Infrared Spectroscopy (vibrational energy) Chemical bonds are not rigid; they are constantly vibrating Different vibrational modes are of higher energy than others, and complex organic molecules have a large number of vibrational modes(complicated! A nonlinear molecule containing n atoms has 3n-6 possible fundamental vibrational modes Luckily, different functional groups(parts of a molecule)exhibit unique chracteristic absorptions This allows us to look at an IR spectrum(% transmission versus wavenumber)and determine which functional groups are present based on the absorption band Wavenumber is the number of waves per centimeter(1/1) Characteristic IR Absorption Bands for Common Functional Groups Class Intensity Assi Alkanes C-H stretch Alkenes 3080-3140 C=C str s由 C-Hout-of-plane bend 3300 s 丝C-h 2100-214 C=C 600-700 =C-H bend 3400 bonded o-H stretch 1540-1870 C=O stretch Aromatic Hydrocarbons m Coplane bendi 1400-1600Infrared Spectroscopy (vibrational energy) • Chemical bonds are not rigid; they are constantly vibrating. • Different vibrational modes are of higher energy than others, and complex organic molecules have a large number of vibrational modes (complicated!). • A nonlinear molecule containing n atoms has 3n–6 possible fundamental vibrational modes! • Luckily, different functional groups (parts of a molecule) exhibit unique chracteristic absorptions. • This allows us to look at an IR spectrum (% transmission versus wavenumber) and determine which functional groups are present based on the absorption bands. • Wavenumber is the number of waves per centimeter (1/λ). Characteristic IR Absorption Bands for Common Functional Groups Class Wavenumber, cm–1 Intensity Assignment Alkanes 2850–3000 s C–H stretch 1450–1470 s 1370–1380 s CH2 and CH3 bend 720–725 m Alkenes 3080–3140 1650 809–990 m =C–H stretch m C=C stretch s C–H out-of-plane bend Alkynes 3300 2100–2140 600–700 s ≡C–H stretch m C≡C stretch s ≡C–H bend Alcohols 3600 var free O–H stretch 3400 s bonded O–H stretch Ethers 1070–1150 s C–O stretch Carbonyls 1540–1870 s C=O stretch Aromatic Hydrocarbons 675–900 1000–1300 1400–1600 s C–H out-of-plane-bend m C–H in-plane bending m C–C stretching 2