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

Content Provider	Springer Nature Link
Author	Kuz'mitskii, V. A.
Copyright Year	2005
Abstract	The 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 (δ $$E_{Q_1 }$$ ≅ +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 δ $$E_{Q_x }$$ = −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 δ $$E_{Q_x }$$ = −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 δ $$E_{Q_x }$$ = +180 cm$^{−1}$, and for the geometry calculated with restrictions on the highest C$_{2v}$ symmetry of the H$_{2}$Ch molecule δ $$E_{Q_x }$$ = +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.
Starting Page	360
Ending Page	370
Page Count	11
File Format	PDF
ISSN	00219037
Journal	Journal of Applied Spectroscopy
Volume Number	72
Issue Number	3
e-ISSN	15738647
Language	English
Publisher	Kluwer Academic Publishers-Consultants Bureau
Publisher Date	2005-01-01
Publisher Place	New York
Access Restriction	One Nation One Subscription (ONOS)
Subject Keyword	chlorin quantum-chemical calculation restricted and unrestricted Hartree-Fock methods AM1 CNDO/S methods Analytical Chemistry Atomic and Molecular Structure and Spectra
Content Type	Text
Resource Type	Article
Subject	Spectroscopy Condensed Matter Physics

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