2236
J . Phys. Chem. 1989, 93, 2236-2243
We may conclude that single-crystal studies in the NIR region at low temperature, when wedded to a detailed local-mode analysis that takes into account the symmetry of the oscillators, can provide information on the strength of hydrogen bonds in crystals.
Acknowledgment. I.M.W. thanks the National Science and
Engineering Research Council of Canada for its continuing support. P.J.M. thanks the Research Corp. for funds to purchase the Displex cryogenic refrigerator. . Registry No. K2CuCI,.2H20, 10085-76-4; Rb2CuCI,.2H20, 1527452-9; CuCI2.2H2O, 10125-13-0.
Resonance Raman Spectra and Normal-Coordinate Analysis of Reduced Porphyrins. 1. Zinc( I I ) Tetraphenylporphyrin Anion Michael Atamian,+ Robert J. Donohoe,I Jonathan S. Lindsey, and David F. Bocian* Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 1521 3 (Received: April 4, 1988; In Final Form: September 26, 1988)
Resonance Raman (RR) spectra are reported for the anion radicals of the Zn(I1) complexes of TPP, TPP-p-d,, T P P - ( ~ ~ s o - ” C ) ~ , and TPP-(15N)4(TPP = tetraphenylporphyrin) as well as for the neutral Cu(I1) complexes of these same isotopic species. Normal-coordinate calculations are performed on the neutral Cu(I1) and the reduced Zn(1I) complexes and an all-valence force field is developed for these molecules. The RR data and normal-mode analysis indicate that reduction of the macrocycle results in a Jahn-Teller distortion along a b,,-like coordinate and that the appropriate symmetry group for describing the vibrations of the anion is DZh.The symmetry lowering that occurs upon porphyrin reduction substantially alters the forms of the normal modes of many of the skeletal vibrations of the macrocycle.
Introduction
tetraphenylbacteriochlorin. Normal-coordinate calculations for these two species are reported in the following paper in this issue.2s Reduced porphyrins play an important role in a wide variety of biological processes. These biologically active pigments contain macrocycles that are reduced via hydrogenation of one or more (1) Svec, W. A. In The Porphyrins; Dolphin, D.. Ed.; Academic Press: pyrrole rings and/or by the addition of an electron. HydrogenNew York, 1978; Vol. V, pp 341-399. ically reduced porphyrins such as chlorins and bacteriochlorins (2) Sibbet, S. S.; Hurst, J. K. Biochemistry 1984, 23, 3007. serve as the prosthetic groups in photosynthetic pigments,] (3) Babcock, G. T.; Ingle, R. T.; Oertling, W. A,; Davis, J. C.; Averill, B. m y e l o p e r ~ x i d a s e , s~lfmyoglobin,~” ~~~ s ~ l f h e m o g l o b i n ,and ~ ~ ~ ~ A.; Hulse, C. L.; Stufkens, D. J.; Bolscher, B. G. J. M.; Wever, R. Eiochim. Eiophys. Acta 1985, 828, 58. bacterial hemes d and d, .2,8-11 One-electron-reduced species are (4) Morell, D. B.; Chang, Y.; Clezy, P. S. Eiochim. Eiophys. Acta 1967, generated in the initial events of light-energy conversion in pho136, 121. tosynthesis.12 As a consequence, there is considerable interest ( 5 ) Andersson, L. A.; Loehr, T. M.; Lim, A. R.; Mauk, A. G. J . Eiol. in the characterization of these various types of reduced species. Chem. 1984, 259, 15340. (6) (a) Peisach, J.; Blumberg, W. E.; Adler, A. Ann. N.Y. Acad. Sci. 1973, Reduction of the porphyrin macrocycle, either via hydrogenation 206, 310. (b) Berzofsky, J. A.; Peisach, J.; Blumberg, W. E. J . Eiol..Chem. or electron addition, significantly alters the vibrational spectrum 1911, 246, 3367. - ~ ~understanding of from that of the unreduced ~ i g m e n t . ’ ~ An (7) Brittain, T.; Greenwood, C.; Barber, D. Eiochim. Eiophys. Acta 1982, these reduction-induced spectral changes is now emerging for the 705, 26. (8) Lemberg, R.; Barrett, J. In Cytochromes;Academic Press: London, chlorins and b a c t e r i o c h l ~ r i n s .In ~ ~contrast, ~~~ the vibrational 1973, pp 233-245. spectra of radical anions of these pigments, or even those of the (9) Timkovich, R.; Cork, M . S.; Gennis, R. B.; Johnson, P. Y. J . Am. anions of the more highly symmetrical parent porphyrins, are not Chem. Soc. 1985, 107, 6060. well characterized. In the case of porphyrinic anions, which serve ( I O ) Chang, C. K.; Barkigia, K. M.; Hanson, L. K.; Fajer, J. J . Am. Chem. SOC.1986, 108, 1352. as prototypical one-electron-reduced species, only a few vibrational ( I 1) (a) Chang, C. K. J . Eiol. Chem. 1985, 260,9520. (b) Chang, C. K.; studies have been reported. Both Ksenofontova et aLz3 and Wu, W. J . Eiol. Chem. 1986, 261, 8593. Yamaguchi et al.24examined the optical absorption and resonance (12) Okamura, M. Y.; Feher, G.;Nelson, N. In Photosynfhesis;Govindjee, Raman (RR) spectra of [ZnTPPI- (TPP = 5,10,15,20-tetraEd.; Academic Press: New York, 1982; Vol. I, pp 195-272. (13) (a) Ozaki,Y.; Kitagawa, T.; Ogoshi, H. Inorg. Chem. 1979,18, 1772. phenylporphyrin). However, the vibrational assignments proposed (b) Ozaki, Y.; Iriyama, K.; Ogoshi, H.; Ochiai, T.; Kitagawa, T. J . Phys. by these two groups are in disagreement for many of the highChem. 