1472
J. Phys. Chem. 1984,88, 1472-1479
of the products being treated more adequately than those of the reactants. On the other hand, taking electron correlation into account at the 6-3 1G level using Mraller-Plesset perturbation theory to second order (MP2) leads to an energy difference more positive by 3.3 kcal mol-' compared to the corresponding HF/ 6-31G calculation (see Table VIIA, i). The energies calculated for the isodesmic bond separation reaction of o-benzyne, both aryne-like and cumulene-like structures, show rather different behavior (see Table VIIA, ii and iii). First, the addition of polarization functions on the carbon gives a slightly less negative instead of a more negative reaction energy. Secondly, although taking electron correlation into account gives a more positive reaction energy for both benzene and o-benzyne, the effect is much more pronounced with o-benzyne; Le., the difference between the MP2/6-31G and HF/6-31G energies is 18.5 or 25.4 kcal mol-' for the aryne-like and cumulene-like structures compared to 3.3 kcal mol-' in the case of benzene. Thirdly, the difference between the HF reaction energies and the experimental value is consistently negative, -7.6 to -9.5 kcal mol-', whereas with benzene the difference lies either side of zero, +3.3 to -3.0 kcal mol-'. Fewer calculations of reaction energies have been carried out for homodesmotic group separation, but those listed in Table VIIB, ii and iii, show features similar to those for isodesmic bond separation.
Undoubtedly many factors determine the extent to which calculated reaction energies agree with experiment. One factor quite apart from those involved in the level of the calculations which may be contributing to the contrast noted above between the results for benzene and o-benzyne is the value adopted for AHfo(o-benzyne) at 298 K. Two groups of investigators have reported +118 f 5 kcal m01-1.1g920A value closer to the upper limit would bring the o-benzyne calculations more into line. The recent MNDO calculations of Dewar et al. tend to bear this out: 138.2 kcal mol-' at the R H F level, and 125.7 and 125.7 kcal mol-' using standard 3 X 3 CI and with the inclusion of all significant configurations, respectively, although 119.9 kcal mol-' was obtained a t the U H F level.5
Acknowledgment. We gratefully acknowledge the generous grant of computer time provided by the Computer Center of the Philadelphia College of Textiles and Science. Registry No. o-Benzyne, 462-80-6; benzene, 71-43-2; 1,3-butadiene, 106-99-0;ethane, 74-84-0; ethylene, 74-85-1;acetylene, 74-86-2; methane, 74-82-8. Supplementary Material Available: Tables IS-IIIS containing comparisons of charge transfer, and u- and r-electron overlap populations, using Mulliken population a n a l y ~ i s '(6 ~ pages). Ordering information is given on any current masthead page.
Resonance Raman Spectra of Oxygen- and Nitrogen-Bridged Iron Octaethylporphyrin Dimers Joseph A. Hofmann, Jr., and David F. Bocian*l Department of Chemistry, University of California, Riverside, California 92521 (Received: July 6,1983)
Resonance Raman (RR) spectra are reported for the oxygen- and nitrogen-bridged iron porphyrin dimers (OEPFe)20 and (OEPFe),N (OEP = octaethylporphyrin) with excitation at a large number of wavelengths throughout the Soret and visible regions of the absorption spectrum. RR bands due to in-plane porphyrin vibrations are assigned by analogy to those observed for OEPFeCl and NiOEP. As is the case for the (TPPFe)2X complexes (TPP = tetraphenylporphyrin; X = 0,N), no evidence is found for dimer vibrational splittings in the porphyrin modes of the (OEPFe)2X species (X = 0,N). RR bands sensitive to the porphyrin core size, vj and ul0, are observed at 1495 and 1627 cm-' for (OEPFe)20and at 1513 and 1647 cm-' for (OEPFe)2N, characteristic of high- and low-spin Fe ions in the two complexes, respectively. The oxidation state marker, v4, is observed at 1377 cm-' for both complexes, characteristic of high-valent Fe ions. Several out-of-plane modes are identified in the spectra of both complexes including the symmetric Fe-X-Fe stretching modes at 391 cm-' (X = 0) and 439 cm-' (X = N). The visible region excitation profiles of the two (OEPFe)2X complexes as well as those of OEPFeCl exhibit evidence of very strong vibronic coupling. A maximum, Q,, is observed in the profiles of all the RR bands of all three complexes 800-900 cm-' to the blue of the Q(0,O) maximum. The Q, peak dominates the Q(0,l) region of the profiles of OEPFeCl with the individual Q(0,l) peaks barely in evidence. The Q,/Q(O,l) intensity ratios are decreased for the RR bands of (OEPFe)2N relative to those of OEPFeCl and further decreased for the bands of (OEPFe)20. The Q, maximum is attributed to the symmetric linear combination of many strongly coupled modes in the electronic excited state, in accord with the theoretical predictions of Shelnutt and OShea. The relative intensities of the Q,and Q(0,l) maxima indicate that the relative vibronic coupling strength in the complexes is OEPFeCl > (OEPFe)2N > (OEPFe)20.
