Conformational Distortions of Metalloporphyrins with Electron-Withdrawing NO 2 Substituents at Different Meso Positions. A Structural Analysis by Polarized Resonance Raman Dispersion Spectroscopy and Molecular Mechanics Calculations Reinhard Schweitzer-Stenner,* ,² Christina Lemke, ‡ Raid Haddad, §,| Yan Qiu, §,| John A. Shelnutt, §,| J. Martin E. Quirke, ⊥ and Wolfgang Dreybrodt ‡ Department of Chemistry, UniVersity of Puerto Rico, Rı ´o Piedras Campus, P.O. Box 23346, San Juan, Puerto Rico 00931, FB1-Institut fu ¨ r Experimentelle Physik, UniVersita ¨ t Bremen, 28359 Bremen, Germany, Biomolecular Materials and Interfaces Department, Sandia National Laboratories, Albuquerque, New Mexico 87185-1349, Department of Chemistry, UniVersity of New Mexico, Albuquerque, New Mexico 87131 and Department of Chemistry, Florida International UniVersity, Miami, Florida 33199 ReceiVed: March 12, 2001; In Final Form: March 21, 2001 The meso substituted Ni(II)(5,15-diNO 2 -octaethylporphyrin) coexists in at least three different conformers in CS 2 . To explore the structural properties of these conformers, we measured the resonance excitation profiles and depolarization ratio dispersions of various prominent Raman lines of Ni(5,15-diNO 2 -octaethylporphyrin) in CS 2 . The data were analyzed by a theoretical approach, which formulates the Raman tensor in terms of vibronic coupling parameters that depend on static deformations along the normal coordinates. The coupling parameters were determined by simultaneously fitting the depolarization ratio dispersion data and the corresponding resonance excitation profiles. We have also performed molecular mechanics calculations to identify all possible stable conformers of the molecule. To quantify the out-of-plane distortions of the calculated structures, we subjected them to normal coordinate deformation analysis (Jentzen, W.; Song, X.-Z.; Shelnutt, J. A. J. Phys. Chem. B 1997, 101, 1684). The results obtained from the Raman data and from molecular modeling are consistent in showing that the most stable conformers are strongly affected by rhombic in-plane (0.3 Å) and ruffling (2.1 Å) and doming (0.5-0.6 Å) out-of-plane distortions. Additionally, smaller contributions from saddling were also obtained (∼0.1 Å). The three conformers detectable from the analysis of the Raman spectra most likely differ in terms of saddling and doming. The lowest-energy calculated conformers all show a horizontal orientation and out-of-plane position of the NO 2 groups with respect to the macrocycle, but the conformers differ in the orientations of the ethyl substituents. Conformers with vertical orientations of the NO 2 groups are calculated to be slightly higher in energy. INDO/s calculations reveal that the horizontal NO 2 group orientation, and to a lesser extent the vertical orientation, gives rise to a strong admixture between porphyrin and NO 2 molecular orbitals, enhancing the above distortions and leading to a break down of the 4-orbital model. A comparison with monosubstituted Ni(II)(5-NO 2 -OEP) reveals that all distortions increase with increasing number of nitro substituents. Altogether, this study demonstrates that meso nitro substitution of metalloporphyrins has a significant impact on electronic as well as structural properties of the ground and excited electronic states. Introduction The structural properties of metalloporphyrins have become a major subject of research over the last 10 years. 1 This particularly concerns the issue of how macrocycle distortions determine physicochemical and functional properties. 2-7 This research is of practical relevance because it aids in designing porphyrins as biosensors 8 and optical switches 9 and also has implications for the understanding of chromophore-protein interactions. 10 Strong evidence has been provided in the meantime that distortions may affect the spin delocalization, redox potential, electronic structure, and vibrational dynamics of the macrocycle 11 as well as the affinity and geometry of axial ligand binding. 1,12 In this context, a strong emphasis has been put on nonplanar distortions, which, in solution and crystals, are mostly caused by steric interactions between peripheral substituents, 1 whereas specific heme protein interactions are the predominant cause in proteins. 6,12,13 While nonplanar distortions have attracted considerable attention, the number of studies dealing with in-plane distortions induced by asymmetrically arranged peripheral substituents and chromophore-protein interactions are rather limited. Jentzen et al. have investigated a series of nonplanar 5,15-meso substituted porphyrins without focusing on the in-plane distor- tions. 14,15 Senge et al. 16 have addressed this issue in their recent comparative analysis of the crystal structures of various deca- and undecasubstituted porphyrins with meso alkyl and aryl groups. They found that 5,15 substitution gives rise to a rectangular elongated core characterized by different N‚‚‚N separations parallel to the 5,15 and 10,20 axes. Interestingly, * Corresponding author. Tel.: 787-764-0000 (ext 2417). Fax: 787-756- 8242. E-mail: rstenner_upr_chemistry@gmx.net. ² University of Puerto Rico. ‡ Universita ¨t Bremen. § Sandia National Laboratories, Albuquerque. | University of New Mexico. ⊥ Florida International University. 6680 J. Phys. Chem. A 2001, 105, 6680-6694 10.1021/jp010936+ CCC: $20.00 © 2001 American Chemical Society Published on Web 06/16/2001