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Related Concept Videos

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...
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
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.
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...
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...
Hybridization of Atomic Orbitals II03:35

Hybridization of Atomic Orbitals II

sp3d and sp3d 2 Hybridization

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

Kiran Bhaskaran-Nair1, Ondrej Demel, Jirí Pittner

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

The Journal of Chemical Physics
|December 3, 2008
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This summary is machine-generated.

We developed a new computational chemistry method, multireference Mukherjee

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Accurate computational methods are crucial for understanding molecular behavior.
  • Static correlation poses challenges for standard quantum chemistry techniques.
  • Multireference methods are needed for strongly correlated systems.

Purpose of the Study:

  • To formulate and implement the multireference Mukherjee's coupled cluster method with connected singles, doubles, and perturbative triples [MR MkCCSD(T)].
  • To assess the performance of the new MR MkCCSD(T) method.
  • To evaluate its applicability to systems with significant static correlation.

Main Methods:

  • Implementation of the MR MkCCSD(T) method within the ACES II program package.
  • Application of the method to the first three electronic states of the oxygen molecule.
  • Calculation of the automerization barrier of cyclobutadiene.
  • Comparison with other multireference coupled cluster (CC) treatments and experimental data.

Main Results:

  • The MR MkCCSD(T) method was successfully formulated and implemented.
  • The method provided accurate results for the oxygen molecule's electronic states.
  • Calculations on cyclobutadiene demonstrated the method's capability for challenging systems.
  • Performance was comparable to other advanced multireference CC methods.

Conclusions:

  • The MR MkCCSD(T) method is a promising approach for accurately treating systems with static correlation.
  • It offers a computationally tractable solution for complex electronic structure problems.
  • This method advances the capabilities of quantum chemistry software like ACES II.