Antiaromatic Character of 16 π Electron Octaethylporphyrins:
Magnetically Induced Ring Currents from DFT-GIMIC Calculations
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Abstract
The
magnetically induced current density susceptibility, also called
current density, has been calculated for a recently synthesized octaethylporphyrin
(OEP) zinc(II) dication with formally 16 π electrons. Numerical
integration of the current density passing selected chemical bonds
yields the current pathway around the porphyrinoid ring and the strength
of the ring current. The current strengths show that the OEP-Zn(II)
dication is strongly antiaromatic, as also concluded experimentally.
The calculation of the ring current pathway shows that all 24 π
electrons participate in the transport of the ring current because
the current splits into inner and outer branches of practically equal
strengths at the four pyrrolic rings. The corresponding neutral octaethylporphyrinoid
without Zn and inner hydrogens is found to be antiaromatic, sustaining
a paratropic ring current along the inner pathway with 16 π
electrons. The neutral OEP-Zn(II) molecule with formally 18 π
electrons is found to be almost as aromatic as free-base porphyrin.
However, also in this case, all 26 π electrons contribute to
the ring current, as for free-base porphyrin. A comparison of calculated
and measured <sup>1</sup>H NMR chemical shifts is presented. The current
strength susceptibility under experimental conditions has been estimated
by assuming a linear relation between experimental shielding constants
and calculated current strengths