Nuclear Magnetic Resonance (NMR) Spectroscopy involves the absorption of radio frequency radiation by atomic nuclei in an applied magnetic field. Any atomic nucleus which possesses either odd mass, odd atomic number, or both has spin angular momentum and a magnetic moment.
The nuclear spin quantum number, I, determines the allowed spin states for a nucleus:
allowed spin states = 2I + 1
For hydrogen (I=½), there are two spin states (2(½) + 1 = 2) for its nucleus: clockwise (+½) or counterclockwise (-½) spin. In the absence of an external magnetic field, these magnetic moments are randomly oriented in all directions. If an external magnetic field is applied, the nuclear spins of the hydrogen nuclei orient themselves either in alignment with or in opposition to the field.
Figure 1. Hydrogen nuclear spin in no external magnetic field (left) and an applied magnetic field (right).
The two spin states for hydrogen nuclei are normally degenerate. However, in the presence of a magnetic field, the two spin states become unequal in energy. Those nuclei whose spin are in alignment with the external magnetic field are lower in energy than those nuclei whose spin are in opposition to the external field.
The energy difference between the two spin states in a magnetic field of 7.0586 Tesla is about 1.20 x 10-4 kJ/mole. Radiation with a frequency of about 300 MHz, which lies in the radio frequency region of the electromagnetic spectrum, corresponds to this energy difference.
Figure 2. Energy differences between hydrogen nuclear spin states.
DE = 2.675 x 108 s-1T-1 * (6.626 x 10-34 / 2p) * 7.0586 T
DE = 1.20 x 10-4 kJ/mol
The two spin states are not equally populated; there is a small excess population in the lower energy spin state. The nuclei in the lower spin state can be excited into the upper spin state by absorption of energy of about 300 MHz (called resonance). This produces a signal which provides the NMR spectrum. If the two spin states become equally populated, then no net spin transitions are observable and no signal is produced. This situation is called saturation. This can happen if a too powerful radio frequency pulse is used.
Hydrogen nuclei are not the only spin active nuclei. The following table contains information about other spin active nuclei important to the organic chemist.
Table 1. Important values for spin active nuclei (field strength = 7.0586 Tesla).
Isotope |
Magnetogyric Ratio |
# of Spin States |
Resonance Frequency |
| 1H | 267.52 | 2 | 300.530 |
| 2H | 41.06 | 3 | 46.133 |
| 13C | 67.26 | 2 | 75.565 |
| 14N | 19.33 | 3 | 21.709 |
| 17O | -36.26 | 6 | 40.742 |
| 19F | 251.7 | 2 | 282.726 |
| 31P | 108.29 | 2 | 121.655 |
| 35Cl | 26.24 | 4 | 29.446 |
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