Fingerprint Region of the Spectrum No two molecules will give exactly the same IR spectrum (except enantiomers) Fingerprint region is between 600-1400 cm-1,and has the most complex vibrations. The region between 1600-3500 cm-1 has the most common vibrations and we can use it to get information about specific functional groups in the molecule. Chapter 12 11
Chapter 12 11 Fingerprint Region of the Spectrum • No two molecules will give exactly the same IR spectrum (except enantiomers). • Fingerprint region is between 600–1400 cm-1 , and has the most complex vibrations. • The region between 1600–3500 cm-1 has the most common vibrations and we can use it to get information about specific functional groups in the molecule
Effect of an Electric Field on a Polar Bond represented as y000© force on in direction of field force on+ in direction of field molecule compressed molecule stretched dipole moment decreased dipole moment increased electric electric field force on- field opposite direction force on- of field opposite direction of field Pearson Prentice Hall.Inc 。 A bond with a dipole moment (as in HF,for example)is either stretched or compressed by an electric field,depending on the direction of the field. Notice that the force on the positive charge is in the direction of the electric field(E)and the force on the negative charge is in the opposite direction. Chapter 12 12
Chapter 12 12 Effect of an Electric Field on a Polar Bond • A bond with a dipole moment (as in HF, for example) is either stretched or compressed by an electric field, depending on the direction of the field. • Notice that the force on the positive charge is in the direction of the electric field (E) and the force on the negative charge is in the opposite direction
The Infrared Spectrometer motor reference beam rotating segmented mirror 日 detector reference monochromator cell 200 glowing wire light source transmitted sample cell sample beam chart recorder Copyright2010 Pearson Prentice Hall,Inc. Chapter 12 13
Chapter 12 13 The Infrared Spectrometer
FT-IR Spectrometer fixed mirror Has better sensitivity. ·Less energy is needed from source. ·Completes a scan in1 IR source to 2 seconds. ·Takes several scans He-Ne laser moving mirror beam splitter and averages them. laser calibration ·Has a laser beam that beam keeps the instrument sample accurately calibrated. detector Copyright2010 Pearson Prentice Hall,Inc. Chapter 12 14
Chapter 12 14 FT–IR Spectrometer • Has better sensitivity. • Less energy is needed from source. • Completes a scan in 1 to 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 because the bond is difficult to stretch: ·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 15
Chapter 12 15 Carbon-Carbon Bond Stretching • Stronger bonds absorb at higher frequencies because the bond is difficult to stretch: ▪ 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