1986, 90, 6105. (c) Ozaki, Y.; Iriyama, K.; Ogoshi, H.; Ochiai, T.: frequency skeletal modes. Kitagawa, T. Ibid. 1986, 90, 6113. To provide a better understanding of the properties of porphyrin (14) Cotton, T. M.; Timkovich, R., Cork, M. S. FEES Lett. 1981, 133, 39. anions, we initiated a detailed vibrational investigation of [ZnT(15) Ching, Y.; Ondrias, M. R.; Rousseau, D. L.; Muhoberac, B. B.; PPI-. In this paper, we report R R spectra and normal-coordinate Wharton, D. C. FEES Lett. 1982, 138, 239. calculations for this complex and the isotopically substituted species (16) (a) Andersson, L. A.; Loehr, T. M.; Chang, C. K.; Mauk, A. G. J . [ ZnTPP-P-d8]-, [ZnTPP-(1SN)4]-,and [ Z ~ T P P - ( ~ ~ S O - ’ ~ C ) ~Am. ] - . Chem. Soc. 1985, 107, 182. (b) Andersson, L. A,; Loehr, T. M.; Sotiriou, C.; Wu, W.; Chang, C. K. J . Am. Chem. Soc. 1986,108,2908. (c) Andersson, As a starting point for the investigation of the anions, a force field L. A . , Sotiriou, C.: Chang, C. K.; Loehr, T. M. J . Am. Chem. Soc. 1987, 109, was developed for CuTPP. This force field was also used for the 258. development of force fields for the two hydrogenically reduced (17) (a) Cotton, 7. M.; Van Duyne, R. P. J . Am. Chem. SOC.1981, 103, porphyrin complexes, copper(I1) tetraphenylchlorin and copper(I1) 6020. (b) Cotton, T. M.; Parks, K. D.; Van Duyne, R. P. J . Am. Chem. Soc. ‘Current address: Department of Chemistry, Michigan State University, East Lansing, MI 48824. *Current address: INC-4, Los Alamos National Laboratory, Los Alamos, NM 87545.
1980, 102, 6399. (c) Cotton, T. M.; Van Duyne, R. P. Eiochem. Eiophys. Res. Commun. 1978, 82, 424. (d) Callahan, P. M.; Cotton. T. M. J . Am. Chem. Soc. 1987, 109, 7001. (18) (a) Fujiwara, M.; Tasumi, M. J . Phys. Chem. 1986, 90, 250. (b) Fujiwara, M.: Tasumi. M. Ibid. 1986, 90, 5646.
0022-3654/89/2093-2236$01.50/00 1989 American Chemical Society
Reduced Porphyrins. 1.
Methods Experimental Procedures. Free-base TPP (chlorin-free) was purchased from Midcentury and used as received. Free-base TPP-(15N)4and TPP-(meso-I3C), were prepared as described by Lindsey et a1.26by using the appropriate pyrrole (pyrrole, Aldrich; pyrrole-I5N, MSD Isotopes, 99.3% isotopic purity) and benzaldehyde (benzaldehyde, Aldrich; benzaldehyde-a-13C, MSD Isotopes, 99.9% isotopic purity). Free-base TPP-P-d, was prepared as described by Shirazi and G ~ f f . ~Insertion ' of Zn(I1) or Cu(I1) into the porphyrin followed standard procedures.28 [ZnTPPIand its isotopically substituted anions were prepared in a Vacuum Atmospheres Model HE-43 glovebox equipped with a Model 493 Dri-Train by using electrochemical instrumentation that has been described elsewhere.2' The samples were generated in N,N-dimethylformamide, DMF (Fisher, stored over 4-A molecular sieves for several weeks and distilled in vacuo), with -0.1 M tetrabutylammonium perchlorate, TBAP (Kodak, recrystallized twice from absolute ethanol and dried at 111 OC in vacuo), serving as supporting electrolyte. The integrity of the anions was confirmed by cyclic voltammetry, coulometry, and visible absorption. In particular, cyclic voltammograms recorded after extended periods of laser irradiation of [ ZnTPPI- indicated negligible photodecomposition. B-state-excitation R R data were obtained for [ZnTPPI- in D M F solutions. The spectra of the anion were acquired either in situ in an airtight electrochemical cell or in isolated quartz capillaries. Unfortunately, high-quality Q-state-excitation R R data are not obtainable for [ZnTPPI- (or ZnTPP) due to interference from fluorescence. Therefore, to facilitate the vibrational assignments, R R spectra were also acquired for the various isotopically substituted complexes of CuTPP, for which Q- and B-state-excitation RR spectra are obtainable. [It should be noted that macrocycle reduction of CuTPP results in phlorin formation and/or decompositi~n.~~ Consequently, it is not possible to obtain the full complement of vibrational data for both neutral and anionic species that contain the same metal ion.] The CuTPP samples were compressed in Na2SO4pellets (3 mg of sample/ 100 mg of NazS04). For comparison, B-state-excitation R R spectra of ZnTPP were also obtained in NaZSO4pellets. The Raman spectrometer and laser systems have been previously described.z0 The incident laser power at the sample was typically 40 mW, and the spectral slit width was -4 cm-l IR spectra of CuTPP and the isotopically labeled species (compressed in KBr disks) were recorded on a Nicolet SDXB FT-IR spectrometer. The spectral resolution was 4 cm-I. IR spectra of [ZnTPPI- were not obtained. Normal-Coordinate Calculations. The G matrices for CuTPP and its respective isotopes were constructed by using an idealized planar geometry for the metalloporphyrinz9in conjunction with (19) (a) Lutz, M . In Advances in Infrared and Raman Speciroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley: New York, 1984; Vol. 11, pp 21 1-300. (b) Robert, B.; Lutz, M. Biochemistry 1986, 25, 2303. (c) Lutz, M.; Hoff, A. L.; Brehamet, L. Eiochim. Eiophys. Acta 1982, 679, 331. (d) Lutz, M. J . Raman Spectrosc. 1974,2,497. (e) Lutz, M. Eiochem. Eiophys. Res. Commun. 1973, 53, 413. (20) (a) Boldt, N. J.; Donohoe, R. J.; Birge, R. R.; Bocian, D. F. J . Am. Chem. SOC.1987. 109.2284. (b) Boldt. N. J.: Bocian. D. F. J . Phvs. Chem. 1988, 92, 581. (c) Donohoe, R.J.; Frank, H.'A.; Bocian, D. F. Piotochem. Photobiol. 1988, 48, 531. (21) Donohoe, R. J.; Atamian, M.; Bocian, D. F. J . Am. Chem. SOC.1987, 109, 5593. (22) Teraoka, J.; Hashimoto, S.; Sugimoto, H.; Mori, M.; Kitagawa, T. J. Am. Chem. SOC.1987, 109, 180. (23) Ksenofontova, N. M.: Maslov, V.G.; Sidorov, A. N.; Bobovich, Ya. S. Opt. Spectrosc. 1976, 40, 462. (24) Yamaguchi, H.; Soeta, A.; Toeda, H.; Itoh, K. J. Electroanal. Chem. 1983, 159, 347. (25) Donohoe, R. J.; Atamian, M.; Bocian, D. F. J . Phys. Chem. following paper in this issue. ( 2 6 ) (a) Lindsey, J. S.; Hsu, H. C.; Schreiman, I. C. Tetrahedron Lett. 1986, 27, 4969. (b) Lindsey, J. S . ; Schreiman, I. C.; Kearney, P. C.; Marguerettaz, A. M. J . Org. Chem. 1987, 52, 827. (27) Shirazi, A,; Goff, H. M. J . Am. Chem. SOC.1982, 104, 6318. (28) Fuhrhop, J.-H.; Smith, K. M. In Porphyrins and Metalloporphyrins; Smith, K. M., Ed.; Elsevier: Amsterdam, 1975; p 798.
