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A student is running a kinetics experiment in a J-Young tube under 5-10 atm of oxygen, much like in this journal article: http://pubs.acs.org/doi/abs/10.1021/ja9061932

  1. Student is seeing reduction in signal from his compound (slightly polar) and his internal standard (non-polar). Obviously paramagnetic oxygen?
  2. The part I can't figure out: student is NOT seeing a reduction in his solvent peak. Solvent is benzene or toluene. I am assuming he is using at least some deuterated solvent.

I have found a lot of literature on the interaction of paramagnetic O2 with membrane proteins, etc., but the paper the student sent to me makes no mention of accounting for this in the kinetics calculations. Assuming the relaxation of his compound and the internal standard are affected similarly by the paramagnetic O2, the relative integration should be correct? Is the relaxation of the solvent just too long to be affected much by paramagnetic O2? The issue with the kinetics runs is that they typically do one scan per time point since the reaction occurs somewhat quickly and they do not want to average over several scans. If the relaxation is short enough can we just do several scans with very short delay times?

asked Jun 02 '11 at 14:03

sararah's gravatar image

sararah
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How much time are you allowing between scans in your kinetics run? Also, what kind of resolution do you have on your system? If you had enough oxygen to change your relaxation significantly I would think it would be enough to slightly change the chemical shift of your system. - vanderwaal (Jun 09 '11 at 10:00)


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For a single scan spectrum, relaxation should not change the relative areas (solvent and/or solute) IF THERE IS NO NOISE in your spectrum.

In the presence of noise, weak and broad peaks tend to exhibit LOWER areas of integration than expected. The reason is that the tails of weak peaks are buried under noise and do not contribute to the integration. Therefore, quantitative comparison of signals that differ substantially in width and intensity often give results that are not consistent with stoichiometry.

If the dynamic range problem and the width variation is not too bad, many curve fitting programs can deal with this problem. I suggest that the integrals should be re-evaluated using a more robust curve fitting program if the intensities and widths of the reference and sample differ substantially. Otherwise, relative areas should be OK.

p.s. I have never heard of a J-Young tube before!

p.p.s. variation in solvent may be due to sample handling inconsistency? Repeating the sample preparation step may identify the variation, if any, due to sample preparation, pipetting, weighing, dilution, exposure to atmospheric humidity, etc.

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answered Jun 03 '11 at 12:00

sekhar%20Talluri's gravatar image

sekhar Talluri
601

updated Jun 04 '11 at 09:37

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To clarify: You state that the student sees a "reduction in signal from his compound" - I assume you mean integrated intensity, and not just amplitude?

The effectiveness of O2 as a relaxation agent depends on its concentration in solution, and that in turn depends on sample temperature as well. For example, the solubility of O2 in benzene at 30 C is ~ 7 mM (Fischer et al, J Chem Eng Data, 2001, 1504-1505) at 1 atm partial pressure of O2. At 10 atm, one might expect ~ 70 mM in solution as a first approximation. In comparison with other relaxation agents at similar concentrations (e.g. Galya et al, Int. J. Polymer Anal. Char., 1996, 293-303), one might expect a very short T1 (likely < 1 s), especially for 1H nuclei. Mattiello and Freeman, J Magn Reson 1998, 514-521 is also instructive. Interaction with O2 will likely provide the dominant relaxation mechanism for all nuclei in the system, including the solvent.

So at any reasonable interval between pulses, full T1 relaxation should be achieved. Strongly enhanced T2 relaxation may pose practical difficulties in integration, per the comment above. This is often seen for e.g. 1H integration with the Cr(acac)3 relaxation agent at 50 mM discussed in the Galya paper above. However, if proper integration limits can be set, or deconvolution applied, this should not much affect the integrated values for the purposes of a kinetic analysis.

One (likely remote) possibility: Depending on the system, if O2 were constrained to the near vicinity of a molecule without bond formation (such that its SOMO's remained intact), it might induce extremely rapid relaxation in the surrounding nuclei and broaden their resonances into invisibility in the NMR spectrum.

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answered Jun 09 '11 at 10:17

Alexander%20Marchione's gravatar image

Alexander Marchione
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