OutcomesOutcomes• State that NMR spectroscopy involves interaction

of materials with low-energy radio-frequency radiation.

• State the need for deuterated solvents.

• Describe the use of TMS (tetramethylsilane) as the standard for chemical shift measurements.

• Analyse carbon-13 NMR spectra to make predictions about the different types of carbon atoms present.

• Predict the chemical shifts of carbons within a given molecule.

Carbon-13 NMR Carbon-13 NMR SpectroscopySpectroscopy

12C atoms do not have nuclear spin, but 1.1% of carbon atoms are 13C and these do have nuclear spin and so produce NMR spectra.

NMR spectra give a lot of valuable information about the chemical environment of C atoms (e.g. the difference between C atoms in C=O, C-N, C≡N, C-C, C=C, etc.).

Samples carried out in solution – to prevent the C/H atoms in the solvent giving a signal Deuterium (isotope of H) is used as it has an even number of nucleons so will produce no NMR signal.

– this makes them behave like tiny magnets. They will match or oppose an external field.

– this makes them behave like tiny magnets.

Both Protons

Neutrons & Protons possess spin

NMR Spectrometry: Spin states

Magnetic field

Proton B

Proton A E

Proton B

Proton A

Radio waves are required to ‘flip’ the nuclei so that all nuclei are no longer aligned with the magnetic field.

Energy

N

S

N

S

Nuclei that oppose the magnetic field have a higher energy level than those aligned with the field.

A low-energy nuclei (aligned with the applied field) will jump to a high energy spin state (opposing the applied field) when given a pulse of RF. (Excitation)

NMR Spectrometry: Excitation & Relaxation

Againstfield

Withfield

When the magnetic field is removed, the nuclei revert back to their original state releasing the energy that was just given to them in the form of radiation. (Relaxation)

The cycle of excitation and relaxation of the nucleus is called resonance – hence the name NMR...nuclear magnetic resonance.

RF signal coil detector coilcan be the same coil!

Induced signal

Chemical shift

• The chemical shift (δ) is measured relative to TMS • TMS is chemically unreactive and is very volatile so is easily

removed from the sample after running the NMR spectrum.

10 0 2 4 6 8

CH3 CH3 Si CH3

CH3

Chemical shift

TMS= 0ppm

δ (ppm)

The carbon in the CH3 group is attached to 3 hydrogens and a carbon.

The carbon in the CH2 group is attached to 2 hydrogens, a carbon and an oxygen.

But which is which?

A table of typical chemical shifts in C-13 NMR spectra

carbon environment

chemical shift (ppm)

C=O (in ketones) 205 - 220

C=O (in aldehydes) 190 - 200

C=O (in acids and esters) 170 - 185

C in aromatic rings 125 - 150

C=C (in alkenes) 115 - 140

RCH2OH 50 - 65

RCH2Cl 40 - 45

RCH2NH2 37 - 45

R3CH 25 - 35

CH3CO- 20 - 30

R2CH2 16 - 25

RCH3 10 - 15

CH3CH2

The external magnetic field experienced by the carbon nuclei is affected by the electronegativity of the atoms attached to them.

The effect of this is that the chemical shift of the carbon increases if you attach an atom like oxygen to it.

3 - methylpentanePredict the number of signals

CyclohexanePredict the number of signals

ButylaminePredict the number of signals

2 - chlorobutanePredict the number of signals

4-pentenoic acid

Predict the number of signals

Predict the number of signals

methyl butanoate

Predict the number of signals

Pentan-3-one