Infrared spectroscopy is an analytical technique that is commonly used by scientists to identify unknown materials and determine the molecular structure of compounds. IR spectroscopy works by sending a beam of infrared light through a sample and measuring how much of the infrared radiation is absorbed at specific wavelengths. Molecules absorb frequencies that are characteristic of their structure, and this absorption provides a molecular fingerprint. By analyzing the infrared absorption spectrum, researchers can determine functional groups and chemical bonds that are present in an unknown compound.

Global IR Spectroscopy: Absorption Bands and Infrared Regions

When infrared light hits a molecule, the energy can cause the bonds between atoms to vibrate. The vibrations occur at frequencies specific to a particular molecular structure or chemical environment. Global IR Spectroscopy absorption bands show up as peaks in the infrared spectrum. There are three main regions of infrared spectra defined by the wavelength ranges: near-IR from 14000 to 4000 cm-1, mid-IR from 4000 to 400 cm-1, and far-IR from 400 to 10 cm-1. The mid-IR region is most commonly used for molecular identification since frequencies within this range correspond to the fundamental vibrations of most chemical bonds.

Global IR Spectroscopy: Common Functional Groups

Certain functional groups have characteristic and distinctive absorption bands in infrared spectroscopy. For example, the O-H stretch of an alcohol or carboxylic acid appears around 3300-2500 cm-1. The C=O carbonyl stretch of aldehydes, ketones, acids, and esters occurs near 1700 cm-1. An alkene C=C stretch is usually observed around 1600-1500 cm-1. Aromatic C-H stretches give a broad signal around 3000-3100 cm-1. Being able to recognize the absorption patterns of common functional groups allows researchers to make educated guesses about molecular structure from an IR spectrum.

Global IR Spectroscopy Sample Preparation Methods

There are two main methods used to prepare samples for IR spectroscopy analysis: transmission and attenuated total reflectance (ATR). In transmission preparation, the sample is mixed with an infrared-transparent matrix like potassium bromide and pressed into a pellet. An IR beam passes straight through the sample. ATR preparation involves placing the pure sample directly on the surface of an optically dense crystal with a high refractive index, like diamond or germanium. The infrared beam passes through the crystal and reflects internally, absorbing at frequencies specific to the sample in contact. ATR allows analyzing solids, liquids, and even gases without further preparation.

Applications in Organic Chemistry

Infrared spectroscopy has widespread applications in organic chemistry research and industrial process monitoring. It is commonly used for compound identification, determination of purity, and analysis of reaction progress and mechanisms. IR can reveal if a multi-step synthesis resulted in the expected product structure. It also allows directly observing how functional groups change during a reaction. When paired with chemometrics, infrared spectroscopy has become a valuable process analytical technology (PAT) tool. Examples include real-time monitoring of polymerization reactions, pharmaceutical tablet coating uniformly, and biodiesel quality. Researchers also employ IR to study biochemical phenomena like protein folding and DNA base-pairing.

Applications in Materials Analysis

Beyond chemistry and biochemistry, infrared spectroscopy provides valuable insight into materials properties as well. Analysis of characteristic absorption peaks allows determining important aspects like crystallinity, defects, impurities, and phase composition in materials. IR is applied to evaluate additive content in plastics and polymers, check curing completion in synthetics like epoxies, and monitor degradation over time in materials like tires and circuit boards. Vibrational techniques are important for semiconductor characterization, studying stress in materials subjected to different environments, and quality control of advanced composites. Infrared imaging techniques coupled with microscopy are becoming valuable tools for materials failure analysis.

Advancements with Fourier Transform IR

A major improvement over dispersion spectroscopy was the development of Fourier transform infrared (FTIR) spectrometers. In FTIR instruments, the infrared light is collected and measured as an interferogram using an interferometer before being mathematically converted into an infrared spectrum with Fourier transformation. This allows measuring the entire mid-IR range simultaneously and provides significant improvements in speed, sensitivity, and signal-to-noise ratio over dispersive instruments. The advancements in FTIR technology have expanded applications across many fields by enabling faster, more detailed analysis of smaller samples. Current developments focus on miniaturized portable designs and coupling IR to other techniques like microscopy and mass spectrometry.

infrared spectroscopy has proved to be a remarkably versatile and powerful analytical method since its development over a century ago. Advances continue to be made to push the capabilities and applications of this vibrational technique. With its selectivity for molecular structure and ability to rapidly analyze a wide range of sample types, infrared spectroscopy remains a fundamental analytical tool across chemistry, materials science, medical research, pharmaceutical industry, and many other fields. The information obtained from IR absorption spectra will surely prove invaluable as scientists tackle the challenges of tomorrow.
 
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