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’
-COOH
-CHO
-C=CH-
ROH
- C - XH
- C - H
TMS
‘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)
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
2 adjacent H’smore possible combinationsget 3 peaks in the ratio 1 : 2 : 1
MULTIPLICITY (Spin-spin splitting)MULTIPLICITY (Spin-spin splitting)
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
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
MULTIPLICITY (Spin-spin splitting)MULTIPLICITY (Spin-spin splitting)
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
MULTIPLICITY (Spin-spin splitting)MULTIPLICITY (Spin-spin splitting)
• 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
CH3 C
O
CH2 CH2 C
O
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
2
3
WHAT IS IT!WHAT IS IT!
C2H3Br3
1
2
WHAT IS IT!WHAT IS IT!
C2H4Br2
1
3
WHAT IS IT!WHAT IS IT!
C2H4O21
1
3
WHAT IS IT!WHAT IS IT!
C4H8O2
2
33
WHAT IS IT!WHAT IS IT!
C3H6O
WHAT IS IT!WHAT IS IT!
C3H6O
1
32
WHAT IS IT!WHAT IS IT!
C4H8O
2
3
3
WHAT IS IT!WHAT IS IT!
C8H16O2
WHAT IS IT!WHAT IS IT!
C11H16
WHAT IS IT!WHAT IS IT!
C8H10
WHAT IS IT!WHAT IS IT!
C8H10
2 3
WHAT IS IT!WHAT IS IT!
C9H12
WHAT IS IT!WHAT IS IT!
C4H8Br2