Infrared spectra of matrix-isolated metal complexes of

3096

J. Phys. Chem. 1983, 87,3096-3101

Infrared Spectra of Matrix-Isolated Metal Complexes of Octaethylporphlne James R. Klncald," Chemlstry Department, Universlfy of Kentucky, Lexlngton, Kentucky 40506

Marek W. Urban, Takeshi Watanabe, and Kazuo Nakamoto Chemlsby Department, krquette Universi?v, Mllwaukee, Wlsconsin 53233 (Received: November 29, 1982; I n Final Form: February 8, 1963)

High-resolution infrared spectra of divalent metal (Mn, Fe, Co, Ni, Cu, Zn) complexes of octaethylporphine (OW)obtained at -15 K in argon matrices or as thin films are reported. The spectra of Ni(0EP) and a number of its isotopicallysubstituted analogues are reported, including those of the meso-deuterated and I5N substituted as well as that of the doubly labeled 16N,meso-deuterated species. A thorough discussion of all observed bands is presented. Justification for selection of fundamental modes is given and sensitivity to metal substitution is discussed in terms of structural and bonding alterations.

Introduction Vibrational spectroscopy has proven itself to be an effective structural probe of metalloporphyrins and hemes via infrared (IR) and resonance Raman (RR) studies. The ability to selectively enhance modes of the heme in dilute aqueous solutions of heme proteins via RR spectroscopy provides a powerful monitor of heme structure in its native biological surroundings.' Full exploitation of this rich source of vibrational spectral information relies on a thorough understanding of the relationships of the vibrational modes to structure. Such relationships may be most effectively established through the detailed investigation of the vibrational spectra of simple, high-symmetry metalloporphyrins. The similarity of peripheral substituent type and overall vibrational spectral patterns of octaethylporphine (OEP) to naturally occurring porphyrins has led several research groups to focus their attention on the vibrational spectra of metal complexes of this readily available synthetic An important contribution was made by Kitagawa and c o - w ~ r k e r s , ~ who ~~~' proposed assignments for all in-plane vibrations based on isotopic substitution and an analysis of combination modes in the RR spectrum of Ni(0EP). A normal coordinate analysis' gave calculated frequencies in reasonable agreement with those observed. In fact, the majority of previous works concentrated on the analysis of the RR data. In the present work we report a comprehensive and detailed analysis of all bands observed in the IR spectra of divalent metal complexes of OEP which have been isolated in an argon matrix at 15 K. The high-resolution infrared spectra of isotopically labeled nickel complexes (15N,d4, and (15N, d4))are also included. On the basis of this new high-quality spectral information, we suggest alternative assignments to those which have been previously proposed.'

-

(1) Asher, S. In Methods Enzymol. 1981, 76, 371-413. (2) Shelnutt, J. A.; O'Shea, D. C.; Yu, N. T.; Cheung, L. E.; Felton, R. H.J. Chem. Phys. 1976, 64, 1156. (3)Ogoshi, H.;Masai, N.; Yoshida, Z.; Takemoto, J.; Nakamoto, K. Bull. Chem. SOC.Jpn. 1971,44, 49. (4)Kitagawa, T.; Abe, M.; Kyogoku, Y.; Ogoshi, H.; Watanabe, E.; Yoshida, Z. J. Phys. Chem. 1976, 80, 1181. ( 5 ) Spaulding, L. D.; Chang,C.; Yu, N. T.; Felton, R. H. J. Am. Chem. SOC.1975, 97,2517. (6)Kitagawa, T.;Abe, M.; Ogoshi, H. J. Chem. Phys. 1978,69, 4516. (7)Abe, M.;Kitagawa, T.; Kyogoka, Y. J . Chem. Phys. 1978,69,4526. 0022-3654/83/2087-3096$01.50/0

