Dependence of the Electronic Spectra of the Chlorin Molecule on the Degree of Alternation of the Macrocycle Bond Lengths

Abstract

539.19The geometric structure of the chlorin molecule (H 2 Ch) has been calculated by the restricted and unrestricted Hartree-Fock (RHF and UHF) methods with an AM1 Hamiltonian. Transformations of this molecule into excited electronic states have been calculated by the CNDO/S method. The RHF method without symmetry restrictions gives a plane structure for the chlorin macrocycle with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and a C 1h symmetry for the molecule as a whole. The level of the first excited state Q 1 of this structure is substantially shifted (δE Q 1 . +4000 cm -1 ) relative to the Q x level of porphin (H 2 P) toward shorter waves, which is in contradiction with the experimental data, according to which this shift is long-wave and is equal to δE Q x = -400 to -550 cm -1 . The optimization of the geometry of H 2 Ch by the UHF method has shown that it has a structure with an 18-member azacyclopolyene with bonds of equal lengths and a D 2h symmetry. For this geometry of H 2 Ch the calculated shift of the Q x level, equal to δE Q x = -70 cm -1 , is bathochromic and the position of the Q y level is practically exactly coincident with the experimental one. For the geometry calculated by the RHF method with restrictions on the D 2h symmetry of the 18-member azacyclopolyene δE Q x = +180 cm -1 , and for the geometry calculated with restrictions on the highest C 2v symmetry of the H 2 Ch molecule δE Q x = +620 cm -1 . The latter result shows that the "natural" requirement for the C 2v symmetry of the H 2 Ch macrocycle, which is frequently used in various calculations, is inadequate to achieve a quantitative agreement between the calculation and experimental data and, in this case, the lengths of the bonds along the 18-member azacyclopolyene are not equal. Keywords: chlorin, quantum-chemical calculation, restricted and unrestricted Hartree-Fock methods, AM1 CNDO/S methods.Introduction. It has been established in the calculations of monobenzoporphyrin molecules (H 2 MBP) carried out in [1] that their geometric and chemical structures significantly influence the energy of the Q and B states, the transitions to which determine the positions of the absorption bands in the visible and near-UV regions of the spectrum. The geometric structure of the molecules considered was optimized by the restricted and unrestricted HartreeFock (RHF and UHF) methods with a semiempirical AM1 Hamiltonian [2]. The results obtained have an important peculiarity. The RHF method without symmetry restrictions gives a structure of H 2 MBP with an alternation of the lengths of the bonds along the 18-member azacyclopolyene and large (,0.5 eV) shifts of its Q levels relative to the Q levels of porphin (H 2 P) toward shorter waves. In contrast to this, the UHF method gives an H 2 MBP structure with equal lengths of the bonds along the 18-member azacyclopolyene and long-wave shifts of the Q levels that are consistent in value with the experimental shifts [3].It should be noted that the data of calculations o...