Infrared Absorption Spectroscopy (IR)

Introduction

Infrared (IR) absorption spectroscopy is the measurement of the wavelength and intensity of the absorption of mid-infrared light by a sample. Mid-infrared light (2.5 - 50 µm, 4000 - 200 cm^-1) is energetic enough to excite molecular vibrations to higher energy levels. The wavelength of many IR absorption bands are characteristic of specific types of chemical bonds, and IR spectroscopy finds its greatest utility for qualitative analysis of organic and organometallic molecules. IR spectroscopy is used to confirm the identify of a particular compound and as a tool to help determine the structure of a newly synthesized molecule (with NMR spectroscopy and mass spectrometry).

Mechanism of IR absorption

The transition moment for infrared absorption is:
R = < X_i | µ | X_j dt >
where X_i and X_j are the initial and final states, respectively, and µ is the electric dipole moment operator:
µ = µ_o + (r-r_e)(dµ/dr) + ... higher terms.
µ_o is the permanent dipole moment, which is a constant, and since < X_i | X_j > = 0, R simplifies to:
R = < X_i | (r-r_e)(dµ/dr) | X_j >
The result is that there must be a change in dipole moment during the vibration for a molecule to absorb infrared radiation.

Examples of infrared active and inactive absorption bands in CO₂

There is no change in dipole moment during the symmetric stretch vibration and the 1340 cm^-1 band is not observed in the infrared absorption spectrum (the symmetric stretch is called infrared inactive). There is a change in dipole moment during the asymmetric stretch and the 2350 cm^-1 band does absorb infrared radiation (the asymmetric stretch in infrared active). A related vibrational spectroscopic method is Raman spectroscopy, which has a different mechanism and therefore provides complementary information to infrared absorption.

Instrumentation

Instrumentation for dispersive and Fourier-transform IR absorption instruments are described in a separate document.

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