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2237 TABLE I: Valence Force Constants for CuTPP" CuN C,N C,C, NCUN CUNC, C,NC, NC,C,
0.786 7.328 5.517 0.120 0.646 1.152 0.597
Stretches, mdyn A-' C,C, 8.329 CBH 5.243 C,C, 8.298 Bends, mdyn NC,C, C,C,C, C,C,C, CaCmCph
CmCph
8, rad-2
2.060 0.597 1.534 0.565
C,C,H C,C&,
C,C,C,-C,C,C, C,NC,-NC,C, C,C,C,-C,C,C,
Bend-Bend, mdyn A rad-2 0.309 NC,C,-C,C,C, 0.309 C,NC,-NC,C, 0.473
C,N-C,N C,N-C,C, C,N-C,C, C,N-C,C, C,N-C,C, C,N-C,C, C,N-C,Cph
Stretch-Stretch, 1.746 1.234 0.1936 0.862 0. 1086 0.3 12 0.092
C,C,-C,C&p C,C,-NC,C,
c,c,-c,c,c, C,N-NC,C, C,N-C,NC, C,C,-C,C,C,
c,c,-c,c,c, C,C,-C,C,N c,c,-c,c,cph
Stretch-Bend, 1.159 1.159 1.159 1.159 1.159 1.037 0.122 0.122 0.186
4.841
mdyn A-' CaCm-CuCm CaCm-Ce C, Cacm-CmcPh cacm-c8c8
c,c,-c,c,
CuCB-C,C, CmCph-CphCph
0.409 2.060
0.473 0.473
1.867 1.234 0.728 0.413 -0.523 0.862 0.728
mdyn rad-'
c,c,-c,c,cph
C,Cp-C,CBH C,C,-C.&,H C,C,-C,C,H C,C,-C,C,H CmCph-CmCphCph CmCph-CaCmCph C,N-NC,C, CuC~-CmCaC,
-0.186' 0.046 0.046 -0.03 1 -0.03 1 0.299 0.299 0.436 0.436
"C,, C, = pyrrole carbons; C, = methine bridge carbon; Cph = phenyl carbon. Interaction between alternate bonds. cStretch and bend have atom but no bond in common.
the structural parameters for toluene.'O The planes of the phenyl rings were taken to be perpendicular to the plane of the porphyrin macrocycle. These approximations greatly simplify the calculations because the molecule has D4,,symmetry under these geometrical constraints. The G matrices for ZnTPP were also constructed by using the idealized D4h geometry with the exception that the metal-nitrogen bonds were lengthened by an amount commensurate with the larger Zn(I1) ion.31 No structural information is currently available for [ZnTPPI-; consequently, a geometry was estimated for this complex by using the semiempirical quantum chemistry force field (QCFF/PI) method of Warshel and I ( a r p l u ~ . ~This ~ - ~calculation ~ predicts that the anion has Dzhsymmetry due to a Jahn-Teller distortion along a b,,-like coordinate. In the DZh distorted geometry, one pair of opposite C,C, bonds is lengthened while the other pair is shortened. The distortion of the C,C, bonds to a given pyrrole ring is in phase with and of slightly larger magnitude than the distortion of the COCObond. On the other hand, the distortions of the corresponding C,C, and C,N bonds are out-of-phase with and slightly smaller than those of the C,C, bond. The R R data confirm the bl,-like distortion (vide infra). Normal-coordinate calculations were also performed on [ZnTPPI- by using the D4,,geometry of the neutral complex. [In this calculation, the F matris is not restricted to D4h symmetry.] The D2h and D4h geometries gave essentially the same vibrational frequencies and potential energy distributions (PEDS). The normal modes of the metallo-TPP complexes were calculated with an all-valence force field by using the vibrational (29) Radonovich, L. J.; Bloom, A,; Hoard, J. L. J . Am. Chem. Soc. 1972, 94, 2073. (30) La Lau, C.; Snyder, R. G. Spectrochim. Acta 1971, 27A, 2073. (31) Scheidt, W. R. In The Porphyrins; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 111, p 482. (32) Warshel, A,; Karplus, M. J . Am. Chem. SOC.1972, 94, 5612. (33) Warshel, A,; Levitt, M . Quantum Chemistry Program Exchange, No. 247, Indiana University, 1974.
