Nuclei with zero I can be regarded as non-spin spheres, and nuclei with I 1/2 can be regarded as spin spheres with uniform charge distribution, and I 1H, 13C, 15N, 19F, 3 1P are all/kloc. Nuclei with I greater than 1/2 can be regarded as spin ellipsoids with uneven charge distribution.
Nuclei are positively charged particles. Nuclei that cannot spin have no magnetic moment. The spinning nucleus has a circulating current, which will generate a magnetic field and form a magnetic moment (μ).
μ=γP
In the formula, p is the angular momentum and γ is the magnetic spin ratio, that is, the ratio of the magnetic moment of the spin nucleus to the angular momentum.
When the spin nucleus is in an external magnetic field with a magnetic field strength of B0, it will move around B0 in addition to the spin, which is very similar to the movement of the gyro, and is called Lamor precession, as shown in Figure 8- 1. The angular velocity ω0 of the precession of spin nuclei is proportional to the external magnetic field B0, and the proportional constant is the magnetic rotation ratio γ. Where v0 is the precession frequency.
ω0=2πv0=γB0
The orientation of microscopic magnetic moment in external magnetic field is quantized. Under the action of external magnetic field, the nucleus with spin quantum number I can only have 2I+ 1 orientations, and each orientation can be represented by a spin quantum number m, and the relationship between m and I is as follows:
m=I，I- 1，I-2…-I
Each orientation of the nucleus represents an energy state of the nucleus in the magnetic field, and its energy can be obtained by the following formula:
The energy of nuclei arranged in the forward direction is lower, and the energy of nuclei arranged in the reverse direction is higher. The energy difference between them is △ e. The nucleus must absorb the energy of △E before it can transition from a low-energy state to a high-energy state. Let the spinning nuclei in the external magnetic field receive electromagnetic radiation of a certain frequency. When the radiated energy is exactly equal to the energy difference between the two different orientations of the spin nucleus, the spin nucleus in the low energy state absorbs the electromagnetic radiation energy and transitions to the high energy state. This phenomenon is called nuclear magnetic resonance (NMR)
At present, 1H NMR * * * vibration is the most studied, and 13C NMR * * * vibration has also developed greatly in recent years. The nuclear magnetic resonance of 1H is called proton magnetic resonance, also referred to as 1H-NMR. 13C nuclear magnetic resonance (carbon-13 nuclear magnetic resonance) is abbreviated as CMR, and also expressed as 13C-NMR.