Chapter 13: Spectroscopy

Interpretting ¹H-NMR Spectra

Let's summarise what can be obtained from a ¹H NMR spectrum:
 

How many types of H ?	Indicated by how many groups of signals there are in the spectra
What types of H ?	Indicated by the chemical shift of each group
How many H of each type are there?	Indicated by the integration (relative area) of the signal for each group.
What is the connectivity ?	Look at the coupling patterns. This tells you what is next to each group

Chemical shift

• The chemical shift is the position on the d scale (in ppm) where the peak occurs.
• Typical d /ppm values for protons in different chemical environments are shown in the figure below.
• There are two major factors that influence chemical shifts (a) deshielding due to reduced electron density (due electronegative atoms) and (b) anisotropy (due to magnetic fields generated by π bonds).

Note that the figure shows the typical chemical shifts for protons being influenced by a single group. In cases where a proton is influenced by more than one group, the effects are essentially cumulative.

Integration

• The area of a peak is proportional to the number of H that the peak represents
• The integral measures the area of the peak
• The integral gives the relative ratio of the number of H for each peak
• Don't under estimate what the integral can tell you!
• Summing the integrals can give you the empirical number of H and can be related to the molecular formula

• The proximity of other "n" H atoms on neighbouring carbon atoms, causes the signals to be split into "n+1" lines.
• This is also known as the multiplicity or splitting of each signal.

An example of an H NMR is shown below.

Based on the outline given above the four sets of information we get are:

5 basic types of H present in the ratio of 5 : 2 : 2 : 2 : 3.
These are seen as a 5H "singlet" (ArH), two 2H triplets, a 2H quartet and a 3H triplet. Each triplet tells us that there are 2H in the adjacent position, and a quartet tells us that there are 3H adjacent.
(Think of it as the lines you see, L = n + 1, where n = number of equivalent adjacent H)
This tells us we that the peaks at 4.4 and 2.8 ppm must be connected as a CH₂CH₂ unit.
The peaks at 2.1 and 0.9 ppm as a CH₂CH₃ unit. Using the chemical shift charts, the H can be assigned to the peaks as below:

© Dr. Ian Hunt, Department of Chemistry