to add a Longitudinal Eddy current Delay LED to the pulse sequence as proposed

To add a longitudinal eddy current delay led to the

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to add a Longitudinal Eddy current Delay (LED) to the pulse sequence as proposed by Gibbs and Johnson [ 43 ], where the signal acquisition is delayed. Another possibility is to increase the gradient recovery time, up to half a millisecond can be used should the system require it. This leaves D as the best candidate to increase. This can be done by heating the sample and running experiments at higher temperatures. For our purposes it has been found that 60 gives a significant enough increase in D to allow for eNMR measure- ments. Increasing temperature from 25 to 60 resulted in a change in D from 1.8 · 10 - 11 m 2 s - 1 to 7.7 · 10 - 11 m 2 s - 1 . Increasing temperature and thus D of the sample has the significant drawback of also increasing the conductivity of the sample as this depends on the viscosity. Investigation and evaluation of changes to these parameters has been in large part what this project has concerned. By a combination of theoretical work and the equations mentioned above as basis and trial and error a set of parameters were chosen. Coupled with increased temperature to 60 the parameters that found to give reproducible results were: E = 200 V Δ = 300 ms δ = 3 ms g = 0 . 45 T/m Finding a reference for convection compensation in eNMR proved to be a far from easy task when working with neat ionic liquids. One requirement is a large number of chemically identical protons (or whichever nucleus is being investigated) as the sample cannot be diluted too much with a reference. Another requirement is that the reference and the ionic liquid are miscible and that they do not coordinate. During the course of this project, no 19 F reference could be found that did not either coordinate to a significant extent to the cation’s imidazolium ring as shown by chemical shift change for the imidazolium peaks or were immiscible in BMIM TfAc. Hexamethyldisilane proved to be a good choice for 1 H reference.
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22 CHAPTER 2. SUMMARY OF RESEARCH 2.3 Results 2.3.1 Diffusion Diffusion experiments were run and evaluated and the resulting fits to the signal decay gave D cation at 60 for each run as: 7 . 70 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) 7 . 68 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) 7 . 50 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) Figure 2.7: 13 of the 24 steps (every second step) of the first diffusion measurement on BMIM TfAc with 5 volume % hexamethyldisilane. Figure 2.8: The exponential fit for peak 2 (see figure 2.2 and figure 2.1 on page 15 ) from the diffusion experiment shown in figure 2.7 .
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2.3. RESULTS 23 D anion was also measured with the same settings and found to be: 8 . 09 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) 7 . 97 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) 8 . 21 ± 0 . 03 · 10 - 11 ( m 2 s - 1 ) This results in the following average and variance: D cation = 7 . 6 ± 0 . 08 · 10 - 11 ( m 2 s - 1 ) D anion = 8 . 9 ± 0 . 10 · 10 - 11 ( m 2 s - 1 ) 2.3.2 eNMR 1 H eNMR results are reported with reference compensation and were run at 60 . For clarity the reference phase shift is also reported.
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