MODELING THE SORET-RESONANT RAMAN INTENSITIES OF METALLOPORPHYRINS AND HEME-PROTEINS .1. NICKEL PORPHINE

A framework is developed for modeling the resonance Raman (RR) intensi ties of metalloporphyrins, with a view toward rationalizing the enhanc ement patterns observed in the spectra of heme proteins. The geometry of the S-2 excited state of nickel(II) porphine is computed using INDO /1s methods, and the structural changes resulting from S-0-S-2 photoex citation are projected onto the ground-state normal modes to calculate the intensity of each Raman-active vibration. The RR intensities deri ve mainly from expansion of the CalphaCm and CbetaCbeta bonds in the e xcited state, with the relative intensities strongly influenced by the phasing between CalphaCm and CbetaCbeta stretching coordinates. Analy sis of the vs overtone shows the INDO predicted geometry changes to be about 25% too large. Results are compared at successive levels of app roximation, demonstrating that inclusion of displacements along; bendi ng coordinates in the excited state are essential, as are frequency-de pendent scaling factors which are determined from the absorption spect rum by the transform approach to RR scattering. Finally, the activatio n of non-totally symmetric modes by an A-term mechanism is modeled by distortion of the excited state along a b(1g) coordinate. Enhancement of the experimentally observed non-totally symmetric modes is correctl y predicted, although quantitative modeling of their intensity require s the inclusion of non-Condon coupling.