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Related Concept Videos

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

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A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to...
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IR Spectroscopy: Molecular Vibration Overview01:24

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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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IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations01:08

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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...
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Bond Energies and Bond Lengths02:49

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Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, that is, the energy necessary to separate the bonded atoms. Separating any pair of bonded atoms requires energy — the stronger a bond, the greater the energy required to break it.
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IR Frequency Region: Alkyne and Nitrile Stretching01:22

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Both alkyne (C≡C) and nitrile (C≡N) functional groups contain triple bonds and show stretching absorptions around the wavenumber range of 2100 to 2300 cm−1 in the diagnostic region of the IR spectra.
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¹H NMR: Long-Range Coupling01:27

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The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
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Local Bond-Stretch Coordinates for Anharmonic Vibrational Computations.

Sebastian Riis Thomsen1, Nicolai Machholdt Høyer1, Mads Greisen Højlund1

  • 1Department of Chemistry, University of Aarhus, Aarhus C DK-8000, Denmark.

The Journal of Physical Chemistry. A
|April 2, 2025
PubMed
Summary
This summary is machine-generated.

The novel local bond-stretch (LBS) method and its variants offer improved vibrational mode analysis. Projected LBS (pLBS) and orthogonal, projected LBS (opLBS) show promise for faster convergence in spectral calculations.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Spectroscopy

Background:

  • Accurate calculation of molecular vibrational spectra is crucial for understanding chemical properties.
  • Traditional methods using normal coordinates can be computationally intensive and less intuitive for localized vibrations.

Purpose of the Study:

  • To introduce and evaluate the local bond-stretch (LBS) method and its variants for computing vibrational spectra.
  • To compare the performance of LBS variants against traditional normal coordinates.

Main Methods:

  • Developed three variants of the local bond-stretch method: pure LBS, projected LBS (pLBS), and orthogonal, projected LBS (opLBS).
  • Employed a vibrational coupled cluster band Lanczos approach for spectral computations.
  • Utilized potential energy surfaces (PESs) generated by the adaptive density-guided approach (ADGA).

Main Results:

  • Demonstrated that LBS variants provide localized, rectilinear vibrational modes.
  • Observed faster convergence with respect to coupling level in PES when using LBS variants compared to normal coordinates.
  • Computed overtone vibrational spectra for water, nitroxyl (HNO), formaldehyde, and 1,3-butadiene.

Conclusions:

  • The local bond-stretch (LBS) method offers an effective approach for analyzing vibrational spectra.
  • Projected LBS (pLBS) and orthogonal, projected LBS (opLBS) variants are particularly promising due to their balance of localization and computational efficiency.