Question

Describe and explain the possible effect on your results of the following experimental errors or variations.

a) you used a solution cell with a 0.1mm spacer for the infrared spectrum,

b) you dissolved your unknown in petroleum ether for the UV analysis

c) There was no deuterochloroform available for the NMR sample so you used ordinary chloroform

Answer 1

c) answer

There are two reasons for using deuterated solvents in NMR experiments:

a) modern NMR spectrometers measure the deuterium absorption of the solvent to stabilize the magnetic field strength. As the observation frequency is field dependent, the deuterium receiver notices a field fluctuation through a change of the observation frequency ("lock frequrency") and can correct the field strength correspondingly. You call this the field/frequency lock. In principle one could use other nuclei for the lock, as was done earlier (e. g. 19F), but deuterium is the most convenient;

b) as there is always much more solvent than substance of interest in the sample to be investigated, one uses a deuterated solvent instead of the ordinary 1H-containing solvent to avoid the huge solvent absorption that would otherwise spoil the 1H-NMR spectrum.