Introduction The structural and electronic properties of a number of metalloporphyrins and heme proteins have been elucidated with resonance Raman (RR) spectro~copy.~-~ These systems produce extremely intense and detailed R R spectra which have been interpreted successfully by using models for vibronically induced (1) Alfred P. Sloan Fellow, 1982-1984. (2) Spiro, T. G. In "Iron Porphyrins"; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading. MA, 1982; Part 11, pp 89-152. (3) Kitagawa, T.; Ozaki, Y.; Kyogoku, Y. A d a Biophys. 1978,1I, 153. (4) Warshel, A. Annu. Rev. Biophys. Bioeng. 1977, 6, 273. ( 5 ) Felton, R. H.; Yu, N.-T. In "The Porphyrins"; Dolphin, D., Ed.; Academic Press: New York, 1978; Vol. 11, Part A, pp 347-388.
0022-3654/84/2088- 1472$01.50/0
scattering from the porphyrin Q(a and p) and B(Soret) electronic Most R R studies of metalloporphyrins have been directed at monomeric species. The only oligomeric systems which have been investigated with R R techniques are the axially bridged dimeric complexes (TPPFe)2012,13 and (TPPFe)2N (TPP = tet(6) Johnson, B. B.; Peticolas, W. Annu. Reu. Phys. Chem. 1976,27,465. (7) Spiro, T. G.; Stein, P. Annu. Rev. Phys. Chem. 1977, 28, 501. (8) Shelnutt, J. A,; OShea, D. C.; Yu, N.-T.; Cheung, L. D.; Felton, R. H . J. Chem. Phys. 1976,64, 1156. (9) Shelnutt, J. A.; Cheung, L. D.; Chang, R. C. C.; Yu,N.-T.; Felton, R. H. J . Chem. Phys. 1977,66, 3387. (10) Shelnutt, J. A.; O'Shea, D. C. J . Chem. Phys. 1978, 69, 5361. (11) Shelnutt, J. A. J. Chem. Phys. 1981, 74, 6644. (12) Adar, F.; Srivastava, T. S. Prog. Natl. Acad. Sci. U.S.A. 1975, 72, 4419.
0 1984 American Chemical Society
0- and N-Bridged Iron Octaethyloporphyrin Dimers
The Journal of Physical Chemistry, Vol. 88, No. 8, 1984 1473
OEPFeCl (OEPFe), 0
0.8
-
0.0
-
0.4
-
0.2
-
(OEPFe),N
1000
800
(cm-')
Figure 2. Infrared spectra of (OEPFe)20 and (OEPFe)*N in KBr pellets. The labeled bands are the asymmetric Fe-X-Fe stretching modes.
proximate natural systems than do the analogous TPP complexes in that the substituent groups are at the C, rather than C, ring positions. The C, phenyl groups of TPP perturb the porphyrin welectronic structure, which results in significantly different Q-B vibronic coupling and R R enhancemenk2 Also, the frequencies of analogous R R bands in OEP and TPP complexes differ by as much as 60 crn-l, rendering the R R frequency-structure correlations developed for C substituted porphyrin^^,^"^^ inapplicable for TPP complexes.27- These complications for the TPP derivatives could potentially obscure subtle perturbations which occur upon dimer formation. In this paper, we first discuss the assignments for the in-plane porphyrin skeletal modes of (OEPFe)20 and (OEPFe)*N in relation to those for OEPFeCl. Next, we discuss RR enhancements for out-of-plane modes. Finally, we examine the Q-state excitation profiles for the complexes and compare these profiles with those observed for other metalloporphyrin complexes.
f-
400
500 600 Wavelength (nrn)
700
Figure 1. Absorption spectra of OEPFeCI, (OEPFe)20, and (OEPFe)2N in CH2CI,.
