REPRESENTATION OF NONPLANAR STRUCTURES OF NICKEL(II) 5,15-DISUBSTITUTED PORPHYRINS IN TERMS OF DISPLACEMENTS ALONG THE LOWEST-FREQUENCY NORMAL COORDINATES OF THE MACROCYCLE

The influence of substituents with increasing steric demands on the st ructure of nickel(II) 5,15-disubstituted porphyrins has been investiga ted with X-ray crystallography, UV-visible absorption spectroscopy, re sonance Raman spectroscopy, molecular energy optimization calculations , and INDO/s molecular orbital calculations. Nickel 5,15-diphenylporph yrin is predicted by molecular mechanics calculations to be a mixture of planar and nonplanar conformers. All of the nickel dialkylporphyrin s (where the alkyl group is propyl, isopropyl, and tert-butyl) are cal culated to be in a predominantly gabled (gab) conformation resulting f rom an alpha alpha orientation of the substituents with respect to the macrocycle. This nonplanar gab distortion is made up of a linear comb ination of distortions along the lowest-frequency out-of-plane macrocy cle normal coordinates of A(2u) (doming) and B-1u (ruffling) symmetry types. A higher energy stable ap conformer is also predicted for dialk ylporphyrins, and its nonplanar structure can be represented as an equ al combination of distortions along the x- and y-components of the low est-frequency E(g) (waving) normal coordinate. The nonplanar structure s calculated by using molecular mechanics have been structurally decom posed into the displacements alone the lowest-frequency normal coordin ate of each symmetry type, and the contributions of each type to the t otal distortion in the calculated structures agree well with contribut ions obtained from structural decompositions of the available X-ray cr ystal structures. The predicted gab distortion is confirmed most convi ncingly by the X-ray crystal structure of [5,15-di-tert-butylporphinat o]nickel(II) which is found to be in a gab alpha alpha conformation. F inally, INDO/s calculations show that the red shifts in the absorption spectra of the nickel disubstituted porphyrins are caused by the incr easing nonplanarity resulting from increasing steric crowding within t he series.