Aromatic Pathways of Porphins,
Chlorins, and Bacteriochlorins
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Abstract
Magnetically induced current densities have been calculated
for free-base porphynoids using the gauge including magnetically induce
current (GIMIC) method. Numerical integration of the current density
passing selected chemical bonds yields current pathways and the degree
of aromaticity according to the magnetic criterion. The ring-current
strengths of the porphins, chlorins, and bacteriochlorins are 1.5–2.5
times stronger than for benzene. The calculations show that the 18π
[16]annulene inner cross is not the correct picture of the aromatic
pathway for porphyrins. All conjugated chemical bonds participate
in the current transport independently of the formal number of π
electrons. The ring current branches at the pyrrolic rings taking
both the outer and the inner route. The NH unit of the pyrrolic rings
has a larger resistance and a weaker current strength than the pyrroles
without inner hydrogens. The traditional 18π [18]annulene with
inactive NH bridges is not how the ring-current flows around the macroring.
The porphins have the strongest ring current of ca. 27 nA/T among
the investigated porphynoids. The current strengths of the chlorins
and bacteriochlorins are 19–24 nA/T depending on whether the
ring current is forced to pass an NH unit or not. The current strengths
of the 3-fold and 4-fold β-saturated porphynoids are 13–17
nA/T, showing that the inner-cross 18π [16]annulene pathway
is not a preferred current route