Nuclear magnetic resonance is mainly caused by the spin movement of atomic nuclei. Different atomic nuclei have different spin motions, which can be represented by the nuclear spin quantum number I. There is a certain relationship between the spin quantum number and the mass number and atomic number of the atom, which can be roughly divided into three situations. The spin nuclear magnetic resonance of hydrogen spectrum nuclei is code-named NMR (Nuclear Magnetic Resonance). The nucleus with I of zero can be regarded as a non-spin sphere. The nucleus with I of 1/2 can be regarded as a spin sphere with uniform charge distribution. The I of 1H, 13C, 15N, 19F and 31P are all uniform. is 1/2, and their nuclei are spin spheres with uniform charge distribution. A nucleus with I greater than 1/2 can be regarded as a spin ellipsoid with uneven charge distribution. Nuclear magnetic resonance phenomenon: Atomic nuclei are positively charged particles. Nuclei that cannot spin have no magnetic moment. Nuclei that can spin have circulating current, which generates a magnetic field and forms a magnetic moment (μ). In the formula, P is the angular momentum, γ is the magnetic spin ratio, which is the ratio between the magnetic moment and the angular momentum of the spin nucleus. When the spin nucleus is in an external magnetic field with a magnetic field strength of H0, in addition to the spin, It will also move around H0. This movement is very similar to the movement of a gyroscope and is called precession. See Figure 8-1. The angular velocity ω0 of the spin core precession is proportional to the external magnetic field strength H0, and the proportionality constant is the magnetic spin ratio γ. where v0 is the precession frequency. The orientation of the microscopic magnetic moment in an external magnetic field is quantized. An atomic nucleus with a spin quantum number I can only have 2I+1 orientations under the influence of an external magnetic field. Each orientation can be represented by a spin magnetic quantum number m. The relationship between m and I is: m=I, I-1, I-2...-I. Each orientation of the atomic nucleus represents an energy state of the nucleus in the magnetic field. Its energy can be calculated from the following formula Out: The arranged nuclear energy is lower, and the reverse arranged nuclear energy is higher. The energy difference between them is ΔE. In order for a nucleus to transition from a low energy state to a high energy state, it must absorb the energy of △E. Let the spin nucleus in the external magnetic field receive electromagnetic wave radiation of a certain frequency. When the energy of the radiation 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 jumps to the high energy state. This phenomenon is called nuclear magnetic resonance, or NMR for short. At present, the most studied one is 1H NMR, and 13C NMR has also made great progress in recent years. 1H NMR is called Proton Magnetic Resonance, or PMR for short, also expressed as 1H-NMR. 13C Nuclear Magnetic Resonance (Carbon-13 Nuclear Magnetic Resonance) is referred to as CMR, also expressed as 13C-NMR. 1H NMR: The spin quantum number of 1H is I=1/2, so the spin magnetic quantum number m=±1/2, that is, the hydrogen atom 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 for the occurrence of nuclear magnetic resonance in 1H is that the radiation frequency of the electromagnetic wave must be equal to the precession frequency of 1H, which conforms to the following formula. How much radiation energy does the nucleus absorb? Equation (8-6) shows that to make v = v0, two methods can be used. One is to fix the magnetic field intensity H0, gradually change the radiation frequency of the electromagnetic wave, and scan. When the v-radiation matches H0, nuclear magnetic resonance occurs. Another method is to fix the radiation frequency of the radiation wave, v-radiation, and then gradually change the magnetic field intensity H0 from low field to high field. When H0 matches v-radiation, NMR will also occur. This method is called sweep. Generally, instruments use the field sweeping method. Under the influence of an external magnetic field, 1H tends to be aligned in the same direction as the external magnetic field, so the number of nuclei in a low-energy state is greater than that in a high-energy state. However, since the energy difference between the two energy levels is very small, the former is smaller than the latter. only have a slight advantage. The 1H-NMR signal is generated by the absorption of the radiant energy of radio frequency electromagnetic waves by these weak excess low-energy nuclei and their transition to high energy levels. If the high-energy state nuclei cannot return to the low-energy state, then as the transition continues, this weak advantage will further weaken until it disappears. At this time, the number of 1H nuclei in the low-energy state is equal to the number of 1H nuclei in the high-energy state. At the same time, , the PMR signal will gradually weaken and eventually disappear. This phenomenon is called saturation.
The 1H nucleus can transform from a high energy state to a low energy state in a non-radiative manner, a process called relaxation. Therefore, saturation does not occur under normal testing conditions. There are two ways of relaxation. The nucleus in a high-energy state transfers energy to the surrounding molecules through alternating magnetic fields. That is, the system releases energy to the environment and returns to a low-energy state. This process is called spin lattice relaxation. Its rate is expressed as 1/T1, and T1 is called the spin lattice relaxation time. Spin lattice relaxation reduces the overall energy of the magnetic core and is also called longitudinal relaxation. The process in which two nuclei with the same precession frequency and different precession orientations within a certain distance interact with each other to exchange energy and change the direction of precession is called spin-spin relaxation. Its rate is represented by 1/T2, and T2 is called spin-spin relaxation time. Spin-spin relaxation does not reduce the overall energy of the magnetic core and is also called transverse relaxation.