rapher~ylporphyrin).'~These dimers attracted attention because of their potential utility as model systems for probing intradimer (exciton) coupling between closely spaced porphyrin units (4.615 and 4.2 A,16 respectively). In this regard, the R R studies revealed that exciton coupling is very weak or nonexistent for both dimers and that the two halfs can be treated as independent monom e r ~ . The ~ ~R, R~study ~ ~ of (TPPFe)2N14bdid, however, elucidate a number of the structural and electronic properties of the complex, which have been of considerable interest1623 due to the unusual nature of the Fe-N-Fe bridge. In particular, R R excitation profiles and enhancement patterns were used to resolve ambiguities in the assignments of the Q bands and to identify charge-transfer states associated with the bridging ligand system. In order to further investigate the electronic and structural properties of dimeric metalloporphyrins, we have examined in detail the R R spectra of (OEPFe)20 and (OEPFe)2N (OEP = octaethylporphyrin). The (OEPFe),X dimers more closely ap(13) Burke, J. M.; Kincaid, J. R.; Spiro, T. G. J . Am. Chem. SOC.1978, 100, 6077. (14) (a) Schick, G. A,; Bocian, D. F. J. Am. Chem. SOC.1980 102,7982. (b) Ibid. 1983, 105, 1830. (15) Hoffman, A. B.; Collins, D. M.; Day, V. W.; Fleischer, E. B.; Srivastava, T. S.; Hoard, J. L. J. Am. Chem. SOC.1972, 94, 3620. (16) Scheidt, W. R.;Surnmerville,D. A.; Cohen, I. A. J. Am. Chem. SOC. 1976.98, 6623. (17) Summerville, D. A.; Cohen, I. A. J. Am. Chem. SOC.1976,98, 1747. (18) Kadish, K. M.; Bottomley, L. A.; Brace, J. G.; Winograd, N. J. J. Am. Chem. SOC.1980, 102, 4341. (19) Kadish, K. M.; Rhodes, R.K.; Bottomley, L. A.; Goff, H. M. Inorg. Chem. 1981, 20, 3195. (20) Bottomley, L. A.; Garrett, B. B. Inorg. Chem. 1982, 21, 1260. (21) Schick, G. A.; Findsen, E. W.; Bocian, D. F. Inorg. Chem. 1982,21, 2885. (22) Tatsumi, K.; Hoffmann, R. J. Am. Chem. SOC.1981, 103, 3328. (23) English, D. R.; Hendrickson, D. N.; Suslick, K. S. Inorg. Chem. 1983, 22, 361.
Experimental Section Octaethylporphyrin was obtained from Midcentury Chemicals (Posen, IL). The OEPFeCl and (OEPFe)20 complexes were prepared by the method of Dolphin et al.30 The (OEPFe)2N complex was prepared in a manner similar to that described for (TPPFe)2N by Summerville and Cohen.I7 Substitution of 54Fe (from 54Fe203;97.08% in 54Fe,Oak Ridge National Laboratories) was accomplished in a manner similar to that outlined in ref 13. The formation of the (OEPFe)20 and (OEPFe)2N complexes was monitored by using UV-vis spectroscopy (Figure 1) and confirmed by the appearance of the asymmetric Fe-X-Fe stretching modes in the IR spectra (Figure 2). All solvents were spectral grade and used without further purification. The R R spectra were recorded with the optics in a 90° scattering configuration on a computer-controlled Spex Industries Ramalog 6 spectrometer equipped with a thermoelectrically cooled Hamamatsu R955 photomultiplier tube and a photon counting detection system. Excitation wavelengths were provided by a tunable dye laser (Coherent Radiation Model 590) utilizing the tuning ranges of Rhodamine 6G, Rhodamine 560, Coumarin 5 15, (24) Spiro, T.G.; Strekas, T. C. J. Am. Chem. SOC.1974, 96, 338. (25) Spiro, T. G.; Burke, J. M. J . Am. Chem. SOC.1976, 98, 5482. (26) Spaulding, L. D.; Chang, C. C.; Yu,N.-T.; Felton, R. H. J. Am. Chem. SOC.1975,97, 2517. (27) Burke, J. M.; Kincaid, J. R.; Peters, S.; Gagne, R. R.; Collman, J. P.; Spiro, T. G. J. Am. Chem. SOC.1978, 100, 6083. (28) Chottard, G.; Battioni, P.;Battioni, J.-P.; Lange, M.; Mansuy, D. Inorg. Chem. 1981, 20, 1718. (29) Strong, J. D.; Kabaska, R. J.; Shupack, S . I.; Spiro, T. G. J. Raman Spectrosc. 1980, 9, 312. (30) (a) Dolphin, D.; Sams, J. R.;Tsin,T.B.; Wong, K. L. J. Am. Chem. SOC.1976, 98, 6970. (b) Ibid. 1978, 100, 1711.
