High Resolution Proton NMR
- More structural details can be deduced using high resolution NMR
- The peaks observed on a high resolution NMR may sometimes have smaller peaks clustered together
- The splitting pattern of each peak is determined by the number of protons on neighbouring environments
The number of peaks a signal splits into = n + 1
(Where n = the number of protons on the adjacent carbon atom)
High resolution 1H NMR spectrum of ethanol showing the splitting patterns of each of the 3 peaks. Using the n+1, it is possible to interpret the splitting pattern
- Each splitting pattern also gives information on relative intensities
- A doublet has an intensity ratio of 1:1 – each peak is the same intensity as the other
- In a triplet, the intensity ratio is 1:2:1 – the middle of the peak is twice the intensity of the 2 on either side
- In a quartet, the intensity ratio is 1:3:3:1 – the middle peaks are three times the intensity of the 2 outer peaks
Integrated spectra
- In 1H NMR, the relative areas under each peak give the ratio of the number of protons responsible for each peak
- The NMR spectrometer measures the area under each peak, as an integration spectra
- This provides invaluable information for identifying an unknown compound
- The 1H NMR of methyl chloroethanoate, ClCH2COOCH3, will show an integration spectra in the peak area ratio of 2:3
- 2 for the protons in the CH2
- 3 for the protons in CH3
Spin-Spin Splitting
- A 1H NMR peak can show you the structure of the molecule but also the peaks can be split into sub-peaks or splitting patterns
- These are caused by a proton's spin interacting with the spin states of nearby protons that are in different environments
- This can provide information about the number of protons bonded to adjacent carbon atoms
- The splitting of a main peak into sub-peaks is called spin-spin splitting
The n+1 rule
- The number of sub-peaks is one greater than the number of adjacent protons causing the splitting
- For a proton with n protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern = n+1
- When analysing spin-spin splitting, it shows you the number of hydrogen atoms on the immediately adjacent carbon atom
- These are the splitting patterns that you need to be able to recognise from a 1H spectra:
1H NMR Peak Splitting Patterns Table
- Splitting patterns must occur in pairs, because each protons splits the signal of the other
- There are some common splitting pairs you will see in a spectrum however you don't need to learn these but can be worked out using the n+1 rule
- You will quickly come to recognise the triplet / quartet combination for a CH3CH2 because it is so common
Common pair of splitting patterns
- A quartet and a triplet in the same spectrum usually indicate an ethyl group, CH3CH2-
- The signal from the CH3 protons is split as a triplet by having two neighbours
- The signal from the CH2 protons is split as a quartet by having three neighbours
- Here are some more common pairs of splitting patterns
Common pairs of splitting patterns
1H NMR spectrum of propane
- The CH2 signal in propane (blue) is observed as a heptet because it has six neighbouring equivalent H atoms (n+1 rule), three either side in two equivalent CH3 groups
- The CH3 groups (red) produce identical triplets by coupling with the CH2 group
Worked example
For the compound (CH3)2CHOH predict the following:
i) the number of peaks
ii) the type of proton and chemical shift (using the Data sheet)
iii) the relative peak areas
iv) the split pattern
Answers:
i) 3 peaks
ii) (CH3)2CHOH at 0.7 - 1.2 ppm, (CH3)2CHOH at 3.1 - 3.9 ppm, (CH3)2CHOH at 0.5 - 5.5 ppm
iii) Ratio 6 : 1 : 1
iv) (CH3)2CHOH split into a doublet (1+1=2), (CH3)2CHOH split into a heptet (6+1=7)