Practical Aspects of Carbon-13 Double Quantum NMR
We want to address an erroneous view that has recently been advanced on 13C double quantum NMR. A recent publication (I) introduced a modified version (I) of the original double quantum NMR experiments; this new version claims to offer an attractive increase in sensitivity by a factor of three, or a decrease in the size of the required data matrix by a factor of four. To avoid disappointment among spectroscopists we wish to make it clear that this modified version offers no gain in sensitivity whatsoever in comparison with the other double quantum experiments (2-6), provided they are properly optimized. It is shown here that the sensitivity of this modified experiment can be even slightly lower. The claim of a reduction of the size of the required data matrix is also shown to be incorrect. In the original two-dimensional double quantum “C experiments, the acquisition time t2 starts immediately after the final “read pulse” (Fig. 1), and the time during which the double quantum coherence evolves is labeled ti. The detected 13C-13C satellites (7) are modulated in amplitude as a function of tl with the double quantum frequencies (2). In the quadrature-Fi-detection version of this experiment (3), results are combined with those of a second experiment which contains a composite 45: pulse (8) in the evolution period. This allows the sign of the modulation frequency to be determined, which simplifies the interpretation of the results. A simpler and more sensitive alternative to obtain quadrature F, detection has been proposed more recently (6); an increase of the flip angle of the final “read pulse” to 135” provides directly satellite signals which are modulated in phase as a function oft,. In either case, data acquisition is started after the final read pulse and the coherence transfer echo (9) is detected (4, 6). The time domain signal of a carbon A, directly coupled to carbon X, is then given by sA(tl, t2) = c exp[-i(nA + &)t,] exp[i& + d&2]
119/117/115Sn low‐abundance single‐transition correlation spectroscopy (LASSY): sensitivity‐enhanced homonuclear correlation experiments for Sn NMR
We report the implementation of our novel rare‐spin homonuclear correlation experiment, namely, Low‐Abundance Single‐transition correlation SpectroscopY (LASSY), for 119/117/115Sn NMR at natural abundance. Our pulse sequence results in diagonal suppressed COSY‐style display and outperforms the optimal homonuclear correlation experiment for rare spins, which involves double quantum evolution (INADEQUATE CR). The new experiment maximizes efficiency both in respect of pulse transformations as well as relaxation effects, and gives rise to a simplified two‐dimensional (2D) spectrum with considerably reduced crowding, exhibiting only one transition in each cross peak, instead of four. Performance optimization of LASSY is carried out in light of the relatively ‘large’ line widths typical of Sn NMR in solution state. The superior performance of the sequence is demonstrated on dimeric tetraorganodistannoxane samples. Copyright © 2010 John Wiley & Sons, Ltd.
Parameters for low energy operation of CESR
We present a detailed design for operation of the Cornell Electron Storage Ring, CESR, in the energy range covering the /spl psi/ resonances and the Charm and Tau thresholds, as well as reaching to the upper Y resonances near 11 GeV. The addition of 18 m of super-ferric wiggler magnets will partially restore low energy beam emittance and damping times. Installation of superconducting quadrupoles in the interaction region and the addition of high performance superconducting RF cavities will enhance performance at all energies. Studies of optics, beam dynamics, wiggler and vacuum system performance, beam stability, and beam-beam effects confirm operation with a luminosity of 3/spl times/ 10/sup 32/ CM/sup -2/ sec/sup -1/ at 1.88 GeV.