Type of Vibration causing IR absorption 3000-2800 (Note: The absorptions can be seen as several distinct peaks in this region.) Si-OCH 3, C=O, Si-CH=CH 2, BF 4-) are the very kind of functional groups chemists are interested in. Therefore carboxylic acids show a very strong and broad band covering a wide range between 2800 and 3500 cm-1 for the O-H stretch. Infrared Spectroscopy • The vibrational IR extends from 2.5 x 10-6 m (2.5 μm) to 2.5 x 10-5 m (25 μm) – the frequency of IR radiation is commonly expressed in wavenumbers – Wavenumber ν: the number of waves per centimeter, with units cm-1 (read reciprocal centimeters) – expressed in wavenumbers, the vibrational IR extends w to m =C-H bend 995 – … 26 CHARACTERISTIC INFRARED ABSORPTION BANDS OF FUNCTIONAL GROUPS Class of Compounds -Absorption, cm 1 Intensity -Assignment Class of Compounds Absorption, cm 1 Intensity Assignment Alkanes and 2850-3000 s C-H stretch Carboxylic Acids 2500-3500 s, broad O-H stretch Alkyls 1450-1470 s C-H bend R-C(O)-OH 1710-1715 s, broad C=O stretch Table 12.1 Characteristic IR Absorptions of Some Functional Groups Absorption (cm—I) 3300-3500 1030-1230 1670-1780 1730 1715 1735 1690 1710 2500-3100 2210-2260 1540 Intensity Medium Medium Strong Strong Strong Strong Strong Strong Strong, broad Medium Strong Functional Group Alkane C-H Alkene Alkyne Alkyl halide C-CI C-Br Alcohol O-H c-o Arene The infrared portion of the electromagnetic spectrum is divided into three regions; the near-, mid- and far- infrared, named for their relation to the visible spectrum. Certain groups of atoms have characteristic vibration frequencies which persist in different compounds. The far-infrared, approximately 400-10 cm-1 (1000–30 μm), lying adjacent to the microwave region, has low energy and may be used for rotational spectroscopy. /Type /Font Infrared absorption data for some functional groups not listed in the preceding table are ... energy near-IR, approximately 14000-4000 cm-1 ... absorbance spectrum can be produced, showing at which IR wavelengths the sample absorbs. Carbonyl Group C=O C–O Carboxylic Acids O-H C=O Nitriles C≡N Nitro Group-NO 2 (br, m) (br, m-w) (s) 1650-1780 ~1200 (s) 2500-3100 (br, s) (s) 2240-2250 (w-m) 1540 cm-1 Functional Group-1 Typical IR Absorptions for Funtional Groups Functional Group molecule. A carboxylic acid functional group combines the features of alcohols and ketones because it has both the O-H bond and the C=O bond. These groups (i.e. Table 1: Principal IR Absorptions for Certain Functional Groups Functional Group Names & Example compounds Absorption Ranges(cm-1) [Look for a single absorption in these regions, unless stated otherwise.] The vibrations of functional groups are so characteristic that a quick glance at an infrared spectrum can give immediate information about the functional groups present in that molecule. molecules. IR Tables, UCSC Table 1. IR spectroscopy in organic and biochemistry: IR spectoscopy can be used to indicate the presence and absence of functional groups in organic compounds. The beauty of the spectrum is the close and accessible relationship between infrared bands and molecular structure. 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