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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

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.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
NMR Spectroscopy: Spin–Spin Coupling01:08

NMR Spectroscopy: Spin–Spin Coupling

The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved in...
Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)

Two NMR-active nuclei bonded to a central atom can be involved in geminal or two-bond coupling. Geminal coupling is commonly seen between diastereotopic protons in chiral molecules and unsymmetrical alkenes, among others.
The central atom need not be NMR-active because its electrons are affected by the electron polarization of the spin-active atoms. However, spin information is transmitted less effectively than in one-bond coupling, and 2J values are usually weaker than 1J values. The energy of...
Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

Coupling interactions are strongest between NMR-active nuclei bonded to each other, where spin information can be transmitted directly through the pair of bonding electrons. While nuclei polarize their electrons to the opposite spins, the bonding electron pair has opposite spins. Configurations with antiparallel nuclear spins are expected to be lower in energy. When coupling makes antiparallel states more favorable, J is considered to have a positive value. The one-bond coupling constant, 1J,...
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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 have a...
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Uncoupled multireference state-specific Mukherjee's coupled cluster method with triexcitations.

Ondřej Demel1, Kiran Bhaskaran-Nair, Jiří Pittner

  • 1J. Heyrovský Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejškova 3, 18223 Prague 8, Czech Republic.

The Journal of Chemical Physics
|October 15, 2010
PubMed
Summary
This summary is machine-generated.

Researchers developed an uncoupled multireference coupled cluster method. This new computational chemistry approach accurately models molecular electronic structures, including complex systems with triple excitations.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • The standard Mukherjee's multireference coupled cluster (MRCC) method is a powerful tool for electronic structure calculations.
  • Previous work established agreement between uncoupled and standard MRCC methods at the singles and doubles excitation level.
  • Extending these methods to include triple excitations is crucial for accurate modeling of complex molecular systems.

Purpose of the Study:

  • To investigate the performance of the uncoupled version of Mukherjee's multireference coupled cluster method with connected triexcitations.
  • To assess the method's accuracy across varying basis set sizes, model space dimensions, and multireference character.
  • To evaluate the impact of including connected triple excitations on the method's performance.

Main Methods:

  • Implementation of the uncoupled Mukherjee's MRCC method with connected triexcitations within the ACES II program package.
  • Systematic computational studies on singlet methylene, the potential energy curve of the fluorine molecule, and the third electronic state of the oxygen molecule.
  • Analysis of the method's performance as a function of basis set size, model space size, and multireference character.

Main Results:

  • The uncoupled MRCC method with connected triexcitations demonstrates reliable performance across diverse molecular systems.
  • The method's accuracy is shown to be robust with respect to variations in basis set size and model space.
  • Comparisons with the standard MRCC method confirm the validity of the uncoupled approach for including triple excitations.

Conclusions:

  • The developed uncoupled multireference coupled cluster method with connected triexcitations is a viable and accurate computational tool.
  • This method provides a computationally efficient alternative for electronic structure calculations, particularly for systems with significant multireference character.
  • The findings support the broader applicability of coupled cluster methods in quantum chemistry for complex molecular problems.