Figure: 12-01 Caption: The electromagnetic spectrum is the range of all possible frequencies, from zero to infinity. In practice, the spectrum ranges from very low radio frequencies to very high energy gamma rays.

Figure: 12-01-01UN Caption: If the bond is stretched, a restoring force pulls the two atoms together toward their equilibrium bond length. If the bond is compressed, the restoring force pushes the two atoms apart. If the bond is stretched or compressed and then released, the atoms vibrate.

Figure: 12-01-02T01 Caption: Bond Stretching Frequencies

Figure: 12-02 Caption: A nonlinear molecule with n atoms has 3n - 6 fundamental vibrational modes. Water has 3(3) - 6 = 3 modes. Two of these are stretching modes, and one is a bending mode.

Figure: 12-04 Caption: Block diagram of an infrared spectrometer.

Figure: 12-06b Caption: Infrared spectrum of n-octane. Notice that the frquencies shown in an routing IR spectrum range from about 600 cm-1 to about 4000 cm-1.

Figure: 12-07 Caption: Comparison of the IR Spectra for n-hexane and 1-hexene. The most important absorptions in the 1-hexene are the C=C stretch at 1642 cm-1, and the unsaturated stretch at 3080 cm-1.

Figure: 12-07-01UN Caption: Alkanes, alkenes, and alkynes also have characteristic C-H stretching frequencies. Carbon-hydrogen bonds involving sp3 hybrid carbons generally absorb at frequencies just below (to the right) 3000 cm-1, while those involving sp2 carbons absorb just above (to the left) 3000 cm-1.

Figure: 12-08a,b Caption: (a) The IR spectrum of 1-octyne shows characteristics absorptions at 3313 cm-1 and at 2119 cm-1. (b) Neither of this absorptions are seen in the spectrum of 1-octyne.

Figure: 12-09 Caption: The IR spectrum of 1-butanol shows a broad, intense O-H stretching absorption centered around 3300 cm-1. The broad shape is due to the diverse nature of the hydrogen bonding interactions of alcohol molecules.

Figure: 12-10 Caption: The IR spectrum of dipropylamine shows a broad N-H stretching absorption centered around 3300 cm-1. Notive the spike in this broad absorption.

Figure: 12-10-01UN1-3 Caption: The C=O stretching vibrations of simple ketones, aldehydes and carboxylic acids occur at frequencies around 1710 cm-1. These frequencies are higher than those for C=C double bonds because the C=O double bond is stronger and stiffer.

Figure: 12-11a,b Caption: (a) 2-heptanone and (b) butyraldehyde. Both show intense carbonyl absorptions near 1710 cm-1.

Figure: 12-12 Caption: IR spectrum of hexanoic acid.

Figure: 12-12-05UN Caption: The delocalization of the pi electrons reduces the electron density of the carbonyl double bond, weakening it and lowering the stretching frequency from 1710 cm-1 to about 1685 cm-1 for conjugated ketones, aldehydes and acids.

Figure: 12-14 Caption: Nitrile triple bond stretching absorptions are at slightly higher frequencies than those of alkyne triple bonds.

Figure: 12-14-04UN Caption: The IR spectrum has distinct regions. The left of the spectrum shows the C-H, O-H, and N-H stretches. The triple bonds absorb around 2200 cm-1 followed by the double bonds to the right at around 1700 cm-1. The region below 1400 cm-1 is called the fingerprint region.