Download 11-4 Infrared Spectroscopy

11-5 Infrared Spectroscopy
IR spectroscopy measures the vibrational excitation of atoms around the bonds
that connect them.
The positions of the absorption lines are related to the types of functional groups
present.
The IR spectrum as a whole is unique for each individual substance.
Absorption of infrared light causes molecular vibrations.
The infrared region is range of the electromagnetic spectrum just below visible
light. Absorption of light of this wavelength causes vibrational excitation of the
bonds in a molecule.
Middle infrared light (λ~2.5-16.7 μm, or 600-4000 cm-1) has energies from 1 to 10
kcal mol-1 and is most useful to the chemist.
Hooke’s law relates the parameters affecting the vibrational frequency of two
weights connected by a spring.
The vibrational frequency of two atoms connected by a bond is also accurately
described by Hooke’s law:
However, the infrared spectrum of a molecule is significantly more complex than
the vibrational frequencies of all of the bonds present.
Various bending motions, and combinations of stretching and bending are also
excited by IR radiation, which leads to complicated patterns.
Fortunately, the vibrational bands of many functional groups appear at
characteristic wavenumbers, and the entire IR spectrum of a given compound is
unique and can be distinguished from that of any other substance.
Functional groups have typical infrared absorptions.
Compare the IR spectra of pentane and hexane:
Above 1500 cm-1 the C-H stretching absorptions typical of alkanes can be seen.
Since no function groups are present, no absorptions are seen in the region from
2840–3000 cm-1.
Below 1500 cm-1, the fingerprint region, C-C stretching and C-C and C-H bending
motions absorb to give complicated patterns.
All saturated hydrocarbons show peaks at 1460, 1380, and 730 cm-1.
Now compare hexane to 1-hexene:
An additional peak at 3080 cm-1 can be seen which is due to the stronger Csp2-H
bond.
The C=C stretching band should appear between 1620 and 1680 cm-1 and is seen
at 1640 cm-1.
The two signals at 915 and 995 cm-1 are characteristic of a terminal alkene.
Several other strong bending modes are characteristic for the substitution
patterns in alkenes:
The O-H stretching absorption is the most characteristic band in the IR spectra of
alcohols. This appears as a broad band over the range 3200–3650 cm-1. This is due
to hydrogen bonding.
Dry, dilute alcohols show a sharp narrow band in the range 3620–3650 cm-1.
Haloalkane C-X stretching frequencies are too low (<800 cm-1) to be useful for
characterization.
11-6 Degree of Unsaturation: Another Aid to
Identifying Molecular Structure
Knowledge of the degree of unsaturation, defined as the numbers of rings and 
bonds present in a molecule, is useful information when determining the
structure of a compound.
A fully saturated hydrocarbon will have 2n+2 hydrogen atoms for every n carbon
atom.
Consider the compounds in the class C5H8. This compound is 4 hydrogens short of
being saturated, so its degree of unsaturation is 4/2 = 2.
All molecules having this formula must have a combination of rings and  bonds
adding up to 2.
The presence of heteroatoms may affect the calculation.
The presence of a halogen atom decreases the number of hydrogens by one.
The presence of a nitrogen atom increases the number of hydrogens by one.
The presence of oxygen or sulfur does not affect the number of hydrogens.
To determine the degree of unsaturation:
Step 1: Hsat = 2nC + 2 – nX + nN
Step 2: Degree of unsaturation = (Hsat – Hactual)/2