J. Phys. Chem. 1986, 90, 61 13-61 18
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Resonance Raman Characterization of Iron-Chiorin Complexes in Various Spin, Oxidation, and Ligation States. 2. Low-Frequency Skeletal Vibrations an4 Iron-Ligand Stretching Modes Y. Ozaki,+ K. Iriyama,+H. Ogoshi,* T. Ochiai,* and T. Kitagawa*s Division of Biochemistry, The Jikei University School of Medicine, Nishi-Shinbashi, Minato-ku, Tokyo 105, Japan, Department of Chemistry and Chemical Engineering, The Technological University of Nagaoka, Nagaoka, Niigata 949-54, Japan, and Institute for Molecular Science, Okazaki National Research Institutes, Myodaiji, Okazaki 444, Japan (Received: April 7, 1986)
The low-frequency region of resonance Raman (RR) spectra of (octaethylch1orinato)iron[Fe(OEC)] in various spin, oxidation, and ligation states and (octaethylch1orinato)coppe.r[Cu(OEC)] has been explored and compared with that of the analogous complexesof octaethylporphyrin (OEP). The Feaxial ligand (L) stretching frequencies of Fe(0EC) derivatives are in general very close to those of the corresponding Fe(0EP) derivatives. The RR bands due to internal modes of pyridine (Py) have been identified at almost the same frequencies between Fe"(OEC)(Py), and Fe"(OEP)(Py),. These facts indicate the nature of the Fe-L bonds remains unaltered upon saturation of one of four C,C, bonds. Excitation profiles of the bound-pyridine modes of Fe11(OEC)(Py)2imply that a d,(Fe)-r*(Py) charge-transfer band is located near 500 nm. With regard to the vibrations of the macrocycle, the 2vj5, and vg frequencies of chlorin derivatives are always lower than those of porphyrin derivatives, while the v7 frequency exhibits the opposite trend. The v7 and VI6 RR bands can be used as markers of a chlorin skeleton.
Introduction In the preceding paper' we characterized resonance Raman (RR) spectra in the 1200-1700-cm-' region of (octaethylch1orinato)iron [Fe(OEC)] complexes having various spin, oxidation, and ligation states. This paper treats the R R spectra in the lW900-cm-' region of the similar complexes and Cu(0EC). In this frequency region, an Fe-axial ligand (L) stretching mode, which reflects the nature of the Fe-L bond most directly, is expected to appear. Regarding the porphyrin vibrations, outof-plane deformation modes, which are sensitive to a geometrical structure of the macrocycle, are also observed in addition to the in-plane modes. Therefore, the RR spectrum in the low-frequency region is considered to be another rich source of structural inf o r m a t i ~ nbut ~ . ~has been much less studied on metallochlorins than on metalloporphyrins. We previously reported the lowfrequency region of Fe(0EC) and Ni(0EC) complexes, obtained with 488.0-nm excitation. As shown herein, this excitation line is not the optimal choice for resonance enhancement of low-frequency modes. Here we discuss vibrational assignments of the low-frequency Raman lines of Cu(OEC), the Fe-L stretching modes of various Fe(0EC) derivatives, and the internal modes of bound pyridine (4)of Fe"(OEC)(Py)Z on the basis of extensive experimental investigation. Experimental Section Cu(OEP), Cu(OEC), Cu( [1SN4]OEC)and Cu(OEC)-d, were synthesized with the methods described el~ewhere.~All solvents were spectroscopic grade and were used without further purification. The methods for preparing the Fe(0EP) and Fe(0EC) complexes and for recording R R spectra were described in the preceding paper.' Frequency calibration was carried out with indene (above 500 cm-') and C C 4 (below 500 cm-I), and the estimated errors of fcequencies were less than 1 cm-' for wellresolved bands. Results Cu(0EC) and Cu(0EP). Figure 1 shows the low-frequency region of the 406.7-nm excited R R spectra of Cu(OEP), Cu(OEC), Cu( ['-?i4]OEC), and Cu(0EC)-d, in tetrahydrofuran (THF) solution. Polarization measurements revealed that only a band a t 751 cm-' of Cu(0EP) is depolarized (dp) and the rest
'*The Jikei University School of Medicine. Technological University of Nagaoka. 'Institute for Molecular Science.
