IR Spectroscopy: Frequencies & Functional Groups

Introduction

Infrared spectroscopy, often referred to as IR spectroscopy or vibrational spectroscopy, is a technique that measures how infrared radiation interacts with matter through processes such as absorption, emission, or reflection. This method is widely used to study and identify chemical substances or functional groups in various states, including solid, liquid, or gas. The process involves the use of an instrument known as an infrared spectrometer, which generates an infrared spectrum. This spectrum can be represented as a graph with infrared light absorbance (or transmittance) on the vertical axis and frequency, wavenumber, or wavelength on the horizontal axis. Wavenumbers are typically expressed in reciprocal centimeters (cm−1), while wavelengths are commonly given in micrometers (μm), with both units being inversely related. dimensional IR is also possible.

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The infrared spectrum is typically divided into three main regions: near-, mid-, and far-infrared, named based on their relation to the visible spectrum. The near-IR, with higher energy, can excite overtone or combination modes of molecular vibrations. The mid-infrared is generally used to study the fundamental vibrations and associated rotational-vibrational structure. Infrared spectroscopy leverages the fact that molecules absorb frequencies that are characteristic of their structure. These absorptions occur at resonant frequencies, meaning the frequency of the absorbed radiation matches the vibrational frequency. The energies are influenced by the shape of the molecular potential energy surfaces, the masses of the atoms, and the associated vibronic coupling.