1474 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
Hofmann and Bocian
TABLE 1: Resonance Raman Bands (cm-') of OEPFeCI, (OEPFe),O, and (OEPFe),N Assignable t o In-Plane Skeletal Modes.
no.a
sym
OEPFeCl
(OEPFe),O
(OEPFe), N
hd+8'
1582 (0.14)' 1494 (0.11) 1376 (0.12) 1256 (PI 1136 (0.17)
1583 (0.12) 1495 (0.13) 1377 (0.13) 1258 (P) 1136 (0.21)
1590 (0.28) 1513 (0.33) 1378 (0.12) 1261 (PI 1138 (0.15)
'23 d > "
1024 (0.34)
1023 (0.42)
1023 (0.30)
963 (P) 733 (P) 66P 345 (0.11) 1627 (0.76) 1211 (0.8) 753 (0.73) 1559 (2.2) 1388 (2.7) 1315 (6.6) 1129 (2.5) 1403 (dp) 1159 (dp)
733 (P) 668e 335 (0.25) 1647 (0.75) 1212 (0.8) 753 (0.75) 1572 (1.3) 1390 (1.4) 1312 (1.1) 1132 (1.1) 1405 (dp) 1159 (dp)
'32
+ '35
'33
+
961 (PI 732 (P) 676e
'35
'7 V8
OI' I'
3
'16
A2g
' I 9 'ZQ '21
.;z B2,
'29 '30
1628 (0.75) 1210 (0.8) 752 (0.75) 1565 (3.2) 1390 (ip) 1311 (6.5) 1125 (ip) 1404 (dp) 1156 (dp)
assigntb
Skeletal mode numbering follows Kitagawa e t al.33 Skeletal mode assignments follow Abe e t al.34 The subscripts used in the assignment labels identify the a- and 0-pyrrole and the methine-bridging carbon atoms in the porphyrin macrocycle. Depolarization ratio, p . The values for the A,, modes are those observed with he, 4067 A (Figure 3). The values for the other modes are those observed with the hex's given in Figure 4. Abbreviations: p, polarized; dp, depolarized, ip, inversely polarized. The large p for this band suggests that the u z 3 (A,,) mode contributes t o the intensity. e Frequency observed in a NaCl/NazS04 pellet; p not determined.
'
and Stilbene 420 dyes and by the discrete lasing outputs of Ar ion (Spectra-Physics Model 164-05) and Kr ion (Coherent Radiation Model CR-2000K) lasers. R R spectra were obtained for both solid (in 1:l NaC1/Na2SO4 pellets) and solution (CS,, CH2Cl,, and CHC1,) samples. Sample concentrations in the pellets were 1.5 mg/250 mg of host. Concentrations in solution were 0.2 mg/mL for B-state excitation and 1.0 mg/mL for Q-state excitation. All solution samples were cooled and flowed to prevent photodecomposition. Sample integrity was monitored by using the R R spectrum and UV-vis spectroscopy. B-state excitation R R spectra were collected at 2-cm-' intervals at rates of 5-12 s/point with a spectral slit width of 3 cm-'. The incident laser power was less than 10 mW. Isotope shifts were measured from spectra recorded at comparable count rates with similar laser powers, but with count intervals of 0.2 cm-I and a spectral slit width of 0.5 cm-'. Q-state excitation R R spectra were collected at 2-cm-' intervals at rates of 4-20 s/point with a spectral slit width of 4 cm-'. The incident laser powers ranged from 15 to 50 mW. Excitation profiles were obtained for both solid (in 1:l NaC1/Na2SO4 pellets) and solution (CS,, CH2ClZ,and CHC13) samples. Solvent or N a 2 S 0 4 bands were used as intensity standards. The other spectral conditions were the same as those given above for Q-state excitation. The profiles were corrected for the differences in the v4 dependence of scattered light, sample reabsorption (solutions), and instrument and photomultiplier responses which result from the frequency difference between a given band and the internal standard. Results and Discussion The R R spectra of OEPFeCl, (OEPFe)20, and (OEPFe)2N in CS2observed with excitation in the B- and Q-band regions are compared in Figures 3 and 4, respectively. The R R spectra observed in the other solvents (CH2C12and CHC13) and for solid samples are identical with those shown except for small (2-3 cm-') systematic shifts in the frequencies of certain modes. As expected, the R R spectra are dominated by high-frequency bands assignable to in-plane porphyrin skeletal vibrations; however, a number of low-frequency bands are also observed, some of which are attributable to out-of-plane modes. The specific assignments for the in- and out-ofaplane modes are given in Tables I and 11, respectively. Depolarization ratios, p, are also given for the R R bands. The values reported for strong1 polarized bands ( p 0.75), and inversely polarized ( p = m) bands are those measured at the visible wavelengths of Figure 4 where these modes dominate the RR spectra, indicative of the vibronic scattering mechanisms operative upon excitation of the less intense Q band^.*-^^^^' A number of depolarized modes and a few anomalously polarized modes are also resonance enhanced with B-state excitation. The depolarized modes observed with A,, 4067 8,exhibit p -0.75. The p values for the anomalously polarized modes are in general less than those observed with
-
(31) Clark, R. J. H.; Stewart, B. Struct. Bonding (Berlin) 1979, 36, 1.
