of low-pass J filters or not noticed because of a coarse digital resolution in the spectra. Clean HMBC is the HMBC technique of choice for molecules notorious for strong coupling among protons, such as carbohydrates, and the new technique is demonstrated on D-mannose. Finally, a fundamental difference between HMBC and H2BC explains why strong-coupling artifacts are much less of a problem in the latter type of spectra.

A structure refinement strategy for NMR crystallography: An improved crystal structure of silica-ZSM-12 zeolite from 29Si chemical shift tensors
Darren H. Brouwer
A strategy for performing crystal structure refinements with NMR chemical shift tensors is described in detail and implemented for the zeolite silica-ZSM-12 (framework type code MTW). The 29Si chemical shift tensors were determined from a slow magic-angle spinning spectrum obtained at an ultrahigh magnetic field of 21.1 T. The Si and O atomic coordinate parameters were optimized to give the best agreement between experimentally measured and ab initio calculated principal components of the 29Si chemical shift tensors, with the closest Si–O, O–O, and Si–Si distances restrained to correspond with the distributions of the distances found in a set of single-crystal X-ray diffraction (XRD) structures of high-silica zeolites. An improved structure for the silica-ZSM-12 zeolite, compared to a prior structure derived from powder XRD data, is obtained in which the agreement between the experimental and calculated 29Si chemical shift tensors is dramatically improved, the Si–O, O–O, and Si–Si distances correspond to the expected distributions, while the calculated powder XRD pattern remains in good agreement with the experimental powder XRD data. It is anticipated that this “NMR crystallography” structure refinement strategy will be an important tool for the accurate structure determination of materials that are difficult to fully characterize by traditional diffraction methods.

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