Modeling the Soret-Resonant Raman Intensities of Metalloporphyrins and Heme Proteins. 1. Nickel Porphine

A framework is developed for modeling the resonance Raman (RR) intensities of metalloporphyrins, with a view toward rationalizing the enhancement 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 photoexcitation are projected onto the ground-state normal modes to calculate the intensity of each Raman-active vibration. The RR intensities derive mainly from expansion of the C α C m and C β C β bonds in the excited state, with the relative intensities strongly influenced by the phasing between C α C m and C β C β stretching coordinates. Analysis of the ν 8 overtone shows the INDO predicted geometry changes to be about 25% too large. Results are compared at successive levels of approximation, demonstrating that inclusion of displacements along bending coordinates in the excited state are essential, as are frequency-dependent scaling factors which are determined from the absorption spectrum by the transform approach to RR scattering. Finally, the activation 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 correctly predicted, although quantitative modeling of their intensity requires the inclusion of non-Condon coupling.