Normal modes energy is characteristic of the atoms in

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Unformatted text preview: 2500 cm-1 N-H, C-H, O-H (stretching) 3300-3600 N-H, O-H 3000 C-H 2500-2000 cm-1 C C and C N (stretching) 2000-1500 cm-1 double bonds (stretching) C=O 1680-1750 C=C 1640-1680 cm-1 Below 1500 cm-1 "fingerprint" region 36 Table 12-1, p. 425 37 Interpretation of IR Spectrum WHAT INFORMATION CAN BE OBTAINED FROM IR? C3C 2O HHH O-H stretch C-H stretch C-C stretch C-O stretch 1. Type of bonds in molecule. 2. Functional groups in molecule 38 Spartan Demonstration 39 < 3000 alkane C-H stretch Stiffer bond = higher spring constant, k > 3000 alkene C-H stretch >> 3000 alkyne C-H stretch Fig. 12-14, p. 424 40 ~1450 alkane C-C stretch ~ 1600 alkene C=C stretch ~ 2150 alkyne C-H stretch Fig. 12-14, p. 424 41 Differences in Infrared Absorptions Molecules vibrate and rotate in normal modes, which are combinations of motions (relates to force constants) Bond stretching dominates higher energy modes Light objects connected to heavy objects vibrate fastest: C-H, N-H, O-H For two heavy atoms, stronger bond requires more energy: C C, C N > C=C, C=O, C=N > C-C, CO, C-N, C-halogen 42 12.8 Infrared Spectra of Some Common Functional Groups Alkanes, Alkenes, Alkynes C-H, C-C, C=C, C C have characteristic peaks absence helps rule out C=C or C C 43 IR: Aromatic Compounds Weak CH stretch at 3030 cm-1 Weak absorptions 1660 - 2000 cm-1 range Medium-intensity absorptions 1450 to 1600 cm-1 See spectrum of phenylacetylene, Figure 12.15 44 IR: Alcohols and Amines OH 3400 to 3650 cm-1 Usually broad and intense NH 3300 to 3500 cm-1 Sharper and less intense than an OH 45 IR: Carbonyl Compounds Strong, sharp C=O peak 1670 to...
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