2238 The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 analysis programs of S c h a ~ h t s c h n e i d e r . ~The ~ first step in the vibrational analysis procedure was to obtain a force field for CuTPP. [A valence force field has previously been reported for this complex by Gladkov and S o l o ~ y o vhowever, ;~~ this force field was obtained by using only deuteriation shifts. We found this force field to be unsatisfactory in that it completely failed to reproduce the observed I5N and meso-I3C isotope shifts.] Under D4h symmetry, metallo-TPP complexes have 55 Raman-active vibrations upon exclusion of out-of-plane modes of the porphyrin macrocycle and the phenyl rings. [Recall, however, that these planes are orthogonal.] These 55 modes transform as follows:
rvtb = 19A1, + 9B1, + 8A2, + 19B2, Of these modes, 10 A,, and 10 BZgvibrations arise from motions of the phenyl substituents. In addition, there are 28 in-plane, IR-active E, modes and 10 in-plane, IR-active Azumodes. Ten of the E, and all 10 of the A,, vibrations arise from motions of the phenyl substituents. The F matrix for the porphyrinic portion of CuTPP was initially constructed by utilizing the previously reported values for the diagonal and appropriate off-diagonal elements for copper porphine.36 The force constants for the phenyl rings were transferred directly from toluene.30 In the refinement procedure, only the force constants of the porphyrinic moiety were allowed to vary. The refined set of porphyrinic valence force constants for CuTPP is given in Table I. These 38 force constants (plus the 18 transferred from toluene) were used to fit 83 experimental R R frequencies with an average error of 3.1 cm-l. The observed IR frequencies were not included in the refinement procedure because only 5 of the 18 porphyrinic E, modes were identified with certainty. Although the average error in the calculated versus observed frequencies is reasonable for CuTPP, the number of observed frequencies used in the fits is relatively small compared with the number of allowed modes. Thus, the overall quality of the force field is clearly open to question. The analysis of spectral data for additional isotopically labeled complexes, such as phenyldeuteriated species, would undoubtedly aid in the further refinement of the force field. In particular, these data may resolve uncertainties in the assignments for the E, modes. Spectral data for phenyl-deuteriated complexes would also allow the assessment of the influence of phenyl ring orientation on the calculated vibrational frequencies. The assumption of orthogonal phenyl and porphyrin rings must to some extent degrade the quality of the force field because certain angle-bending G matrix elements are constrained to zero for this geometry. This constraint is relaxed as the phenyl rings are rotated toward the porphyrin plane. Despite the potential utility of additional isotope data, many of the RR-allowed vibrations (particularly the Bzgmodes) are not observed. Consequently, the total number of observed frequencies will be less than the number of allowed modes even with the inclusion of such data. The second step in the vibrational analysis procedure was an attempt to adapt the force field of CuTTP to fit the R R data for ZnTPP. Unfortunately, the number of observed vibrational modes for the latter complex and its isotopes is small (-32); consequently, the reliability of such a modified force field is questionable. Because the number of frequencies observed for [ZnTPPI- and its isotopes (-69) is considerably greater than t h e number observed for the neutral species, we instead chose to directly adapt the force field of CuTPP to the Zn(I1) anions. Under DZhsymmetry, there are still 55 Raman-active, in-plane vibrations. However, these modes are now classified as follows:
Atamian et al. TABLE 11: Valence Force Constants for [ZnTPPr Stretches, mdyn 8,-' ZnN C"
CUC,
c,c, C,H CaCmc
(IJII)
(IIJV)
0.850 7.380 6.275 6.904 5.243b 7.114
0.850 7.303 6.319 7.238 5.243b 8.353 5.170
CmCPh
NZnN ZnNC, C,NC, NC,C,
0.120 0.646 1.152 0.597
Bends, mdyn NC,C, CC ,C ,, CC ,C ,, CuCmCph
8, rad-26 2.060 0.597 1.534 0.565
C&H C,C,C,
0.409 2.060
Bend-Bend, mdyn 8, radw2 0.309 C,NC,-NC,C, 0.309 cec8c8-cuc~c~ 0.473 NC,Cp-C,C,C, 0.473 C,NC,-NC,C, 0.473 cmcuc~~cuc8~~
Stretch-Stretch, mdyn 8,-'
(II,IV)
(1 I I 1) 0.808 1.543 0.280e 0.523 -0.620' -0.754 5
C,N-C,N C,N-C,C, C,N-C,C, C,N-C,C, C,N-C,C, C,N-C,C, CoN-CmCPh
CuCm-CuCm CmCm-Co C, CuCm-CmCP, cacm-c8c@ cac,9-cuc,5 cuc~-cb'c~
1.712 1.768 0.647' 0.804 -0.620' -0.534 0.245
0.115 1.417 0.7 15 0.390 0.100 -0.500 0.523
CmCPh-CPhCPh
1.755 0.390 0.100 -0.500 0.804 0.653
Stretch-Bend, mdyn rad-' cuc8-cuc,c@
C,C,-NC,C, c,c,-cuc8c8
C,N-NC,C,g C,N-C,NC, CuCm-CuCmCa CaCm-CmCmC, CC , -,CC ,N , cucm-cucmcPh
cucm-cacmcPh
C,C,-C,C,H C,C,-C,C,H C,C,-C,C, C,C,-C,C,H CmCPh-CmCPhCPh CmCPh-CuCmCPh
C,N-NC,C, CuC,-CmCuC, cmcPh-cPhcPhcPh
(1,111)
(1I, I v 1
1.190* 1.190' 1.190' 1.190' 1.190* 1.169* 0.122 0.122 0.186 -0.186 0.046 0.046 0.03 1 0.031 0.299 0.299 0.436 0.436
1.190* 1.190* 1.190* 1.190' 1.190* 1.010* 0.122 0.122 0.186 -0.186 0.046 0.046 0.03 1 0.031 0.299 0.299 0.436 0.436
0.43 1 *
The A, symmetry block is comprised of former A,, and B,, modes,
"c,, c, = pyrrole carbons; C, = methine bridge carbon; c p h = phenyl carbon. Roman numerals (1,111) and (IIJV) refer to pairs of symmetry-equivalent (DZh),nonadjacent pyrrole rings. bTransferred from CuTPP. C ' ,C, bonds are designated according to the pyrrole ring that contains the a-carbon. dAll force constants of this type were transferred directly from CuTPP except for those indicated by an asterisk 'Interaction between alternate bonds. ,Stretch and bend have atom but no bond in common.
