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Ideally, an unpaired electron shows a single peak in the EPR spectrum due to the transition between the two spin energy states. However, coupling interactions can occur between the spins of the unpaired electron and any neighboring spin-active nuclei. This hyperfine coupling results in hyperfine splitting, where the EPR signal is split into multiplets. The signals split into 2nI + 1 peaks, where n is the number of equivalent nuclei and I is the nuclear spin. These splitting patterns provide...
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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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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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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-Orbit versus Hyperfine Coupling-Mediated Intersystem Crossing in a Radical Pair.

Sam R May1, Clàudia Climent1, Zhen Tao1

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Hyperfine coupling (HFC) becomes crucial in multichromophore systems, influencing intersystem crossing (ISC) rates. HFC dominates over spin-orbit coupling (SOC) as distance increases, particularly in radical pair mechanisms.

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Area of Science:

  • Photochemistry
  • Quantum Chemistry
  • Chemical Physics

Background:

  • Spin-orbit coupling (SOC) typically dominates singlet-triplet state interactions in single chromophores.
  • Hyperfine coupling (HFC) gains significance in multichromophoric systems, especially for radical pair mechanisms.

Purpose of the Study:

  • To investigate the interplay between SOC and HFC in a pyrene/N,N-dimethylaniline radical pair.
  • To determine how intermolecular distance affects the relative contributions of SOC and HFC to intersystem crossing (ISC).

Main Methods:

  • Time-dependent density-functional theory (TD-DFT) calculations.
  • Calculation of SOC and HFC for the first singlet and triplet charge-transfer states.
  • Analysis of electronic structure and coupling mechanisms as a function of donor-acceptor distance.

Main Results:

  • SOC between singlet and triplet states decays to zero with increasing intermolecular distance.
  • HFC remains relatively constant despite changes in intermolecular distance.
  • A transition occurs around 4 Å where HFC becomes the dominant factor in ISC rate.

Conclusions:

  • HFC is a critical factor in controlling ISC rates in radical pair systems at larger intermolecular distances.
  • The findings highlight the importance of considering HFC in multichromophoric systems for understanding photophysical processes like ISC.