Why d2o in nmr

Post a Comment. You can identify exchangeable protons in your proton NMR spectrum with a very simple technique called a "D2O shake". For some reason this simple technique, used frequently years ago, seems to be used so much less today. All you do is run a 1H NMR spectrum of your sample then put a single drop of D2O in the tube, shake it, and run another spectrum. The exchangeable protons will exchange with the deuterium in the D2O and disappear from the spectrum. The D2O does not have to be miscible with the solvent.

Below is a partial proton spectrum of menthol in CDCl3. The bottom trace is the spectrum before the addition of D2O and the top trace after 1 drop of D2O was shaken with the sample.

More by Daniel Hemmler. More by Philippe Schmitt-Kopplin. More by Silke Sophie Heinzmann. Cite this: Anal. Article Views Altmetric -. Citations 3. Supporting Information. Cited By. This article is cited by 3 publications. Johanna-Barbara Linse , Jochen S. The Journal of Chemical Physics , 19 , The amount of splitting tells you about the number of hydrogens attached to the carbon atom or atoms next door to the one you are currently interested in.

The number of sub-peaks in a cluster is one more than the number of hydrogens attached to the next door carbon s. So - on the assumption that there is only one carbon at om with hydrogens on next door to the carbon we're interested in. Assume that you know that the compound above has the molecular formula C 4 H 8 O 2.

Treating this as a low resolution spectrum to start with, there are three clusters of peaks and so three different environments for the hydrogens. The hydrogens in those three environments are in the ratio Since there are 8 hydrogens altogether, this represents a CH 2 group and two CH 3 groups. What about the splitting? So what is this compound?

You would also use chemical shift data to help to identify the environment each group was in, and eventually you would come up with:. Where is the -O-H peak? This is very confusing! Different sources quote totally different chemical shifts for the hydrogen atom in the -OH group in alcohols - often inconsistently. For example:. The problem seems to be that the position of the -OH peak varies dramatically depending on the conditions - for example, what solvent is used, the concentration, and the purity of the alcohol - especially on whether or not it is totally dry.

If you measure an NMR spectrum for an alcohol like ethanol, and then add a few drops of deuterium oxide, D 2 O, to the solution, allow it to settle and then re-measure the spectrum, the -OH peak disappears!

By comparing the two spectra, you can tell immediately which peak was due to the -OH group. The reason for the loss of the peak lies in the interaction between the deuterium oxide and the alcohol. All alcohols, such as ethanol, are very, very slightly acidic. The hydrogen on the -OH group transfers to one of the lone pairs on the oxygen of the water molecule.

The fact that here we've got "heavy water" makes no difference to that. The negative ion formed is most likely to bump into a simple deuterium oxide molecule to regenerate the alcohol - except that now the -OH group has turned into an -OD group.