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Related Concept Videos

Atomic Nuclei: Nuclear Relaxation Processes01:23

Atomic Nuclei: Nuclear Relaxation Processes

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In the absence of an external magnetic field, nuclear spin states are degenerate and randomly oriented. When a magnetic field is applied, the spins begin to precess and orient themselves along (lower energy) or against (higher energy) the direction of the field. At equilibrium, a slight excess population of spins exists in the lower energy state. Because the direction of the magnetic field is fixed as the z-axis,  the precessing magnetic moments are randomly oriented around the z-axis.
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Spin–Spin Coupling Constant: Overview01:08

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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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¹H NMR: Interpreting Distorted and Overlapping Signals01:02

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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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The spin state of an NMR-active nucleus can have a slight effect on its immediate electronic environment. This effect propagates through the intervening bonds and affects the electronic environments of NMR-active nuclei up to three bonds away; occasionally, even farther. This phenomenon is called spin–spin coupling or J-coupling. Coupling interactions are mutual and result in small changes in the absorption frequencies of both nuclei involved. While nuclei of the same element are involved...
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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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NMR Spectrometers: Resolution and Error Correction01:14

NMR Spectrometers: Resolution and Error Correction

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When magnetic nuclei in a sample achieve resonance and undergo relaxation, the signal detected in NMR is an approximately exponential free induction decay. Fourier transform of an exponential decay yields a Lorentzian peak in the frequency domain. Lorentzian peaks in an NMR spectrum are defined by their amplitude, full width at half maximum, and position, where the peak width is governed by the spin-spin relaxation time alone. In real experiments, however, the applied magnetic field is rendered...
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Spin Saturation Transfer Difference NMR SSTD NMR: A New Tool to Obtain Kinetic Parameters of Chemical Exchange Processes
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New Gradient Correction Scheme for Electronically Nonadiabatic Dynamics Involving Multiple Spin States.

Yinan Shu1, Linyao Zhang2, Dihua Wu1

  • 1Department of Chemistry and Supercomputing Institute, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States.

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|April 20, 2023
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Summary

This study introduces a new time-derivative-matrix scheme to avoid calculating nonadiabatic coupling vectors (NACs) in trajectory surface hopping (TSH) simulations for intersystem crossing. This method enhances computational efficiency for TSH calculations.

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Area of Science:

  • Computational Chemistry
  • Quantum Chemistry
  • Theoretical Chemistry

Background:

  • Trajectory surface hopping (TSH) calculations are crucial for simulating non-adiabatic processes in molecules.
  • The fully adiabatic basis is recommended for TSH, but conventional methods require computing nonadiabatic coupling vectors (NACs) for intersystem crossing.
  • Calculating NACs in the molecular-Coulomb-Hamiltonian (MCH) basis negates efficiency gains from overlap-based and curvature-driven algorithms.

Purpose of the Study:

  • To develop a novel computational scheme that circumvents the need for explicit NAC computation in TSH simulations.
  • To enable more efficient TSH calculations for intersystem crossing processes.

Main Methods:

  • A new time-derivative-matrix scheme was developed.
  • This scheme avoids the explicit calculation of nonadiabatic coupling vectors (NACs).

Main Results:

  • The time-derivative-matrix scheme successfully circumvents the requirement for NACs in TSH simulations of intersystem crossing.
  • This approach preserves the advantages of efficient TSH algorithms.

Conclusions:

  • The proposed time-derivative-matrix scheme offers a more computationally efficient approach to trajectory surface hopping for intersystem crossing.
  • This advancement can accelerate simulations of complex chemical dynamics.