0022-3654/86/2090-6113$01.50/0
of the bands are polarized (p). R R spectra of these complexes excited at 441.6 and 488.0 nm were of lower quality than those shown in Figure 1. This confirms that Raman excitation into the Soret region provides the greatest enhancement for most of the low-frequency mode^.^^^ The R R bands of Ni(0EP) in the 100-900-cm-' region were previously assigned on the basis of the observed isotope shifts and normal coordinate calculations.6 The R R bands of Cu(0EP) can be assigned by analogy to Ni(0EP). Correspondence of R R bands between Cu(0EP) and Cu(0EC) allows us to infer the vibrational assignments for Cu(0EC); bands at 764, 744, 7 16, 676, 353, and 339 cm-' are assigned to the modes similar to vj3 + v34, v I 6 ,v33 v35,v7, 2vj5, and v8, respectively. Although the V I 6 mode is expected to show a large deuteration shift,6 the 744cm-I band showed negligible shift. This may suggest that the 744-cm-' band is associated with vlS. Although VI6 is used in this paper, this should be thought tentative regarding the OEC complexes as will be discussed later. The band around 470 cm-I may arise from an out-of-plane mode as pointed out by Choi et aL7 The vg mode of Ni(0EP) was recently reassigned to the band at 260 cm-I,8 and in accord, the Raman band of C u ( 0 E C ) a t 267 cm-' is assigned to the vg-like mode. It was stressed for metalloporphyrins that R R spectra in the vg frequency region change sensitively with the peripheral substituents.8 With regard to differences between chlorin and porphyrin, the bands of Cu(0EC) at 826, 566, and 400 cm-l are not seen for Cu(0EP). The 566-cm-' band of Cu(0EC) may be associated with the like mode, because Ni(0EP) with D4hsymmetry gives an IR band at 550 cm-1,6and Ni(0EP) with D M symmetry gives a Raman band at 551 cm-Ie9 It is noteworthy that the v33 vj4, v16, and 2vj5 bands show a downward shift upon a change from
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(1) Ozaki,Y.; Iriyama, K.; Ogoshi, H.; Ochiai, T.; Kitagawa, T., preceding paper in this issue. (2) (a) Kitagawa, T.; Ozaki,Y.; Kyogoku, Y. Adu. Biophys. 1978, ZZ, 153. (b) Kitagawa, T.; Ozaki, Y. Struct. Bonding (Berlin), in press. (3) Spiro, T. G. In Iron Porphyrins; Lever, A. B. P., Gray H. B., Eds.; Addison-Wesley: Reading, MA 1983; Vol. 2, p 91. (4) Ozaki, Y.; Kitagawa, T.; Ogoshi, H. Inorg. Chem. 1979, 18, 1772. (5) Ogoshi, H.; Watanabe, E.; Yoshida, Z.; Kincaid, J.; Nakamoto, K. Inorg. Chem. 1975, 14, 1344. (6) (a) Kitagawa, T.; Abe, M.; Ogoshi, H. J . Chem. Phys. 1978,69,4516. (b) Abe, M.; Kitagawa, T.; Kyogoku, Y. Ibid. 1978, 69, 4526. (7) Choi, S.; Spiro, T. G. J . Am. Chem. SOC.1983, 105, 3683. (8) Lee, H.; Abe, M.; Pandey, R. K.; Leung, H.-K.; Smith, K. M.; Kitagawa, T. J . Mol. Struct., in press. (9) Spaulding, L. D.; Chang, C. C.; Yu, N.-T.; Felton, R. H. J . Am. Chem. SOC.1975, 97, 2517.
0 1986 American Chemical Society
Ozaki et al.
6114 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986
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Figure 1. Low-frequency region of the 406.7-nm excited RR spectra of Cu(OEP), Cu(OEC), Cu( [1JN4]OEC),and Cu(0EC)-d, in THF. Through all the RR spectra presented in this paper, Raman bands marked with S are due to solvent. Instrumental conditions are as follows: laser power, 20 mW; slit width, 7 cm-I; scan speed, 25 cm-'/min; time constant, 4.0 s.