-
(32) Kitagawa, T.; Ogoshi, H.; Watanabe, E.; Yoshida, Z. J . Phys. Chem. 1975, 79, 2629. (33) Kitagawa, T.; Abe, M.; Ogoshi, H.J . Chem. Phys. 1978.69, 4516. (34) Abe, M.;Kitagawa, T.; Kyogoku, Y. J. Chem. Phys. 1978,69,4526. (35) Bernstein, H.J. Philos. Trans. R. SOC.London, Ser. A 1979,293,287.
1476 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
Hofmann and Bocian
of (OEPFe)2N with &, 4067 8, exhibits a strong band at 439 cm-' (Figure 3) which we assign as u, (Fe-N-Fe) on the basis of a 6-cm-' upshift in frequency which occurs upon substitution of 54Fe into the complex (Figure 5, right). This isotope shift is the same as that observed for (TPPFe)2N and represents 75% of the theoretical value expected on the basis of the change in reduced mass of the mode (assuming the bridging system of (OEPFe)2N is linear, as it is known to be for the TPP derivativeL6).Examination of the B- and Q-state R R spectra of (OEPFe)20 and its 54Fe analogue failed to reveal any band assignable as us( Fe-0-Fe). This mode was eventually identified in the R R spectrum obtained with A,, 4545 %, as a moderately intense band at 391 cm-' which upshifts 5 cm-' with S4Fesubstitution (Figure 5, left). It should be noted that this band is not the same as the 398-cm-I mode observed with A,, 4067 8, (Figure 3). The 5-cm-' isotope shift for v,(Fe-0-Fe) in (OEPFe)20 is the same as observed for 450 400 450 400 (TPPFe)20 and represents 75% of the expected value (assuming FREQUENCY (cm-') FREQUENCY (cm-9 the bridging ligand system of (OEPFe)20 is slightly bent, as it Figure 5. Low-frequency RR spectra of solid (OEPFe)20 and (OEP175'). is known to be for the TPP derivative;lS LFe-0-Fe Fe)*N in 1:l NaC1/Na2S04 pellets showing the isotope shifts of the The frequencies of the symmetric and asymmetric Fe-X-Fe symmetric Fe-X-Fe stretching modes. Note that the frequency observed stretches can be used to calculate the force constants for the for the band of solid (OEP56Fe)2Nin the pellet is slightly different from the solution value (Figure 3, Table 11). See Experimental Section for vibrations. The frequencies correspond to an Fe-X stretching force spectral conditions. constant of 4.8 mdyn/8, (X = N ) and 4.1 mdyn/A (X = 0) with stretch-stretch interaction constants of 1.6 and 0.9 mdyn/8,, 8, (Figure 3). These bands are relatively strong in the spectra respectively. The values are larger than their conterparts in the of (OEPFe),O and (OEPFe)2N but weak for OEPFeCI. One (TPPFe)2X complexes, This may reflect a slight shortening of strong band is observed in the spectrum of the latter complex at the Fe-X bonds upon substitution of the less bulky C,-ethyl groups 360 cm-l which has previously been assigned to the F e C l stretch?6 for the bulkier C,-phenyl groups. In the case of the p-oxo comThe low-frequency bands of the two dimeric complexes (except plexes, the C,-ethyl substituents may also allow the Fe-0-Fe u s ) exhibit p >0.125, indicating a significant contribution of azz linkage to bend slightly more than do the C,-phenyl groups. A to their scattering tensors. We have assigned these bands as slightly more bent structure for the (OEPFe)20 complex could out-of-plane modes involving the porphyrin skeleton and FeX-Fe also explain the anomaly in the frequencies of the symmetric and bridges on the basis of the considerations given below (Table 11). asymmetric stretches for the p-oxo complexes of OEP and TPP ( 1 ) Skeletal Deformations. R R bands are observed for the relative to those of the two p-nitrido complexes. The symmetric FeOEP complexes at 466, 400, and 359 cm-'. Bands at these stretching frequencies of both the p-oxo and p-nitrido complexes frequencies have been observed in a number of other metalloof OEP (391 and 439 cm-') are higher than those of their TPP porphyrin systems by Choi and Spiroe3' These workers assigned counterparts (363 and 424 cm-'). The asymmetric stretching the 466- and 400-cm-I bands as E,-type pyrrole folding modes frequency of (OEPFe)2N (940 c d ) is also higher than that of and the 359-cm-' band