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Related Concept Videos

¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

1.3K
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...
1.3K
Spin–Spin Coupling Constant: Overview01:08

Spin–Spin Coupling Constant: Overview

985
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...
985
Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule01:10

Interpreting ¹H NMR Signal Splitting: The (n + 1) Rule

1.4K
In the AX proton spin system, proton A can sense the two spin states of a coupled proton X, resulting in a doublet NMR signal with two peaks of equal (1:1) intensity. When proton A is coupled to two equivalent protons (AX2 spin system), the spin states of each X can be aligned with or against the external field, creating three possible scenarios. This results in a 1:2:1  triplet signal, where the central peak corresponds to the chemical shift of A and is twice as large or intense as the...
1.4K
¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

5.3K
When protons A and X are coupled, their nuclear spin energy levels are slightly modified. This is because the energy required to excite proton A to a spin state parallel to proton X is slightly different from the energy required for it to become anti-parallel to spin X. Consequently, there are two possible excitation frequencies for A (A1 and A2), depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2, indicated...
5.3K
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

1.9K
The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
1.9K
Spin–Spin Coupling: Three-Bond Coupling (Vicinal Coupling)01:22

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

1.1K
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...
1.1K

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Isotopic Effect in Double Proton Transfer Process of Porphycene Investigated by Enhanced QM/MM Method
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(T) Correction for Multicomponent Coupled-Cluster Theory for a Single Quantum Proton.

Dylan Fowler1, Kurt R Brorsen1

  • 1Department of Chemistry, University of Missouri, Columbia, Missouri65203, United States.

Journal of Chemical Theory and Computation
|November 23, 2022
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Summary
This summary is machine-generated.

Perturbative corrections improve multicomponent coupled-cluster theory with single and double excitations (CCSD) calculations. These enhanced methods accurately predict proton affinities and energies for systems involving a single quantum proton.

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Area of Science:

  • Quantum chemistry
  • Theoretical chemistry
  • Computational physics

Background:

  • Coupled-cluster theory is a powerful method for electronic structure calculations.
  • Multicomponent methods are necessary for systems with significant electron-nuclear interactions.
  • Proton affinities and absolute energies are key chemical properties.

Purpose of the Study:

  • To derive and implement (T) and [T] perturbative corrections for multicomponent coupled-cluster theory with single and double excitations (CCSD).
  • To assess the accuracy of these corrected methods for calculating proton affinities and absolute energies.
  • To introduce an approximation for computational efficiency by considering only specific contributions from mixed electron-nuclear excitations.

Main Methods:

  • Derivation of (T) and [T] perturbative corrections within the multicomponent CCSD framework.
  • Application and benchmarking of the developed methods to systems with a single quantum proton.
  • Comparison of results with standard multicomponent CCSD calculations.

Main Results:

  • Multicomponent CCSD methods incorporating (T) or [T] perturbative corrections demonstrate higher accuracy compared to standard multicomponent CCSD.
  • The enhanced methods provide more reliable calculations of proton affinities and absolute energies.
  • An approximation focusing on specific mixed electron-nuclear excitation contributions was successfully introduced.

Conclusions:

  • Perturbative corrections significantly enhance the accuracy of multicomponent CCSD for chemical property calculations.
  • The developed methods offer a more precise approach for studying systems with strong electron-nuclear coupling.
  • The introduced approximation provides a computationally viable route to accurate results.