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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 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...

UV–Vis Spectroscopy: Woodward–Fieser Rules

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UV–Visible absorption spectra of conjugated dienes arise from the lowest energy π&VeryThinSpace;→&VeryThinSpace;π* transitions. The light-absorbing part of the molecule is called the chromophore, and the substituents directly attached to the chromophore are called auxochromes. A strong correlation exists between the absorption maxima, λmax, and the structure of a conjugated π system. The Woodward–Fieser rules predict the value of λmax for a given structure by adding the...

UV–Vis Spectroscopy: Molecular Electronic Transitions

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In Ultraviolet–Visible (UV–Vis) spectroscopy, the absorption of electromagnetic radiation is used to probe the electronic structure of molecules. This technique provides insights into molecular electronic transitions, particularly the movement of electrons between different molecular orbitals. Radiation is absorbed if the energy of the electromagnetic radiation passing through the molecule is precisely equal to the energy difference between the excited and ground states. During this...

¹H NMR: Interpreting Distorted and Overlapping Signals

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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.
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UV–Vis Spectroscopy of Conjugated Systems

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Organic compounds with conjugated double bonds show strong absorption features in the UV–visible region of the electromagnetic spectrum attributed to π&VeryThinSpace;→&VeryThinSpace;π* electronic excitations. Generally, a UV–vis absorption spectrum is recorded as a plot of absorbance vs wavelength. The wavelength of maximum absorbance, which manifests as a peak in the absorption spectrum, is denoted as λmax.
One of the factors influencing λmax is the extent of conjugation in...

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Updated: Jan 17, 2026

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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Analytic Computation of Vibrational Circular Dichroism Spectra Using Configuration Interaction Methods.

Brendan M Shumberger¹, T Daniel Crawford¹

¹Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, United States.

The Journal of Physical Chemistry. A

|January 15, 2026

Summary

This summary is machine-generated.

This study introduces analytic-gradient methods for atomic axial tensors in vibrational circular dichroism (VCD) simulations. New methods improve stability and reveal discrepancies between common theories, impacting VCD spectral predictions.

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Area of Science:

Computational Chemistry
Spectroscopy
Quantum Chemistry

Background:

Vibrational Circular Dichroism (VCD) spectroscopy is crucial for determining molecular chirality.
Accurate VCD spectral simulations require precise computation of atomic axial tensors (AATs).
Existing methods for AAT computation can face numerical stability issues.

Purpose of the Study:

To derive and implement analytic-gradient methods for AATs in VCD simulations.
To enhance the numerical stability of gradient calculations using noncanonical perturbed orbitals and frozen-core capabilities.
To investigate the impact of singly excited determinants and CI-coefficient optimization on VCD spectra.

Main Methods:

Development of analytic-gradient methods for Configuration Interaction with Doubles (CID) and Singles and Doubles (CISD) excitations.
Implementation utilizing noncanonical perturbed orbitals for improved gradient stability.
Validation against finite-difference approaches and comparison with Hartree-Fock (HF) and Møller-Plesset perturbation (MP2) theories.

Main Results:

Successful implementation of analytic-gradient methods for CID and CISD AAT calculations.
Demonstrated improved numerical stability in gradient computations.
Observed significant sign discrepancies between HF/MP2 and CID/CISD methods for four out of five tested molecules.
Identified discrepancies between CID and CISD methods for a specific molecule.

Conclusions:

The new analytic-gradient methods provide a robust approach for VCD spectral simulations.
Singly excited determinants and CI-coefficient optimization play a significant role in VCD spectral accuracy.
Discrepancies highlight the importance of choosing appropriate theoretical methods for accurate VCD predictions.