*4.3.4.2 Proton nuclear magnetic resonance spectroscopy (1H-NMR)*

Resonance absorption peaks are generated after hydrogen protons absorb electromagnetic waves of different frequencies in an external magnetic field. 1 H-NMR possesses high sensitivity, easy measurement, and wide application. <sup>1</sup> H-NMR spectrum can provide structural information of chemical shifts (*δ)*, coupling constants (*J*) that indicate the coupling relationships between different hydrogen nucleus, and the number of protons (the peak area is proportional to the number of protons that cause the absorption).

Because of the different surrounding chemical environment, the 1 H nuclei possess different magnetic cloud densities and magnetic shielding effects caused by the rotation around the nucleus, and then different types of 1 H nuclear resonance signals appear in different regions. Tetramethylsilane (TMS) is usually used as a reference compound. Compared with the general compounds, the shielding effect of protons and carbons on the methyl groups is stronger in TMS. Therefore, regardless of the hydrogen spectrum


**Table 1.**

*Chemical shifts of common deuterated solvents (TMS is an internal standard).*

**Figure 3.** *1 H-NMR chemical shift range of common hydrogen protons.*

#### **Figure 4.**

*13C-NMR chemical shifts of common carbon signals.*

or the carbon spectrum, the absorption peaks generated by the general compounds appear in the lower field than TMS, that is to say, *δ* values generated by common compounds is positive. The chemical shifts of the 1 H-NMR spectrum is mostly in the range of *δ*0–20. Some typical chemical shifts of 1 H nuclei are shown in **Figure 3** [4].

In addition to the normal 1 H-NMR spectrum technique, there are some auxiliary techniques that assist in structural analysis, such as selective decoupling, heavy hydrogen exchange, addition of reaction reagents, and dual irradiations.