as an A2,-type deformation of the C,ethyl (TPPFe)2N (910 cm-'); however, the frequencies of this mode groups. We have assigned these bands in the FeOEP derivatives in the two p o x 0 complexes are nearly the same (875 and 872 by analogy. cm-l) . There are several possible mechanisms for R R enhancement Q-State Excitation Profiles. The Q-state excitation profiles of the out-of-plane skeletal modes of the FeOEP c o m p l e x e ~ . ~ ~ ~for ~ several R R bands of each polarization observed for OEPFeCl The A2, deformation of the C,-ethyl groups could gain activity and (OEPFe)20 in CS2 and for solid (OEPFe)2N are shown in as a result of the severe distortion of the porphyrin cores from Figures 6, 7, and 8, respectively. The profiles were constructed planarity due to the presence of a single axial ligand to the metal from 30-60 values of &. The errors in the intensity measurements ions. A C4"doming of the planar D4hporphyrin core renders Azu are 15% for OEPFeCl and (OEPFe)20 and f10% for (OEPmodes totally symmetric (Al), and R R enhancement is possible Fe)2N. These uncertainties result in an error in the peak positions with B-state excitation. The E, pyrrole folding modes cannot gain in the profiles of flOO cm-' for the former complexes and f150 R R activity by this mechanism. However, these modes can encm-' for the latter. Excitation profiles obtained for OEPFeCl and hance through vibronic mixing of an out-of-plane A2, charge(OEPFe)20 in the other solvents (CH2C12and CHC13) and for transfer state with the E, porphyrrin r** states (A2,XE, = E,; solid samples and for (OEPFe)2N in the three solvents were Ddhor AIXE = E, C40).Porphyrin Fe charge-transfer states of essentially identical with those shown. The profile of solid (OEAzu symmetry are possible in all three FeOEP complexes as are PFe)2N is displayed because photodecomposition problems in charge-transfer transitions involving the metal and axial ligands. solution14bprecluded obtaining as detailed a profile as was desired The substantially larger R R activity for the out-of-plane skeletal on a single sample. modes of (OEPFe)20 and (OEPFe)2N suggests that chargeExcitation profiles have been reported for a number of other transfer transitions involving the Fe-X-Fe system could be remetalloporphyrin systems, and the factors contributing to their sponsible for a significant fraction of the enhancement. As was general features are well understood.8-1L~13~14b~38-46 The profiles previously noted, a transition localized in the Fe-N-Fe bridge occurs around 400 nm. The analogous transition for the Fe-0-Fe linkage is thought to be near 450 nm.13,14bA detailed discussion (38) Verma, A. L.; Mendelsohn, R.; Bernstein, H. J. J. Chem. Phys. 1974, of these charge-transfer transitions can be found in ref 14b. 61, 383. (39) Collins, D. W.; Champion, P. M.; Fitchen, D. B. Chem. Phys. Lett. ( 2 ) Symmetric Fe-X-Fe Stretch. The symmetric Fe-X-Fe 1976, 40, 416. stretches of (TPPFe),X are observed as a strong R R band at 424 (40) Collins, D. W.; Fitchen, D. B.; Lewis, A. J. Chem. Phys. 1973, 59, cm-I with A, -400 nm (X = N)14band as a moderately intense 5714. band at 363 cm-' with bx450-550 nm (X = O)." These modes (41) Sunder, R.; Mendelsohn, R.; Bernstein, H. J. J . Chem. Phys. 1975, 63,'573. are thought to gain RR activity via the out-of-plane charge-transfer (421 Mendelsohn,R.; Sunder, S.;Verma, A. L.; Bernstein, H.J. J . Chem. transitions localized in the Fe-X-Fe bridges. The R R spectrum Phys. 1975, 62, 37. (43) Verma, A. L.;Bernstein, H. J. J . Chem. Phys. 1974, 61, 2560. (44) Spiro, T. G.; Strekas, T.C. Proc. Natl. Acad. Sci. U.S.A. 1972,69, (36) Kitagawa, T.;Abe, M.; Kyogoku, Y.; Ogoshi, H.; Watanabe, E.;
-
Yoshida, Z. J . Phys. Chem. 1976,80, 1 1 8 1 . (37) Choi, S.; Spiro, T. G. J. Am. Chem. SOC.1983, 105, 3683.
*I**
LOLL.
(45) Champion, P. M.; Albrecht, A. C. J. Chem. Phys. 1980, 72, 6498.
The Journal of Physical Chemistry, Vol. 88, No. 8. 1984 1477
0- and N-Bridged Iron Octaethyloporphyrin Dimers
n
>
a
i
. 1 : I
: :
4 1210 (dp)
1156 (dp)
2.
"
.4
1.
