1H Nuclear Magnetic Resonance Spectroscopy Tutorial
Nuclear Magnetic Resonance Spectroscopy, or NMR Spectroscopy, can be used to identify any isotope unless the isotope has both an even number of protons and an even number of neutrons.
The nuclei of many elements, such as 1H, spin generating a magnetic field.
When placed in a strong external field they align themselves either with or against the field.
When the sample is irradiated with radio waves the energy absorbed corresponds to the difference between these two magnetic alignments.
1H NMR Spectroscopy (proton nuclear magnetic resonance spectroscopy) is used to identify the structure of organic (carbon) compounds.
1H NMR spectra provide information about:
The Number of Signals: each chemically different proton in a structure is also magnetically different.
CH3 groups are chemically different to CH2 groups and to CH groups.
eg, CH3CH2CH2Cl contains 3 chemically different groups of protons:
CH3 CH2 adjacent to CH3 CH2 covalently bonded to Cl
The Relative Areas of Each Signal: the strength of the NMR signal is proportional to the number of protons giving rise to that signal, or, the area under the absorption curve is proportional to the number of protons.
The Position of the Signal with respect to an internal standard (chemical shift): tetramethylsilane, (CH3)4S or TMS, is often used as an internal standard since almost all proton signals appear downfield from the TMS signal.
On the δ scale, the TMS signal is 0 ppm, and on the τ scale, the TMS signal is 10 ppm. The two scales are related in that δ + τ = 10.
Tables of chemical shifts are derived from measurements of a large number of samples and represent a "normal" range.
Proton signals affected by highly electronegative elements are shifted further downfield.
The Influence of One Proton on Adjacent Protons (magnetic coupling or spin-spin splitting): neighbouring protons in chemically different groups may either reinforce or subtract from the effective force of the applied field, the net effect of this is to split the proton signal.
The proton signal will be split into n + 1 peaks by n adjacent protons.
Number of Adjacent Chemically Different Protons
Number of Peaks in a Signal n + 1
Name Given to Signal
0 + 1 = 1
1 + 1 = 2
2 + 1 = 3
3 + 1 = 4
4 + 1 = 5
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Example: Number of Signals
How many signals are predicted to be in the 1H NMR spectrum of ethanol?
Draw the structural formula for ethanol, C2H5OH.
Identify the groups of chemically different protons.
There are 3 groups of protons:
the protons in the CH3 group
the protons in the CH2 group
the proton in the O-H group
Predict the number of signals
Number of signals = number of chemically different groups of protons = 3 signals
Using the data provided below, predict the positions of each of the signals in ethanol, CH3-CH2-OH.
4.6 - 6.0
4.0 - 5.4
2 - 3
7 - 8
1.7 - 2.2
7.8 - 8.3
1 - 6
4 - 9
9 - 10
0 - 1
10.5 - 12
-2 - -0.5
4 - 9
6 - 8.5
1.5 - 8.5
For ethanol, CH3CH2OH, we expect 3 proton signals corresponding to each of the 3 chemically different groups of protons.
Using the table of δ values above, we can get some idea of the likely chemical shifts for each proton type, keeping in mind that the chemical shifts of protons adjacent to the electronegative oxygen atom will be shifted downfield.