Effects Of Peripheral Substituents Research Articles

Effects of Peripheral Substituents and Axial Ligands on the Electronic Structure and Properties of Iron Phthalocyanine

The effects of peripheral substituents and axial ligands on the electronic structure and properties of iron phthalocyanine, H(16)PcFe, have been investigated using a DFT method. Substitution by electron-withdrawing fluorinated groups alters the ground state of H(16)PcFe and gives rise to large changes in the ionization potentials and electron affinity. For the six-coordinate adducts with acetone, H(2)O, and pyridine, the axial coordination of two weak-field ligands leads to an intermediate-spin ground state, while the strong-field ligands make the system diamagnetic. The electronic configuration of a ligated iron phthalocyanine is determined mainly by the axial ligand-field strength but can also be affected by peripheral substituents. Axial ligands also exert an effect on ionization potentials and electron affinity and can, as observed experimentally, even change the site of oxidation/reduction.

The effect of peripheral substituents in metalloporphyrins on their catalytic activity in Lyons system

Manganese porphyrin catalysts with different number of halogens atoms substituted at the phenyl and pyrrole rings were investigated in the Lyons system in oxidation of cyclooctane with molecular oxygen (as air) to cyclooctanone and cyclooctanol without the use of sacrificial co-reductant. The catalytic activity of metalloporphyrins increases with the increase of the redox potential of metalloporphyrins in the investigated system and shows almost linear relationship with the number of the halogens on porphyrin macrocycle. On the basis of obtained results the new reaction mechanism is discussed and proposed.

Steric and Electronic Effects of Substituents on the Yield of 5,15-Substituted Octaalkylporphines

The electronic and steric effects of peripheral substituents on the yield of 5,15-diphenyloctaalkylporphines were studied. It was found that the electronic nature of substituents in the starting benzaldehydes exerts almost no yield effect. At the same time, there is some optimal size of substituents, which provides the highest possible yield of porphyrins.

A First-Principles Quantum Chemical Study of Coenzyme F430: Interplay of Skeletal Stereoisomerism and Conformation in the Stabilization of Nickel(I)

Nonlocal density functional theory calculations, with full geometry optimization, are reported for the Ni(I) and low-spin Ni(II) forms of high-fidelity models of coenzyme F430, the nickel tetracorphinoid cofactor of methylcoenzyme M reductase (MCR), and its 12,13-diepimer. The diepimer appears to exhibit the conformational characteristics of a typical hydroporphyrin in terms of a strong tendency to adopt highly ruffled conformations and short Ni(II)−N bond distances. In contrast, for native F430, the steric effects of peripheral substituents impose a potent planarizing influence on the ring system. The relative inability to ruffle implies that the N4 core of F430 cannot contract sufficiently to optimally coordinate a small low-spin Ni(II) ion. This appears to be the key factor that results in the stabilization of the larger Ni(I) and high-spin Ni(II) ions by the F430 ligand environment. The optimized Ni−N bond distances for the Ni(I)−F430 model compound are 198, 200, 203, and 214 pm and span an extremely wide range of 16 pm, which qualitatively reproduces the central feature of the experimental EXAFS results. Understandably, these bond distances are similar to those found in a crystallographic study of a six-coordinate Ni(II) form of MCR. The relatively long Ni−N distances in the optimized geometry of the low-spin Ni(II) form of the F430 model compound provide a natural explanation for the enhanced axial ligand affinity of F430 and its greater tendency to switch to the high-spin Ni(II) form, relative to its diepimer. Consistent with experiment, the calculations also predict that Ni(II)−diepi-F430 is thermodynamically more stable than native Ni(II)−F430. However, for the Ni(I) oxidation level, the two epimers are predicted to be equienergetic. In qualitative agreement with electrochemical measurements, the adiabatic ionization potential of Ni(I)−F430 is about 0.2 eV higher than that of Ni(I)−diepi-F430, again reflecting a unique destabilization of low-spin Ni(II) by the F430 ligand. Finally, the nickel center in Ni(I)−F430 is truly Ni(I): it carries approximately 82% of the molecular unpaired spin, compared to a nickel spin population of only 56% for Ni(I)−diepi-F430.

Qy-excitation resonance Raman spectra of chlorophyll a and bacteriochlorophyll c/d aggregates. Effects of peripheral substituents on the low-frequency vibrational characteristics.