(34) Schachtschneider, J. H. Vibrational Analysis of Polyaromic Molecules; Shell Development Co.: Emeryville, CA, 1962; Vol. 1-111. (35) (a) Gladkov, L. L.; Solovyov, K. N. Spectrochim. Acta 1985. 41A, 1437. (b) Gladkov, L. L.; Solovyov, K. N. Ibid. 1985, 41A, 1443. ( c ) Gladkov, L. L.; Solovyov, K. N. Ibid. 1986, 42A, 1. (36) Susi, H.; Ard, J. S. Spectrochim. Acta 1977, 33A, 561.
and the B,, block contains former A,, and B,, modes. Ten phenyl modes are included in each symmetry block. The normal modes of [ZnTPPI- were initially calculated by using the CuTPP force field. In the refinement procedure, all of the angle-bending and bend-bend interaction force constants were held fixed as were certain stretch-bend interaction constants. In total, 36 porphyrinic
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2239
Reduced Porphyrins. 1
p p. 4
N
900 900
1300
1700
Raman Shift (cm-1) Figure 1. High-frequency regions of the B-state-excitation(bX = 413.1 nm) RR spectra of (a) CuTPP, (b) CuTPP-P-ds, (c) CuTPP-(meso13C)4,and (d) CUTPP-('~N),.The mode due to Na2S04is marked with
the symbol #.
1300
1700
Raman Shift (cm-') Figure 2. High-frequency regions of the Q-state-excitation(A, = 530.9 nm) RR spectra of (a) CuTPP, (b) CuTPP-P-ds, (c) CuTPP-(meso13C)4,and (d) CuTPP-(lSN),. The mode due to Na2SO4is marked with
the symbol # .
force constants were allowed to vary, while 37 others were fixed (plus the 18 transferred from toluene). The refined set of porphyrinic force constants for [ZnTPPI- is given in Table 11. This set of force constants reproduced the 69 observed frequencies for the anionic species with an average error of 4.4cm-I.
Results and Discussion Vibrational Spectra and Assignments. The high-frequency regions of the B- and Q-state-excitation R R spectra of CuTPP and its various isotopes are shown in Figures 1 and 2, respectively. The high-frequency regions of the B-state-excitation R R spectra of ZnTPP and the [ZnTPPI- complexes are shown in Figure 3. The observed and calculated frequencies, isotope shifts, and PEDS for CuTPP and [ZnTPPI- are listed in Tables Ill and IV, respectively. All calculated Raman-allowed in-plane vibrations are listed in the tables. The absence of a listed frequency and/or isotope shift indicates that the band is not observed (due to negligible resonance intensity enhancement) or could not be clearly identified (due to poor signal-to-noise and/or overlapping bands). The mode designations previously used by Spiro and co-workers3' are also included in Table 111. Deuteriation shifts are not included for certain of the modes listed in Tables I l l and IV because the vibrational eigenvectors are substantially different in the protonated and deuteriated complexes and a one-to-one correlation is not possible. The calculated and observed frequencies of these modes of the neutral and anionic species are listed in Table V. Comparison of Figures 1 and 3 reveals that B-state excitation of [ZnTPPI- results in resonance enhancement of more modes than does B-state excitation of CuTPP (or ZnTPP). Essentially all of the [ZnTPPI- modes are polarized. The additional R R bands that appear in the spectra of the anion are too numerous to arise from AI, vibrations that are not enhanced in the neutral complex. Instead, the new bands are due to vibrations that are non-totally symmetric under Ddhsymmetry and become totally symmetric due to the symmetry lowering that occurs upon re(37) Stein, p.; Ulman, A.; Spiro, T. G. J . Phys. Chem. 1984, 88, 369.
900
1300
1700
Raman Shift (cm-') Figure 3. High-frequency regions of the B-state-excitation RR spectra of (a) ZnTPP, (b) [ZnTPPI-, (c) [ZnTPP-@-ds]-,(d) [ZnTPP-(mesoI3C)J, and (e) [ZnTPP-(1SN)4]-.All spectra shown were obtained in DMF solutions with X, = 457.9 nm with the exception of that of ZnTPP, which was obtained from a compressed Na2S04pellet with A, = 413.1 nm. Solvent or Na2S04modes are marked with the symbol #.
duction of the macrocycle. Comparison of the Q-state-excitation RR spectra of CuTPP with the B-state-excitation spectra of [ZnTPPI- reveals that modes of B,, symmetry become totally symmetric upon macrocycle reduction. This interpretation is
2240
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Atamian et al.
