Relaxation processes in NMR spectroscopy

Relaxation processes in NMR spectroscopy

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Example for the determination of T1 using the inversion recovery method

The figure shows a series of proton-decoupled 29Si spectra of symmetrical tetrafluorodimethyldisilane, the vd times are indicated on the y-axis.

The splitting of the signals is caused by the interaction of the silicon cores with fluorine, and a signal from the calibration substance TMS can also be seen.

Evaluation of the relaxation time measurement

The relationship between the signal intensities and the relaxation time T1 is given for the ir as follows:

ln(I.0I.z)=ln(2I.0)tT1I.0... maximum measurable intensity I.z... Intensity at time t

In the first step, the integral intensities of the signals of interest are determined in all spectra of the measurement series, in the example at approx. -3 ppm.

Tab. 1
Signal intensities as a function of the times vd
vd in s151015202225283035404560
Intensity I.z-1,90-1,45-1,25-0,450,060,200,400,600,700,921,151,251,65

The maximum intensity I0was determined in the experiment and is 2.15.

The graphic plot ln(I.0I.z) against t yields a straight line whose slope T1 is determined.

The application Iz versus t yields a diagram from which T1 at the zero crossing of the e-function t=T1·ln2 can be estimated.

If you look closely at the sample spectra, you can see that with a vd of 20 s only a very small signal can be seen. One can therefore estimate that no more signal appears at approx. 19 s, i.e. I.z = 0. Since at this point t=T1·ln2 holds, we get for T1 the following value:

T1 = 27 s


That 29Si signal of the symmetrical tetrafluorodimethyldisilane has a relaxation time T1 from 28s ± 1s.

Video: Thermal Analysis online training course (May 2022).