Experimental Section Octaethylporphine (OEP) and its I5N analogue were prepared by the method described by Dolphin and coworkers.8 The correspondingmeso-deuterated derivatives (OEP-d4and 15N-OEP-d4)were obtained by exchange of the methine hydrogens in D2S04during 18 h at room temperature. The deuterated compounds were isolated by neutralization, extraction into CHC13,chromatography on alumina, and recrystallization from toluene. The metal complexes were prepared by the dimethylformamide methodag The complexes were chromatographed on 10% deactivated alumina (Fisher A-540) by using chloroform/toluene (1/1by volume) as eluent. The nonfluorescent fractions were combined, the solvent was removed under reduced pressure, and the complex was recrystallized from toluene. The purity of the complexes was checked by thin-layer chromatography and visible spectroscopy. The complexes were vaporized from a Knudsen cell at -430 K and codeposited with respective gases on a CsI plate which was cooled to -15 K by a CTI Model 21 closed-cycle helium refrigerator. A detailed design of the cell is described elsewhere.'O IR spectra were measured on a Beckman Model 4260 infrared spectrophotometer at a 5 cm-'/min scan speed at 2.0-cm-' resolution between 1700 and 650 cm-I. Rotation-vibration bands of standard molecules and polystyrene bands were used for calibration of frequency. Results and Discussion The IR spectra of Ni(0EP) and its meso-deuterated analogue are shown in Figures 1 and 2. The general improvement in the quality of the spectra in proceeding from the room temperature pellet to thin films to matrix-isolated sample is clearly evident. Most importantly, a number of weaker bands are more readily observed in the matrix spectra, especially those that are in close proximity to stronger bands and that are sometimes obscured in the room temperature spectra. The advantage of the matrix isolation technique in detecting weaker bands and resolving overlapping bands in a particular region is especially important in determining which bands are as(8) Paine, J. B.; Kirshner, W. B.; Moskowitz, D. W.; Dolphin, D. J. Org. Chem. 1976,41, 3857. (9) Adler, A.; Longo, F. R.; Kampas, F.; Kim, J. J . Inorg. Nucl. Chem. 1970.32. 2443. (10)Tevault, D.;Nakamoto, K. Inorg. Chem. 1976, 15, 1282.

0 1983 American Chemical Society

The Journal of physical Chemistry, Voi. 87, No. IS, 1983 3097

I R Spectra of M(0EP)

TABLE I: Observed Frequencies of NiOEP and Isotopically Labeled Analogues ~~

~

~

Ni( OEP) 1604 1575 1501 1473 1456 1396 1378 1323 1275 1231 1153 1133 1119 1069 1061 1021 996 959 927 84 6 834 7 54 7 26 703 605

aIsN 0 0 0 0 0 0

0 0 3 8 12 2 0 0 0 5

0 3 0 0 0 4 4 0

Ni( OEP-d,) 1564 1494 1473 1455 1392 1380 1322 1267 1185 948 1151 1121 1069 1062 1024 996 964 919 682 766 7 26 634 (KBr)

A 15N

~~

(11) Sunder, S.; Bernstein, H. J.Raman Spectrosc. 1976, 5 , 351.

designation

0 0 0 0

0 0 0 0 8 8

11 2 0 0 0 5

0 3 0 6 4

sociated with the porphyrin macrocycle. In contrast to the situation encountered during the interpretation of the RR spectra of metalloporphyrins, in which case only those modes associated with the porphyrin core are enhanced, the selection of in-plane IR-active fundamental modes (&) is complicated by the appearance of out-of-plane (A2J modes and most seriously by vibrations associated with the peripheral ethyl groups. Detailed consideration of all observed bands, the sensitivity of each band toward isotopic substitution and toward variation of the central metal ion, and careful comparison with previously established ethyl group vibrations are required in order to support assignments and eliminate ambiguities. Furthermore, consideration of the observed isotopic shifts in parallel with the apparent metal sensitivity of various bands may provide for a better understanding of the structural origin of vibrational frequency shifts. The spectra of matrix-isolated OEP complexes of a number of other metals have also been obtained and are shown in Figure 3. The frequencies for Ni(0EP) and the frequency shifts observed upon isotopic substitution and metal substitution are given in Tables I and I1 along with an indication of the principal contributions from internal coordinates based on normal coordinate calc~lations.~J~ The labeling system follows that commonly used to describe metalloporphyrin vibrational modes. The designations C,, C,, C,, and C, refer to the carbon atoms adjacent to the nitrogen atom, at the /3 pyrrole position, the methine bridges, and the substituent, respectively. The symbols u, 6, and T refer to stretching, bending, and outof-plane bending while pw, pr, pt, etc. are common group frequency designations for the ethyl group vibrations.12 Further justification for these assignments is provided by the following considerations. As was pointed out by previous worker^,^ the 1690-cm-’ band is not a fundamental and can be correlated with the band which appears a t 1352 cm-’ in the spectra of the deuterated analogues. In agreement with previous reports, we identify the highest frequency fundamental as a very weak band appearing at -1600 cm-’ ( Y ~ ~ )This . region is somewhat obscurred in the matrix spectra by unidentified