TABLE 111: Observed and Calculated Vibrational Frequencies for CuTPP frequency, cm-'
AW,
Ad8
obsd
AI, 1
2 3 4 5 (A) 6 (u2) 7 (B) 8 (~g) 9 (~4) I O (C) 11 (D) I2(u5) 13 (E) 14 ( ~ g ) 15 (F) 16 (Y,) 17 (G) 18 (vg) 19 (09)
1600 1564 1477 1367 1238 1080 1031 1006 889 640 392 202
BI, 20 21 22 23 24 25 26 27 28
(40)
1585 (~12) 1504 ( u I 3 ) 1357d (ut5) 1077d (vI4) 1006d (q6) 780d (~11)
('17)
A2%
29 (VI9) 30 ( u z 0 ) 31 ( ~ 2 1 ) 32 ( Y ~ 33 34 35 36 B2s
31 38 39 40 41 42 43 44 (v2*) 45 46 ( + I ) 41 48 49 50 51 52 53 54 55 ( u 3 5 )
153Id 1340 ~ 1235d ) 1010 849
1372d 1269d
calcd 3113 3064 3063 306 1 1600 1564 1499 1478 1370 1241 1180 1083 1030 1008 997 889 655 388 199 3113 1582 1502 1355 1078 1003 765 260 189 3108 1530 1335 1242 1016 840 489 27 1 3107 3064 3063 3061 1600 1500 1481 1371 1249 1227 1180 1065 1028 IO00 884 696 451 410 125
obsd
0 18' 38 8 2' 312 +I
+3 1 9
c 44'
calcd 784 0 0 0 0 19 2 32 6 4 0 318 +4 +I2 +2 6 1 7 3 784 9 47 5
e 1
+3
e
obsd
0 6
3 1 8
12 3 2
c 1
+2c 2
2
+9'
28 8' 5
799 0 0 0 1 1
35 0
9 25 1 e 0 e +1 +3
e 9
25 20 I
0 0 0 0 0 3 0
1' 1
1
0 11
7 4
1
0 3 1 8 4 0
6 4 4 1
6
1
1c
801 2 144 +I3 150 34 21 7
obsd
0 3 4 2
1
7
AI5N
calcd
c
0 16 4 9 1 3 II 0 0 0 23 0 8 2 7 0 0 0 0 0 0 1 0 3 9 13 0 0 4 2 0 6
0'
calcd
PED (CuTPP)"
0 0
98% vC,H 99% VCphH 99% VCphH 98%~CphH 79% YCphCph, 14% 6CphH 39% uC,C,, 35% uc,c, 59% 6CphH. 35% VCphCph 48% vC,C,, 28% K , N , 28% vC,C, 36% L O N ,32% uC,C, 59% VCmCph, 15% UC,N 74% JCphH, 11% VCphCph 56%GC,H 33% 6CphCphCph, 28% GCphH, 19% YCphCph 30% 6CphCphCph, 12% &C,H, 11% Uc,c, 59% UCphCph 28% uC,C,, 12% YCphCphr 12% 6c,c@c, 57% 6CphCPhCPh, 12% 6c,c,c, 23% GC,C,C,, 18% K u N , 16% uC,C,, 12% bC,NC, 19% vc,cph, 17% vC,c,, 15% 6c,C@c,, 10% 6CphCphCph
0 0 0 0 0 3 8 3 0 I 0 3 5 2 0 4 2 0 0
1
98% uC,H 71% uC,C,, 17% uC,N, 14% 6C,C,C, 75% vC,C,, 16% vC,C,, 14% 6C,H 79% uC,N, 12% BC,C,C, 60% 6C,H, 23% uC,C, 83% vC,C,, 17% 6CpH 57% 6C,C,C,, 32% GC,NC, 76% GNC,C, 48% uCUN,41% GC,C,C,
0 0 0 15 9 1 4 O
99% vC,H 92% uC,C,, 15% uC,C,,14% GC,C,C, 64% GC,H, 46% K,Cp 78% vC,N, 14% vC,C, 51% wC,C,,27% 6CpH 47% GC,C,Cp, 29% GC,C,C, 44% 6NC,C,, 24% 8c,c,cph 69% 6c,c,cph, 24% BNC,C,
1
1 18
2 5' 8'
0 2 9 0
+lC 4
0 0 0 0 1 0 1
4
1 1 1 I
2 6 6 0 9 1
0 1 1 1
2 1
99% uCBH 99% UCphH 99% UCphH 98% UCphH 82% YCphCph, 27% 6CphH 54% 6CphH, 32% UCphCph 59% vc,c,, 21% vc,c,, 20% dC,C,C, 40% vC,C,, 22% vC,N, 18% BC,C,C,, 16% GC,CpC, 48% VC,cph, 27% VC,N 58% GCOH, 20% vC,N 74% 6CphH, 11% VCphCph 18% VC,N, 18% VCphCph 59% 6CphCphCph, 44% VCphCph 64% YCphCph 45% GC,C,C,, 15% vC,C,, 12% 6C,C,C, 38% fiCphCphCph, 12% 6C,C@C@, 11% 6c,c,c, 22% YC,C,, 19% uC,C,, 19% GC,C,C, 19% 6CUNC,, 17% VCmCphr 16% 6NC,CB, 14% CphCphCph 43% GCuNC,, 29% GC,C,C,
"Mode descriptors are as follows: u = stretch and 6 = in-plane deformation. C,, C, = pyrrole carbons; C, = methine bridge carbon; c p h = phenyl carbon. bNomenclature of ref 37. 'Observed shift uncertain due to overlapping bands and/or spectral noise. dObserved frequency uncertain due to overlapping bands and/or spectral noise. e Normal coordinate changes substantially in isotopic species. See Table V.
confirmed by the normal-coordinate calculations and is consistent with the predictions of the QCFF/PI calculations. The Occurrence of a b,, (versus a b2J Jahn-Teller distortion is also in accord with a prediction based upon inspection of the nodal pattern of the eg* redox orbital.38 ( 3 8 ) Gouterman, M. In The Porphyrins: Dolphin, D., Ed.; Academic Press: New York. 1978; Vol. I, p 9 3 .
Comparison of the Normal Modes ofthe Neutral and Anionic Complexes. The computed vibrational eigenvectors of certain AI, and Bl, vibrations of CuTPP are shown in Figure 4, while those of certain A,, and B,, modes are shown in Figure 5 . For comparison, the computed vibrational eigenvectors of a number of the A, (Alg and B1, in D4h symmetry) and B,, (A,, and Bag in D4h symmetry) vibrations of [ZnTPPI- are shown in Figures 6 and 7 , respectively. In the figures, displacements are shown only for
Reduced Porphyrins. 1.
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989 2241
No. Obs.
Calc.
Y
>
-i No. 3 0 Obs. 1531 Calc. 1530
NO. 31-j-3 Obs. 1340 Calc. 1335
d-\
NO. 32 Obs. 1235 Calc. 1242
c'5
NO. 13 i, j Obs. 1350 Calc. 1346
No.
Obs. Calc
ii
Figure 6. Vibrational eigenvectors of selected A, modes of [ZnTPPI-.
