COMPLETE DENSITY-FUNCTIONAL NORMAL-COORDINATE ANALYSIS OF DICHLOROSILICON PORPHYRAZINE

Dichlorosilicon porphyrazine is investigated as an example for tetrapy rrolic macrocycles with the methods of density functional theory (DFT) . In the optimized ground-state geometry the major difference from por phyrins is the shorter C-alpha-N-b bond compared to the C-alpha-C-m bo nd. This leads to a smaller cavity for a central ion in porphyrazine i n comparison with porphyrin. A complete normal coordinate analysis for Cl(2)SiPz is presented. The computed frequencies for the infrared-act ive modes show good agreement with experimental data of different meta l porphyrazines. The high-frequency modes are interpreted as coupled l ocal modes whereas most low-frequency vibrations are collective modes that may be described according to the model of vibrations of a contin uous ring or a solid disk, respectively. In a transition region at abo ut 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 t he in-plane vibrations the mode assignments comply rather well with th e results of a previous empirical normal coordinate analysis for porph yrin. 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 coord inate analysis for porphyrin. On the basis of the presented results th e assignments for several modes in tetrapyrroles are revisited.