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Related Concept Videos

IR Spectroscopy: Molecular Vibration Overview01:24

IR Spectroscopy: Molecular Vibration Overview

1.9K
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...
1.9K
IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

1.2K
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...
1.2K
¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

¹H NMR of Conformationally Flexible Molecules: Temporal Resolution

807
At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
807
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

878
In bromoethane, the three methyl protons are coupled to the two methylene protons that are three bonds away. In accordance with the n+1 rule, the signal from the methyl protons is split into three peaks with 1:2:1 relative intensities. The methylene protons appear as a quartet, with the relative intensities of 1:3:3:1.
Qualitatively, any spin plus-half nucleus polarizes the spins of its electrons to the minus-half state. Consequently, the paired electron in the hydrogen–carbon bond must...
878
IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations01:08

IR Spectrum Peak Splitting: Symmetric vs Asymmetric Vibrations

913
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...
913
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

1.0K
Spin systems where the difference in chemical shifts of the coupled nuclei is greater than ten times J are called first-order spin systems. These nuclei are weakly coupled, and their chemical shifts and coupling constant can generally be estimated from the well-separated signals in the spectrum.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
1.0K

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Vibrational Spectra of a N719-Chromophore/Titania Interface from Empirical-Potential Molecular-Dynamics Simulation, Solvated by a Room Temperature Ionic Liquid
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Convergent Concordant Mode Approach for Molecular Vibrations: CMA-2.

Nathaniel L Kitzmiller1,2, Mitchell E Lahm1, Laura N Olive Dornshuld1

  • 1Center for Computational Quantum Chemistry and Department of Chemistry, University of Georgia, Athens, Georgia 30602, United States.

Journal of Chemical Theory and Computation
|December 13, 2024
PubMed
Summary
This summary is machine-generated.

The concordant mode approach (CMA) enhances quantum chemical computations for molecular vibrational frequencies. New CMA methods achieve high accuracy with reduced computational cost, making complex calculations more accessible.

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Interfacial Molecular-level Structures of Polymers and Biomacromolecules Revealed via Sum Frequency Generation Vibrational Spectroscopy
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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Molecular Spectroscopy

Background:

  • Accurate calculation of molecular vibrational frequencies is crucial for understanding molecular properties and reactions.
  • Existing computational methods face limitations in system size and theoretical rigor.
  • The concordant mode approach (CMA) offers a promising strategy to overcome these limitations.

Purpose of the Study:

  • To advance the concordant mode approach (CMA) hierarchy for molecular vibrational frequency computations.
  • To benchmark CMA performance against high-level coupled cluster singles and doubles with perturbative triples (CCSD(T))/cc-pVTZ calculations.
  • To develop efficient methods for achieving high accuracy in vibrational frequency calculations.

Main Methods:

  • Utilized second-order Møller-Plesset perturbation theory (MP2)/cc-pVTZ for generating normal modes (Level B) within CMA.
  • Developed a convergent CMA-2 method employing Hartree-Fock (HF) and MP2 or density functional theory (DFT) data.
  • Introduced ξ parameters to select sparse off-diagonal force field elements for explicit evaluation at higher levels (Level A).

Main Results:

  • CMA-0A with MP2/cc-pVTZ reproduced 1501 benchmark frequencies with a mean absolute error (MAE) of 0.11 cm-1.
  • CMA-2 achieved an average maximum absolute error of 0.17 cm-1 with a modest cost increase (33%).
  • The new CMA methods successfully computed frequencies for diverse vibrations in 1-(1H-pyrrol-3-yl)ethanol.

Conclusions:

  • The enhanced CMA hierarchy significantly improves the accuracy and efficiency of molecular vibrational frequency calculations.
  • MP2/cc-pVTZ is an excellent choice for generating Level B normal modes in CMA.
  • The CMA-2 method provides a robust and computationally feasible pathway to high-accuracy vibrational frequencies.