RUFFLING EFFECTS ON PORPHYRIN VIBRATIONS - NORMAL-MODE ANALYSIS FOR NICKEL OCTAETHYLTETRAPHENYLPORPHINE FROM RESONANCE RAMAN AND IR-SPECTRAOF ISOTOPOMERS

Resonance Raman [RR] and infrared spectra are reported for nickel octa ethyltetraphenylporphine (NiOETPP) and its N-15, meso-C-13, and phenyl -d20 isotopomers. Assignments were made of most of the in-plane skelet al modes and the substituent modes using a normal-coordinate calculati on for a reference planar porphyrin. For the calculation, the force fi eld was transferred from nickel tetraphenylporphine (NiTPP)1 and nicke l octaethylporphine (NiOEP).2 The effects of steric crowding and the a ssociated saddle distortion in NiOETPP are manifested in large, ca. 70 cm-1, downshifts in a number of porphyrin skeletal modes, relative to the frequencies calculated for a planar skeleton. Three prominent low -frequency RR bands are assigned to the out-of-plane modes gamma15, ga mma16, and delta4, which have B2u symmetry in the idealized D4h point group.3 This is the same symmetry as that of the saddling distortion. The gamma16 mode, which involves tilting of the pyrrole rings, gives r ise to one of the strongest bands in the Soret-excited RR spectrum.