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Related Concept Videos

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...
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.
Coupled Reactions01:17

Coupled Reactions

Cellular processes such as building and breaking down complex molecules occur through stepwise chemical reactions. Some of these chemical reactions are spontaneous and release energy, whereas others require energy to proceed. Cells often couple the energy-releasing reaction with the energy-requiring one to carry out important cell functions. 
Energy in adenosine triphosphate or ATP molecules is easily accessible to do work. ATP powers the majority of energy-requiring cellular reactions. Cells...
Hybridization of Atomic Orbitals II03:35

Hybridization of Atomic Orbitals II

sp3d and sp3d 2 Hybridization
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...
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,...

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Multireference Mukherjee's coupled cluster method with triexcitations in the linked formulation: Efficient

Kiran Bhaskaran-Nair1, Ondrej Demel, Jirí Pittner

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

The Journal of Chemical Physics
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We developed a new multireference Mukherjee

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Accurate electronic structure calculations are crucial for understanding molecular properties.
  • Existing multireference coupled cluster methods face challenges in accuracy and computational cost.
  • Static correlation effects are important in describing certain electronic states and molecular systems.

Purpose of the Study:

  • To formulate and implement a new multireference Mukherjee's coupled cluster method with triexcitations (MR MkCCSDT).
  • To assess the accuracy and performance of the MR MkCCSDT method.
  • To compare the new method with existing multireference coupled cluster treatments and experimental data.

Main Methods:

  • Formulation of the linked version of the multireference Mukherjee's coupled clusters method with triexcitations (MR MkCCSDT).
  • Implementation of the MR MkCCSDT method within the ACES II program package.
  • Application and validation of the method to the electronic states of the oxygen molecule, methylene, and twisted ethylene.

Main Results:

  • The MR MkCCSDT method demonstrates accuracy comparable to single-reference coupled cluster with triples ( CCSDT) calculations.
  • The newly developed MR MkCCSDT method shows excellent agreement with experimental data for the studied systems.
  • The MkCCSD(T) method shows promise for systems with significant static correlation, especially with small model spaces.

Conclusions:

  • The MR MkCCSDT method provides a highly accurate approach for electronic structure calculations, particularly for systems with strong static correlation.
  • The MR MkCCSDT method is a valuable addition to the suite of multireference coupled cluster methods.
  • The MkCCSD(T) method is a computationally efficient and accurate alternative for specific applications requiring treatment of static correlation.