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Experience in interpreting infrared spectra requires memorizing the characteristic absorptions of various functional groups, as these characteristic absorptions form the basis for analyzing spectra. For an infrared spectrum that has already been obtained: (1) First, determine the type of carbon skeleton of the compound based on the spectrum. The degree of unsaturation is calculated using the formula: Degree of unsaturation = F + 1 + (T – O)/2, where F represents the number of atoms with a valence of 4 (mainly carbon atoms), T represents the number of atoms with a valence of 3 (mainly nitrogen atoms), and O represents 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 the threshold: Absorptions above 3000 cm^-1 indicate unsaturated C-H stretching vibrations; these could correspond to alkenes, alkynes, or aromatic compounds, while absorptions below 3000 cm^-1 generally represent 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 between 2200–2100 cm^-1, alkenes between 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, further analysis of the fingerprint region, namely the frequency range of 1000–650 cm^-1, should be carried out 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 connect the peaks related 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. The analysis process is basically like this. As for sample preparation and the use of infrared spectroscopy software, detailed explanations can be found in standard organic chemistry textbooks, so I won’t go into further detail here. This is a troublesome issue; just jot down one or two points whenever you can: 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 vibrations (2250–2100 cm^-1); C-H stretching vibration of alkynes (around 3300 cm^-1). 4. Aromatic hydrocarbons: 3100–3000 cm^-1 for the C-H stretching vibrations of the aromatic ring; 1600–1450 cm^-1 for the C=C framework vibrations; 880–680 cm^-1 for the out-of-plane bending vibrations of C-H bonds. An important characteristic of aromatic compounds is that four peaks with varying intensities typically appear at 1600, 1580, 1500, and 1450 cm^-1. 880–680 cm^-1: absorption due to out-of-plane bending vibrations of C-H bonds, which varies depending on the number and position of substituents on the benzene ring. In the infrared spectroscopy analysis of aromatic compounds, the absorptions in this frequency range are often used to identify isomers. 5. Alcohols and phenols: The main characteristic absorption bands are those resulting from 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, presenting as a sharp absorption peak; whereas for intermolecular hydrogen bonds, the O-H stretching vibration takes place at 3500–3200 cm^-1, resulting in a broad absorption peak ； C-O stretching vibration: 1300–1000 cm^-1; O-H out-of-plane bending: 769–659 cm^-1. 6. Ethers: Characteristic absorptions include stretching vibrations in the range of 1300–1000 cm^-1; for aliphatic ethers, this range is 1150–1060 cm^-1, with a strong absorption peak. For aromatic ethers, there are two C-O stretching vibration absorptions: at 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 for aldehydes is at 1750–1700 cm^-1 (C=O stretching); there are also absorptions at 2820 and 2720 cm^-1 (C-H stretching of the aldehyde group). For ketones, the absorption is at 1715 cm^-1, due to strong C=O stretching vibrations; if the carbonyl group is conjugated with a double bond or an aromatic ring, the absorption frequency decreases. 8. Carboxylic acids: For carboxylic acid dimers, there are wide, strong O-H stretching absorptions in the range of 3300–2500 cm^-1. There are also C=O absorptions at 1720–1706 cm^-1, and C-O stretching absorptions at 1320–1210 cm^-1. There is also an out-of-plane bending vibration of the bonded O-H bonds at 920 cm^-1. 9. Esters: For saturated aliphatic esters (except formates), the C=O absorption band is in the range of 1750–1735 cm^-1. For saturated esters, the C-C(=O)-O band is in the range of 1210–1163 cm^-1, with strong absorption. 10. Amines: Absorptions due to N-H stretching vibrations occur in the range of 3500–3100 cm^-1. Absorptions due to C-N stretching vibrations occur 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 the out-of-plane bending vibration occurs in the range of 900–650 cm^-1. 11. Nitriles: The spectral characteristics of nitriles include absorption in the region corresponding to triple bond stretching vibrations, with weak to moderate intensity. For aliphatic nitriles, the absorption is in the range of 2260–2240 cm^-1; for aromatic nitriles, it’s in the range of 2240–2222 cm^-1. 12. Amides: Absorptions due to N-H stretching vibrations occur in the range of 3500–3100 cm^-1. C=O stretching vibrations occur at 1680–1630 cm^-1, while N-H bending vibrations occur at 1655–1590 cm^-1. C-N stretching vibrations occur in the range of 1420–1400 cm^-1. 