ID Transient NOE using DPFGSE

This technique differs from the old NOE difference experiment we looked at in Chapter 5, where we selectively saturate one resonance (make Mz = 0) over a long period of time (the mixing time) whereas other spins are perturbed and reach a steady-state level of 

The traditional ID NOE experiment (Section 5.12) involves irradiating with low-power radio frequency at the resonant frequency of one peak in the H spectrum in order to equalize the populations of the two states ( saturation ). This saturated state is maintained by continued irradiation until the perturbation of populations of nearby nuclei in the molecule reaches a steady state and does not change any further. Then a 90° pulse is applied and an FID is recorded to measure the amount of perturbation on the nearby nuclei. As the enhancement of signals is quite small (a few percent), it is necessary to record a control spectrum with irradiation away from any peaks in the spectrum, and then subtract the control spectrum from the NOE spectrum. There are a number of disadvantages to this approach  

In any difference spectrum, the conditions (temperature, RF power, sensitivity, magnetic field, and vibration) must be identical in the two experiments in order to get perfect subtraction of the signals that are not affected. This subtraction is always imperfect as the two spectra are recorded at different times, so there are always big subtraction artifacts in the difference spectrum. 

The magnitude of the NOE is proportional to the inverse sixth power of the distance between two nuclei only for very short times between the perturbation and the measurement of the effect on other nuclei. The magnitude of the steady-state NOE is dependent on many other competing relaxation processes, so it cannot be used as an accurate measure of distance. To accurately measure distances, you need to measure the transient NOE with a number of different times between perturbation and measurement ( mixing times ) and measure the initial slope of the curve as the effect increases with time. 

The selectivity of continuous-wave (C W) irradiation is limited, and in crowded regions of the spectrum nearby peaks are also affected. This sometimes makes the results ambiguous. 

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