nuclear magnetic resonance

Nuclear Magnetic Resonance (NMR) can observe the splitting of the spin energy levels of overlapping atomic nuclei in the external magnetic field and the energy transition caused by nuclear spin resonance absorption in the RF frequency range.

Zeeman found at the end of the 19th century that the spectral lines would be further split in the external magnetic field; In 1924, Pauli proposed that charged particles have spin angular momentum, which can induce magnetic moments; Zaviosky used CrCl in 1944 and observed that

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Electron paramagnetic resonance; Block and Purcell first observed NMR signals in 1946; The spin property proposed by Pauli in theory was later proved to be a quantum property under Dirac quantization.

The energy level splitting caused by the reaction between the external magnetic field and the nuclear magnetic moment is a Zeeman effect, which is proportional to the intensity of the external magnetic field. The final NMR energy level is determined by the interaction between smaller magnetic moments, which provides detailed information about the molecular structure.

The resonance frequency detected by NMR spectrum is a collection of interactions between magnetic moments, which is very sensitive to nuclear bonds and chemical structures, and gives important information about the surrounding molecular structures. Molecular dynamics information can be obtained by analyzing the relaxation changes after peak shape or resonant transition.

Magnetic moment of nucleus μ It is expressed by the following equation:

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In the formula, the angular momentum of the nucleus is proportional to the size of P γ Is the spin magnetic ratio, which has different values according to different nuclei. I is the nuclear spin quantum number determined by internal composition. For elements with odd atomic number and even atomic weight, such as 2D (I=1) and 6Li (I=1), the value of I is taken as integer 1, 2 and 3; For elements with odd atomic weights, such as 1H (1=1/2), 13C (I=1/2), 7Li (I=3/2), I takes half integer values 1/2, 3/2, etc. The z-axis component can be taken as – 1, – I+1,…, 1-1, I, quantized in the (21+1) direction.

Assuming that the z axis is the direction of the magnetic field, the Zeeman effect can be expressed as follows [Eq. (4.74)]. Where, m1 represents the z-axis component of the nuclear spin quantum number [25, 26]. Zeeman effect is the interaction between nuclear magnetic moment and external magnetic field, which varies with the type of nucleus. The magnetic moments can be arranged in ascending order.

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The interaction between nuclear spins is very complex, which can provide a lot of information about the internal structure of molecules [Eq. (4.75)]. A basic understanding of these effects is required to accurately interpret the results of NMR spectroscopy.

The interaction mentioned above enables HZeeman to reach hundreds of megahertz, but the interaction is not caused by magnetic moment. HQuadrupolar must be considered when interpreting the results of solid-state NMR analysis, because it has an impact on the NMR resonance frequency. When the nuclear spin quantum number is greater than 1, the nucleus shows an asymmetric structure. Charged non spherical nuclei are generally in the range of several megahertz due to the magnetic field, the quadrupole interaction in the solid sample lattice and the nuclear orientation. These interactions must be considered when analyzing samples using 6Li (I=1) and 7Li (I=3/2) NMR. HFermi contact is the interaction of electron spin and nuclear spin existing in the nucleus, which appears in paramagnetic materials. HDipolar is a dipole interaction between nuclear spin and adjacent nuclear spin, which is usually as high as tens of kilohertz and directional. HCSA is an interaction involving chemical shifts, which are commonly used in NMR spectroscopy. It indicates the tendency of the electron cloud to shield the external magnetic field, and the value is directional and varies with the orientation and structure of the electron cloud. HJ coupling is the direct and indirect interaction between two nuclear spins generated by electron pairs. Unlike dipole interaction, J-coupling has no directivity and only hundreds of Hertz, which is generally considered unimportant in solid-state NMR experiments, but can provide useful information about the connectivity of neighboring atoms.

In liquid samples with high molecular activity, directional spin interactions cancel each other, and the details of molecular orientation are lost, but high-resolution analysis can be performed. In solid samples, the interaction accumulates with the change of molecular orientation, and a wide peak up to hundreds of kilohertz can be observed. These peaks contain a lot of information, but the analysis process is too complex.

When the angle between the orientation direction and the magnetic field is θ, Interaction between HCSA and HDipolar and (1-3cos2 θ) Is proportional. In liquid state, due to the remarkable Brownian motion of molecules, it can usually be averaged and eliminated to obtain high-resolution results. For the powder morphology, the powder particles are uniformly distributed along all directions, and the nuclear spin interaction forms a wide distribution. By analyzing the relationship between 0 and interaction order of magnitude, at magic angle 54.74 °, (1-3cos2 θ) Is eliminated. This can be confirmed by setting the angle to 54.74 ° and then performing Magic Angle Spinning (MAS). Even in solid samples, high resolution results can be obtained by counteracting the directional nuclear spin interaction. High resolution solid-state NMR spectroscopy has some advantages, but it will lose the detailed information about the nuclear spin interaction. Since there are many selective detection and analysis methods for nuclear spin interaction, it is important to choose an appropriate method based on the sample and the required information.

The NMR of lithium battery involves the analysis of the structure and composition of different components, such as electrolyte, binder and electrode materials. It can also observe the structural changes of electrode components during charging and discharging. Figure 4.68 shows LiNi0.8

NMR spectra of different lithium removal amount during Co0.15Al0.05O2 charging [27].

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