Low-frequency (80-700 cm-1) Qy-excitation resonance Raman (RR) spectra are reported for thin-solid-film aggregates of several chlorophyll (Chl) a and bacteriochlorophyll (BChl) c/d pigments. The pigments include Chl a, pyrochlorophyll a (PChl a), methylpyrochloropyllide a (MPChl a), methylbacteriochloropyllide d (MBChl d), [E,M] BChl cS, [E,E] BChl cF, and [P,E] BChl cF. The BChl c/d's are the principal constituents of the chlorosomal light-harvesting apparatus of green photosynthetic bacteria. Together, the various Chl a's and BChl c/d's represent a series in which the peripheral substituent groups on the chlorin macrocycle are varied in systematic fashion. All of the Chl a and BChl c/d aggregates exhibit rich low-frequency vibrational patterns. In the case of the BChl c/d's, certain modes in the very low-frequency region (100-200 cm-1) experience exceptionally strong Raman intensity enhancements. The frequencies of these modes are qualitatively similar to those of oscillations observed in femtosecond optical experiments on chlorosomes. The RR data indicate that the low-frequency vibrations are best characterized as intramolecular out-of-plane deformations of the chlorin macrocycle rather than intermolecular modes. The coupling of the out-of-plane modes in turn implies that the Qy electronic transition(s) of the aggregate have out-of-plane character. The RR spectra of the BChl c/d's also reveal that the nature of the alkyl substituents at the 8 and 12 positions of the macrocycle plays an important role in determining the detailed features of the low-frequency vibrational patterns. The frequencies of the modes are particularly sensitive to larger substituent groups whose conformations may be more easily perturbed in the tightly packed aggregates. These factors also make aggregates of pigments containing larger substituents more susceptible to structural, electronic, and vibrational inhomgeneities. Collectively, the RR studies of the various pigments delineate the factors which influence the low-frequency vibrational characteristics of chlorosomal aggregates.

Electronic effects of peripheral substituents in porphyrins: x-ray photoelectron spectroscopy and ab initio self-consistent field calculations

The electronic effects of a wide variety of peripheral substituents in free base prophyrins have been evaluated using X-ray photoelectron spectroscopy (XPS) and all-electron ab initio calculations. Both methods have identified the 1s energies of the central nitrogens as excellent sensors of the electronic effects of peripheral substituents. Core level photoelectron spectra are reported fornine porphyrins, including unsubtituted prophyrin, octaethylporphyrin, and seven tetraphenylporphyrins

A study of substituent effects in quadridentate schiff base complexes with nickel(II). 13C NMR, electronic spectra and polarographic measurements

The effect of peripheral substituents on the electron distribution of a Ni(II) complex with a quadridentate Schiff base ligand has been investigated through 13C NMR, electronic spectroscopy and half-wave potential measurements. It has been demonstrated that electron-withdrawing groups such as COCH 3 and COOC 2H 5 decrease the electron density on the Ni(II) atom, and that this effect is responsible for the stabilization of the O 2 adducts of the corresponding Co(II) complexes at ambient temperature.

NMR studies of low-spin ferric complexes of natural porphyrin derivatives. 1. Effect of peripheral substituents on the .pi. electronic asymmetry in biscyano complexes

NMR studies of low-spin ferric complexes of natural porphyrin derivatives. 1. Effect of peripheral substituents on the .pi. electronic asymmetry in biscyano complexes

Resonance Raman spectra of the nickel, cobalt, and copper chelates of mesoporphyrin IX dimethyl ester

Resonance Raman spectra have been obtained for the Ni, Co, and Cu chelates of mesoporphyrin IX dimethyl ester in solution using the tunable Rhodamine 6G dye and Ar+ lasers. Several bands show variation of intensity and depolarization ratio with exciting wavelength. Excitation profiles for some isolated bands peak at the maxima of the corresponding α electronic absorption bands. The variation of depolarization ratio for some anomalously polarized bands (ρl > 3/4) has been interpreted in terms of lowering of the symmetry of metalloporphyrins from D4h. The rigorous symmetry of these chelates is proposed as Cs because of the effect of peripheral substituents. A vibrational assignment is suggested for some of the metal sensitive modes.

Porphyrins: XIII: Fluorescence spectra and quantum yields

Fluorescence spectra, quantum yields, natural radiative lifetimes, and absorption oscillator strengths are reported for a number of porphyrins in benzene solution. Emission yields for free base, Mg-, and Zn-phthalocyanine in 1-chloronaphthalene solution are also reported. The phthalocyanines are considerably more fluorescent than the porphyrins, and this is attributed mainly to their shorter radiative lifetimes. The heavy atom effect on fluorescence is strong with yields in the order H 2 ∼ Mg > Zn ⪢ Cd. The effect of peripheral substituents is small, both on absorption and fluorescence. Four free base porphyrins studied all showed yields of about 10%, natural radiative lifetimes about 120 nsec, and oscillator strengths about 0.03. Closed-shell metal derivatives have natural radiative lifetimes roughly half those for the free bases, independent of the atomic number of the metal. A simple theoretical model of borrowing from the B band gives a good explanation for the difference between metal and free-base radiative lifetimes.