IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will absorb weakly or not at all. force on in direction of field force on+ E in direction of field E molecule compressed molecule stretched dipole moment decreased dipole moment increased electric electric field force on- field opposite direction of field force on- opposite direction of field Chapter 12 11
Chapter 12 11 IR-Active and Inactive • A polar bond is usually IR-active. • A nonpolar bond in a symmetrical molecule will absorb weakly or not at all. =>
An Infrared Spectrometer motor rotating segmented mirror detector reference cell monochromator glowing wire light source transmitted sample cell chart recorder Chapter 12 12 =>
Chapter 12 12 An Infrared Spectrometer =>
FT-IR Spectrometer Uses an interferometer. Has better sensitivity. Less energy is needed from source. Completes a scan in 1-2 seconds. Takes several scans and averages them. Has a laser beam that keeps the instrument accurately calibrated. Chapter 12 13
Chapter 12 13 FT-IR Spectrometer • Uses an interferometer. • Has better sensitivity. • Less energy is needed from source. • Completes a scan in 1-2 seconds. • Takes several scans and averages them. • Has a laser beam that keeps the instrument accurately calibrated. =>
Carbon-Carbon Bond Stretching Stronger bonds absorb at higher frequencies: >C-C 1200cm-1 >C=C 1660cm-1 >C=C 2200 cm-1 (weak or absent if internal) Conjugation lowers the frequency: >isolated C=C 1640-1680cm-1 >conjugated C=C 1620-1640cm-1 >aromatic C=C approx.1600 cm-1 Chapter 12 14
Chapter 12 14 Carbon-Carbon Bond Stretching • Stronger bonds absorb at higher frequencies: ➢C-C 1200 cm-1 ➢C=C 1660 cm-1 ➢CC 2200 cm-1 (weak or absent if internal) • Conjugation lowers the frequency: ➢isolated C=C 1640-1680 cm-1 ➢conjugated C=C 1620-1640 cm-1 ➢aromatic C=C approx. 1600 cm-1 =>
Carbon-Hydrogen Stretching Bonds with more s character absorb at a higher frequency. >sp3 C-H,just below 3000 cm-1 (to the right) >sp2 C-H,just above 3000 cm-1 (to the left) >sp C-H,at 3300 cm-1 => Chapter 12 15
Chapter 12 15 Carbon-Hydrogen Stretching Bonds with more s character absorb at a higher frequency. ➢sp3 C-H, just below 3000 cm-1 (to the right) ➢sp2 C-H, just above 3000 cm-1 (to the left) ➢sp C-H, at 3300 cm-1 =>