Interpreting 1 H nmr spectra

Interpreting 1H nmr spectra

L.O.:Intrepet 1H nmr spectra using the n+1 rule

CHEMICAL SHIFTCHEMICAL SHIFT

Approximatechemical shifts The actual values depend on the environment

13 12 11 10 9 8 7 6 5 4 3 2 1 0

‘deshielding’

-C=CH-

- C - XH

- C - H

‘shielding’ Low

‘deshielding’ High Bonding to electronegative atoms (O, N)

LOWLOW RESOLUTION RESOLUTION

LOW RESOLUTION SPECTRUM OF 1-BROMOPROPANE

• low resolution NMR gives 1 peak for each environmentally different group of protons

• Strengths of the absorption are proportional to number of equivalent 1H atoms. It is measured by the are under each peak. Integration.

Look at low resolution 1H NMR of EtOH

The simplified NMR spectrum of EtOH shows three single peaks.

A detailed, high-resolution spectrum of EtOH shows that some peaks are split into a number of subsidiary peaks. This splitting is caused by spin-spin coupling between protons on neighbouring atoms.

High resolution 1H NMR of EtOH.

The ‘n +1’ rule

The n.m.r. absorption of a proton which has n equivalent neighbouring protons will be split into n + 1 peaks.

O adjacent H’sThere is no effect

1 adjacent Hcan be aligned either with or against the fieldthere are only two equally probable possibilitiesthe signal is split into 2 peaks of equal intensity

MULTIPLICITY (Spin-spin splitting)MULTIPLICITY (Spin-spin splitting)

2 adjacent H’smore possible combinationsget 3 peaks in the ratio 1 : 2 : 1

3 adjacent H’seven more possible combinationsget 4 peaks in the ratio 1 : 3 : 3 : 1

Number of peaks = number of chemically different H’s on adjacent atoms + 1

1 neighbouring H 2 peaks “doublet” 1:1

2 neighbouring H’s 3 peaks “triplet” 1:2:1

3 neighbouring H’s 4 peaks “quartet” 1:3:3:1

4 neighbouring H’s 5 peaks “quintet” 1:4:6:4:1

Signals for the H in an O-H bond are unaffected by hydrogens on adjacent atoms - get a singlet

• the area under a signal is proportional to the number of hydrogen atoms present• an integration device scans the area under the peaks• lines on the spectrum show the relative abundance of each hydrogen type

By measuring the distances between the integration lines one canwork out the simple ratio between the various types of hydrogen.

before integration after integration

INTEGRATIONINTEGRATION

NOTICE THAT THE O-H SIGNAL IS ONLY A SINGLET

INTEGRATIONINTEGRATION

HOW TO WORK OUT THE SIMPLE RATIOS• Measure how much each integration line rises as it goes of a set of signals• Compare the relative values and work out the simple ratio between them• In the above spectrum the rises are in the ratio... 1:2:3

IMPORTANT: It doesn’t provide the actual number of H’s in each environment, just the ratio

Measure the distance between the top and bottom lines.

Compare the heights from each signal and make them into a simple ratio.

When is a hydrogen chemically different?

NMR SPECTROSCOPYNMR SPECTROSCOPY

TWO SIGNALSQuartet and triplet :- ratio of peak areas = 3 : 2

Carbons 1 & 4 are the similar and so are carbons 2 & 3 so there are only two different chemical environments.The signal for H’s on carbon 2 is a quartet - you ignore the two neighbours on carbon 3 because they are chemically identical.

BUTANE

1 2 3 4

When is a hydrogen chemically different?

NMR SPECTROSCOPYNMR SPECTROSCOPY

TWO SIGNALSQuartet and triplet :- ratio of peak areas = 3 : 2

Carbons 1 & 4 are the similar and so are carbons 2 & 3 so there are only two different chemical environments.The signal for H’s on carbon 2 is a quartet - you ignore the two neighbours on carbon 3 because they are chemically identical.

TWO SIGNALSboth singlets :- ratio of peak areas = 2 : 1

Hydrogens on OH groups only give singlets. The signal for H’s on each carbon are not split, because- H’s on the neighbouring carbon are chemically identical... and- H’s on adjacent OH groups do not couple.

BUTANE

ETHANE-1,2-DIOL

1 2 3 4

An nmr spectrum provides several types of information :-

number of signal groups tells you the number of different proton environmentschemical shift the general environment of the protonspeak area (integration) the number of protons in each environment multiplicity how many protons are on adjacent atoms

In many cases this information is sufficient to deduce the structure of an organic molecule but other forms of spectroscopy are used in conjunction with nmr.

NMR SPECTROSCOPY - NMR SPECTROSCOPY - SUMMARYSUMMARY

NMR spectra of –OH and –NH protons

o They are usually broad

o The is usually no splitting pattern.

D2O Shake

CH3CH2OH + D2O → CH3CH2OD + HOD

1H NMR TASK 4For each of the following compounds, draw the molecule predict the number of signals predict the relative intensity of each signalpredict the approximate chemical shift (of each signala)propanoic acid b)propanal c)2-chloropropaned)2-methylbutanee)methylpropene f)methyl propanoate

CH2 CH2 C

O CH2 CH3

Task 5-9

Interpreting 1H nmr spectra

L.O.:Intrepet 1H nmr spectra using the n+1 rule

WHAT IS IT!WHAT IS IT!

C2H5Br

C2H3Br3

C2H4Br2

C2H4O21

C4H8O2

C8H16O2

C11H16

C4H8Br2

Interpreting 1 H nmr spectra

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