-

assignment

TABLE 11: Observed Frequency Shifts for M(0EP) Complexesa

Ni Co Cu Fe Mn Znb 1575 10 21 24 24 38 metalsensitive 1501 7 18 1 5 -25c -25c metalsensitive 1473 1 1 2 0 ethylgroup 1456 0 1 0 0 2 5 7 1396 1 1 0 -1 0 -1 0 ethyl group 1378 0 0 1 1 1 ethyl group 1322 2 4 1275 - 2 -1 -1 3 4 14 9 metal sensitive 1231 -1 -1 0 1153 -1 -1 -1 1133 0 -4 -3 +8 d 1119 1 2 2 6 3 1069 -1 0 0 0 ethyl group 1061 -1 0 0 -1 -3 2 5 2 ethyl group 1021 0 1 12 metalsensitive 2 8 5 16 996 0 0 0 ethyl group 959 - 1 -1 4 16 11 metalsensitive 927 0 5 3 1 -2 846 0 0 4 8 1 834 1 3 2 10 6 metal sensitive 754 0 2 742 1 - 2 -2 0 2 4 4 d 726 - 3 - 3 703 3 2 0 -3 0 a Ni(0EP) - M(0EP). From KBr spectrum. Overlapped with ethyl group vibration. Not observed.

impurity bands which are concentrated relative to the bands of the complex during the matrix isolation procedure. The highest frequency fundamental contains major contributions from the pyrrole (C,-C,) stretching coordinate and may therefore be consistent with weak IR intensity. Previous workers assigned this band to v(C,-C,).’ Bands observed at 1575 (v3& and 1501 cm-l (u3J are assigned to core vibrations and are correlated with bands observed at 1564 and 1494 cm-I in the spectrum of the deuterated analogue. The two strong bands which are observed at 1473 and 1456 cm-l, which show no isotope or metal sensitivity, are assigned as ethyl group vibrations rather than to core modes. In general, the highest frequency “group vibrations” (excluding u(C-H)) associated with the ethyl group are 6,(CH3) and 6(CHz) which are normally observed as medium-trong bands between 1450

3008

Kincaid et al.

The Journal of Physical Chemistiy, Vol. 87, No. 16, 1983

_

I

w

THIN FILM

-E

KBr PELLET

I

no0

I

1

1500

I

, 5, Boo

1

I

1100

I

900

I

I

700

I

I

I

500

I

300

CM-' Flgure 1. Infrared spectra of Ni(0EP): (A) Ar matrix, (B) thin film,

(C)KBr pellet.

Spectral bandpass = 2.0 cm-'

and 1475 cm-' based on extensive compilations.12 The 1396-cm-' band which becomes more apparent in the matrix spectrum is assigned as a fundamental mode (v4J. It exhibits only slight deuterium sensitivity in agreement with previous reports.' The symmetric bending frequency for the -CH3moiety of the ethyl group is usually observed as a medium-intensity band within a rather narrow frequency range of 1375-1385 cm-'. In contrast, the bands associated with the CH2group which are usually designated as pw and pt are normally weak and may occur within a rather wide frequency range. For example, in the case of ethylbenzene,126,(CH3) is observed as a mediumintensity band at 1377 cm-' while p,(CH2) is observed as a weak band at 1245 cm-'. We assign the bands observed in the spectrum of Ni(0EP) at 1378 and 1323 cm-l to these alkyl group vibrations. As expected, no shifts are observed for these bands upon meso deuteration, 15N substitution, or variation of the metal. The region between 1300 and 900 cm-' contains a large number of strong bands. It is in this region that some modes containing large contributions from in-plane wagging of the mekine hydrogens are expected to appear. The relatively large frequency shifts which are expected to occur as well as the redistribution of internal coordinates upon meso deuteration seriously complicates selection of

