Abstract

GORDILLO-ROMAN, Bárbara. Radical stabilization of meso porphyrinoids: Gleaning theoretical data for the rational design of porphyrinyl prodrugs. Rev. latinoam. quím [online]. 2012, vol.40, n.3, pp.148-163. ISSN 0370-5943.

This article is engaged with the search for a confident theoretical model to study the stabilization energies and geometry of free radicals stabilized by a porphin ring and/or a p-X-substituted phenyl ring, where X is either an electron donating or withdrawing atom or group. DFT calculations at the B3LYP/6-31G(d,p) or UB3LYP/6-31G(d,p) levels of theory were applied to homolog series of saturated porhyrins 2(a-h) - 5(a-h) and porphyrinoids 3⋅(a-h), 4⋅(a-h) series, respectively. Calculated radical stabilization energy (RSE) values for the various p-X-phenyl substituents of benzyl radical 6⋅(a-h) series obtained through an isodesmic reaction model (Scheme 1) provided a quite good correlation (r = 0.989) with the radical total effect (TE) reported by Wu (Wu et al., 1996). The TE effect is generally accepted as a scale to study radical stabilization due to spin delocalization, since it is in a very good agreement with the experimental ESR hyperfine coupling constants (hfc) of benzyl radicals (Dust and Arnold, 1983). Likewise, isodesmic RSE values for porphyrinoid 3⋅(a-h) series provided a good correlation (r = 0.940) with RSE values for 6⋅. However, RSE values for porphyrinoid 4β⋅(a-h) series deviate from the expected correlation trend (r = 0.731), not to mention 4α⋅(a-h) series (r = 0.660). Relative absorption wavenumbers (Δλ, nm) at the λ_max bands of porphyrin series [λ(3^.) -λ(3)] and [λ(4^.) -λ(4)] indicate large porphyrinoid bathochromic effects (ca. 200 - 300 nm).

Keywords : porphyrin; theoretical calculations; free radicals; bathochromic effect.