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Related Concept Videos

Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

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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...
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Vicinal or three-bond coupling is commonly observed between protons attached to adjacent carbons. Here, nuclear spin information is primarily transferred via electron spin interactions between adjacent C‑H bond orbitals. This generally favors the antiparallel arrangement of spins, so 3J values are usually positive.
The extent of coupling depends on the C‑C bond length, the two H‑C‑C angles, any electron-withdrawing substituents, and the dihedral angle between the...
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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...
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Spin–Spin Coupling: One-Bond Coupling01:17

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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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The stability of equilibrium configurations is an important concept in physics, engineering, and other related fields. In simple terms, it refers to the tendency of an object or system to return to its equilibrium position after being disturbed. The stability of an equilibrium configuration can be analyzed by considering the potential energy function of the system and examining its behavior near the equilibrium point.
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Understanding the stability of equilibrium configurations is a fundamental part of mechanical engineering. In any system, there are three distinct types of equilibrium: stable, neutral, and unstable.
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Modular Approach to Selected Configuration Interaction in an Arbitrary Spin Basis: Implementation and Comparison of

Andrew W Prentice1, Jeremy P Coe1, Martin J Paterson1

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A new modular selected configuration interaction (SCI) code accurately predicts potential energy surfaces. This computational chemistry tool generates compact wave functions, offering significant efficiency gains over traditional methods.

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Area of Science:

  • Quantum Chemistry
  • Computational Physics

Background:

  • Accurate prediction of molecular properties requires computationally intensive methods like Full Configuration Interaction (FCI).
  • Existing methods face challenges in balancing accuracy with computational cost, especially for larger systems.

Purpose of the Study:

  • To develop a modular Selected Configuration Interaction (SCI) code for efficient and accurate quantum chemical calculations.
  • To enable fair comparison of different wave function construction criteria through a flexible code structure.

Main Methods:

  • Development of a modular SCI code building upon the Monte-Carlo Configuration Interaction (MCCI) framework.
  • Implementation of energy- and coefficient-driven selection schemes in both Slater determinant (SD) and Configuration State Function (CSF) bases.
  • Extension to state-averaged regimes for improved handling of complex electronic structures.

Main Results:

  • All implemented SCI methods accurately predicted potential energy surfaces for bond-breaking scenarios.
  • Achieved highly compact wave functions compared to FCI, with minimal nonparallelity errors (within chemical accuracy).
  • Demonstrated efficiency by utilizing only 0.02% of the FCI CSF space for adaptive SCI.

Conclusions:

  • The developed modular SCI code provides a computationally efficient alternative to FCI for accurate electronic structure calculations.
  • The code's modularity facilitates the implementation and comparison of various selection strategies.
  • The use of CSF basis offers advantages in reduced dimensionality and guaranteed spin purity.