bands corresponding to fundamental modes. In addition, several strong ethyl group vibrations are also expected to occur in this frequency regional2 In order to facilitate proper selection of core modes, it is useful to consider the sensitivity toward 15N substitution of bands in the deuterated compound. Thus, the doubly labeled compound Ni(15N-OEP-d4)has been prepared and its frequencies have been compared with those of the deuterated complex. The strong band observed at 1275 cm-' is assigned as a fundamental core mode (v4J and correlated with the medium-intensity band observed at 1267 cm-' in the spectrum of the deuterated analogue. Although this band diminishes in intensity in the room temperature spectrum upon deuteration, no such drastic intensity differences are noted in the matrix spectrum. A similar deuterium shift has been observed for this band in the spectra of the corresponding cobalt ~omp1exes.l~ The 1231-cm-' band (vu), which exhibits a shift of 3 cm-' upon 15Nsubstitution, disappears upon meso deuteration and is replaced by a band at 1185 cm-l. The 1231-, 1153-, and 1133-cm-' bands in the spectrum of Ni(0EP) exhibit substantial 15Nshifts. In contrast, only two bands in this region in the spectrum of Ni(0EP-d,) exhibit significant shifts upon 15Nsubstitution; these occur at 1185 and 1151 cm-'.

(12) Bellamy, L. J. 'The Infrared Spectra of Complex Molecules", Methuen: London, 1954.

61, 77.

(13) Urban, M. W.; Nakamoto,K.;Kincaid, J. Inorg. Chim. Acta 1982,

The Journal of phvsical Chemistry, Voi. 87, No. 16, 1983 3000

IR Spectra of M(OEP)

ARGON MATRIX

c' lli! J

1700

I

I

I500

I

I

1300

I

1

1100

1

KBr PELLET

I

900

I

1

700

I

1

1

I

500

CM" Flgure 2. Infrared spectra of Ni(0EP-d,): (A) Ar matrix, (B) thin film, (C) KBr pellet. Spectral bandpass = 2.0 cm-'

The weak band at 1133 cm-' (vu) observed in the spectrum of Ni(OEP), which disappears upon deuteration, was not reported previously.' Furthermore, it exhibits a quite large (12 cm-') shift upon 15Nsubstitution. Although it has very weak intensity in several of the metal complexes of OEP, we assign it as an E, fundamental core mode containing a substantial contribution from both u(C,C,H) and u(C,-N). This assignment is supported by the data for the copper complexes of porphine and porphine-d4 in which case a band observed at 1150 cm-' in the spectrum of copper porphine disappears upon meso deuteration and is replaced by a band at 938 cm-' in the spectrum of the deuterated comp~und.'~ The 1119-cm-' band is correlated with a band observed at 1121 cm-l in the spectrum of the d4analogue. The slight sensitivity to 15N substitution as well as the observed splitting of this band upon oxygenation of the Mn(0EP) complex15implicate this band as a core mode. However, little information is available concerning the extent to which modes associated with the peripheral substituents may couple with modes associated with the porphine core. This band may arise from such coupling. Further, detailed (14) Cladkow, L. L.; Gradyushko, A. T.;Shulga, A. M.Solovyov, K. N.; Starukhin, A. S. J . Mol. Struct. 1978, 47, 463. (15) Watanabe,T.;Ama, T.; Nakamoto, K., submitted for publication.

