RELATIONSHIPS BETWEEN STRUCTURAL PARAMETERS AND RAMAN FREQUENCIES FOR SOME PLANAR AND NONPLANAR NICKEL(II) PORPHYRINS

Molecular mechanics calculations, based on a force field derived from a previously published normal coordinate analysis for NiOEP (1) and the X-ray structure of NiOEP B triclinic form, have been used to predict the structures of NiTPP (2), NiP(3), and several recently synthesized nickel porphyrins which bear alkyl or phenyl groups at all 12 peripheral positions (4-10). These calculations predict that porphyrins 4 (NiOMTPP),5 (NiOETPP), 6 (NiOPTPP), 7 (NiDPP),9 (NiTC6TPP), and 10 (NiTC7TPP) adopt a saddle conformation with varying degrees of nonplanarity of the macrocycle. To evaluate the accuracy of these calculations, a single-crystal X-ray structure of the porphyrin with the most nonplanar calculated structure (NiOPTPP) was determined. The agreement between observed and calculated structures is good, both in terms of the general molecular conformation and more subtle structural differences such as a decrease in core size and increase in the C-alpha-NC-alpha bond angle compared to planar nickel porphyrins. The structural parameters obtained from these calculations were then used to investigate the relationship between core size and the frequency of the structure-sensitive Raman lines (nu-4, nu-3, nu-2, and nu-19). The linear relationship between core size and Raman frequency has a positive slope, in contrast to the negative slope exhibited by the usual core size-Raman frequency relationship. Thus it is not the core size that determines the frequencies of the structure-sensitive Raman lines. On the other hand, the linear correlation between the C-alpha-NC-alpha angle and Raman frequency has a negative slope for both the nickel porphyrins used in this study and the metalloporphyrins of the conventional core-size relationship, suggesting a more direct relationship between this angle and the marker line frequencies.