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Related Concept Videos

¹H NMR Signal Multiplicity: Splitting Patterns01:13

¹H NMR Signal Multiplicity: Splitting Patterns

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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...
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Spin–Spin Coupling: Two-Bond Coupling (Geminal Coupling)01:20

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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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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

¹H NMR: Interpreting Distorted and Overlapping Signals

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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...
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¹H NMR: Complex Splitting01:13

¹H NMR: Complex Splitting

2.1K
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...
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Spin–Spin Coupling: One-Bond Coupling01:17

Spin–Spin Coupling: One-Bond Coupling

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

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

3.0K
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...
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Coupled cluster Green function: Model involving single and double excitations.

Kiran Bhaskaran-Nair1, Karol Kowalski2, William A Shelton1

  • 1Cain Department of Chemical Engineering, Louisiana State University, Baton Rouge, Louisiana 70803, USA.

The Journal of Chemical Physics
|April 17, 2016
PubMed
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This summary is machine-generated.

We developed a parallel Green

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Area of Science:

  • Computational chemistry
  • Quantum chemistry
  • Electronic structure theory

Background:

  • Coupled-cluster (CC) methods are essential for accurate electronic structure calculations.
  • Green function methods provide insights into excitation energies and ionization potentials.
  • Parallel implementations are crucial for tackling large quantum systems.

Purpose of the Study:

  • To develop and present a parallel implementation of the coupled-cluster Green function formulation with single and double excitations (GFCCSD).
  • To accurately determine the frequency-dependent self-energy Σ(ω) within this framework.
  • To validate the approach by calculating ionization potentials for benchmark systems.

Main Methods:

  • Developed a parallel algorithm for the Green function coupled-cluster singles and doubles (GFCCSD) method.
  • Employed approximations to preserve the pole structure of GFCCSD, reducing computational cost.
  • Analyzed the block structure of the self-energy for systems with varying local correlation.

Main Results:

  • Successfully implemented a parallel GFCCSD approach.
  • The method accurately calculates the frequency-dependent self-energy.
  • Calculated ionization potentials for benchmark systems show good agreement with experimental data.
  • Observed a correlation between system's local correlation strength and the block structure of the self-energy.

Conclusions:

  • The parallel GFCCSD implementation is accurate and efficient for electronic structure calculations.
  • The method provides a reliable way to compute ionization potentials.
  • The observed block structure offers insights into electron correlation effects in different systems.