Cu(0EP) to Cu(0EC) but the u7 band shows an upward shift. The 15N isotopic frequency shifts in this frequency region were thought to be small by analogy to Ni(OEP),6 but it was unexpected that the deuteration shifts of Cu(0EC) were so small, although the deuteration shifts in the 1200-1700-cm-' region were confirmed to be as e ~ p e c t e d . ~ Ferric Fe(0EC) and Fe(0EP) Derivatives. Figure 2a shows the low-frequency region of the 406.7-nm excited R R spectra of the five-coordinate ferric high-spin complexes in the THF solution: FelI1(OEP)F (A), Fe"'(0EP)Cl (B), and F p ( 0 E P ) B r (C). These R R spectra are distinctive of the three complexes, despite the fact that their R R spectra in the 1200-170acm~'region are quite alike. The R R spectra of Fe"'(0EP)I and [Fe11'(OEP)]20 in THF solution were also obtained (not shown). The relative intensities of the 804(p) ( Y 6 ) and 753(dp) (q6) cm-' bands change largely among the three complexes. A band at 736(p) cm-' does not have a counterpart in the R R spectrum of Ni(OEP), but Ni(0EP) showed an IR band at 726 cm-'assignable to u47 (E,): Therefore, the 736-cm-' band of Fe"'(0EP)F is presumably due to the u47 mode. Weak features at 468 and 428 cm-I may be due to outof-plane modes.' FelIr-L stretching modes were previously located at 606, 364, and 279 cm-l for Fe"'(OEP)F, Fe"'(OEP)Cl, and Fe"'(OEP)Br, respectively.I0J1 These modes are clearly identified in the present spectra, although the Fe-F and Fe-C1 stretching bands at 607 and 364 cm-I, respectively, are overlapped with porphyrin bands. We failed to detect the FeIII-1 stretching mode of Fe"'(0EP)I with any of the 406.7-, 441.6-, or 488.0-nm excitations. Figure 2b shows the R R spectra of the five-coordinate ferric high-spin chlorin complexes: Fe"'(0EC)F (A'), Fe"'(0EC)Br (C') and Fe111(OEC)(2-MeIm)(D') (2-MeIm: 2-methylimidazole) in T H F solution. The R R spectrum of Fe"'(0EC)I was also obtained (not shown). The v6 band around 800 cm-' is generally obscure but the u7 band is clearly observed near 670 cm-I, and the latter frequency is slightly higher for OEC complexes than for OEP complexes as was so for the Cu complex. A shoulder of the u7 mode near 693 cm-' is characteristic of the Fe"'(0EC) complexes. A band of Fe"'(0EC)F at 746 cm-' presumably corresponds to the 753-cm-' band ( V I 6 mode) of Fe"'(OEP)F, because the u16 mode shows a downward shift by 7 cm-l upon a change from Cu(0EP) to Cu(0EC). The Fe"LL stretching mode of FeII'(0EC)F and Fe"'(0EC)CI was previously identified at 608 and 361 cm-', re~pectively.~In the present study we assign a band of Fe"'(0EC)Br at 272 cm-' (10) Kitagawa, T.; Abe, M.; Kyogoku, Y.; Ogoshi, H.; Watanabe, E.; Yoshida, Z. J. Phys. Chem. 1976, 80, 1181. (11) Kincaid, J.; Nakamoto, K. Specfrosc. L e f f . 1976, 9, 19.
