The aromatic pathways of porphins, chlorins and bacteriochlorins

Abstract

The aromatic pathways of free-base porphins, chlorins and bacteriochlorins have been studied by calculating the nuclear magnetic shieldings at selected points outside the molecules. The strengths of the induced ring currents for a given magnetic Ðeld have been obtained by using the aromatic ring current shieldings (ARCS) method. We found that pyrrolic rings with an inner hydrogen have a local ring current of the same magnitude as the ring current for the pyrrole molecule. The local ring current for pyrrolic rings without an inner hydrogen is between half and one fourth of the pyrrole value depending on the porphyrin. The units of C 2 H 2 the pyrrolic rings in free-base porphin have recently been suggested to function as exocyclic bridges, but the present study indicates that this is not the case. The suggested 18p-[16]annulene inner cross aromatic pathway does not exist until all pyrrolic rings are saturated in the b-positions. Free-base trans-porphin was found to have the largest ring-current susceptibility among the molecules studied. Porphyrins for which the aromatic pathway has to pass the nitrogen of a pyrrolic unit with an inner hydrogen have signiÐcantly smaller ring currents than free-base trans-porphin. We also show that the 1H NMR shielding of the inner hydrogens is a sensitive measure of the strength of the ring current for the studied molecules.

Ring Currents in the Porphyrins: A Four-Orbital Model

“…[4, 5, 7±10] Ring currents have been studied theoretically by calculating the response of the electron distribution to an external magnetic field. This includes direct calculation of long-range shieldings at points outside the molecule [12] and of shieldings at the centres of rings, that is, the nucleus-independent chemical shifts (NICS) [11] that have made such a valuable contribution to the systematisation of ring currents, both diamagnetic (aromatic, diatropic) and paramagnetic (antiaromatic, paratropic).…”

“…Indirect evidence of currents in the porphin ring comes from experimental proton chemical shifts [13] and from calculations of the magnetic shieldings at chosen points within the molecule. [11,12] Here we present direct visualisations of the p current density of free-base porphin, chlorin and magnesium porphin. We show how a recently developed orbital analysis [14,15] of current density unifies the interpretation of ring currents with that of the electronic spectrum in terms of a fourorbital model.…”

“…[4, 5, 7±10] Ring currents have been studied theoretically by calculating the response of the electron distribution to an external magnetic field. This includes direct calculation of long-range shieldings at points outside the molecule [12] and of shieldings at the centres of rings, that is, the nucleus-independent chemical shifts (NICS) [11] that have made such a valuable contribution to the systematisation of ring currents, both diamagnetic (aromatic, diatropic) and paramagnetic (antiaromatic, paratropic).With modern distributed gauge-origin methods in quantum chemistry, it is now possible to obtain accurate magnetic properties of large molecules, and to calculate direct visualisations of current densities induced by an external magnetic field, including the p electron ring currents in conjugated cyclic systems. [16±22] The present calculations were performed with the 6-31G** basis by means of coupled Hartree ± Fock theory within the ipsocentric [14,15] CTOCD-DZ (continuous transformation of origin of current density diamagnetic zero) formulation, [16,17] in which the induced current density at a given point is calculated with that point as the origin of vector potential.…”

“…[6] A notable feature of the induced circulation in porphin is the bifurcation of the current across both types of pyrrole subunit, with no evidence of the local ring currents that were proposed in some models. [11,12] The NICS values [11] (centre: À 16.5; rings A, C: À 15.2; rings B, D: À 4.5 ppm) correctly predict a diamagnetic circulation around the centre of the molecule and distinguish between the two types of pyrrole unit, but this sampling of point shieldings does not distinguish ring currents from bifurcated currents. The sense of circulation depends on whether (facing in the direction of flow) the bifurcated current is greater on the right (rings A, C; apparent local diatropicity) or on the left (rings B, D; apparent local paratropicity).…”

Synthesis and spectroscopic characterization on 4-(2,5-di-2-thienyl-1H-pyrrol-1-yl) benzoic acid: A DFT approach

“…Pyrroles are components of more complex macrocycles, including the porphyrins of heme, the chlorins, bacteriochlorins, chlorophyll, porphyrinogens [1]. Pyrrole is a 5-membered aromatic heterocycle, like furan and thiophene.…”