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¹H NMR of Conformationally Flexible Molecules: Temporal Resolution00:52

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At room temperature, the chair conformer of cyclohexane undergoes rapid ring flipping between two equivalent chair conformers at a rate of approximately 105 times per second. These two chair conformers are in equilibrium. The rapid ring flipping results in the interconversion of the axial proton to an equatorial proton and an equatorial to the axial proton. Such interconversions are too rapid and cannot be detected on the NMR timescale. Hence, the NMR spectrometer cannot distinguish between the...
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Characterization of Deformational Isomerization Potential and Interconversion Dynamics with Ultrafast X-ray Solution

Natalia E Powers-Riggs1, Benedikt O Birgisson2, Sumana L Raj1

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Metal dimer complexes with rigid ligands exhibit two ground state isomers due to competing dispersion forces and steric strain. Ultrafast spectroscopy reveals equilibrium restoration in 2.3 ps, aiding computational modeling.

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Area of Science:

  • Inorganic Chemistry
  • Physical Chemistry
  • Computational Chemistry

Background:

  • Dimeric complexes with d8 square planar metal centers and rigid bridging ligands serve as models for understanding dispersion forces and steric strain.
  • The [Ir2(dimen)4]2+ complex (dimen = para-diisocyanomenthane) exhibits two ground state deformational isomers due to structural distortions counteracting attractive forces.

Purpose of the Study:

  • To investigate the equilibrium distribution and exchange dynamics between the two ground state deformational isomers of the [Ir2(dimen)4]2+ complex.
  • To provide a critical test for density functional approximations in modeling bridged d8-d8 metal complexes.

Main Methods:

  • Ultrafast X-ray solution scattering (XSS) and optical transient absorption spectroscopy (OTAS) were employed to probe isomer dynamics.
  • Selective photoexcitation of individual isomers using femtosecond visible light pulses.

Main Results:

  • Distinct electronic excitations allowed for selective excitation and tracking of ground state population dynamics.
  • Restoration of ground state equilibrium was observed within 2.3 picoseconds.
  • Density functional theory calculations accurately reproduced experimental observations when dispersion interactions were included and a hybrid functional was used.

Conclusions:

  • The study elucidates the interplay of dispersion forces and steric strain in dictating the structure of metal dimer complexes.
  • Ultrafast spectroscopy provides powerful insights into the dynamics of isomer interconversion.
  • Hybrid density functional theory with dispersion corrections is effective for modeling such d8-d8 metal complexes.