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Figure 1. Hydrogen nuclear spin in no external magnetic field (left) and an applied magnetic field (right). 重点+难点
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 1.41 Tesla is about 2.39 x 10-5 KJ/mole. Radiation with a frequency of about 60 MHz, which lies in the radiofrequency region of the electromagnetic spectrum, corresponds to this energy difference.
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Figure 2. Energy differences between hydrogen nuclear spin states.
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 60 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
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observable and no signal is produced. This situation is called saturation. 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 = 1.41 Tesla).
Magnetogyric Isotope Ratio γ (radians/Tesla)
1 2
# of Spin States
2 3 2 3 6
Resonance Frequency ν (MHz)
60.0 9.2 15.1 4.3 8.1 56.4 24.3 5.9
H H C
267.53 41.1 67.28
13 14 17
N
O F 251.7 108.3
19 31 35
2 2 4
P
Cl
3.2 The chemical shift of
Resonance Frequency

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