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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.
MO Theory and Covalent Bonding02:40

MO Theory and Covalent Bonding

The molecular orbital theory describes the distribution of electrons in molecules in a manner similar to the distribution of electrons in atomic orbitals. The region of space in which a valence electron in a molecule is likely to be found is called a molecular orbital. Mathematically, the linear combination of atomic orbitals (LCAO) generates molecular orbitals. Combinations of in-phase atomic orbital wave functions result in regions with a high probability of electron density, while...
Molecular Orbital Theory II03:51

Molecular Orbital Theory II

Molecular Orbital Energy Diagrams
Molecular Orbital Theory I02:35

Molecular Orbital Theory I

Overview of Molecular Orbital Theory
¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

A proton M that is coupled to a proton X results in doublet signals for M. However, NMR-active nuclei can be simultaneously coupled to more than one nonequivalent nucleus. When M is coupled to a second proton A, such as in styrene oxide, each peak in the doublet is split into another doublet.
Splitting diagrams or splitting tree diagrams are routinely used to depict such complex couplings. While drawing splitting diagrams, the splitting with the larger coupling constant is usually applied first.
¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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.
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are slanted or...

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry

Published on: April 8, 2020

A companion perturbation theory for state-specific multireference coupled cluster methods.

Francesco A Evangelista1, Andrew C Simmonett, Henry F Schaefer

  • 1Center for Computational Chemistry and Department of Chemistry, University of Georgia, Athens, GA 30602, USA.

Physical Chemistry Chemical Physics : PCCP
|June 4, 2009
PubMed
Summary

A new Mukherjee multireference perturbation theory (Mk-MRPT2) method was developed for accurate electronic structure calculations. This method effectively models complex molecular systems, aiding in determining precise ab initio energy limits.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Accurate electronic structure calculations are crucial for understanding molecular properties and reactions.
  • Multireference coupled cluster methods are powerful but computationally demanding.
  • Developing efficient and accurate post-Hartree-Fock methods is an ongoing challenge.

Purpose of the Study:

  • To develop and implement a new perturbation theory method, Mukherjee multireference perturbation theory (Mk-MRPT2), based on a partitioning scheme.
  • To assess the performance and applicability of the Mk-MRPT2 method for challenging chemical systems.
  • To demonstrate the utility of Mk-MRPT2 in multireference focal point extrapolations for determining accurate ab initio energy limits.

Main Methods:

  • A partitioning scheme was applied to the state-specific Mukherjee multireference coupled cluster method.
  • A production-level code for the derived Mukherjee multireference perturbation theory (Mk-MRPT2) was implemented.
  • The method was applied to the F(2) dissociation and the electronic states of meta-benzyne, utilizing large basis sets (up to 766 atomic orbitals).

Main Results:

  • The effectiveness of the Mk-MRPT2 method was demonstrated through calculations on the F(2) dissociation and meta-benzyne.
  • The implementation supports large-scale computations, handling systems with up to 766 atomic orbitals.
  • Mk-MRPT2 proved particularly useful for multireference focal point extrapolations.

Conclusions:

  • The developed Mk-MRPT2 method provides an effective and computationally feasible approach for accurate electronic structure calculations.
  • Mk-MRPT2 is a valuable tool for studying challenging multireference systems and achieving high-accuracy energy predictions.
  • The method shows significant promise for future applications in computational quantum chemistry.