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To analyze an infrared spectrum that has already been obtained, one can follow these steps: (1) First, determine the type of carbon skeleton of the compound based on the spectrum. Calculate the degree of unsaturation using the formula: Degree of unsaturation = F + 1 + (T – O)/2, where F is the number of atoms with a valence of 4 (mainly carbon atoms), T is the number of atoms with a valence of 3 (mainly nitrogen atoms), and O is the number of atoms with a valence of 1 (mainly hydrogen atoms). For example, in benzene (C6H6), the degree of unsaturation = 6 + 1 + (0 – 6)/2 = 4; this corresponds to 3 double bonds plus one ring, for a total of 4 degrees of unsaturation； (2) Analysis of C-H stretching vibration absorptions in the 3300–2800 cm-1 region ； Using 3000 cm-1 as a boundary: values above 3000 cm-1 correspond to absorption due to the stretching vibrations of unsaturated C-H bonds, and these may indicate the presence of alkenes, alkynes, or aromatic compounds; whereas values below 3000 cm-1 generally represent absorption from saturated C-H stretching vibrations ； (3) If absorption is present at a value slightly above 3000 cm-1, then the absorption features of the stretching vibrations of unsaturated carbon-carbon bonds should be analyzed in the frequency range of 2250–1450 cm-1. Specifically: alkynes are detected at 2200–2100 cm-1, alkenes at 1680–1640 cm-1, and aromatic compounds at 1600, 1580, 1500, and 1450 cm-1. If it has been determined that the compound is an alkene or an aromatic compound, then the fingerprint region, namely the frequency range of 1000–650 cm-1, should be analyzed further to determine the number and position of substituents (cis/trans, ortho, meta, para) ； (4) After the type of carbon skeleton is determined, the functional groups of the compound are identified based on other functional groups, such as the characteristic absorptions of C=O, O-H, C-N, etc ； (5) During analysis, it is important to relate the peaks corresponding to various functional groups in order to accurately determine their presence. For example, the three peaks at 2820, 2720, and 1750–1700 cm-1 indicate the presence of an aldehyde group. So far, the analysis is basically complete; all that’s left is to memorize some common and frequently used key values! 1. Alkanes: C-H stretching vibration (3000–2850 cm-1), C-H bending vibration (1465–1340 cm-1). In general, the C-H stretching vibrations of saturated hydrocarbons are below 3000 cm-1, with absorption frequencies approaching 3000 cm-1. 2. Olefins: Olefin C-H stretching (3100~3010 cm-1), C=C stretching (1675~1640 cm-1), olefin C-H out-of-plane bending vibration (1000~675 cm-1). 3. Alkynes: Stretching vibration (2250–2100 cm-1); C-H stretching vibration of alkynes (around 3300 cm-1). 4. Aromatic hydrocarbons: 3100–3000 cm-1 for C-H stretching vibrations in the aromatic ring; 1600–1450 cm-1 for C=C backbone vibrations; 880–680 cm-1 for out-of-plane C-H bending vibrations. An important characteristic of aromatic compounds is the presence of four peaks at wavelengths around 1600, 1580, 1500, and 1450 cm-1, with varying intensities. 880–680 cm-1: absorption due to out-of-plane bending vibrations of C-H bonds; this value varies depending on the number and position of substituents on the benzene ring. In the infrared spectroscopy analysis of aromatic compounds, the absorption in this frequency range is often used to identify isomers. 5. Alcohols and phenols: The main characteristic absorption bands are those corresponding to the stretching vibrations of O-H and C-O bonds. For free hydroxyl groups, the stretching vibration of O-H occurs at 3650–3600 cm-1, resulting in a sharp absorption peak; whereas for intermolecular hydrogen bonds, the O-H stretching vibration takes place at 3500–3200 cm-1, giving rise to a broad absorption peak ； C-O stretching vibration: 1300–1000 cm-1; O-H out-of-plane bending: 769–659 cm-1. 6. Characteristic absorptions of ethers: Stretching vibrations in the range of 1300–1000 cm-1. Aliphatic ethers: 1150–1060 cm-1, with one strong absorption peak. Aromatic ethers: Two C-O stretching vibration absorptions: 1270–1230 cm-1 (for Ar-O stretching) and 1050–1000 cm-1 (for R-O stretching). 7. Aldehydes and ketones: The main characteristic absorption of aldehydes is at 1750–1700 cm-1 (C=O stretching); additional absorptions occur at 2820 and 2720 cm-1 (C-H stretching of the aldehyde group). For ketones, the absorption is at 1715 cm-1, with a strong C=O stretching vibration; if the carbonyl group is conjugated with an alkene or aromatic ring, the absorption frequency decreases. 8. Carboxylic acids: Dimeric carboxylic acids show broad, strong O-H stretching absorptions in the range of 3300–2500 cm-1. C=O absorption occurs at 1720–1706 cm-1, while C-O stretching vibrations are observed at 1320–1210 cm-1. Out-of-plane bending vibration of the bonded O-H bond occurs at 920 cm-1. 9. Esters: The C=O absorption band for saturated aliphatic esters (except formates) is in the range of 1750–1735 cm-1. The C-C(=O)-O band for saturated esters shows strong absorption in the range of 1210–1163 cm-1. 10. Amines: N-H stretching vibration absorptions occur in the range of 3500–3100 cm-1. C-N stretching vibration absorptions are seen in the range of 1350–1000 cm-1. The deformation vibration of N-H is similar to the scissoring vibration of CH2; its absorption band is in the range of 1640–1560 cm-1, while out-of-plane bending vibrations occur at 900–650 cm-1. 11. Nitriles: The spectral characteristics of nitriles include absorption in the region corresponding to triple bond stretching, with weak to moderate intensity. For aliphatic nitriles, the absorption is at 2260–2240 cm-1; for aromatic nitriles, it’s at 2240–2222 cm-1. 12. Amides: N-H stretching vibration occurs at 3500–3100 cm-1. C=O stretching vibration is observed at 1680–1630 cm-1. N-H bending vibration is seen at 1655–1590 cm-1. C-N stretching vibration occurs at 1420–1400 cm-1. 13. Organic halides: C-X stretching vibrations: For aliphatic compounds, C-F has an absorption range of 1400–730 cm-1; C-Cl ranges from 850–550 cm-1; C-Br ranges from 690–515 cm-1; C-I ranges from 600–500 cm-1