13. Organic halides: C-X stretching vibrations: For aliphatic compounds, C-F has absorptions in the range of 1400–730 cm^-1; C-Cl has absorptions in the range of 850–550 cm^-1; C-Br has absorptions in the range of 690–515 cm^-1; C-I has absorptions in the range of 600–500 cm^-1. Please indicate that this content is taken from the Fluorosilicon Forum at http://www.dowpont.com/bbs/. The URL of this post is: http://www.dowpont.com/bbs/viewthread.php?tid=6871. It’s important to memorize the characteristic absorptions of various functional groups, as they form the basis for interpreting spectroscopic data. For an infrared spectrum that has already been obtained: (1) First, determine the type of carbon skeleton of the compound based on the spectrum. The degree of unsaturation is calculated using the formula: Degree of unsaturation = F + 1 + (T – O)/2, where F represents the number of atoms with a valence of 4 (mainly carbon atoms), T represents the number of atoms with a valence of 3 (mainly nitrogen atoms), and O represents 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 the threshold: Absorptions above 3000 cm^-1 indicate unsaturated C-H stretching vibrations; these could correspond to alkenes, alkynes, or aromatic compounds, while absorptions below 3000 cm^-1 generally represent 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 between 2200–2100 cm^-1, alkenes between 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, further analysis of the fingerprint region, namely the frequency range of 1000–650 cm^-1, should be carried out 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 connect the peaks related 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. The analysis process is basically like this. As for sample preparation and the use of infrared spectroscopy software, detailed explanations can be found in standard organic chemistry textbooks, so I won’t go into further detail here. This is a troublesome issue; just jot down one or two points whenever you can: 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 vibrations (2250–2100 cm^-1); C-H stretching vibration of alkynes (around 3300 cm^-1). 4. Aromatic hydrocarbons: 3100–3000 cm^-1 for the C-H stretching vibrations of the aromatic ring; 1600–1450 cm^-1 for the C=C framework vibrations; 880–680 cm^-1 for the out-of-plane bending vibrations of C-H bonds. An important characteristic of aromatic compounds is that four peaks with varying intensities typically appear at 1600, 1580, 1500, and 1450 cm^-1. 880–680 cm^-1: absorption due to out-of-plane bending vibrations of C-H bonds, which varies depending on the number and position of substituents on the benzene ring. In the infrared spectroscopy analysis of aromatic compounds, the absorptions in this frequency range are often used to identify isomers. 5. Alcohols and phenols: The main characteristic absorption bands are those resulting from 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, presenting as a sharp absorption peak; whereas for intermolecular hydrogen bonds, the O-H stretching vibration takes place at 3500–3200 cm^-1, resulting in a broad absorption peak ； C-O stretching vibration: 1300–1000 cm^-1; O-H out-of-plane bending: 769–659 cm^-1. 6. Ethers: Characteristic absorption bands include stretching vibrations in the range of 1300–1000 cm^-1; for aliphatic ethers, these bands are found in the range of 1150–1060 cm^-1, with a strong absorption peak. For aromatic ethers, there are two C-O stretching vibration absorption bands: 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 band for aldehydes is in the range of 1750–1700 cm^-1 (C=O stretching); additional absorption bands appear at 2820 and 2720 cm^-1 (C-H stretching of the aldehyde group). For ketones, the absorption band is at 1715 cm^-1, with a strong C=O stretching vibration; if the carbonyl group is conjugated with a double bond or an aromatic ring, the absorption frequency decreases. 8. Carboxylic acids: For carboxylic acid dimers, there is a broad, strong O-H stretching absorption band in the range of 3300–2500 cm^-1. There are also C=O absorption bands at 1720–1706 cm^-1, and C-O stretching absorption bands at 1320–1210 cm^-1. Additionally, there is an out-of-plane bending vibration of the bonded O-H bonds at 920 cm^-1. 9. Esters: For saturated aliphatic esters (except formates), the C=O absorption band is in the range of 1750–1735 cm^-1. For saturated esters, the C-C(=O)-O absorption band is in the range of 1210–1163 cm^-1, with a strong absorption signal. 10. Amines: N-H stretching vibration absorption occurs in the range of 3500–3100 cm^-1; C-N stretching vibration absorption occurs in the range of 1350–1000 cm^-1. The deformation vibration of N-H is similar to the scissoring vibration of CH2, with its absorption band in the range of 1640–1560 cm^-1. Out-of-plane bending vibration occurs in the range of 900–650 cm^-1. 11. Nitriles: The spectral characteristics of nitriles include absorption bands in the region corresponding to triple bond stretching vibrations, with weak to moderate intensity. For aliphatic nitriles, these bands are in the range of 2260–2240 cm^-1; for aromatic nitriles, they are in the range of 2240–2222 cm^-1. 12. Amides: N-H stretching vibration occurs in the range of 3500–3100 cm^-1; C=O stretching vibration occurs in the range of 1680–1630 cm^-1. N-H bending vibration occurs in the range of 1655–1590 cm^-1, while C-N stretching vibration occurs in the range of 1420–1400 cm^-1. 13. Organic halides: C-X stretching vibrations: For aliphatic compounds, C-F has absorption bands in the range of 1400–730 cm^-1; C-Cl has bands in the range of 850–550 cm^-1; C-Br has bands in the range of 690–515 cm^-1; C-I has bands in the range of 600–500 cm^-1