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Figure 2. (a) Low-frequency region of the 406.7-nm excited RR spectra of Fe"'(0EP)F (A), Fe"'(0EP)CI (B), and Fe"'(0EP)Br (C) in THF. (b) Corresponding spectra of Fe"'(0EC)F (A'), Fe';'(OEC)Br (C'), and
Fe11'(OEC)(2rMeIm)(D') in THF. Through all the RR spectra presented in this paper, Raman bands marked with P and C are due to the laser plasma line and a Raman cell, respectively. Bands near 605 cm-' with a single asterisk contain contributions from both sample and solvent. Instrumental conditions are the same as those in Figure 1. and that of Fe"'(0EC)I at 248 cm-' (not shown) to Fel''-Br and Fe"'-I stretching modes, respectively, because these bands are specific to individual complexes and their frequencies are close to those of the corresponding I R bands5 Figure 3 shows the low-frequency region of the 406.7-nm excited R R spectra of the six-coordinate ferric complexes: Fel"(OEP)(CH,OH), in a 1/1 mixture of C6H,/CH,OH (A), Fe"'(OEP)(Me2SO), (Me2SO: dimethyl sulfoxide) in 1/ 1 C6H6/Me2S0 (B), Fe11'(OEP)(Im)2 (Im: imidazole) in THF (C), Fe111(OEC)(CH30H)2in 1/ 1 C 6 H 6 / C H 3 0 H (A'), Fe"'(OEC)(Me,SO), in 1/ 1 C6H6/Me2S0(B'), and Fer1'(OEC)(Im)2 in T H F (C'). The methanol and MezSO complexes and Im complexes adopt the high- and low-spin states, respectively. The R R spectra in the 1200--1700-cm-' region are distinctive of the two spin states. Interestingly, however, the low-frequency R R spectra of the methanol and Me2S0 complexes are closer to those of the low-spin Im complexes than to those of the high-spin complexes shown in Figure 2 for both the OEP and OEC complexes. With regard to the porphyrin structure, the six-coordinate complexes are usually planar regardless of a spin state but the five-coordinate complexes are pyramidal. Accordingly, the results shown in Figure 3 suggest that the RRlspectrum in the low-frequency region of both chlorins and porphyrins is more sensitive to the geometrical structure of the macrocycle rather than to the spin state of the iron ion. A good correspondence of R R bands between Fe"'(0EP)(CH30H)2 and Cu(0EP) suggests the assignments of the R R bands of Fe"'(OEP)(CH,OH), at 805(p), 752(dp), 673(p), 361(p), 345(p), and 269(p) cm-' to v6, q 6 , u 7 , 2u35, v g , and v g modes, respectively. A band near 735(p) cm-I of the Fe"'(0EP) complexes is extra compared with the R R spectrum of Cu(0EP).
Low-Frequency Vibrations of Metallochlorins
The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 6115
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Figure 4. Low-frequency region of the 406.7-nm excited RR spectra of Fe"(OEP)(Z-MeIm) (A) and Fe"(OEC)(Z-MeIm) (A') in THF and FeII-ligand stretching region of the 441.6-nm excited RR spectra of Fen(OEP)(1,2-Me21m)(B) and Fe1'(OEC)(1,2-MeZIm)(B') in CH2C12. Raman bands with double asterisks (B and B') are due to 1,2-Me21m. Instrumental conditions for (A) and (A') are the same as those in Figure 1 and those for (B) and (B') are as follows: laser power, 40 mW: slit
width, 7 cm-'; scan speed, 25 cm-'/min; time constant, 4.0 s.
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Figure 3. (a) 406.7-nm excited RR spectra in the low-frequency region Fe"'(OEP)(CH,OH), (A) in a 1/1 (V/V) mixture Of CH@H/C6H, Fe1''(OEP)(Me2S0), (B) in that of Me2SO/C6H6,and Fe"'(OEP)(Im), (C) in THF. (b) Corresponding spectra of Fe111(OEC)(CH30H)2(A') in a 1/1 (v/v) mixture of CH@H/C6H6, Fe11'(OEC)(MezSO)2(B') in that of Me2SO/C6H6,and Fell'(OEC)(Im)z (C') in THF. Bands near 605 cm-I with a single asterisk are the overlap of two bands due to sample and solvent, respectively. The intense feature near 670 cm-' (B and B') and a shoulder at 697 cm-' (B') marked with a single asterisk contain a large contribution from Me2S0. Instrumental conditions are the same as those in Figure 1. Of
This band probably corresponds to the v47 mode similar to the 736(p)-cm-' band of Fe"'(0EP)X. Choi et aL7 proposed the detailed assignments for the low-frequency RR bands of Fe"'(OEP)(Im)2 and assigned the bands at 468(p), 362(p), and 258(p) cm-' to out-of-plane modes. Upon a change from the porphyrin to chlorin, the v7 band is shifted upward by 4 cm-' while the 2v35 and v8 bands are shifted downward by -5 cm-'. Ferrous Derivatives. Figure 4 shows the RR spectra of fivecoordinate ferrous high-spin complexes: FeI1(OEP)(2-MeIm) (A), Fe"(OEP)( 1,2-Me21m)(B) (1,2-MqIm: 1,2-dimethylimidazole), FeI1(OEC)(2-MeIm) (A'), and Fe"(OEC)(1,2-Me21m) (B'). The v, mode of Fe"(OEC)(Z-MeIm) is shifted upward by 5 cm-l from the corresponding band of the O E P complex (668 cm-') and is accompanied with a shoulder at 686 cm-'. The band of Fe"(OEP)(2-MeIm) a t 207 cm-' (A) is assigned to the Fe"-L stretching mode. This frequency is somewhat sensitive to solvent: 206, 205, and 208 cm-l for the CH2C12, C6H6,I2and DMFL3 solutions, respectively. The corresponding band of Fe"(0EC)(2-MeIm) is seen at 21 1 cm-' (A'). The overall spectral patterns of the 1,2-Me21m complexes are very similar to those of the 2-MeIm complexes, but the Fe-L stretching Raman band is observed at 188 and 187 cm-' for the OEP and OEC complexes (12) 1795. (13)
Stein, P.; Mitchell, M.; Spiro, T.G. J. Am. Chem. SOC.1980, 102, Teraoka, J.; Kitagawa, T. J . Eiol. Chem.