2242
The Journal of Physical Chemistry, Vol. 93, No. 6, 1989
Atamian et al.
TABLE IV: Observed and Calculated Vibrational Frequencies for [ZnTPP-]
frequency, cm-I hLSN
A”C,
Ad8
obsd calcd obsd calcd obsd calcd obsd calcd A*
1
2 3 4 5 6 7 8 9 IO
1595 1549‘ 1534‘
1478 11 1439 12 13 1350 14 1234‘ 15 16 1067c 17 18 19 1020 20 1004 21 22 23
24 25 26 27 28
388
3112 3111 3058 3056 3055 1599 +3’ 1547 12 1534 20 1498 1477 24 1443 30 1349 1346 31 3 1235 1 I79 d 1072 1041 d 1030 d 1024 1009 2 997 905 +2 759 d 655 387 26 1 200 192
0 0 0 0 0 2’ 12 IO
1
25 32 7 27 2 0
7‘ +4’
I2 7 +4*
2 2 5 6 2
2 1 6
1
11
7
+3*
1 1
BI,
3107 29 30 3107 31 3058 32 3056 33 3055 34 1599 35 1514 36 1500 37 1492c 1493 38 1386 39 1321 40 1297c 1290 41 1252 1249 1212 42 43 1179 44 1092 1053 45 46 1029 1003 47 48 884 888 49 842 834 50 688 51 490 52 440 455 53 413 54 27 1 55 123
789 792 0 0 0 0 9 25
9 12 1 5 4 0 11
9 0 3 0 0 2 2 2 IO 12 4 0 1 0
798 799 0 0 0 0 9 3 14
0 0 0 0 0 0 9 2 IO 6 18 9 3
1 4
2’ 3 4 0 6 0 +2’
4
0
0
8’
1
3
d d 6 d
0
8
0 0 0 0 1
0 +2 d d
IO 20
0 0 2 1 0
20 15 1 0 7 7 0 0 1
0 2 2 0 0 8 7 2 0
d d
3 0 1 1
0 0
1
0
0 0 0 0 0 0 0 1 0 4 5 9 O 3 0 5 0 0 4 2 9 3 8
1
1
6
2 0
4 0 3 0
0
0
0
d d
1
1
6
PED ([ZnTPPI-)“ 99% vCBH (11,IV) 99% Y C ~ (1,111) H 99% VCphH 99% VCphH 99% UCphH 82% YCphCph, 26% 6CphH 41% uC,C, (IJII), 35% vC,C, (IIJV), 12% vC,C, (IJII), 12% vC&, (1,111) 36% vC,C, (IIJV), 18% vC,C, (IJII), 13% uC,N (IJII), 10% vC,N (11,IV) 63% 6CphH, 39% UCphCph 35% uC,C, (II,IV), 17% vC,N (I,III), 15% 6CBH (I1,IV) 23% uC,C, (IIJV), 20% K,C, (IIJV), 20% vC,N (IJII), 19% C,C, (IIJV) 34% uC,C, (IIJV), 25% YC,N (II,IV), 23% YC,N (I,III), 14% YC&, (I1,IV) 37% VC,C, (I,III), 21% uC,C, (LIII), 17% YCmCphr13% vC,N (IIJV) 49% Uc,cph 74% 6CphH, 12% VCphCph 23% 6C,H (IIJV), 22% vC,C, (IIJV), 20% vC,C, (IIJV), 15% uC,N (IIJV) 56% 6CBH (IJII), 33% vC,&, (1,111) 50% YCphCph, 27% 6CphH 37% vC,H (II,IV), 20% uC,C, (II,IV), 19% uC,C, (1,111) 33% 6CphCphCph 46% vcphcph, 10% vc,cp (1,111) 16% 6C,C,Cp 16% 6C,NC, (ILIV), 16% 6c,c,cph (IIJV), 15% GC,NC, (IJII), 13% GNC,C, (II,IV) 57% 6CphCphCph 29% 6C,C,C,, 9% vZnN (IIJV), 9% vZnN (1,111) 73% 6c,c,cph 16% vC,C, (1,111). 11% CNC,C, (I1,IV) 26% vZnN (IIJV), 20% vZnN (IJII), 20% GC,C,C, (IJI), 18% BNC,C, (IJI), 17% 6C,C,C, (IIJV) 98% u C ~ H(I1,IV) 98% v C ~ H(1,111) 99% UphH 99% UCphH 99% UCphH 82% UCphCph, 15% 6CphH 42% vC,C, (I,III), 31% vC,C, (I,III), 28% YC,C, (I1,IV) 41% 6CphH, 26% VCphCph 48% vC,C, (II,IV), 13% vC,N (1,111) 38% uC,C, (II,IV), 35% 6CPH (IIJV) 39% vc,c, (I,III), 30% v c m c p h 81% uC,N (II,IV), 20% uC,C, (I1,IV) 85% vC,N (IJII), 11% vC,C, (1,111) 31% 6C,H (I,III), 21% 6CPH (I1,IV) 74% 6CphH, 11% UcphCph 19% 6COH (II,IV), 14% vC,C, (I1,IV) 23% vC,C, (I,III), 23% 6COH (I,III), 16% uC,C, (II,IV), 13% 6C,H (I1,IV) 46% YCphCphr 36% 6CPhCphCph 59% YCphCph, 11% VCphH 23% 6C,H (II,IV), 16% 6C,H (IJII), 13% 6C,C,C, 24% 6C,H (I,III), 17% 6CpH (I1,IV) 48% 6CPhCPhCPh. 13% 6c,c,c, 24% GC,C,Cph, 22% GC,C,C, (1,111). 19% BC,C,C, (I1,IV) 15% GC,C,Cp (II,IV), 13% 6C,C,C, (I,III), 11% vC,C, (I,III), 11% vC,C, (1,111) 15% 6CphCphCphr 15% vc,cph 69% 6c,c,cph, 15% 6c,c,c, (I,III), 14% 6c,c,c, (II,Iv) 31% GC,C,C,, 23% GZnNC, (IIJV), 22% GZnNC, (1,111)
Mode descriptorsare as follows: Y = stretch and 6 = in-plane deformation. C,, C, = pyrrole carbons; C, = methine bridge carbon; c p h = phenyl carbon. Observed shift uncertain due to spectral noise. Cobservedfrequency uncertain due to overlapping bands and/or spectral noise. “Normal coordinate changes substantially in isotopic species. See Table V. those atoms whose motions contribute significantly to the normal mode (10% or greater of the maximum atomic displacement in a given mode). Examination of the eigenvectors for the modes of the neutral and anionic complexes allows an assessment of the effects of reduction on the normal-coordinate composition. C,C, and C,C, Modes. Four C,C, and two C,C, vibrations are Raman-allowed for both the neutral and anionic complexes. These modes all occur at frequencies above 1440 cm-I. For the neutral complex, modes no. 6 (Alg),21 (Big), 30 (A2g),and 43
(B2J contain the largest amount of C,C, character (Figures 4 and 5; Table 111). However, only modes no. 21 and 30 are relatively pure C,C, vibrations. The other two C,C, modes, no. 6 and 43, contain substantial amounts of C,C, and C,C, character, respectively. In the case of the two C,C, vibrations, no. 8 (Alg) and 22 (Big), the latter mode is relatively pure while the former contains significant amounts of C,C, and C,N character (Figure 4). Reduction of the macrocycle results in a change in composition of the eigenvectors of certain C,C, vibrations but not others. In
The Journal of Physical Chemistry, Vol. 93, No. 6,1989 2243
Reduced Porphyrins. 1
No. Obs. Calc. 1 5 1 4
Calc.
