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=0,C=N>CC,C O C-N, C-halogen
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 º C, C º N > C=C, C=O, C=N > C-C, CO, C-N, C-halogen
Infrared Spectra of Some Common Functional Groups
Infrared Spectra of Some Common Functional Groups
Infrared Spectra of Hydrocarbons lanes Alkenes =C-H 3020-3100 cm-1 C-H 2850-2960cm-1 cm RCH=CHo 910 and 9 90 cm C-C-800-1300cm-1 RoC=CH2 890 cm 0msn· Boos COl Alkynes C≡C-2100-2260cm1 C-H330m-1
Infrared Spectra of Hydrocarbons
IR: Alcohols and amines ·O-H3400to3650cm Usually broad and intense ·N-H3300to3500cm Sharper and less intense than an o-h 0-H 3400-3650 cm-(broad, intense omson Amines -N-H 3300-3500 cm-I(sharp, medium intensity @ Thomson-Brooks Cole
IR: Alcohols and Amines • O–H 3400 to 3650 cm-1 – Usually broad and intense • N–H 3300 to 3500 cm-1 – Sharper and less intense than an O–H
IR: Aromatic Compounds Weakc-h stretch at 3030 cm-l Weak absorptions 1660-2000 cm-range Medium-intensity absorptions 1450 to 1600 cm-l See spectrum of phenylacetylene, Figure 12. 15 Aromatic compounds C-H 3030 cm-l(weak) Ring 1660-2000 cm-I(weak) 1450-1600cm1( medium) Thomson· Brooks cole
IR: Aromatic Compounds • Weak C–H 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