1981, 256, 3969.
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Figure 5. (a) Low-frequencyregion of the 406.7-nm excited RR spectra of FeI1(0EP)(4-CHO(Py)), (A), Fe"(OEP)(Py), (BJ, Fe"(OEP)(Im)2 (C), and Fe"(OEP)(n-C4H9NH2), (D) in THF. (b) Corresponding spectra of Fe11(OEC)(4-CHO(Py))2(A'), Fe"(OEC)(Py), (Bl'), Fell(OEC)(Im)2 (C'), and Fe"(OEC)(n-C4H,NH2)2(D') in THF. (B,) and (B;) are the 488.0-nm excited RR spectra of Fe"(OEP)(Py), and Fe"(OEC)(Py), in CH2CI2,respectively. Instrumental conditions are the same as those in Figure 1. in CH2C12,respectively. This frequency is appreciably lower than that for the C6H6solution (195 cm-') reported by Stein et a1.I' for Fe"(OEP)(1,2-Me21m). In Figure 5a is shown the low-frequency region of the 406.7-nm excited RR spectra of ferrous low-spin porphyrin complexes:
Ozaki et al.
6116 The Journal of Physical Chemistry, Vol. 90, No. 23, 1986 a
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Figure 6. RR spectra of Fe"(OEP)(Py), and Fe"(OEC)(Py), in CH2CI2 with 488.0-nm excitation. Bands marked b and fare due to bound and free pyridine, respectively. Bands with a single asterisk contain contri-
butions from both sample and solvent. Instrumental conditions: laser power, 100 mW; slit width, 7 cm-I; scan speed, 50 cm-'/min; time constant, 2.0 s.
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/' Fe"( OEP) (4-CHO( Py)) (Py : pyridine) (A), Fe"(0EP) (Py), F e b r (B1),Fe11(OEP)(Im)2(C), and Fe"(0EP)(n-C4H9NH2), (D) in Feb n THF. The frequencies and relative intensities of R R bands are p(oEp)120 close to those of Ni(OEP)6 and Cu(OEP), and accordingly the , , FebECX2-Mehl R R bands at 848,801,770,749,671,360,345, and 264 cm-l of Ferr(OEP)(4-CHO(Py))2are assigned to v7 v35,vg, v33 v ~ ~ , q 6 , u7, 2U35, v8 and v9 modes, respectively. The R R spectra of the ferrous low-spin complexes are distinguishable from those of ferrous high-spin complexes, because R R frequencies are generally FeIBX1-h) higher for the low-spin complexes than for the high-spin complexes as in the case of the high-frequency region.' Within the category Fe@X4CHOPy)2 of the low-spin complexes, the frequencies of four bands near 769 F&")z (u33 + v3& 748 (VI& 670 (v7),and 260 (v9) cm-'change sensitively F d 0 2 with axial ligands, and it will be discussed later. F&'X+C&N-r2)2 Figure 5b displays similar spectra of low-spin chlorin complexes: Wavenumber (cm-0 Fe"( OEC) (4-CHO( Py)) 2 (A'), Fe"( OEC) (Py) (B ]'), Fe"(OEC)(Im)2 (C'), and Fe"(OEC)(n-C4H9NHZ), (D'). It is seen Figure 8. RR frequencies of v16 (-750 cm-I), v7 (-670 cm-I), 2vj5 (-350 cm-I), and vs (-340 cm-I) modes for Cu(P) (P: OEP or OEC) again that the v33 + v34 (-765 cm-I), and vI6 (-740 cm-I) and various Fe(P) derivatives: (-) data for the OEP complexes; (---) frequencies are lower but the v7 (-670 cm-l) frequency is higher data for the OEC complexes. with the chlorin complexes than with the porphyrin complexes, although they are less sensitive to the axial ligand for the chlorin Fe"(OEP)(Py), and Fe"(OEC)(Py),, although some of the complexes. porphyrin-ring vibrations show a distinct frequency difference. The RR bands of Fe"(OEP)(Py), at 183 cm-l (B,) and of Ligand internal modes were also compared for y-Pic and 