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Infrared (IR) Spectroscopy: Overview

Infrared (IR) Spectroscopy: Overview

When electromagnetic radiation passes through a material, atoms or molecules transition from a lower to a higher energy state by absorbing radiation corresponding to the energy difference between the two states. The absorption of infrared (IR) radiation causes transitions between vibrational energy levels in a molecule. Therefore, IR spectroscopy is a useful analytical tool for determining the molecular structure of molecules.
Different compounds display unique properties due to their...

IR Spectroscopy: Molecular Vibration Overview

IR Spectroscopy: Molecular Vibration Overview

When Infrared (IR) radiation passes through a covalently bonded molecule, the bonds transition from lower to higher vibrational levels. The fundamental vibrational motions that result in infrared absorption can be classified as stretching or bending vibrations.
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...

IR Frequency Region: X–H Stretching

IR Frequency Region: X–H Stretching

In IR spectroscopy, signals produced by the X−H bonds (such as C−H, O−H, or N−H) can be observed in the frequency range of 2700–4000 cm–1. The C−H stretching vibration forms sharp bands in the region 2850–3000 cm–1. The presence of the O−H stretching vibration leads to the forming of an absorption band in the frequency range 3650–3200 cm−1. At the same time, N−H stretching can be confirmed by absorption bands in the 3500–3100 cm−1 range. Even though both O−H and N−H bonds vibrate at a similar...

IR Spectrum

IR Spectrum

When infrared (IR) radiation passes through a molecule, the bonds stretch or bend by absorbing the radiation. This absorption creates the molecule's absorption spectrum, which is the plot of its percentage transmittance versus wavenumber.
Transmittance is defined as the ratio of the radiant power passing through a sample to that from the radiation's source. Multiplying the transmittance by 100 gives the percent transmittance (%T), which varies between 100% (no absorption) and 0% (complete...

IR Frequency Region: Alkene and Carbonyl Stretching

IR Frequency Region: Alkene and Carbonyl Stretching

Double bonds in alkenes and carbonyl compounds exhibit stretching frequencies in the diagnostic region of the IR spectrum. In addition, alkenes exhibit vinylic C–H stretching and C–H out-of-plane bending absorptions that are useful for identifying substitution patterns.
Stretching frequencies are affected by several factors, such as resonance, inductive effects, ring strain, dipole moment, and hydrogen bonding. Consequently, the stretching frequency of the carbonyl double bond varies in...

IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations

IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations

Identical bonds within a polyatomic group can stretch symmetrically (in-phase) or asymmetrically (out-of-phase). Similar to hydrogen bonding, these vibrations also influence the shape of the IR peak. Generally, asymmetric stretching frequencies are higher than symmetric stretching frequencies. For example, primary amines exhibit two distinct IR peaks between 3300–3500 cm−1 corresponding to the symmetric and asymmetric N-H stretching, while secondary amines exhibit a single stretching vibration...

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High-definition Fourier Transform Infrared (FT-IR) Spectroscopic Imaging of Human Tissue Sections towards Improving Pathology

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Line shape distortion effects in infrared spectroscopy.

Miloš Miljković¹, Benjamin Bird, Max Diem

¹Laboratory for Spectral Diagnosis, Department of Chemistry and Chemical Biology, Northeastern University, Boston, MA 02115, USA.

|July 20, 2012

Summary

This study examines how reflective and absorptive components distort infrared absorption spectra, leading to asymmetric bands and altered peak shapes. Understanding these spectral distortions is crucial for accurate infrared spectroscopy analysis.

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Area of Science:

Spectroscopy
Physical Chemistry

Background:

Infrared absorption spectra are fundamental for chemical analysis.
Spectral band shapes can be distorted by various physical phenomena.

Purpose of the Study:

To explore phenomena causing distortions in infrared absorption spectra.
To analyze the mixing of reflective and absorptive band shape components.

Main Methods:

Analysis of infrared absorption spectra.
Investigation of anomalous dispersion and its effect on band shapes.
Examination of the resonance Mie (RMie) mechanism.

Main Results:

Distortions result in asymmetric bands with negative contributions at high wavenumbers.
Observed band shifts occur toward lower wavenumbers, confounding intensity measurements.
Extreme distortions via the RMie mechanism can cause peak splitting or inversion.

Conclusions:

The interplay between reflective and absorptive components significantly impacts observed infrared spectra.
Understanding these distortions is essential for accurate spectral interpretation and analysis.
The RMie mechanism represents a significant factor in complex spectral feature formation.