Resonance Raman Spectrum of Distorted Porphyrin Radical Cation Reveals Orbital Mixing Ching-Yao Lin, Songzhou Hu, Thomas Rush III, and Thomas G. Spiro* Department of Chemistry, Princeton UniVersity Princeton, New Jersey 08544 ReceiVed June 24, 1996 We report striking reversals of vibrational frequency shifts in radical cations of Ni(II) porphines bearing n-propyl or tert- butyl substituents on the meso-carbon atoms (Figure 1). These reversals are evidence that the two highest occupied molecular orbitals (HOMO) are mixed by out-of-plane distortions of the porphyrin. The n-propyl molecule is known to be planar, 1 while the tert-butyl molecule is severely distorted by the steric clashes between the substituents and the adjacent pyrrole rings. 2,3 Such distortions are known to destabilize the HOMO, 3 as evidenced by diminution of the porphyrin oxidation potential 4 and of the energies of the π-π* electronic transitions. 4,5 However, the present results call attention to the electronic reorganization which accompanies the distortion. Such reorganization may play a role in modulating electron transfer to or from tetrapy- rrolic centers in biology, which are subject to distortion by the surrounding protein. 6 The two Ni(II) porphyrins undergo clean one-electron oxida- tions, as evidenced by isosbestic visible absorption spectra (Figure 2). The B and Q absorption energies are substantially lower for NiT(t-Bu)P than for NiT(n-Pr)P, as is the oxidation potential (0.65 and 0.85 V Vs SCE, determined by CV). In both cases, however, oxidation leads to blue-shifted B bands and to broad low-energy absorptions, characteristic of porphyrin cation radicals. 7 Resonance Raman (RR) spectra (Figure 3) in the 1300-1600-cm -1 region are assigned to porphyrin skeletal vibrations Via the polarization characteristics of the bands, and with reference to the previous RR study of the neutral molecules by Jentzen et al. 3 As expected for B-band excitation, 8 the spectra are dominated by totally symmetric skeletal modes, ν 2-4 . The mode frequencies are substantially lower for NiT(t-Bu)P than for NiT(n-Pr)P, an effect of the porphyrin distortion. 3 The same three totally symmetric modes can readily be identified in the radical cation spectra, and are shifted very differently in the two molecules. Most striking is the reversal in the ν 3 shift: down 41 cm -1 in NiT(n-Pr)P but up 50 cm -1 in NiT(t-Bu)P. ν 2 shifts oppositely, but by much smaller amounts: up 8 cm -1 in NiT(n-Pr)P and down 5 cm -1 in NiT(t-Bu)P. Meanwhile ν 4 is nearly unshifted in NiT(n-Pr)P but undergoes a large downshift, 37 cm -1 , in NiT(t-Bu)P. To investigate the origin of these differences, we carried out INDO calculations 9-11 on the two porphyrins, focusing on the two frontier orbitals. In the planar NiT(n-Pr)P, these are the standard a 1u and a 2u orbitals (Figure 1). 12 The latter lies highest, consistent with the expected * To whom correspondence should be addressed. (1) Renner, M. W.; Furenlid, L. R.; Barkigia, K. M.; Fajer, J. Proceedings of the Nineteenth DOE Solar Photochemistry Research Conference, 1995. (2) Senge, M. O.; Ema, T.; Smith, K. M. J. Chem. Soc., Chem. Commun. 1995, 7, 733-734. (3) Jentzen, W.; Simpson, M. C.; Hobbs, J. D.; Song, X.; Ema, T.; Nelson, N. Y.; Medforth, C. J.; Smith, K. M.; Veyrat, M.; Mazzanti, M.; Ramasseul, R.; Marchon, J.-C.; Takeuchi, T.; Goddard, W. A., III; Shelnutt, J. A. J. Am. Chem. Soc. 1995, 117, 11085-11097. (4) Barkigia, K. M.; Chantranupong, L.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1988, 110, 7566-7567. (5) Barkigia, K. M.; Renner, M. W.; Furenlid, L. R.; Medforth, C. J.; Smith, K. M.; Fajer, J. J. Am. Chem. Soc. 1993, 115, 3627-3635. (6) (a) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. J. Mol. Biol. 1984, 180, 385-398. (b) Deisenhofer, J.; Epp, O.; Miki, K.; Huber, R.; Michel, H. Nature 1985, 318, 618-624. (c) Allen, J. P.; Feher, G.; Yeates, T. O.; Komiya, H.; Rees, D. C. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 5730-5734. (d) Iwata, S.; Ostermeier, C.; Ludwig, B.; Michel, H. Nature 1995, 376, 660-669. (e) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.; Yamaguchi, H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa, S. Science 1996, 272, 1136-1144. (f) Hildebrandt, P.; Stockburger, M. Biochemistry 1989, 28, 6710-6721. (g) Hobbs, J. D.; Shelnutt, J. A. J. Protein Chem. 1995, 14, 19-25. (h) Ravikanth, M.; Chandrashekar, T. K. In Structure and Bonding 82; Clarke, M. J., Goodenough, J. B., Ibers, J. A., Jφrgensen, C. K., Mingos, D. M. P., Nelands, J. B., Palmer, G. A., Sadler, P. J., Weiss, R., Williams, R. J. P., Eds.; Springer-Verlag: Berlin, Heidelberg, 1995; pp 105-197. (7) (a) Fajer, J.; Borg, D. C.; Forman, A.; Dolphin, D.; Felton, R. H. J. Am. Chem. Soc. 1970, 92, 3451-3459. (b) Seth, J.; Palaniappan, V.; Bocian, D. F. Inorg. Chem. 1995, 34, 2201-2206. (8) Spiro, T. G.; Li, X.-Y. In Biological Applications of Raman Spectroscopy; Spiro, T. G., Ed.; John Wiley & Sons: New York, 1988; Vol. 3, pp 1-38. (9) INDO calculations of the molecular orbitals was performed with the program ARGUS, 10 which employs the INDO/S semiempirical Hamiltonian, paramatrized for first-row transition metal ions. 11 Structure parameters were taken from ref 1 for NiT(n-Pr)P, while for NiT(t-Bu)P, a 4° tilt of the pyrrole rings was added to the molecular-mechanics-derived ruffled structure 3 (see text). (10) (a) Thompson, M. A.; Schenter, G. K. J. Phys. Chem. 1995, 99, 6374. (b) Thompson, M. A.; Glendening, E. D.; Feller, D. J. Phys. Chem. 1994, 98, 10465. (11) Zerner, M.; Loew, G.; Kirchner, R.; Mueller-Westerhoff, U. J. Am. Chem. Soc. 1980, 102, 589 and references therein. Figure 1. The two frontier orbitals for NiT(n-Pr)P (left) and NiT(t- Bu)P (right), calculated Via INDO. 11 Filled and open circles represent oppositely oriented p z orbitals, and the sizes are proportional to the orbital coefficients. Figure 2. Absorption spectral changes of NiT(n-Pr)P (left) and NiT- (t-Bu)P (right) upon one-electron oxidation. The bold lines indicate the traces of the cation radicals. Experimental conditions: OTTLE cell 17 with sample in CH2Cl2/tetrabutylamonium perchlorate; applied potential, 0.90 and 0.75 V Vs SCE for NiT(n-Pr)P and NiT(t-Bu)P, respectively. 9452 J. Am. Chem. Soc. 1996, 118, 9452-9453 S0002-7863(96)02120-8 CCC: $12.00 © 1996 American Chemical Society