Proton positions relaxation

Figure 14 Measured relative molar shifts (a) 8 = the dielectric relaxation time, and (b) 8 = the intramolecular proton magnetic relaxation rate, of aqueous alkali-metal halide solutions these are plotted on the vertical axis against negative ion radii on the horizontal axis and against positive ion radii on the third axis...

Heteronuclear coherence transfer periods of HSQC or HMQC experiments are often incorporated into semi-constant-time proton chemical shift evolution to reduce the effective relaxation and thus improve sensitivity. This method was further extended by Bazzo et to utilise N chemical shift evolution period of HMQC-NOESY experiment for further reduction of the proton effective relaxation rate. This is achieved by changing the position of refocusing pulse in the middle of the heteronuclear MQ evolution period when evolution is encoded. As the result the MQ evolution period that is... 

Interspin Distance. The interspin distance between the electron spin (which is assumed to be located at the metal nucleus) and the protons of metal-bound waters have in most studies been inferred from crystal structure data. However, proton positions are usually not determined directly by x-ray diffraction, and distances may differ in crystal and solution phases. Noting that complexes that have similar correlation times and the same number of bound waters often exhibit proton relaxivities that differ by factors of as much as three, Atashkin et al. and Caravan et aV have explored the possibility that these differences reflect variability in the interspin distance (which enters as r ). They used pulsed ENDOR to measure the Gd-water proton distance in glasses of Gd (aq) and five Gd contrast agents. They find a single distance, r(Gd-H) = 0.31 0.01 nm, for all of these species. 

The whole sequence of successive pulses is repeated n times, with the computer executing the pulses and adjusting automatically the values of the variable delays between the 180° and 90° pulses as well as the fixed relaxation delays between successive pulses. The intensities of the resulting signals are then plotted as a function of the pulse width. A series of stacked plots are obtained (Fig. 1.40), and the point at which the signals of any particular proton pass from negative amplitude to positive is determined. This zero transition time To will vary for different protons in a molecule. 

We can see at once that each proton behaves differently, because it has its individual relaxation time Tx depending on the delay signals may be negative, positive, or have zero intensity. The T, values can be computed using spectrometer software. 

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