Question

Which of the following solvents are appropriate for NMR spectroscopy? Why?

Acetone
Water
Benzene-d6
`CDCl_3`
`CF_3OOH`
Dioxane-d8

Answer 1

Only the deutero-solvents are suitable for `""^1H` `"NMR spectroscopy"`.

Explanation:

When `""^1H` `NMR` experiments are performed, the diamagnetism of all `""^1H` nuclei in the SAMPLE are interrogated. If we use solvents that already contain `""^1H`, then these signals WiLL DOMINATE the spectrum. (Why? because of their concentration.) Sometimes, a solvent can be used that contains few `""^1H` nuclei, i.e. per-fluoroacetic acid. The chemical shift of the proton here would occur well downfield, leaving a window for the desired spectrum.

Modern NMR spectrometers also use the deuterium signal as a means to lock the frequency (the deuterium frequency is thus the reference frequency; mind you, these days, magnetic fields tend to be very stable, and spectra can often be run unlocked).

Of course, `""^31P` and `""^11B` NMR spectroscopies are now routinely performed. Researchers in these fields often add a small quantity of deutero solvent to their reaction mixture, and do the experiment for the heteroatom (in which the proton resonance will be invisible, why?).

Deutero solvents (apart from chloroform-d) tend to be horrendously expensive, and the more expensive ones (i.e. `"toluene"-d_8`, `"methylene chloride"-d_2`, `"THF"-d_8`, `"benzene"-d_6`) tend to collected after an experiment, and then recycled by distillation (which of course is a very good practice both in terms of economy and the environment).