6.3.2 (d) TMS and solvents

(d) (i) the use of Tetramethylsilane, TMS, as the standard for chemical shift measurements

(ii) the need for deuterated solvents, e.g. CDCl₃, when running an NMR spectrum

(iii) the identification of O–H and N–H protons by proton exchange using D₂O

We've already looked at what TMS is, why it is needed, and why it is a good substance for this use.

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Not every substance is in a convenient form when it is made.

Many will need to be dissolved.

But this requires a solvent and most of these contain Hydrogen atoms that will show up in your spectrum even though they're not part of the substance you want to study.

Worse, many contain O-H groups (eg water, alcohols) which will provide extra peaks on a proton NMR which can show up almost anywhere in a spectrum.

The problem with this is that OH groups are generally very small and can occur in the middle of an existing multiplet - confusing you as to whether it is a hextet or a pentet etc.

As the name suggests, these are solvents which contain Deuterium (²H ) rather than Protium (¹H ).

You may recall that only nucleii with odd Mass Numbers provide peaks in NMR.

So, using a deuterated solvent removes the problem of rogue peaks that don't come from the substance that you are studying.

If your substance dissolves in the none polar substance Trichloromethane (CHCl₃), then the deuterated alternative (CDCl₃) would be better

As previously stated, OH groups are a problem - even if they are part of the substance rather than the solvent.

If we didn't know that this substance was ethanol then the peak at 2.6 might be a lot of things.

Adding a drop of D₂O and shaking removes that peak and so confirms that it was an OH.

This trick also works with NH peaks.