Journal of physical chemistry vol:99 issue:28 pages:11101-11114
Dichlorosilicon porphyrazine is investigated as an example for tetrapyrrolic macrocycles with the methods of density functional theory (DFT). In the optimized ground-state geometry the major difference from porphyrins is the shorter C-alpha-N-b bond compared to the C-alpha-C-m bond. This leads to a smaller cavity for a central ion in porphyrazine in comparison with porphyrin. A complete normal coordinate analysis for Cl(2)SiPz is presented. The computed frequencies for the infrared-active modes show good agreement with experimental data of different metal porphyrazines. The high-frequency modes are interpreted as coupled local modes whereas most low-frequency vibrations are collective modes that may be described according to the model of vibrations of a continuous ring or a solid disk, respectively. In a transition region at about 700-800 cm(-1) for in-plane modes and around 500 cm(-1) for out-of-plane modes, a description with both models is possible. For most of the in-plane vibrations the mode assignments comply rather well with the results of a previous empirical normal coordinate analysis for porphyrin. On the other hand, the DFT results for most of the out-of-plane modes of porphyrazine do not agree well with an empirical normal coordinate analysis for porphyrin. On the basis of the presented results the assignments for several modes in tetrapyrroles are revisited.