NMR

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NMR and Chromatography

13.1 NMR Spectroscopy

- 1H and 13C have nuclear spin

13.2 1H NMR

- different environments cause a chemical shift of the absorption

- groups next to more electronegative groups have resonances at a higher ppm

- CCl4 or CDCl3 are used as solvents:

do not contain 1H atoms and therefore do not interfere with spectrum

- TMS is mixed with sample and acts as a reference point (given value of 0) it is used because

silicon has low electronegativity (TMS resonates well above anything on spectrum)

gives 1 strong, sharp absorption caused by 12 equivalent 1H atoms

non toxic

cheap

does not react with sample

has a low boiling point

- low res: strengths of absorptions are proportional to no. of equivalent 1H atoms

- his res: splitting of absorptions is caused by influence of 1H atoms bonded to neighbouring atoms = coupling effect

- equivalent H atoms do not couple with each other

- NMR absorption of a proton that has n equivalent neighbouring protons is split into n+1 peaks.

13.3 13C NMR

- much easier to read as they only consist of a series of singlet peaks

- C attached to electronegative elements again occur at higher chemical shifts

- TMS still used as reference point at 0ppm (no C to interfere with spectrum!)

- not possible to determine how many C there are in each resonance using integration

Summary

- NMR is used to analyse organic molecules

- each chemical environment is responsible for a different resonance (shift)

- 1H NMR:

signal intensities proportional to no. of H atoms

may be split as a result of coupling

n equivalent adjacent H atoms = n+1 peaks

- 13C NMR:

signal intensities have nothing to do with no. of C atoms (making it easier to read!)

coupling doesn't happen