Simulation of Non-Condon Enhancement and Interference Effects in the Resonance Raman Intensities of Metalloporphyrins Ranjit Kumble, ² Thomas S. Rush, III, Milton E. Blackwood, Jr., Pawel M. Kozlowski, and Thomas G. Spiro* Department of Chemistry, Princeton UniVersity, Princeton, New Jersey 08544 ReceiVed: February 12, 1998; In Final Form: June 2, 1998 The room-temperature resonance Raman (RR) spectra of nickel(II) porphine have been simulated for excitation within the Q-band optical transition, taking into account both coordinate-independent (Condon) as well as coordinate-dependent (non-Condon) contributions to the Raman scattering amplitude of each vibration. Computation of scattering intensities for all three classes of Raman-active modes (polarized, depolarized, and inversely polarized) observed in metalloporphyrin spectra was made possible in this fashion. Vibronic parameters were evaluated at the INDO level from the dependence of electronic transition moments and transition energies upon nuclear coordinates; the transform theory of RR scattering was then applied to compute the relative intensities of vibrational modes. The calculated enhancement patterns are in excellent agreement with experimental results. Three vibrational modes of b 1g symmetry (ν 10 , ν 11 , and ν 16 ) are identified as being the primary Jahn-Teller distortion coordinates: a substantial fraction of the total internal-mode reorganization energy for S 0 - S 1 photoexcitation is predicted to arise from symmetry-lowering. Interference between Condon and non-Condon scattering amplitudes is found to be a major determinant of the intensities of several polarized vibrations as well as those depolarized modes which are Jahn-Teller active. The dependence of scattering intensity on excitation wavelength for such modes is correctly predicted, indicating that the relative amplitudes and phasing of Condon and non-Condon contributions for each vibration are reliably determined. I. Introduction Successful correlation of the structure and function of heme proteins has been an important goal toward understanding their diverse role in biological processes. There is a continuing interest in learning how factors such as the conformation of the heme chromophore, its substituents, and surroundings act as key determinants of protein function. Vibrational spectros- copy has been extensively employed to probe the structural properties of heme proteins: resonance Raman (RR) spectros- copy in particular has provided a highly selective and sensitive means to investigate the structure and environment of chro- mophoric groups. 1 The interpretation of heme protein RR spectra have been greatly facilitated by the detailed understand- ing of the normal mode structure of porphyrins that has emerged from studies of model systems. 2-4 This knowledge has enabled heme vibrational frequencies to be correlated with structural differences arising, for example, from changes in redox state or ligation state of the chromophore. A powerful application of this approach was illustrated in the time-resolution of structural changes accompanying ligand release in hemoglobin. 5 In addition to effects on vibrational frequencies, subtle structural changes can have significant influence upon the intensities of RR bands. 6 Analysis of RR intensities therefore can potentially provide deeper insights into structural factors than are available simply from examination of vibrational frequencies alone. In the case of the heme chromophore, however, the sources of RR scattering intensities with regard to the electronic and vibrational properties of the porphyrin macrocycle remain somewhat poorly understood. Although extensive analysis of the RR intensities of porphyrins 7-10 and heme proteins 11,12 have been performed, these treatments have primarily concentrated on investigating optical properties (such as spectral broadening mechanisms), 12 vibronic interactions, 7-10 and excited-state dynamics. 12 The nature of the ground-state vibrational eigenvectors and the electronic wave functions of the ground and excited states are fundamental determinants of RR intensities, yet only a few investigations to date have sought to connect these factors with the observed patterns of resonant enhancement. 13-15 A primary objective of recent studies from this research group has been to establish a quantitative under- standing of the origins of metalloporphyrin RR intensities and their sensitivity to structural variations. Important insights into the sources of resonant activation of each vibrational mode can be obtained from simulation of RR intensities. This approach serves to link models for the vibrational and electronic structure of the chromophore of interest, and provides a stringent criterion for testing their validity. 6,16 The degree of resonant enhancement of Raman- active vibrations directly reflects the ability of each mode to promote structural reorganization in the resonant state (Condon activity) and/or perturb the transition dipole moment of the optical transition (non-Condon activity). 17 For resonance with strongly allowed transitions, such as the near-ultraviolet Soret band of the heme group, the former mechanism dominates and only the Franck-Condon (FC) active vibrations are enhanced. This mechanism is also referred to as A-term scattering enhancement and can be modeled by determining the projection of molecular geometry changes in the resonant excited state * Author to whom correspondence should be addressed. ² Present address: Department of Chemistry, The University of Penn- sylvania, Philadelphia, PA 19104-6323. 7280 J. Phys. Chem. B 1998, 102, 7280-7286 S1089-5647(98)01122-5 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/22/1998