Spectral analysis is a method that employs modern physical techniques such as ultraviolet spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy (including proton NMR and carbon NMR), and mass spectrometry to study the chemical structure of organic compounds. Different testing methods yield distinct structural information and play varying roles in structural elucidation, which can complement each other. Among them, proton nuclear magnetic resonance (¹H NMR) spectroscopy is used to obtain the number and types of hydrogen atoms in the structure of organic compounds. For instance, the type of each group of hydrogen nuclei can be determined from the chemical shift value (δ value). ### Principles of Proton Nuclear Magnetic Resonance (¹H NMR) Spectroscopy Proton nuclear magnetic resonance spectroscopy is a type of magnetic resonance spectroscopy imaging, which applies the nuclear magnetic resonance effect of hydrogen-1 in molecules to nuclear magnetic resonance spectroscopy. The principle of ¹H NMR spectroscopy is as follows: Hydrogen nuclei are magnetic. When a compound containing hydrogen is irradiated with electromagnetic waves, the hydrogen nuclei will absorb the energy of electromagnetic waves at specific frequencies, generating a nuclear magnetic resonance phenomenon. A nuclear magnetic resonance spectrometer can record relevant information. Hydrogen atoms in different chemical environments absorb electromagnetic waves at different frequencies when resonance occurs, exhibiting different chemical shifts (denoted by δ). Moreover, the area of the absorption peak is proportional to the number of hydrogen atoms. Thus, from the ¹H NMR spectrum, we can analyze the number of types of hydrogen atoms in different chemical environments in an organic molecule and their relative quantities. That is, the number of peak groups corresponds to the number of types of hydrogen atoms in different chemical environments in the organic compound, and the ratio of peak areas is equal to the ratio of the number of hydrogen atoms. The following is the ¹H NMR spectrum of organic compound A. There are 3 groups of absorption peaks in the spectrum, indicating that there are 3 types of hydrogen atoms in different chemical environments in molecule A. The peak area ratio is 3:2:1, so the ratio of the numbers of the 3 types of hydrogen atoms is 3:2:1. Combined with the possible structure of A, it can be known that the structural formula of the organic compound should be CH₃CH₂OH (ethanol), while CH₃OCH₃ (dimethyl ether) has only one type of hydrogen atom in a single chemical environment, so its spectrum has only one group of peaks. ### Factors Affecting Chemical Shift in ¹H NMR Spectroscopy The factors affecting the chemical shift in ¹H NMR spectroscopy include: the electronegativity of substituents, the s-p hybridization of the connected carbon atoms, the ring current effect of cyclic conjugated systems, the magnetic anisotropy of adjacent bonds, the influence of electric dipoles and van der Waals forces of adjacent groups, the influence of the medium, and the influence of hydrogen bonds. ### What Do the Number of Peak Groups in ¹H NMR Spectroscopy Represent? The number of peak groups in ¹H NMR spectroscopy represents the number of types of hydrogen atoms in different chemical environments. The area of each group of peaks in the resonance spectrum, indicated by an integral curve or printed values, is proportional to the number of corresponding protons. The sample tubes for nuclear magnetic resonance are special ones made of high-quality heat-resistant glass, requiring no magnetism, straight tube walls, and uniform thickness. Source: Compiled from online sources such as Chemistry Komachi