0.
absorption profile I
Figure 6. Q-State excitation profiles for representative RR bands of OEPFeCl in CS2. The intensities are plotted on a linear scale of mag-
nitude lo4 relative to the 796-cm-' band of CS2, and the frequencies are given in cm-I. The profiles are displaced vertically and plotted by using alternate filled ( 0 )and open (0)circles for clarity of presentation. The visible region of the absorption profiles is also shown. Abbreviations used: p, polarized; dp, depolarized; ap, anomalously polarized; ip, inversely polarized. obtained thus far for Cg-substituted porphyrins are generally characteristic of weak vibronic c o ~ p l i n g . ~ In - ~this ~ - ~limit, the profile of each RR band exhibits two maxima, one at the position of the electronic origin and the other at the position of the appropriate vibronic satellite, Q(0,l). The Q(0,l) maxima are ~~
(46)
Champion, P. M.; Albrecht, A. C . J. Chem. Phys.
~~
1979, 71, 1110.
,
I
l
19.0
l
>\
,
,
1
18.0
4
1
I
l
l
!
17.0
,
I 16.0
FREOUENCY (crn-'X 10.3)
Figure 7. Q-State excitation profiles for representative RR bands of
(OEPFe)20 in CS2. See caption of Figure 6 for details. normally more intense due to nonadiabatic Q-B coupling;Bhowever, the intensity skewing can be attenuated by Jahn-Teller effect^.^ Metallotetraphenylporphyrinsgenerally exhibit the effects of strong vibronic c o ~ p l i n g . ' ~ JIn ~ Jthis ~ ~ case, the profiles are more complicated, displaying supernumary peaks due to helping mode effects and secondary maxima at the positions of the Q(0,2) vibronic satellites. There are also anomalies in the intensities of the various peaks in the profile. The Q-state excitation profiles for all three FeOEP complexes (Figure 6-8) show evidence of very strong vibronic coupling. This coupling as far as we know is stronger than any which has been
1478 The Journal of Physical Chemistry, Vol. 88, No. 8, 1984
1047 (dp)
X0.76
1672 ( I D )
XO.6
A ##
1212 (dp)
d
A-
Am--
1198 (dp)
x1.s
1132 (ap)
J 753 (dp)
absorption profile
J
l
l
21.0
"
~
'
20.0
"
l
l
r
19.0
~
'
l
18.0
l
'
\ '
'
17.0
FREQUENCY ( c r n - l x l O - J )
Figure 8. Q-State excitation profiles for representative RR bands of solid (OEPFe)2N in a 1:l NaC1/Na2S04 pellet. The intensities are plotted on a linear scale of magnitude IO2 relative to the 990-cm-I band of Na,S04, and the frequencies are given in cm-l. The frequencies observed for the solid are slighly different (2-3 cm-I) from those observed in solution, but the profiles are labeled with the solution values to facilitate comparison with Figure 3 and Table I. See caption of Figure 6 for remaining details.
Hofmann and Bocian observed previously for metalloporphyrins, including tetraphenylporphyrin complexes. The profile of every band for all three FeOEP complexes shows a maximum 800-900 cm-' to the blue of the Q(0,O) peak (we will refer to this maximum as Q,). Similar behavior has been observed for a mode near 400 cm-' in TPPCC19 and TPPCu,'O but only for this one mode. (A comparable lowfrequency mode is not observed for the FeOEP complexes.) The Q, maximum is the dominant feature at higher energy in the profiles for OEPFeCl. The Q(0,l) peaks for the RR bands of this complex are barely in evidence, appearing as weak shoulders on the Q, maximum. (It should be noted that very limited excitation profiles have been reported previously for OEPFeC1;32 however, the small number of A, values precluded observation of the Q, feature.) The Profiles for (OEPFe)2N shows somewhat better resolved Q(0,l) peaks than do those for OEPFeCl. This is best seen for the high-frequency bands at 1647, 1590, and 1405 cm-'. The Q(0,l) maxima are more apparent still in the profiles observed for (OEPFe)*O. In addition to the Q(O,O), Q(O,l), and Q, maxima, a number of the bands exhibit peaks in the Q(0,2) region. These maxima are clearly evident in the profiles of the 1375- and 1024-cm-' bands of OEPFeCl, which appears to exhibit the strongest vibronic coupling effects (see below). The Q(0,2) maxima are also observed in the profiles of the 1647- and 1590-cm-I bands of (OEPFe)2N. The Q(0,2) maxima are less apparent in the profiles of (OEPFe)20, which shows weaker vibronic coupling than the other two complexes, although the coupling is still quite strong. The complexity of the excitation profiles for the FeOEP complexes precludes a rigorous theoretical interpretation of the vibronic coupling mechanisms. The Q, maximum observed in the profiles of essentially all the RR bands indicates that a number of modes are strongly coupled to one another. Nevertheless, the general features and trends observed in the profiles of the FeOEP complexes are similar to those predicted by the theoretical model