IR Spectrum Introduction:
Infrared spectroscopy identifies and quantifies functional groups present in a compound non-destructively. It relies on the absorption of infrared radiation by the sample, leading to the excitation of molecular vibrations. An IR spectrum table and chart prove to be valuable tools in interpreting infrared spectra by listing typical IR absorption frequencies for various functional groups.
IR Spectrum Table:
A comprehensive list of typical IR absorption frequencies for various functional groups is an IR spectrum table. The table includes information on the type of bond, the frequency of the absorption, and the intensity of the absorption, which can be used to identify functional groups present in an unknown compound. The table organizes by functional group, making it easy to find the absorption frequencies for a particular bond.
Functional Group Classification:
Classification of functional groups occurs based on the type of bond that is present in the molecule. The most common functional groups include alcohols, amines, carbonyls, carboxylic acids, and ethers. Each of these functional groups has a characteristic absorption frequency in the IR spectrum. This makes it possible to identify the presence of the group in an unknown compound.
IR Spectrum Chart:
An IR spectrum chart graphically represents the IR spectrum of a compound, showing the absorption frequency on the x-axis and the intensity of the absorption on the y-axis. The chart helps identify the functional groups present in a compound and determines the sample’s purity. It is often used with the IR spectrum table to interpret the spectrum.
Interpretation of IR Spectrum:
Interpretation of an IR spectrum involves identifying the functional groups present in the sample. The major absorption bands are the first to identify, typically located in the range of 4000-400 cm-1, with the strongest bands at the lower frequencies. The major absorption bands correspond to the functional groups present in the sample.
Determining the functional groups responsible for the bands follows identifying the major absorption bands. The IR spectrum table lists the typical absorption frequencies for each functional group. This makes it possible to compare absorption frequencies in the spectrum with those in the table, which determines the functional groups present in the sample.
In addition to identifying functional groups, the IR spectrum also determines the purity of a sample. The presence of impurities can cause additional absorption bands in the spectrum, which identify and determine the level of impurities present in the sample.
IR Spectrum Table with Compound Class, Group, Frequency, and Absorption Values:
| Frequency Range | Absorption (cm-1) | Appearance | Group | Compound Class |
| 4000-3000 cm-1 | 3700-3584 | medium, sharp | O-H stretching | alcohol |
| 3550-3200 | strong, broad | O-H stretching | alcohol | |
| 3500 | medium | N-H stretching | primary amine | |
| 3400 | ||||
| 3400-3300 | medium | N-H stretching | aliphatic primary amine | |
| 3330-3250 | ||||
| 3350-3310 | medium | N-H stretching | secondary amine | |
| 3300-2500 | strong, broad | O-H stretching | carboxylic acid | |
| 3200-2700 | weak, broad | O-H stretching | alcohol | |
| 3000-2800 | strong, broad | N-H stretching | amine salt | |
| 3000-2500 cm-1 | ||||
| 3000-2500 cm-1 | 3333-3267 | strong, sharp | C-H stretching | alkyne |
| 3100-3000 | medium | C-H stretching | alkene | |
| 3000-2840 | medium | C-H stretching | alkane | |
| 2830-2695 | medium | C-H stretching | aldehyde | |
| 2600-2550 | weak | S-H stretching | thiol | |
| 2400-2000 cm-1 | ||||
| 2400-2000 cm-1 | 2349 | strong | O=C=O stretching | carbon dioxide |
| 2275-2250 | strong, broad | N=C=O stretching | isocyanate | |
| 2260-2222 | weak | CΞN stretching | nitrile | |
| 2260-2190 | weak | CΞC stretching | alkyne | |
| 2175-2140 | strong | S-CΞN stretching | thiocyanate | |
| 2160-2120 | strong | N=N=N stretching | azide | |
| 2150 | C=C=O stretching | ketene | ||
| 2145-2120 | strong | N=C=N stretching | carbodiimide | |
| 2140-2100 | weak | CΞC stretching | alkyne | |
| 2140-1990 | strong | N=C=S stretching | isothiocyanate | |
| 2000-1900 | medium | C=C=C stretching | allene | |
| 2000 | C=C=N stretching | ketenimine | ||
| 2000-1650 cm-1 | ||||
| 2000-1650 cm-1 | 2000-1650 | weak | C-H bending | aromatic compound |
| 1870-1540 | ||||
| 1818 | strong | C=O stretching | anhydride | |
| 1750 | ||||
