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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...

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 Frequency Region: Fingerprint Region

IR Frequency Region: Fingerprint Region

IR spectra are divided into two main regions: the diagnostic region and the fingerprint region. The diagnostic region of the spectrum lies above 1500 cm−1. The absorptions resulting from single-bond vibrations of the N–H, C–H, and O–H stretch at higher wavenumbers and appear on the left side of the spectrum. The stretching absorptions of the C≡C and C≡N occur between 2100–2300 cm−1. In contrast, those arising from stretching absorptions of the C=O, C=N, and C=C occur between 1600–1850 cm−1.
The...

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 Spectrum Peak Broadening: Hydrogen Bonding

IR Spectrum Peak Broadening: Hydrogen Bonding

The vibrational frequency of a bond is directly proportional to its bond strength. As a result, stronger bonds vibrate at higher frequencies, while weaker bonds vibrate at lower frequencies. The stretching vibration of the strong O–H bond in alcohols and phenols (very dilute solution or gas phase) appears as a sharp peak at 3600–3650 cm−1.
However, the extent of hydrogen bonding influences the observed stretching frequency and band broadening. Intermolecular or intramolecular hydrogen bonding...

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Infrared Degenerate Four-wave Mixing with Upconversion Detection for Quantitative Gas Sensing

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Vibrationally resolved sum-frequency generation with broad-bandwidth infrared pulses.

L J Richter, T P Petralli-Mallow, J C Stephenson

|December 20, 2007

Summary

We developed a new method for sum-frequency generation (SFG) spectroscopy. This technique allows for rapid, high-quality spectral data acquisition without scanning, improving vibrational analysis of surfaces.

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Area of Science:

Surface science
Spectroscopy
Nonlinear optics

Background:

Sum-frequency generation (SFG) spectroscopy is a powerful technique for studying surfaces and interfaces.
Traditional SFG methods often require scanning the infrared (IR) frequency, which can be time-consuming and limit data acquisition speed.

Purpose of the Study:

To introduce a novel, rapid, and high signal-to-noise ratio method for vibrationally resolved sum-frequency generation (SFG) spectroscopy.
To demonstrate the application of this new SFG technique for surface analysis.

Main Methods:

A broad-bandwidth infrared (IR) pulse is mixed with a narrow-bandwidth visible pulse.
The generated SFG signal is dispersed using a spectrograph.
Data is acquired in parallel using a scientific-grade CCD detector.

Main Results:

The developed procedure enables rapid data acquisition over a 400-cm(-1) spectral region without IR frequency scanning.
High signal-to-noise ratio spectra were achieved.
The method was successfully applied to study a self-assembled monolayer of octadecanethiol.

Conclusions:

The novel SFG procedure significantly enhances the speed and efficiency of vibrational spectroscopy for surface studies.
This technique offers a valuable tool for in-situ analysis of molecular monolayers and interfaces.