studies employing specifically deuterated peripheral substituents may help elucidate this coupling. The 10611069-cm-' doublet, which exhibits no shift upon either deuteration or 15Nsubstitution, is assigned to p,(CHJ of the peripheral ethyl groups. Similarly, the bands observed at 1021 and 959 cm-' in the spectrum of Ni(0EP) and at 1024 and 964 cm-' in the spectrum of the deuterated compound may be reasonably assigned to p,(CH3) and v(C-C) of the ethyl group. The 995-cm-' band which shifts upon 15Nsubstitution by 5 cm-' and which exhibits no shift upon deuteration is assigned as a core fundamental mode. Similarly, the band observed at 927 cm-' (A(15N)= 3 cm-'), which occurs at 919 cm-' in the deuterated analogue and also exhibits a 3-cm-I shift upon 15N substitution, is assigned as a fundamental core mode. The assignments proposed in Tables I and I1 differ substantially from those of previous works. The bands assigned to ethyl group vibrations exhibit insignificant frequency shifts upon lSNor deuterium substitution and are also not metal sensitive. In addition, every band observed in each of the isotopically labeled species has been accounted for. Thus, in no case has a strong band been selected as a fundamental in the spectrum of only one isotopic species if there are observed similar bands in the spectrum of the other isotopic species. For example, the 1021-cm-I band observed in all isotopically labeled ana-

The Journal of Physical Chemistry, Vol. 87, No. 16, 1983

3100

,

h

I

S

1600

8

4

lion

c

im

c

1

ionn

1

I

ROO

I

'

I

I

L

hOO c H - ~ 400

Flgure 3. Infrared spectra of M(OEP), Ar matrix. Spectral bandpass = 2.0 cm-'.

logues is assigned to an ethyl group mode and is not selected as a fundamental core mode. Similarly, the 995-cm-' band, which exhibits a -5-cm-' I5N shift, is selected as a core mode for all of the isotopically labeled compounds as well. This is in contrast to previous work which assigned the 995-cm-' band as a fundamental mode in the natural abundance and 15N analogue but correlated it with a 943-cm-I band in the spectrum of the deuterated analogue. In view of the similar sensitivity to 15Nsubstitution and similar intensities, we prefer the present assignment. In fact, it is this 943-cm-' band (observed at 948 cm-' in the matrix spectrum) which exhibits a 15Nshift of magnitude comparable to those bands observed at 1153 and 1133 cm-' in the spectrum of the natural abundance complex. One of these latter two bands disappears upon deuteration. In agreement with previous work, we assign the 927-cm-' band as a core mode frequency but, in contrast to the previous correlation of this band to a band reported at 843 cm-' (which we do not observe), we correlate this band to one observed at 919 cm-' in the spectrum of the deuterated analogue. This is also supported by similar 15N shifts in the two analogues. Although in the previous work the strong 1231-cm-' ) not selected as a fundamental core mode, band ( v * ~was we prefer this assignment based on its disappearance upon deuteration, a 3-cm-' I5N shift, and its metal sensitivity. Furthermore, this band is observed to split in the spectrum of Mn(0EP) upon ~xygenation.'~ There are a number of additional core modes associated with in-plane deformations of the macrocycle which are

Kincaid et al.

