¹H Nuclear Magnetic Resonance Spectroscopy Tutorial

Key Concepts

• 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:

1. 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
2. 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.
3. 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.
4. 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	0 + 1 = 1	singlet
   1	1 + 1 = 2	doublet
   2	2 + 1 = 3	triplet
   3	3 + 1 = 4	quartet
   4	4 + 1 = 5	quintet

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Example: Number of Signals

1. Draw the structural formula for ethanol, C₂H₅OH.
   

   	H |		H |
   H-	C	-	C	-O-H
   	| H		| H

2. Identify the groups of chemically different protons.
   

   H |   H |   H- C - C -O-H   | H   | H

   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

3. Predict the number of signals

   Number of signals = number of chemically different groups of protons = 3 signals

Example: Signal Area

signal area is proportional to the number of protons in each chemically different group.

The area under the CH₃ signal will be 3 times greater than the area under the OH signal and ³/₂ times greater than the area under the CH₂ signal.

In order of increasing area under the signal:

OH < CH₂ < CH₃

Example: Chemical Shift

Proton Type	δ (ppm)	τ (ppm)
R-CH3	0.9	9.1
R-CH2-R	1.3	8.7
R3-CH	2.0	8.0
C=C-H	4.6 - 6.0	4.0 - 5.4
C≡C-H	2 - 3	7 - 8
C=C-CH2-	1.7 - 2.2	7.8 - 8.3
C-OH	1 - 6	4 - 9
R-CH2-OH	3.6	6.4
H-C=O	9 - 10	0 - 1
-COOH	10.5 - 12	-2 - -0.5
R-NH2	1- 5	4 - 9
H	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.

Example: Magnetic Coupling (Spin-Spin Splitting)

CH₃ group is adjacent to a CH₂ group so the signal for CH₃ will be split into 2 + 1 = 3 peaks (a triplet).

CH₂ group is adjacent to a CH₃ and an OH group, 4 protons in total, so the signal will be split into 4 + 1 = 5 peaks (a quintet).

OH group is adjacent to a CH₂ group so will be split into 2 + 1 = 3 peaks (a triplet).

Example: ¹H NMR Spectrum of Ethanol