molecules. 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 molecule. Characteristic IR Absorption Peaks of Functional Groups* Vibration Position (cm-1) Intensity* Notes Alkanes C-H stretch 2990 – 2850 m to s Alkenes =C-H stretch 3100 – 3000 m C=C stretch 1680 – 1620 (sat.) 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. 1650 – 1600 (conj.) IR Tables, UCSC Table 1. The table below lists the characteristic IR absorptions of some common functional groups. At the same time they also show the stake-shaped band in the middle of the spectrum … IR spectroscopy in organic and biochemistry: IR spectoscopy can be used to indicate the presence and absence of functional groups in organic compounds. 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. These groups (i.e. 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. 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.] Type of Vibration causing IR absorption 3000-2800 (Note: The absorptions can be seen as several distinct peaks in this region.) The beauty of the spectrum is the close and accessible relationship between infrared bands and molecular structure. 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. 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 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. 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 Certain groups of atoms have characteristic vibration frequencies which persist in different compounds. w to m =C-H bend 995 – … Functional groups tend to absorb at predictable regions of the spectrum. IR SPECTRUM OF A CARBOXYLIC ACID. /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. Si-OCH 3, C=O, Si-CH=CH 2, BF 4-) are the very kind of functional groups chemists are interested in. Spectrum … molecules C=O bond common functional groups functional groups tend to at! 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