expected to occur below 800 cm-'. Definitive assignments for these are not possible at this time since most are expected to be very weak in intensity and the appearance of strong out-of-plane AzUmodes also occurs in this frequency region. Thus, definitive assignment of in-plane deformation modes must await calculation of out-of-plane vibrations for this system. A number of bands may be described as metal sensitive. ) 1501 cm-' (v3J Thus, the bands observed at 1575 ( v ~ and in the spectrum of the nickel complex, which shift by 9 and 7 cm-l upon deuteration, exhibit shifts as large as 25 cm-' (MnOEP) upon variation of the metal (Table 11). Similarly, the band observed at 1231 cm-' ( v ~in~ the ) spectrum of Ni(OEP), which shifts to a much lower frequency upon deuteration, implying substantial contribution from 6(C,C,H), exhibits significant metal sensitivity. One metal sensitive band, which is observed at 996 cm-' ( v ~ in ~ )the nickel complex, is evidently associated with pyrrole deformation (A(15N)= 5 cm-') but contains little contribution from methine bridge internal coordinates (no shift upon deuteration). On the other hand, those modes which contain largest contributions from v(C,-N), on the basis of large 15Nshifts, are least sensitive to variation of the central metal. These include two bands observed at 1153 (A(I5N) = 8 cm-l) and 1133 cm-' (A(I5N) = 12 cm-') in the spectrum of the nickel complex. Several factors may be of importance in determining metal sensitivity of observed bands. Previous theoretical work indicates that stabilization of the aZua-bonding orbital by metal ion substitution, as reflected in blue shifts in the electronic absorption spectra, results in an increase in bond energy of the ring bonds and consequent increases in force constants and vibrational f r e q u e n ~ i e s . ~ ~ ' ~ In addition, discontinuities in the observed vibrational frequencies may occur as a result of occupation of the cr antibonding d,zg orbital. Thus, large frequency shifts may be observed for those complexes which contain one or two electrons in this orbital."J8 The structural consequences of this occupation have been dealt with previo~sly.'~ Basically, partial or full population of the dXz+orbital leads to either core expansion or extrusion of the metal ion from the plane of the porphinato ligand. The most significant effect of core expansion, which is usually measured by the center to nitrogen distance (C,-N), is to induce alterations in the strength of the methine bridge bonds. In fact, potential energy calculations by Warshel provide convincing evidence that core expansion is accompanied by methine bridge stretching and deformation.20 As has been discussed by Spiro and co-workers, the methine bridge force constants may be lowered by reduction in a overlap if the pyrrole rings are tilted out of the mean plane.21 This tilting may arise in two different ways. In domed metalloporphyrins, which are normally five coordinate, the metal lies out of the mean plane and the pyrrole units swivel about the methine bridge bonds to maintain metal-nitrogen orbital overlap; the result is that the plane of the nitrogen atoms lies above the mean plane of the porphinato ligand. A second mechanism exists for pyrrole tilting which is more relevant to the present investigation. In those metalloporphyrins whose optimum M-N bond length is (16) Gouternan, M.J. Chem. Phys. 1959,30, 1139. (17) Ogoshi, H.; Watanabe, E.; Yoshida, Z.; Kincaid, J.; Nakamoto, K. J . Am. Chem. SOC.1973,95, 2845. (18) Kincaid, J.; Nakamoto, K. J. Inorg. Nucl. Chem. 1975, 37, 85. (19) Scheidt, W. R.In 'The Porphyrins"; Dolphin, D. Ed.; 1978, Vol. 3, Academic Press, New York, pp 463-511. (20) Warshel, A. Annu. Reu. Biophys. Bioeng. 1977, 6, 273. (21) Spiro, T. G.;Stong, J. D.; Stein, P. J. Am. Chem. SOC.1979,101, 2648.

J. Phys. Chem. 1983, 87,3101-3105

shorter than the center to nitrogen distance ((2,-N) for an unconstrained planar porphyrin, estimated by Hoard to be 2.01 A,22the porphinato ligand may distort to give the so-called ruffled structure of DW symmetry.lg In this case the pyrrole units swivel about the metal-nitrogen bonds giving a square-planar array of nitrogens which lie in the average plane of the porphinato ligand while permitting a shortening of the metal-ligand distances d(M-N). The most familiar example of the effect of ruffling is provided by Ni(0EP) which can be crystallized in two forms; one has a planar porphyrin core and d(M-N) = 1.958 A while the other has a severely ruffled core with d(M-N) = 1.929 A. Several bands in the RR spectrum of the planar form are shifted to lower frequencies in the spectrum of the ruffled form.23 In the present study, the nickel complex, which has been shown to exist as the D& form in an argon matrix or as a thin film at -15 K,23exhibits the highest frequencies for all of the metal-sensitive bands. Only slightly lower frequencies are observed for Co(OEP), implying that no significant degree of distortion has occurred. The frequency shifts for the copper (d9)and zinc (dlO) complexes are easily accounted for by core expansion which is associated with occupation of the d,z.+ orbital, the frequency for Zn(0EP) being even lower than that of Cu(0EP). Ruffling evidently plays only a minor role in frequency lowering for these species since, at least in the case of tetraphenylporphinatozinc(II),Zn(TPP), the porphinato ligand has been shown to be ~ 1 a n a r . l ~ The very low frequencies observed for the metal-sensitive bands of the manganese complex may be explained by partial occupation of the d,~-~z for this high-spin, d5 configuration. Furthermore, relative to copper (d9),this effect is magnified by the expansion of d orbitals resulting from decreased effective nuclear charge. Again, these lowered frequencies must be attributed to core expansion (22) Hoard, J. L. In "Porphyrins and Metalloporphyrins"; Smith, K. M., Ed.; Elsevier: New York, 1975; pp 317-76. (23) Scheuermann, W.; Nakamoto, K. J. Mol. S t r u t . 1978, 48,285.

