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How to parse the infrared spectrum? Please read it

(1) Calculate the degree of unsaturation formula according to the molecular formula: degree of unsaturation Ω=n4+1+(n3-n1)/2 where: n4: the number of atoms whose valence is 4 (mainly C atoms), n3: the valence is The number of atoms with 3 valence (mainly N atoms), n1: the number of atoms with valence of 1 (mainly H, X atoms)

(2) Analyze the absorption of CH stretching vibration in the region of 3300~2800cm-1; taking 3000 cm-1 as the boundary: higher than 3000cm-1 is the absorption of CH stretching vibration of unsaturated carbon, which may be alkene, alkyne, aromatic compound; but less than 3000cm-1 is generally saturated CH stretching vibration absorption;

(3) If there is absorption slightly higher than 3000cm-1, the stretching vibration absorption characteristic peaks of unsaturated carbon-carbon bonds should be analyzed in the frequency range of 2250~1450cm-1. Among them, alkyne: 2200~2100 cm-1, alkene: 1680~1640 cm-1 Aromatic ring: 1600, 1580, 1500, 1450 cm-1 If it has been determined to be an alkene or aromatic compound, the fingerprint area, that is, the frequency region of 1000~650 cm-1, should be further analyzed to determine the number of substituents Number and position (cis, reverse, adjacent, between, right);

(4) After the carbon skeleton type is determined, the functional group of the compound is determined according to the characteristic absorption of the functional group;

(5) During analysis, attention should be paid to linking the relevant peaks describing each functional group to accurately determine the existence of the functional group. For example, the three peaks at 2820, 2720 and 1750~1700cm-1 indicate the existence of aldehyde groups.

Memorize health value

  1. Alkanes: C-H stretching vibration (3000-2850cm-1) C-H bending vibration (1465-1340cm-1) Generally saturated hydrocarbon C-H stretching vibrations are below 3000cm-1, which is close to the frequency absorption of 3000cm-1.
  2. Olefins: olefin C-H stretching (3100~3010cm-1), C=C stretching (1675~1640 cm-1), olefin C-H out-of-plane bending vibration (1000~675cm-1).
  3. Alkynes: Alkynes C-H stretching vibration (near 3300cm-1), triple bond stretching vibration (2250~2100cm-1).
  4. Aromatics: C-H stretching vibration on aromatic ring is 3100~3000cm-1, C=C skeleton vibration is 1600~1450cm-1, C-H out-of-plane bending vibration is 880~680cm-1.

Important characteristics of aromatics: 4 peaks of varying intensities may appear at 1600, 1580, 1500 and 1450 cm-1. C-H out-of-plane bending vibration absorbs 880~680cm-1, which varies with the number and position of substituents on the benzene ring. In the infrared spectrum analysis of aromatic compounds, it is often used to discriminate isomers.

  1. Alcohol and phenol: the main characteristic absorption is the stretching vibration absorption of OH and CO, the stretching vibration of free hydroxyl OH: 3650~3600cm-1, which is a sharp absorption peak, the intermolecular hydrogen bond OH stretching vibration: 3500~3200cm-1, Broad absorption peak; CO stretching vibration: 1300~1000cm-1, OH out-of-plane bending: 769-659cm-1
  2. Ether characteristic absorption: 1300~1000cm-1 stretching vibration, aliphatic ether: 1150~1060cm-1 a strong absorption peak Aromatic ether: 1270~1230cm-1 (for Ar-O stretching), 1050~1000cm-1( Scale for RO)
  3. Aldehydes and ketones: The characteristic absorption of aldehydes: 1750~1700cm-1 (C=O stretching), 2820, 2720cm-1 (aldehyde group CH stretching) Fatty ketones: 1715cm-1, strong C=O stretching vibration absorption, If the carbonyl group is conjugated with an ethylenic bond or an aromatic ring, the absorption frequency will decrease
  4. Carboxylic acid: carboxylic acid dimer: 3300~2500cm-1 wide and strong OH stretch absorption 1720~1706cm-1 C=O stretch absorption 1320~1210cm-1 CO stretch absorption, 920cm-1 bonded OH bond Out-of-plane bending vibration
  5. Ester: C=O absorption band of saturated fatty acid ester (except formate): 1750~1735cm-1 region Saturated ester C-O band: 1210~1163cm-1 region is strong absorption
  6. Amine: NH stretching vibration absorption 3500~3100 cm-1; CN stretching vibration absorption 1350~1000 cm-1; NH deformation vibration is equivalent to CH2 shear vibration absorption: 1640~1560cm-1; out-of-plane bending vibration absorption 900 ~650cm-1.
  7. Nitrile: Three-bond stretching vibration zone, with weak to moderate absorption. Aliphatic nitrile 2260-2240cm-1 Aromatic nitrile 2240-2222cm-1
  8. Amide: 3500-3100cm-1 N-H stretching vibration

1680-1630cm-1 C=O stretching vibration

1655-1590cm-1 N-H bending vibration

1420-1400cm-1 C-N telescopic

  1. Organic halide: aliphatic C-X stretch: C-F 1400-730 cm-1, C-Cl 850-550 cm-1, C-Br 690-515 cm-1, C-I 600-500 cm-1

Infrared Spectrum Song

Infrared can be divided into far, middle and near, medium red characteristic fingerprint area, 1300 demarcation, pay attention to the difference in the horizontal axis. Look at the picture to know the infrared instrument, to understand the solid, liquid and gas state. The sample preparation method of the source of the sample is related to the physical and chemical properties.

