Line width measurements, giving relaxation times

Equally Informative are the deuterium spin-spin relaxation times l2 (usually obtained from line width measurements), especially when studying low deuterltim concentrations. Due to the sensitivity of line widths to subtle changes In correlation times, NMR can give a detailed picture of rotational motion of specifically deuterlumr-labeled reporter groups. This technique has, for Instance, been used to study Internal motions of deuterium-labeled molecules bound to proteins (2.9.11,13). 

Figure B2.4.6. Results of an offset-saturation expermient for measuring the spin-spin relaxation time, T. In this experiment, the signal is irradiated at some offset from resonance until a steady state is achieved. The partially saturated z magnetization is then measured with a kH pulse. This figure shows a plot of the z magnetization as a fiinction of the offset of the saturating field from resonance. Circles represent measured data the line is a non-linear least-squares fit. The signal is nonnal when the saturation is far away, and dips to a minimum on resonance. The width of this dip gives T, independent of magnetic field inliomogeneity.

We have already hinted at two ways to measure T. In cases where the observed line width is due solely to effective (field-inhomogeneity-caused) spin-spin relaxation, the reciprocal of the halfwidth of the peak is T [Eq. (3.12)]. Also, in Section 3.4 we noted that the envelope of the FID signal follows exponential decay governed by T. In principle, we could measure the amount of time (t W2) required for the FID signal to decay to exactly half its initial magnitude (M, = i Mq), at which point Eq. (3.14b) gives... 

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