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Related Concept Videos

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...
IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration01:16

IR Spectroscopy: Hooke's Law Approximation of Molecular Vibration

A covalently bonded heteronuclear diatomic molecule can be modeled as two vibrating masses connected by a spring. The vibrational frequency of the bond can be expressed using an equation derived from Hooke's law, which describes how the force applied to stretch or compress a spring is proportional to the displacement of the spring. In this case, the atoms behave like masses, and the bond acts like a spring.
According to Hooke's law, the vibrational frequency is directly proportional to the...
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)

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 involved orbitals. The...
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,...
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...
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...

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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Zero-point vibrational corrections to isotropic hyperfine coupling constants in polyatomic molecules.

Xing Chen1, Zilvinas Rinkevicius, Zexing Cao

  • 1Department of Chemistry and State Key Laboratory of Physical Chemistry of Solid Surfaces, Xiamen University, Xiamen 361005, China.

Physical Chemistry Chemical Physics : PCCP
|November 11, 2010
PubMed
Summary

This study highlights the importance of zero-point vibrational corrections for accurate isotropic hyperfine coupling constants in polyatomic systems. These corrections are crucial for understanding molecular properties and refining computational methods.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Molecular Spectroscopy

Background:

  • Isotropic hyperfine coupling constants (hcc) are vital for understanding electronic structure.
  • Previous studies often neglected zero-point vibrational corrections (ZPVCs).
  • ZPVCs can significantly impact calculated hccs in polyatomic systems.

Purpose of the Study:

  • To investigate the significance of ZPVCs on isotropic hccs.
  • To evaluate ZPVCs for the allyl radical and its derivatives.
  • To provide guidelines for identifying significant ZPVCs.

Main Methods:

  • Density functional theory (DFT) using the restricted-unrestricted approach.
  • Calculation of ZPVCs for isotropic hccs.
  • Analysis of ZPVCs in the allyl radical system.

Main Results:

  • ZPVCs were found to be significant for isotropic hccs.
  • Guidelines were developed to identify hydrogens with significant ZPVCs.
  • The study re-examines computational procedures for hcc determination.

Conclusions:

  • ZPVCs are a critical factor in accurate hcc calculations.
  • The findings necessitate a re-evaluation of current computational practices.
  • Experimental hccs should be used cautiously for benchmarking.