|
1. |
The 13C FID and spectrum are derived from a sample of CH3I. The spectrum is a quartet so it is clear that the spectrum is accumulated with no proton decoupling. |
|
i) |
The pulse angle is 30o so increasing this by a factor of 3 will mean that a 90o pulse is delivered to the sample. The response of the sample is a sinusoidal function of the pulse angle. Sin(30) = ½ and Sin(90) = 1 so the intensity of the signal in the FID and in the spectrum will be increased by a factor of 2. |
|
ii) |
The pulse angle is 30o so increasing this by a factor of 6 will mean that a 180o pulse is delivered to the sample. The response of the sample is a sinusoidal function of the pulse angle. Sin(180) = 0 so the intensity of the signal in the FID and in the spectrum will be zero. Both the FID and the spectrum would contain only noise. |
|
iii) |
The pulse angle is 30o so increasing this by a factor of 12 will mean that a 360o pulse is delivered to the sample. The response of the sample is a sinusoidal function of the pulse angle. Sin(360) = Sin(0) = 0 so the intensity of the signal in the FID and in the spectrum will be zero. Both the FID and the spectrum would contain only noise. |
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iv) |
The signal-to-noise ratio (S/N) of the spectrum increases as the square root of the number of acquisitions that are added together. If 100 acquisitions are added the S/N increases by a factor of Ö
100 = 10. |
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v) |
A viscous solvent will slow the rate of molecular tumbling and this in turn increases the efficiency of relaxation. Nuclei would be expected to relax more quickly and this would be reflected in a change in the appearance of the FID and the spectrum. The signal in the FID would decay to zero more rapidly (ie in less time); signals in the spectrum would be broader. |
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vi) |
Irradiation of the proton spectrum before (but not during) the acquisition would provide an NOE enhancement to the carbon spectrum (without decoupling carbon from protons). So the intensity of both the FID and the spectrum would be increased. |
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vii) |
Saturation of the 13C spectrum would mean that the 13C spectrum would not be observable, providing there was no significant delay between the saturation and recording the FID. Nuclei take several T1's to recover following saturation. |
|
viii) |
Irradiation of the proton spectrum before and during the acquisition would provide an NOE enhancement to the carbon spectrum as well as decoupling the protons from carbon. Both the FID and spectrum would be increased in intensity due to the NOE. In addition the spectrum would no longer show a quartet splitting (due to proton coupling) but it would appear as a singlet. The shape of the FID would also be changed because rather than 4 lines contributing frequency components to the FID, there would be only one. The shape of the FID would be considerably simplified to a single decaying sine wave. |
|
2. |
The 1H FID and spectrum are derived from a sample of CHCl3. |
|
i) |
The response of the sample is a sinusoidal function of the pulse angle. Sin(90) = 1 and Sin(150) = Sin(30) = ½ so the intensity of the signal in the FID and in the spectrum will be decreased by a factor of 2. |
|
ii) |
Addition of a small amount of a paramagnetic salt will increase the efficiency of relaxation. Nuclei would be expected to relax more quickly and this would be reflected in a change in the appearance of the FID and the spectrum. The signal in the FID would decay to zero more rapidly (i.e. in less time); signals in the spectrum would be broader. |
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iii) |
The signal-to-noise ratio (S/N) of the spectrum increases as the square root of the number of acquisitions that are added together. If 16 acquisitions are added the S/N increases by a factor of Ö
16 = 4. |
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iv) |
If an FID was accumulated with a 270o pulse, it would be a mirror image (about the zero level in the FID) of the FID accumulated with a 90o pulse. This means that at every point where the 90o FID was above zero, the 270o FID would be below zero and addition of the 90o and 270o FID's would mean that the signals would exactly cancel - only noise would remain. The spectrum would contain only noise. |
|
v) |
A viscous solvent will slow the rate of molecular tumbling and this in turn increases the efficiency of relaxation. Nuclei would be expected to relax more quickly and this would be reflected in a change in the appearance of the FID and the spectrum. The signal in the FID would decay to zero more rapidly (ie in less time); signals in the spectrum would be broader. |
|
vi) |
The FID as presented has clearly decayed to zero well before the accumulation of the FID has stopped. By the end of the acquisition time, there is no signal only noise being accumulated. Increasing the acquisition time by a factor of two will double the time for which the FID signal is accumulated however the real signal in the FID will still decay at the same rate so the real signal will be proportionally less of the FID and there would be proportionally more noise. In the spectrum, the S/N would be decreased. |
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vii) |
Saturation of the 1H spectrum would mean that the 1H spectrum would not be observable, providing there was no significant delay between the saturation and recording the FID. Nuclei take several T1's to recover following saturation. |
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viii) |
If 16 FIDs were accumulated then each transformed and the spectra added, there would be no difference to the situation where 16 FID's were added and the summed FID was transformed. In practice since the FT actually takes some time it is more time-economical to minimise the number of tiems that the FT has to be performed. |