Electronic structure and spectra of NH-isomers of isobacteriochlorin

Abstract

ev UDC 539.19Using the restricted and unrestricted Hartree-Fock method (RHF and UHF) with the AM1 Hamiltonian, we have calculated the geometric structure of the NH-isomers of isobacteriochlorin and its meso-cyano derivative. We have used the CNDO/S method for all the considered structures to calculate the transitions to excited electronic states. We have analyzed the effect of taking into account electron correlation on the results of the calculations of the chemical and geometric structure and the electronic spectra. For the a isomer with the imino hydrogens opposite each other, we obtained a planar structure corresponding to literature x-ray diffraction data. For the aromatic c isomer and other isomers with the imino hydrogens adjacent to each other, the calculations lead to a substantial nonplanarity of the macrocycle (defined mainly by the tilt of the pyrrole rings) and also deviation of the NH bond from the plane of the pyrrole ring. The ground-state energies of the a and c isomers have close values, which generally explains the observed NH tautomerism. We calculated the dipole moments of the isomers in the ground state and the excited state. The results of the calculation of the characteristics of the excited states with UHF geometry on the whole seem to be preferred over those with RHF geometry. We show that only for the aromatic c isomer can we describe the electronic absorption spectrum within a four-orbital model.

Effect of bridging aza-substitution on electronic structure and spectral-luminescent properties of isobacteriochlorin NH-isomers

“…These maxima are situated at 516 and 569 nm for H 2 TAiBC tt . Based on the calculations given below and the literature [17] and by analogy with other tetrapyrroles, it can be confirmed that these maxima do not belong to two separate electronic transitions. There are two possibilities.…”

“…Luminescence of octaethyl H 2 iBC and its metal complexes have been studied [6,7], with most attention paid to polarized fluorescence spectra of the metal complexes [7] (the free base does not phosphoresce). Derivatives of H 2 iBC that are stabilized by adding two methyls to each of the β-carbons for both hydrogenated pyrrole rings, H 2 -2,2,7,7,12,13,17,18-octamethyl-iBC (H 2 OMiBC), and the analogous compound with a cyano group in the meso-position, H 2 -5-cyano-2,2, 8,8,12,13,17, COMiBC), have been synthesized [8,9]. The spectroscopic properties of these compounds were characterized [10][11][12][13][14][15][16], in particular, NH-isomerism in these types of molecules was observed.…”

“…The spectroscopic properties of these compounds were characterized [10][11][12][13][14][15][16], in particular, NH-isomerism in these types of molecules was observed. Quantum-chemical calculations of the geometric and electronic structures and electronic spectra of the NH-isomers of H 2 iBC and its meso-cyano derivative have been reported [17].…”

“…A total of 400 singly excited electronic configurations [20 occupied molecular orbitals (MO) × 20 unoccupied MO] was calculated. The calculation methods have been described in detail [17].…”

“…Scheme II shows the structures of the possible NH-isomers of H 2 TAiBC: Quantum-chemical calculations were carried out for these to the same extent as for the analogous isomers of H 2 iBC that were previously examined [17]. From here on, reference [17] will not be cited, as a rule, when comparing data for H 2 TAiBC and H 2 iBC. Scheme II shows that isomers a and b in addition to d and e are chemically equivalent pairs.…”