No. Obs. Calc. 1 2 4 9
Calc. 1 2 1 2
Calc.
II
Figure 7. Vibrational eigenvectors of selected B,, modes of [ZnTPPI-. TABLE V Modes of CuTPP and [ZnTPPr Whose Eigenvectors Are Substantiallv Altered uwn Deuteriation"
obsd Bl8
1077 780
4 8
CuTPP calcd 1078 765 1227 1065 884
[ZnTPPI-
A,
obsd
calcd
1067
1072 1041 1024 759 1386 1321 1212 1092 1053 888 834 455 413
1020 BI,
CuTPP-P-dE obsd 770
calcd 769 757 1100 987 825
[ZnTPP-P-d,]obsd calcd 1035 767 76 1 757 755 1330 1265 1118 978 886 875 834 801 425 398 398
" Frequency in cm-I. particular, C,C, modes no. 7 (A,) and 35 (BIJ of the anion (Figures 6 and 7;Table IV) are similar to modes no. 21 (BIJ and 30 (A2,) of the neutral complex, whereas modes no. 8 (A,) and 37 (Big) are quite different from modes no. 6 (Al,) and 43 (B2& In the case of the C,C, vibrations, reduction completely alters the forms of the eigenvectors relative to those of the neutral species. One of the C,C, vibrations, no. 10 (A,), is localized on one pair of opposite pyrrole rings (Figure 6 ) while the other is mixed to such an extent that no mode can be definitively associated with this displacement. C,C, and Cfl Modes. Four C,C, and four C,N vibrations are Raman-allowed for both the neutral and anionic complexes. These modes are spread throughout the frequency range 1000-1450 cm-l. For the neutral complex, modes no. 9 (Alg), 25 (B,,), 33 (A2,!, and 44 (B2,) contain the largest amount of C,C, character (Figures 4 and 5 ; Table 111). However, only mode no. 25 is a relatively pure C,CP vibration. The other three C,C, modes, no. 9,25, and 44, contain substantial amounts of C,N and hydrogen atom deformation. In the case of the four CONvi-
brations, only modes no. 23 (BiE)and 32 (A2.&are relatively pure. As was previously noted, mode no. 9 contains a substantial amount of C,C, as well as C,N character. No B2, C,N mode can be clearly identified. As is the case for the C,C, and C,C, vibrations, reduction of the macrocycle significantly alters the normal-mode composition of most of the C,C, and C,N motions (Figures 6 and 7; Table IV). In the anion, there are no modes that are predicted to contain more than 40% C,C, character. Only two modes contain more than 25% C,N character, nos. 40 and 41. Of these two vibrations, only mode no. 41 resembles a mode of the neutral complex. Other Modes. With the exception of the C H stretches and certain of the CmCPhstretches and C H deformations, the remainder of the calculated normal vibrations of both the neutral and anionic complexes contain contributions from a large number of internal coordinates (Tables I11 and IV). Certain of the vibrations appear to change form upon reduction while others do not. However, the complicated nature of the motions precludes any meaningful assessment of the effects of reduction on these modes.
Conclusions The normal-coordinate calculations reported herein indicate that reduction of the porphyrin macrocycle results in a substantial change in the composition of many of the vibrational eigenvectors. These changes are due to a bl,-like Jahn-Teller distortion. To date, there has been no measurement of the magnitude of the Jahn-Teller splitting in the anion; however, this splitting has been estimated to be in the range 70-200 cm-1.39,40 Such a splitting would be large enough that DZhsymmetry would govern the vibrational selection rules. Consequently, the interpretation of the vibrational spectra of the anion in terms of that of the parent neutral complex is inappropriate. Acknowledgment. We thank Anne Marguerettaz for preparing TPP-(meso-I3C), and TPP-/3-d8 and Richard Wagner for preparing TPP-(I5N),+ This work was supported by Grants GM36238 (J.S.L.) and GM-36243 (D.F.B.) from the National Institute of General Medical Sciences. R.J.D. is a recipient of a National Research Service Award from the same Institute (GM11744). Registry No. [ZnTPPI-, 34465-10-6; CuTPP, 14172-91-9; D,, 7782-39-0; 13C, 14762-74-4; lSN, 14390-96-6. (39) Felton, R. H.; Linschitz, H.J . Am. Chem. SOC.1966, 88, 1 1 13. (40) Maslov, V. G. Opt. Spectrosc. 1974, 37, 580.
