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IR Spectroscopy: Molecular Vibration Overview01:24

IR Spectroscopy: Molecular Vibration Overview

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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...
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Infrared (IR) Spectroscopy: Overview01:09

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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.
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IR and UV–Vis Spectroscopy of Aldehydes and Ketones01:29

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Infrared spectroscopy, also known as vibrational spectroscopy, is mainly used to determine the types of bonds and functional groups in molecules. In aldehydes and ketones, the carbonyl (C=O) bond shows an absorption around 1710 cm-1. The C=O bond vibration of an aldehyde occurs at lower frequencies than that of a ketone. In addition to the C=O absorption in an aldehyde, the aldehydic C–H bond also gives two peaks in the 2700–2800 cm-1 range. This absorption, coupled with the...
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IR Frequency Region: Fingerprint Region01:03

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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...
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¹H NMR of Labile Protons: Temporal Resolution01:10

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Protons bonded to heteroatoms such as nitrogen and oxygen exhibit a range of chemical shift values. This is due to the varying degree of hydrogen bonding between the proton and the heteroatom in other molecules. The extent of hydrogen bonding affects the electron density around the proton, thereby giving different chemical shift values for the protons in the proton NMR spectrum.
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IR Frequency Region: X–H Stretching01:24

IR Frequency Region: X–H Stretching

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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...
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How Vibrational Notations Can Spoil Infrared Spectroscopy: A Case Study on Isolated Methanol.

Dennis F Dinu1,2,3, Kemal Oenen2, Jonas Schlagin2

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This study clarifies methanol's complex infrared spectrum using matrix isolation and computational methods. Deuteration and Sankey diagrams help assign spectral features, resolving long-standing debates in molecular spectroscopy.

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Area of Science:

  • Molecular Spectroscopy
  • Quantum Chemistry
  • Physical Chemistry

Background:

  • Methanol's infrared spectrum has remained incompletely understood for a century.
  • Previous assignments of spectral features, particularly resonances, have been ambiguous.
  • Standard spectroscopic notation often fails to adequately describe methanol's vibrational modes.

Purpose of the Study:

  • To experimentally and computationally unravel the complex mid-infrared spectrum of methanol.
  • To unambiguously assign all fundamental vibrations and resonances.
  • To propose improved methods for spectral analysis and notation.

Main Methods:

  • Isolation of single methanol molecules in solid argon and neon matrices.
  • Acquisition of infrared spectra for natural, partially deuterated, and fully deuterated methanol.
  • Calculation of vibrational wavenumbers using the vibrational configuration interaction (VCI) approach, including anharmonicity and mode-coupling.

Main Results:

  • Unambiguous assignment of all fundamental vibrations and resonances in the mid-infrared spectrum.
  • Demonstration that increasing deuteration simplifies the spectrum and aids band assignment.
  • Identification of shortcomings in current spectroscopic notation for describing molecular vibrations.

Conclusions:

  • The study successfully demystifies debated resonances in methanol's infrared spectrum.
  • Sankey diagrams are proposed as a superior method for unambiguously identifying spectral features.
  • This work provides a comprehensive understanding of methanol's vibrational spectroscopy.