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2D NMR: Overview of Heteronuclear Correlation Techniques01:18

2D NMR: Overview of Heteronuclear Correlation Techniques

163
Heteronuclear correlation spectroscopy is an analytical technique that investigates the coupling between different types of nuclei, often a proton and an X-nucleus, such as carbon-13 or nitrogen-15. This method is commonly used in nuclear magnetic resonance (NMR) spectroscopy to gain insights into complex chemical compounds' structural and compositional aspects. A typical heteronuclear correlation spectrum displays X-nucleus chemical shifts on one axis and a proton spectrum on the other...
163
2D NMR: Overview of Homonuclear Correlation Techniques01:16

2D NMR: Overview of Homonuclear Correlation Techniques

174
Homonuclear correlation spectroscopy (COSY) is a powerful technique used in Nuclear Magnetic Resonance (NMR) spectroscopy to study the correlations between nuclei of the same type within a molecule. It provides information about scalar couplings between adjacent nuclei, which helps determine connectivity and structural information. There are several COSY variants, each with its unique strengths and experimental parameters.
COSY90 is the standard two-dimensional (2D) COSY experiment that...
174
2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)01:19

2D NMR: Heteronuclear Single-Quantum Correlation Spectroscopy (HSQC)

662
Heteronuclear single-quantum correlation spectroscopy (HSQC) is a 2D NMR technique that reveals one-bond correlations between hydrogen and a heteronucleus. The HSQC experiment is similar to the heteronuclear correlation experiment (HETCOR) but is more sensitive. In the HSQC spectrum, the proton chemical shift is plotted on the horizontal F2 axis, while the 13C chemical shift is plotted on the vertical F1 axis. The corresponding proton and 13C spectra are also shown. The HSQC contour plot does...
662
Hückel's Rule Diagram of π MOs: Frost Circle01:08

Hückel's Rule Diagram of π MOs: Frost Circle

4.4K
The Frost circle or the inscribed polygon method is a graphical method for determining the relative energies of π molecular orbitals (MOs) for planar, fully conjugated, and monocyclic compounds. This method was first described by A. A. Frost and Boris Musulin in 1953.
A Frost circle is constructed by drawing a polygon whose number of edges is equal to the number of carbons of the given cyclic system, with one of the vertices pointing down. Then, a circle is drawn enclosing the polygon so...
4.4K
Valence Bond Theory and Hybridized Orbitals02:38

Valence Bond Theory and Hybridized Orbitals

19.0K
According to valence bond theory, a covalent bond results when: (1) an orbital on one atom overlaps an orbital on a second atom, and (2) the single electrons in each orbital combine to form an electron pair. The strength of a covalent bond depends on the extent of overlap of the orbitals involved. Maximum overlap is possible when the orbitals overlap on a direct line between the two nuclei.
A σ bond (single bond in a Lewis structure) is a covalent bond in which the electron density is...
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The Energies of Atomic Orbitals03:21

The Energies of Atomic Orbitals

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In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are also observed. The attractive and repulsive forces are dependent on the distance between the particles, as well as the sign and magnitude of the charges on the individual particles. When the charges on the particles are opposite, they attract each other. If both particles have the same charge, they repel each other.
23.8K

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Updated: Jun 13, 2025

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
12:11

Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

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Unveiling hidden dynamic correlations in CASSCF correlation energies by Hartree-Fock nodes.

Martin Šulka1, Katarína Šulková1, Matúš Dubecký2

  • 1Advanced Technologies Research Institute, Faculty of Materials Science and Technology in Trnava, Slovak University of Technology in Bratislava, Bottova 25, 91724 Trnava, Slovakia.

The Journal of Chemical Physics
|September 17, 2024
PubMed
Summary

A new method precisely separates electron correlation energy into dynamic and non-dynamic parts. This quantum chemistry approach reveals that CASSCF methods include unwanted dynamic correlation, challenging traditional interpretations.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Accurately partitioning electron correlation energy is crucial in quantum chemistry.
  • Existing methods struggle to precisely distinguish dynamic and non-dynamic correlation contributions.
  • The Hartree-Fock (HF) Slater determinant node and fixed-node diffusion Monte Carlo (FNDMC) offer a novel basis for this separation.

Purpose of the Study:

  • To apply a new HF-node/FNDMC-based method for decomposing correlation energy.
  • To investigate the correlation energy contributions within the Complete Active Space Self-Consistent Field (CASSCF) method.
  • To analyze selected molecular systems including BH, FH, F2, and H2-H2.

Main Methods:

  • Utilized an original method based on the HF Slater determinant node.
  • Employed the stochastic projector fixed-node diffusion Monte Carlo (FNDMC) technique.
  • Applied this decomposition to CASSCF calculations for small molecules.

Main Results:

  • CASSCF correlation energies contain an unexpected, system-dependent dynamic correlation component.
  • This finding challenges the common assumption that CASSCF primarily captures non-dynamic correlation.
  • The new method successfully identified this extraneous dynamic correlation.

Conclusions:

  • The HF-node/FNDMC electron correlation energy decomposition method is a valuable tool.
  • It aids in identifying limitations of methods that cannot precisely dissect dynamic and non-dynamic correlation.
  • This technique enhances the understanding of electron correlation in quantum chemical calculations.