| 1815-1785 | strong | C=O stretching | acid halide | |
| 1800-1770 | strong | C=O stretching | conjugated acid halide | |
| 1775 | strong | C=O stretching | conjugated anhydride | |
| 1720 | ||||
| 1770-1780 | strong | C=O stretching | vinyl / phenyl ester | |
| 1760 | strong | C=O stretching | carboxylic acid | |
| 1750-1735 | strong | C=O stretching | esters | |
| 1750-1735 | strong | C=O stretching | δ-lactone | |
| 1745 | strong | C=O stretching | cyclopentanone | |
| 1740-1720 | strong | C=O stretching | aldehyde | |
| 1730-1715 | strong | C=O stretching | α,β-unsaturated ester | |
| 1725-1705 | strong | C=O stretching | aliphatic ketone | |
| 1720-1706 | strong | C=O stretching | carboxylic acid | |
| 1710-1680 | strong | C=O stretching | conjugated acid | |
| 1710-1685 | strong | C=O stretching | conjugated aldehyde | |
| 1690 | strong | C=O stretching | primary amide | |
| 1690-1640 | medium | C=N stretching | imine / oxime | |
| 1685-1666 | strong | C=O stretching | conjugated ketone | |
| 1680 | strong | C=O stretching | secondary amide | |
| 1680 | strong | C=O stretching | tertiary amide | |
| 1650 | strong | C=O stretching | δ-lactam | |
| 1670-1600 cm-1 | ||||
| 1670-1600 cm-1 | 1678-1668 | weak | C=C stretching | alkene |
| 1675-1665 | weak | C=C stretching | alkene | |
| 1675-1665 | weak | C=C stretching | alkene | |
| 1662-1626 | medium | C=C stretching | alkene | |
| 1658-1648 | medium | C=C stretching | alkene | |
| 1650-1600 | medium | C=C stretching | conjugated alkene | |
| 1650-1580 | medium | N-H bending | amine | |
| 1650-1566 | medium | C=C stretching | cyclic alkene | |
| 1648-1638 | strong | C=C stretching | alkene | |
| 1620-1610 | strong | C=C stretching | α,β-unsaturated ketone | |
| 1600-1300 cm-1 | ||||
| 1600-1300 cm-1 | 1550-1500 | strong | N-O stretching | nitro compound |
| 1372-1290 | ||||
| 1465 | medium | C-H bending | alkane | |
| 1450 | medium | C-H bending | alkane | |
| 1375 | ||||
| 1390-1380 | medium | C-H bending | aldehyde | |
| 1385-1380 | medium | C-H bending | alkane | |
| 1370-1365 | ||||
| 1400-1000 cm-1 | ||||
| 1400-1000 cm-1 | 1440-1395 | medium | O-H bending | carboxylic acid |
| 1420-1330 | medium | O-H bending | alcohol | |
| 1415-1380 | strong | S=O stretching | sulfate | |
| 1200-1185 | ||||
| 1410-1380 | strong | S=O stretching | sulfonyl chloride | |
| 1204-1177 | ||||
| 1400-1000 | strong | C-F stretching | fluoro compound | |
| 1390-1310 | medium | O-H bending | phenol | |
| 1372-1335 | strong | S=O stretching | sulfonate | |
| 1195-1168 | ||||
| 1370-1335 | strong | S=O stretching | sulfonamide | |
| 1170-1155 | ||||
| 1350-1342 | strong | S=O stretching | sulfonic acid | |
| 1165-1150 | ||||
| 1350-1300 | strong | S=O stretching | sulfone | |
| 1160-1120 | ||||
| 1342-1266 | strong | C-N stretching | aromatic amine | |
| 1310-1250 | strong | C-O stretching | aromatic ester | |
| 1275-1200 | strong | C-O stretching | alkyl aryl ether | |
| 1075-1020 | ||||
| 1250-1020 | medium | C-N stretching | amine | |
| 1225-1200 | strong | C-O stretching | vinyl ether | |
| 1075-1020 | ||||
| 1210-1163 | strong | C-O stretching | ester | |
| 1205-1124 | strong | C-O stretching | tertiary alcohol | |
| 1150-1085 | strong | C-O stretching | aliphatic ether | |
| 1124-1087 | strong | C-O stretching | secondary alcohol | |
| 1085-1050 | strong | C-O stretching | primary alcohol | |
| 1070-1030 | strong | S=O stretching | sulfoxide | |
| 1050-1040 | strong, broad | CO-O-CO stretching | anhydride | |
| 1000-650 cm-1 | ||||
| 1000-650 cm-1 | 995-985 | strong | C=C bending | alkene |
| 915-905 | ||||
| 980-960 | strong | C=C bending | alkene | |
| 895-885 | strong | C=C bending | alkene | |
| 850-550 | strong | C-Cl stretching | halo compound | |
| 840-790 | medium | C=C bending | alkene | |
| 730-665 | strong | C=C bending | alkene | |
| 690-515 | strong | C-Br stretching | halo compound | |
| 600-500 | strong | C-I stretching | halo compound | |
| 900-700 cm-1 | ||||
| 900-700 cm-1 | 880 ± 20 | strong | C-H bending | 1,2,4-trisubstituted |
| 810 ± 20 | ||||
| 880 ± 20 | strong | C-H bending | 1,3-disubstituted | |
| 780 ± 20 | ||||
| (700 ± 20) | ||||
| 810 ± 20 | strong | C-H bending | 1,4-disubstituted or | |
| 1,2,3,4-tetrasubstituted | ||||
| 780 ± 20 | strong | C-H bending | 1,2,3-trisubstituted | |
| (700 ± 20) | ||||
| 755 ± 20 | strong | C-H bending | 1,2-disubstituted | |
| 750 ± 20 | strong | C-H bending | monosubstituted | |
| 700 ± 20 | benzene derivative |