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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.
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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.
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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.
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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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Double resonance techniques in Nuclear Magnetic Resonance (NMR) spectroscopy involve the simultaneous application of two different frequencies or radiofrequency pulses to manipulate and observe two distinct nuclear spins. One important application of double resonance is spin decoupling, which selectively suppresses coupling with one type of nucleus while observing the NMR signal from another nucleus, simplifying the spectrum and enhancing resolution.
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¹H NMR: Pople Notation01:09

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State-Specific Coupled-Cluster Methods for Excited States.

Yann Damour1, Anthony Scemama1, Denis Jacquemin2,3

  • 1Laboratoire de Chimie et Physique Quantiques (UMR 5626), Université de Toulouse, CNRS, UPS, 31000 Toulouse, France.

Journal of Chemical Theory and Computation
|May 15, 2024
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Summary
This summary is machine-generated.

We evaluated the ΔCCSD method for calculating molecular excited states, finding it effective for doubly excited states but generally less accurate than EOM-CCSD for other types. State-specific orbitals offered minor improvements.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • The standard Equation-of-Motion Coupled-Cluster with Single and Double excitations (EOM-CCSD) method struggles with doubly excited states.
  • The ΔCCSD method, utilizing non-Aufbau determinants, offers an alternative for targeting excited states, particularly doubly excited ones.

Purpose of the Study:

  • To benchmark the accuracy and consistency of the ΔCCSD method against EOM-CCSD for various types of molecular excited states.
  • To assess the performance of ΔCCSD beyond doubly excited states, including doublet-doublet transitions and singly excited states of closed-shell systems.

Main Methods:

  • Comparison of ΔCCSD and EOM-CCSD methods for computing excitation energies.
  • Utilized a dataset of 276 excited states from the quest database.
  • Employed a minimalist two-determinant coupled-cluster approach for singly excited states in closed-shell systems.

Main Results:

  • ΔCCSD shows effectiveness for doubly excited states but generally underperforms EOM-CCSD for other excitation types.
  • Mean absolute errors (MAEs) for doublet-doublet transitions were 0.10 eV (ΔCCSD) vs. 0.07 eV (EOM-CCSD).
  • MAEs for singly excited states were 0.15 eV (ΔCCSD) vs. 0.08 eV (EOM-CCSD), with higher multiconfigurational character contributing to ΔCCSD's lower accuracy.

Conclusions:

  • ΔCCSD is a viable method for doubly excited states where EOM-CCSD falters.
  • For most other excited states, EOM-CCSD remains the more accurate and consistent choice.
  • State-specific optimized orbitals provide marginal improvements for ΔCCSD accuracy.