Answers
Revision Questions #11

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1.

The details of the saturation recovery method and the inversion recovery method for measuring T1 can be found in handouts 23 and 36. Both methods rely on the fact that the spin system is initially perturbed so that the populations are not at equilibrium and then the sample is examined at time intervals following the removal of the perturbation. In the saturation recovery method the perturbation arises from saturation so the populations of the a and b states are equalised. In the inversion recovery sequence, the populations are inverted by a 180o pulse so there is actually a greater population in the upper state than in the more stable lower state. The accuracy of the inversion recovery sequence is higher because the range of data is effectively double that of the saturation recovery experiment. In the saturation recovery experiment, populations recover from being equal to their equilibrium values. In the inversion recovery experiment, populations recover from inverted populations through zero to equilibrium. To measure T1 values for multiple species in the same solution, either experiment can be used. In the saturation recovery experiment, each signal would be examined in turn and a full saturation recovery analysis performed on the signals one by one. In the inversion recovery experiment, the 180o pulse inverts all signals simultaneously so the recovery of all signals can be monitored by a single series of experiments.

2.

i)

Dioxan and water both give rise to singlet resonances in the 1H NMR. Using the inversion recovery pulse sequence, a number of spectra would be recorded for different values of the delay t. At t = 0, both signals would be inverted and at t = ¥ both signals would be normally phased and at their maximum intensity. At values of t which are in between, the signals would recover from their inverted state to upright (at different rates). A plot of signal intensity vs. t would give an exponential recovery curve for both signals and T1 can be calculated by fitting the curve to the equation : St = S¥ (1 - 2e-t/T1 ) Where: St is the intensity of the signal at time t S¥ is the intensity of the signal at time ¥ T1 is the relaxation time of the nucleus.

ii)

The signals from dioxan and water will have different relaxation times. This means that their recovery in the inversion recovery experiment will take place at different rates. There will be a value of t at which the water signal will be zero. There will be a different value of t at which the dioxan signal will be zero. The signal from water can be eliminated by acquiring spectra with t chosen so that the water signal is exactly nulled. Providing T1 is signficantly different for the two species, the dioxan spectrum will be clearly visible when the water signal is at zero.

3.

The DEPT spectra identify protonated carbons and give some information about whether they are CH, CH2 or CH3. From the molecular formula, there are 20 carbons. In the basic spectrum, 18 signals are clearly visible and this implies that 2 signals must have overlapping (unresolved) signals or there may be some chemically equivalent carbons in the molecule. Note that in all of the DEPT spectra, the signal for the solvent (CDCl3) disappears since its carbon doesn't have any protons attached. From the DEPT135, all of the resonances are phased upwards which implies that there are no CH2 groups in the molecule (all protonated carbons are CH or CH3) and there at least 8 CH or CH3 carbons in the molecule (allowing for some degeneracy or close overlap of signals). From the DEPT90 experiment, there are at least 6 CH groups, the fact that one of the signals is consistently taller would imply that one of the signals is degenerate and is really from two carbons. By comparison with the DEPT135, there are at least 3 CH3 groups (possibly 4 given the fact that one of the CH3 signals is consistently about twice the intensity of the other signals. From the DEPT45, there are at least 8 protonated carbons and by comparison with the basic spectrum there are at least 8 non protonated carbons. For information, the complete structure of acronycine is :

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