NMR

Question 1

what is TMS and how is it used in NMR?

Answer 1

Tetramethylsilane (TMS) is used as the standard reference chemical against which all chemical shifts are measured.

Question 2

why are deuterated solvents needed when running an NMR spectrum? give an example of a deuterated solvent.

Answer 2

e.g. CDCl3 (deuterated trichloromethane)
they do not produce signals in the frequency ranges used in H and C NMR spectroscopy because deuterium atoms have an even number of nucleons

Question 3

what is a ‘D2O shake’ and what does it do?

Answer 3

adding D2O (deuterium oxide) to the mixture being put through a proton NMR spectrum. any OH/NH peaks will disappear as the OH/NH protons are replaced by deuterium atoms.

Question 4

what are the 6 columns used in the H NMR table?

Answer 4

chemical shift (delta)
environment
splitting
no. of adj. H
relative peak area
conclusion

Question 5

what is the process of combined analysis using NMR, IR and mass spec?

Answer 5

1. look at mass spec graph (M+ peak to find the Mr)
2. work out the molecular formula using the empirical formula and the Mr
3. look at IR spec to work out the functional groups
4. identify carbon environments using C NMR spectrum (two column table: chemical shift and environment)
5. identify hydrogen environments using H NMR spectrum (6 column table)
   ** don’t forget to check if RPA has been simplified by checking total no. of H against molecular formula **
6. check structure of molecule against fragment ions in mass spec graph