The magnetic nucleus undergoes spin level splitting under the action of an external magnetic field. When absorbing external electromagnetic radiation, a transition of the nuclear spin level will occur, resulting in nuclear magnetic resonance. In organic compounds, the electron cloud density around the various hydrogen nuclei in different structures and positions is different, resulting in a difference in resonance frequency, that is, a shift in the resonance absorption peak, referred to as a chemical shift.
Figure 1. Chemical shift.
Atoms (or groups) with high electronegativity will increase the chemical shift; on the contrary, the electron-donating group can increase the electron cloud density around the hydrogen nucleus, and the resonance absorption peak will move to a high field, and the chemical shift will decrease.
The influence of the hydrogen bond on the chemical shift of the hydroxyl proton is related to the strength of the hydrogen bond and the nature of the electron donor of the hydrogen bond. In most cases, the hydrogen bond produces a de-shielding effect, which shifts the δ value of 1H to a low field.
When the distance between the substituent and the resonant nucleus is less than the van der Waals radius, the electron cloud around the substituent and the electron cloud around the resonant nucleus repel each other, and the density of the electron cloud around the resonant nucleus is reduced, so that the shielding effect of protons is significantly reduced. The proton peak moves to a low field, which is called the van der Waals effect.
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