Infrared Spectroscopy Basics

Study Notes

Infrared Spectroscopy

Infrared (IR) spectroscopy deals with the infrared region of the electromagnetic spectrum.
It is a type of absorption spectroscopy, meaning it measures the amount of light absorbed by a sample at different wavelengths.
The wavelength range of IR radiation spans 14000–10 cm−1 (0.8–1000 μm).
The most useful IR region is between 4000 - 670 cm−1.
IR spectroscopy is based on the principle that molecules absorb infrared radiation at specific frequencies that correspond to the vibrational modes of the molecule.
When a molecule absorbs IR radiation, its bonds vibrate at a higher energy level, causing a change in the electric dipole moment of the molecule.
The intensity of an absorption band in an IR spectrum depends on the change in the electric dipole moment during vibration.
Vibrations within a molecule are primarily categorized into two types: stretching and bending.
Stretching vibrations involve changes in the interatomic distance along the bond axis. There are symmetric and asymmetric stretching vibrations.
Bending vibrations involve changes in the angle between two bonds. There are four types of bending vibrations: rocking, scissoring, twisting, and wagging.
The frequency of a vibrational mode is determined by the force constant of the bond and the reduced mass of the atoms involved.
IR spectroscopy can be used for identifying functional groups, analyzing the structure of molecules, and studying the kinetics and thermodynamics of chemical reactions.
The IR spectrum of a molecule can be used to distinguish between different isomers, since isomers have different vibrational modes.
The IR spectrum of a molecule can also be used to monitor the progress of a chemical reaction, since the IR spectrum will change as reactants are converted to products.

IR spectroscopy components

IR spectroscopy typically requires several components:
- Sources: Produce IR radiation for interaction with the sample.
- Monochromators: Separate radiation into different wavelengths. This helps to narrow down the frequencies of the radiation used to get a specific spectrum.
- Sample preparation: Prepare the sample for analysis in a way that allows it to interact with the IR radiation.
- Detectors: Detect the IR radiation that passes through the sample.

IR Chart

The IR chart is a graphic representation of the absorption frequencies of different functional groups.
The chart helps identify and interpret the presence of specific functional groups in the sample.
It is divided into two regions:
- The functional group region: Contains characteristic absorption bands for various functional groups.
- The fingerprint region: Contains a complex pattern of absorption bands and bands representing vibrations of the whole molecule.

Factors influencing IR frequencies

Force constant: Represents the bond strength. A stronger bond (higher force constant) results in a higher vibrational frequency.
Reduced mass: Lower reduced mass results in higher vibrational frequency.