NMR Spectra Analysis

Once the NMR digital console has completed a compound's spectrum, it is sent to a desktop computer for analysis by the NMR operator. NMR spectra usually are displayed like those below for methyl acetate, with the applied field strength increasing from left to right and the intensity of absorption plotted on the vertical axis:

Spin Splitting

methyl acetate

Internal Standard

In order that we get consistent positions for absorptions we add an internal standard, usually TMS (tetramethylsilane, (CH3)4Si). The protons in this compound are all identical and are assigned a chemical shift of zero. It is selected for both 1H and 13C spectra because its single peak occurs upfield of almost all other absorptions.

Tetramethyl silane,TMS

 

 

 

 

Chemical Shift

The chemical shift is the name given to the position in the spectrum at which an absorption occurs. The horizontal scale is measured in delta or parts per million (ppm), 1 ppm is 1 part per million of the spectrometer frequency. For example, if we used a 200 MHz spectrometer, each ppm would be equal to a separation of 200 Hz. The reason for this is that there are many different spectrometers available, operating at many different frequencies, and so to eliminate any variability from one instrument to another this system is used. So what causes chemical shift? A spectrum contains one chemical shift peak for each unique carbon atom in 13C NMR, or one peak for each set of equivalent protons in 1H NMR. The position of one peak relative to another in proton NMRis dictated by the electron density around the protons. The more electron density near a proton, the more shielding of the external magnetic field that can occur, which causes the peak to appear further upfield (to the right) in the spectrum. If the absorbing nucleus is neighboring a highly electronegative element or an electron withdrawing group such as a carbonyl group, the electron density around the nucleus is diminished and the peak is brought into resonance further downfield ( to the left).

 

Spin-Spin Splitting

Spin-spin splitting generally occurs in 1H NMR spectra and appears in the form of an absorption with multiple peaks. This is caused by the coupling of neighboring nuclear spins. Only neighboring non-equivalent protons can cause splitting. The number of neighboring non-equivalent protons plus one (n+1) determines the number of peaks present in the multiplet. The distance between individual peaks in a multiplet is called the coupling constant (J), measured in hertz. They fall in the range 0-18 Hz. Typically, the coupling constant for two groups of protons in an open-chain, alkyl system is 6-8 Hz. Often, NMR spectra contain many multiplets, so coupling constants can help determine which multiplets are related, because the coupling constant for two adjacent groups of protons is exactly the same. The table below shows some common coupling patterns:

 

Number of equivalent adjacent protons
Type of multiplet observed
0
Singlet
1
Doublet
2
Triplet
3
Quartet
4
Quintet
5
Sextet
6
Septet