4Fe"(OEC)(Py), at 184 cm-' (Bl') disappeared in the spectra CH,==CH(Py) complexes. Their R R frequencies were again very excited at 441.6 nm, but upon excitation at 488.0 nm a new band close between the Fe"(0EP) and Fe"(0EC) derivatives. For appeared at 178 cm-l for the OEP complex (Bz)and a t 179 cm-I Fe"(0EP) (4-CH2=CH( Py)), and Fe'YOEC) (4-CHz==CH(Py)) for the OEC complex (Bi). The new bands exhibited a downward in CHzClz the C=C stretching mode of the vinyl group was shift of 5 cm-I when deuterated Py was used. Accordingly, they observed at 1632 cm-I. are assigned to the Py-FeII-Py symmetric stretching mode, in good The UV-visible absorption spectra and excitation profiles of agreement with the reported frequencies for Fe1I(0EP)(Py), (176 bound-Py modes of Fe"(OEP)(Py), and Fe"(OEC)(Py), in cm-I)l4 and Fe"(MP)(Py), (179 cm-I)l5 (MP: mesoporphyrin CH2C12are shown in Figure 7. The excitation profiles of all the IX dimethyl ester). The measurements on Fe"(OEP)(Pip), and modes of bound Py of Fe"(OEP)(Py), show a maximum near 488 Fe"(OEC)(Pip), (Pip: piperidine) revealed for the first time that nm. With regard to the excitation profiles of the OEC complex, the PipFe'I-Pip symmetric stretching mode is located at 164 and three Raman bands exhibit two maxima near 480 and 500 nm, 163 cm-'for the OEP and OEC complexes, respectively. In similar although one R R band gives only the former maximum. measurements on y-Pic complexes (y-Pic: y-picoline), the yPic-Fe"-y-Pic symmetric stretching mode was found at 161 cm-l Discussion for both the O E P and OEC complexes. Difference between Metalloporphyrin and Metallochlorin. Figure 6 compares the 488.0-nm excited R R spectra of FeT1Excitation into the Soret region provides high-quality low-fre(OEP)(Py), and Fe"(OEC)(Py), in CH2C12. R R bands marked quency R R spectra of metallochlorins that can be directly comb and f are assigned to the vibrations of bound and free pyridine, pared with those of metalloporphyrins. Figure 8 compares the respectively. These assignments were made by comparing the R R frequencies of VI69 v7, 2v35,and vg modes between the OEC spectra shown in Figure 6 with those of Fe"(OEP)(Py-d5),, and OEP complexes treated in this study. Upon a change from Fe11(OEC)(Py-dS)2,and free Py (not shown), Particularly striking porphyrin to chlorin, the v16 mode is shifted downward but the is that frequencies of bound-Py modes are very close between v7 mode is shifted upward. Since this trend is observed for all complexes studied, the two bands can be used for diagnosis to (14) Schick, G . A.; Bocian, D. F. J . Am. Chem. SOC.1984, 106, 1682. discriminate between chlorin and porphyrin skeletons. (15 ) (a) Spiro, T.G . ;Burke, J. M. J . Am. Chem. Soc. 1976,98,5482. (b) The v7 mode corresponds to the breathing-like vibration of the Wright, P. G.;Stein, P.; Burke, J. M.; Spiro, T. G . J. Am. Chem. Soc. 1979, 16-membered ring including C,, N, and C, atoms.6 The vibra101, 3531. tional mode itself should not be different between the OEP and (16) Coffey, S., Ed. Chemistry of Carbon Compounds, 2nd ed.; Elsevier Scientific: Amsterdam, 1976. OEC complexes, but the electronic conjugation of the inner ring
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