# rate determination by 1H NMR spectroscopy

Currently I investigate a reaction where a starting material A reacts with an excess of thiophenol B towards products C and D. As shown by recurrent TLC analysis of the reaction, the formation of D is slower than the one of C, while A and B are still detectable.

Perhaps the reactions are consecutive, i.e.

$\ce{A + B ->[k_1] C ->[k_2] D}$

or they are simultaneous, i.e.

$\ce{D <-[k_2] A + B ->[k_1] C}$.

A, B, C, and D are distinguishable by 1H NMR spectroscopy; NMR data of the purified compounds A, B, C, and D were recorded successfully.

Based on a recurrent 1H NMR spectroscopy, I would like to use the integrations of the signals to determine the rate constants; perhaps there is evidence for the first case (similar to Bodenstein principle). The advancement of the thioetherifications alter the multiplicities of the signals observed, and simultaneously indicator signals vanish.

Question: Prior to get work on my data, where may I find a tutorial of reactions with known starting material(s)/product(s), determined rate constant(s) and NMR data-used-to-determine-these-rate-constants?

• So you want to measure the kinetic of the reaction with NMR, measuring 1D spectra continuously while the reaction happens in the NMR tube (or you run a regular reaction and take samples for NMR), or? From your question you seem to know what you want to do, what is the part you're unsure about? Do you want to know how to set up the NMR experiment, how to extract the rates or how to analyze the rates and determine the mechanism? – Mad Scientist Mar 9 '14 at 15:47
• Md Scientist is right, this is very straight forward, provided you are not generating any persistent free radicals during the reaction. Go to Journal of Chemical Education and poke around for a simple one if you need practice. – Lighthart Mar 10 '14 at 19:53

As mentioned in the comments, J. Chem. Ed is a good resource for NMR kinetics experiments. I have had success with the experiment published by J. Orvis in J. Chem. Ed 80(7), 2003, 803 link. This publication describes the synthesis of trans-dichlorotetraamminecobalt(III) and its subsequent reaction with water to produce cis-aquachlorotetramminecobalt(III). There are several advantages to this experiment as a model study:

• The synthesis is straightforward and does not require materials unlikely to be already in the laboratory, although it is critical that the directions are followed precisely.
• The aquation reaction occurs within about one hour - slow enough to easily monitor via NMR but not so slow as to take up precious instrument time
• The reaction rate is temperature sensitive, which makes it a good model for extracting thermodynamic properties from the reaction

My undergraduate students are able to complete the synthesis about 50% of the time, and the yields using the published procedure are about 400 mg, so one successful synthesis is suitable for a number of NMR experiments.