Abstract on IR Spectroscopy

Fundamental Principle:
The frequency of Infrared (IR) electromagnetic radiation is on the same order of magnitude
as that of bond stretching and bending. When the frequency of the IR radiation striking a
molecule matches the stretching frequency or bending frequency of a bond in the molecule
an absorption will occur.

Bond stretching frequencies depend on two things:

1. The masses of the atoms that are bonded together (light atoms vibrate faster,
e.g. C-H bond vibrates much faster than C-O bond)

2.

The bond strength (stronger the bond the higher the energy needed to stretch it
and therefore the higher the frequency at which it vibrates, e.g. triple bonds >
double bonds > single bonds)

The important information gained from an IR spectrum is:

1. Absorption bands tell us what kind of bonds are present, i.e. what functional group

2. Since each molecule has its own unique IR spectrum it can act as a fingerprint in
confirming the identity of a molecule

Infrared Stretching Correlations-Illustrating the dependency of the
absorption frequency on the two points listed above

Type of Bond

Frequency Range (cm-1)

Example

Y-H

3600-2800

O-H > N-H > C-H

Y≡Z

2300-2100

C≡C, C≡N

Y=Z

1850-1350

C=O, C=C, N=O

Y-Z

1300-900

C-C, C-N, C-O