Infrared Absorption Spectroscopy

Infrared Absorption Spectroscopy (IR) Introduction IR 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 IR absorption bands are characteristic of specific types of chemical bonds, and IR spectroscopy finds its greatest utility for identification of organic and organometallic molecules.

Mechanism of IR absorption The transition moment for infrared absorption is: R = < Xi | u | Xj dt > where Xi and Xj are the initial and final states, respectively, and u is the electric dipole moment operator: u = uo + (r-re)(du/dr) + ... higher terms. uo is the permanent dipole moment, which is a constant, and since < Xi | Xj > = 0, R simplifies to: R = < Xi | (r-re)(du/dr) | Xj > 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.

Infrared Absorption Spectrometers Introduction This document describes dispersive and Fourier-transform spectrometers that are used in infrared absorption spectroscopy.

Dispersive infrared spectrometers Common light sources are tungsten lamps, Nernst glowers, or glowbars. Dispersive IR spectrometers use a grating monochromator to select wavelengths and are commonly used when a single wavelength is desired to monitor the kinetics of a reaction or as a GC or LC detector. Schematic of a dispersive IR absorption spectrometer

Fourier-transform Infrared (FTIR) Modern IR instruments more commonly use Fourier-transform techniques with a Michelson interferometer. Picture of an FTIR absorption spectrometer

Infrared Absorption Bands Introduction IR absorption spectroscopy uses mid-infrared light (2.5 - 50 µm, 4000 - 200 cm-1) to detect specific types of chemical bonds in a sample for identification of organic and organometallic molecules.

Table of characteristic IR bands Group

Bond

Appox. Energy (cm-1)

hydroxyl

O-H

3610-3640

amines

N-H

3300-3500

aromatic rings

C-H

3000-3100

alkenes

C-H

3020-3080

alkanes

C-H

2850-2960

nitriles

C=-N

carbonyl

C=O

1650-1750

amines

C-N

1180-1360

2210-2260

Near-Infrared Absorption Spectroscopy (NIR) Introduction NIR spectroscopy is the measurement of the wavelength and intensity of the absorption of near-infrared light by a sample. Near-infrared light spans the 800 nm - 2.5 µm (12,500 - 4000 cm-1) range and is energetic enough to excite overtones and combinations of molecular vibrations to higher energy levels. NIR spectroscopy is typically used for quantitative measurement of organic functional groups, especially O-H, N-H, and C=O. Detection limits are typically 0.1% and applications include pharmaceutical, agricultural, polymer, and clinical analysis.

Instrumentation

The components and design of NIR instrumentation are similar to uv-vis absorption spectrometers. The light source is usually a tungsten lamp and the detector is usually a PbS solid-state detector. Sample holders can be glass or quartz and typical solvents are CCl4 and CS2. The convenient instrumentation of NIR spectroscopy compared to IR spectroscopy makes it much more suitable for on-line monitoring and process control.