Learn about saturated hydrocarbons first in the picture, and look at the peak shape below 3,000.

2960 and 2870 are methyl groups, and 2930 and 2850 methylene peaks. 1470 Hydrocarbon Bend, 1380 Methyl Display. Two methyl groups have the same carbon, 1380 divided in half. Swinging 720 in the face, the long chain methylene can also be distinguished.

The alkene hydrogen stretches over three thousand, excluding frequency doubling and halocarbons. The peak of terminal olefin is strong, only one hydrogen is not obvious. Compounds, and the bonds are biased, ~1650 will appear.

The alkene hydrogen is easy to deform outside the plane, and there are strong peaks below 1000. 910 terminal hydrogen, and one hydrogen 990.

The cis-dihydrogen 690, the trans-form is shifted to 970; the single hydrogen has a peak of 820, and the interference cis-form is difficult to determine.

The alkyne hydrogen stretches three thousand three, and the peak intensity is large and sharp. The triple bond stretches 2,200, and the alkyne hydrogen swings 680.

Aromatics breath is very special, 1600~1430, 1650~2000, the substitution method is clearly distinguished. 900 to 650, bends out of the plane to determine aromatic hydrogen. The absorption of pentahydrogen has two peaks, 700 and 750; the tetrahydrogen is only 750, and the dihydrogen is adjacent to 830; the dihydrogen is substituted with three peaks, 700, 780, and 880 isolated hydrogen alcohol phenolic hydroxyl groups are easy to associate, and three thousand three strong peaks . C-O stretches and absorbs greatly, and it is easy to distinguish between the two. 1050 is a primary alcohol, 1100 is a secondary alcohol, 1150 is a tertiary alcohol, and 1230 is a phenol.

1110 ether chain extension, pay attention to exclude ester acid alcohol. If it is closely connected to the π bond, the two absorptions should be spotted, 1050 symmetrical peaks, 1250 antisymmetric. If the benzene ring has a methoxy group, the hydrocarbon stretches 2820. Methylene dioxane ring has a strong peak at 930, ethylene oxide has three peaks, 1260 ring vibration, 900 up and down antisymmetric, around 800 is the most characteristic. Acetal, special ether, 1110 non-ketal. Acid anhydrides also have C-O bonds, but there is a difference between open-chain cyclic anhydrides. The width of the open-chain peak is one thousand one, and the cyclic anhydride moves to 1250.

The carbonyl stretches 1.7 and 2720 aldehyde groups. The wave number of the electric absorption effect is high, and the conjugate shifts to the low frequency. Tension promotes rapid vibration, and the double bond outside the ring can be compared.

Two thousand five to three thousand three, carboxylic acid hydrogen bond peak shape is wide, 920, blunt peak is obvious, carboxyl dimer acid can be determined, acid anhydride coupling, bimodal 60 tightly separated, chain anhydride high frequency, high cyclic anhydride Frequency is weak. Carboxylate, coupled, carbonyl stretches out double peaks, 1600 antisymmetric, 1400 symmetrical peaks.

1740 ester carbonyl, what acid can see the carbon and oxygen show. 1180 formate, 1190 for propionic acid, 1220 for acetate, 1250 for aromatic acid. 1600 rabbit ear peak, often phthalic acid.

Nitrogen and hydrogen stretch three thousand and four, and each hydrogen has a distinct peak. Carbonyl extended amide I, 1660 has a strong peak; N-H metamorphic amide II, 1600 minutes. Primary amine frequency is high and easy to overlap, secondary acyl solid is 1550; carbon nitrogen stretch amide III, 1400 strong peak is obvious.

The amine tip often has interference, N-H stretches three thousand three, tertiary amine has no peak secondary amine single, and primary amine double peak is small and sharp. 1600 hydrocarbon bends, aromatic secondary amine thousand five partial. Shake in about 800 or so noodles to make sure it’s best to become salt. Stretching and bending are close to each other, primary amine salt has 3000 strong peaks wide, secondary amine salt, tertiary amine salt, 2700 up and down can be distinguished, imine salt, more pitiful, only visible around 2000.

The nitro group stretches and absorbs greatly, and the connecting group can be clarified. 1350 and 1500 are divided into symmetrical and antisymmetric. Amino acid, as internal salt, wide peak shape from 3100 to 2100. 1600, 1400 acid radical development, 1630, 1510 hydrocarbon bending. Hydrochloride, carboxyl group, sodium salt protein three thousand three.

The mineral composition is messy, and the vibration spectrum is far from the red end. Ammonium salts are simpler, with few and broad absorption peaks. Pay attention to hydroxyl water and ammonium. First remember several common salts: 1100 is sulfate, 1380 is nitrate, 1450 is carbonate, and about one thousand is phosphoric acid. Silicate, a broad peak, 1000 is really spectacular.

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