The nuclear magnetic vibration is mainly caused by the spin motion of the nucleus. Different nuclei have different spin motions, which can be expressed by the spin quantum number I of the nucleus. The spin quantum number has a certain relationship with the atomic mass number and atomic number, which can be roughly divided into three situations. The spin NMR (nuclear magnetic resonance) of the nuclear of hydrogen spectrum is encoded. 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. Nuclear magnetic resonance phenomenon The nucleus is a positively charged particle. Nuclei that can't spin have no magnetic moment, while nuclei that can spin have circulation, which will generate magnetic field and form magnetic moment (μ). 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 intensity of H0, it will move around H0 in addition to the spin, which is very similar to the movement of the gyro and is called precession, as shown in Figure 8- 1. The angular velocity ω0 of the precession of spin nuclei is directly proportional to the external magnetic field intensity H0, and the proportional constant is the magnetic rotation ratio γ. Where v0 is the precession frequency. 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 expressed by a spin quantum number m, and the relationship between m and I is: m=I, I- 1, I-2…-I, and each orientation of the nucleus represents the nucleus in this field. 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. The nuclear magnetic resonance of 1H: the spin quantum number of 1H is I= 1/2, so the spin quantum number m = 1/2, that is, the hydrogen nucleus should have two orientations in the external magnetic field. See Figure 8-2. The two orientations of 1H represent two different energy levels. Therefore, the condition that NMR vibrates at 1H is that the radiation frequency of electromagnetic wave must be equal to the precession frequency of 1H, that is, the following formula is satisfied. Is the radiation energy absorbed by the nucleus large? Equation (8-6) shows that there are two ways to make V-ray =v0. One is to fix the magnetic field intensity H0 and gradually change the radiation frequency V of electromagnetic waves to scan. When V matches H0, nuclear magnetic resonance will occur. Another method is to fix the radiation frequency V-ray of radiation wave, and then gradually change the magnetic field intensity H0 from low field to high field. When H0 matches V-rays, nuclear magnetic resonance will also occur. This method is called field scanning. The general instrument adopts the method of sweeping the field. Under the action of external magnetic field, 1H tends to align positively with the external magnetic field, so the number of nuclei in the low-energy state is more than that in the high-energy state. However, due to the small energy difference between the two energy levels, the former is only slightly superior to the latter. The signal of 1H-NMR is generated by these weak residual low-energy nuclei absorbing the radiation energy of RF electromagnetic waves and jumping to high energy level. If the high-energy nucleus cannot return to the low-energy state, this weak advantage will be further weakened until it disappears with the continuous transition. At this time, the number of 1H nuclei in low energy state is equal to the number of 1H nuclei in high energy state, and the signal of PMR will gradually weaken until it finally disappears. This phenomenon is called saturation. 1H nucleus can be transformed from high energy state to low energy state by non-radiation method. This process is called relaxation, so there will be no saturation under normal test conditions. There are two ways to relax. The nucleus in the high energy state transfers energy to the surrounding molecules through the alternating magnetic field, that is, the system releases energy to the environment and returns to the low energy state itself. This process is called spin lattice relaxation. Its rate is expressed by 1/T 1, and T 1 is called spin lattice relaxation time. Spin lattice relaxation reduces the total energy of the magnetic core, which is also called longitudinal relaxation. The process in which two nuclei with the same precession frequency and different precession directions interact, exchange energy and change precession direction is called spin-spin relaxation. Its rate is expressed by 1/T2, and T2 is called spin-spin relaxation time. Spin-spin relaxation does not reduce the total energy of the magnetic core, which is also called lateral relaxation.