3101

since, in the case of the Mn(TPP), the porphyrin core assumes a planar onf figuration.'^ Consideration of the data for the iron complex provides further evidence of the importance of ruffling in determining frequencies of metal-sensitive bands. This complex takes the intermediate spin state and consequently has an empty d,z+ orbital.lg Nevertheless, large frequency shifts are seen for the two high-frequency bands (1575 and 1501 cm-l in the nickel complex). The short Fe-N bonds expected for this four-coordinate species give rise to severe ruffling of the porphinato ligand,lgthus accounting for the substantial shifts which are observed. The effect of ruffling on lower frequency modes (below 1100 cm-') is relatively small. Although any of the mechanisms discussed above may be invoked to explain the observed metal sensitivity of various bands, core expansion associated with d,~-~z occupancy appears to be most important.21 In any case, both the core expansion and pyrrole tilting mechanisms predict that largest frequency shifts will be observed for those modes which contain the greatest contributions from methine bridge stretching and deformation. It is therefore satisfying that those bands which exhibit greatest metal sensitivity may be reasonably assigned to v(Ca-C,) on the basis of relative frequencies and deuterium shifts. Acknowledgment. The authors thank Professor Thomas Spiro of Princeton University for providing the sample of (NaN02-15N).Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the Research Corporation, and to the Graduate School of the University of Kentucky for partial support of this work. The work performed at Marquette University was supported by the National Science Foundation Grants PCM8114676 and CHE8205522. Registry No. Ni(OEP),24803-99-4;Ni(16N-OEP),86119-71-3; Ni(OEP-d4),55835-59-1;Ni(16N-OEP-d4), 86119-72-4;Co(OEP), 17632-19-8;Cu(OEP),14409-63-3;Fe(OEP),61085-06-1;Mn(OEP), 51321-25-6;Zn(OEP),17632-18-7.

Time-Resolved Resonance Raman Observatlon of Tetrafluoro-p -benzosemiquinone Anion Radlcal' G. N. R. Tripathi and Robert H. Schuler' Radiation Lebofatory and Department of Chemistry, University of Nohe Dame, Notre Dame, Indiana 46556 (Received: November 29, 1982; In Final Form: January 25, 1983)

Time-resolvedresonance Raman spectroscopyhas been used to examine tetrafluoro-p-benzosemiquinone radical anion produced in the pulse radiolytic oxidation of tetrafluorohydroquinone in aqueous solution. This radical is much more reactive than p-benzosemiquinone and is observed to decay on the millisecond time scale in both Raman and pulse radiolytic experiments. For the Raman experiments excitation was on the red edge of the moderately strong absorption band of this radical at 430 nm. Two resonance-enhancedRaman bands are exhibited at 1556 and 1677 cm-' and are assigned to the in-phase CO and symmetrical CC stretch vibrations. These frequenciesare considerably higher than the corresponding values of 1435 and 1620 cm-' observed in this radical's protonated counterpart. The relatively large increase in the CO stretch frequency, in particular, indicates that fluorination induces a substantial increase in the quinoid character of this radical.

The p-benzosemiquinone radical anion (PBSQ;

C6H402--) and related radicals have been studied exten-

sively by optical absorption and electron spin resonance spectroscopy employing chemical, electrochemical, pho-

0022-3654/83/2087-3101$01.5O/Q0 1983 American Chemical Society