of Shelnutt and O'Shea'O in which strong coupling between only two modes is considered. This model predicts that as vibronic coupling becomes strong, the splitting between the Q(0,O) and Q(0,l) maxima in the profile of each mode is decreased from the value expected on the basis of the ground-state vibrational frequency, and peaks due to Q(0,l) of both modes appear in the profile of a given mode. As the coupling is further increased, a single maximum at the same position in both profiles begins to dominate the Q(0,l) region. This maxmum is due to the symmetric linear combination of the two modes in the electronic excited state. The occurrence of the Q, peak in the profiles of the three FeOEP complexes is similar to this predicted behavior. For these molecules, however, Q, apparently obtains intensity from the symmetric combination of many coupled modes. The Q,/Q(O,l) intensity ratios suggest that the relative vibronic coupling strength in the complexes is OEPFeCl > (OEPFe)2N > (OEPFe)20. The theoretical modello also predicts that strong coupling results in Q-state absorption spectra where the Q(0,l) satellites are as strong or stronger than the Q(0,O) band. This is clearly the case for the FeOEP complexes (Figure 1). Conclusions The RR studies of (OEPFe)20 and (OEPFe)2N indicate that perturbations induced in the porphyrin a-electronic structure by dimer formation are negligible. The substitution of nitrogen for oxygen in the Fe-X-Fe bridge does, however, alter the electronic structure of the metal ions, resulting in a low-spin configuration for X = N and a high-spin configuration for X = 0. The bond order in the Fe-N-Fe linkage is also slightly greater than in the FeO-Fe bridge, as is evidenced by the slightly greater Fe-X force constant. The RR excitation profiles for the two dimeric (OEPFe)2X complexes as well as for OEPFeCl are indicative of very strong vibronic coupling. The occurrence of the Q, maximum 800-900 cm-I to the blue of the Q(0,O) maximum in the profiles of all of the bands for all three complexes suggests that this peak may be a general characteristic of strong vibronic coupling in metalloporphyrin systems. The observation of strong coupling effects in
J. Phys. Chem. 1984,88, 1479-1481 the excitation profiles of the FeOEP complexes further suggests that these effects may be important for other C,+ubstituted porphyrins, particularly ferric heme systems which exhibit relatively weak Q(0,O) absorption bands4’ The R R studies reported here also emphasized the importance of collecting detailed profiles before attempting to interpret the trends in these data.
Acknowledgment. Acknowledgment is made to the donors of ~~~~~
~
~
1479
the Petroleum Research Fund, administered by the American Chemical Society, the Cottrell Research Grants Program of the Research Corporation, the Committee on Research, University of California, Riverside, and the National Institute of General Medical Sciences (GM-30078-01A1) for support of this research. Partial funding for the Raman spectrometer was provided by the Biomedical Support Grant Program of the U.S. Public Health Service.
~
(47) Makinen, M.; Chung, A. K. In ‘Iron Porphyrins”; Lever, A. B. P., Gray, H. B., Eds.; Addison-Wesley: Reading, MA, 1982; Part I, pp 141-235.
Registry No. OEPFeC1,28755-93-3; (OEPF+O, 39393-88-9; (OEPFe)2N, 88524-95-2.
Methyl- and Dimethylketene: He I Photoelectron Spectra and Vertical Ionization Potentials Calculated by Using Perturbation Corrections to Koopmans’ Theorem D. P. Chong,* N. P. C. Westwood, Department of Chemistry, University of British Columbia, Vancouver, B.C., Canada V6T 1 Y6
and S. R. Langhoff NASA Ames Research Center, Moffett Field, California 94035 (Received: August 22, 1983)
He I photoelectron spectra are reported for the monomethyl and dimethyl derivatives of ketene, H,CCO. The spectra, which beyond the first structured ionization potential show a complicated series of bands in the 12-18-eV range, have been fully assigned by using calculations employing perturbation corrections to Koopmans’ theorem.
Introduction Simple derivatives of ketene are unstable with respect to dimerization or decomposition, but it has been shown by and others4 that it is possible to generate species of the type XHC= C+ and ~~c-c-0(X = C H ~ ~, 1and , B,.) in the gas phase for spectroscopic observation. In most cases the technique of ultraviolet photoelectron spectroscopy (UPS) has been used to monitor the reactions, although more recently microwave and Fourier transform infrared have proved to be s~ccessful.~Of the above species the monomethyl and dimethyl derivatives are by far the more stable, and consequently microwave and infrared spectra have been reported.@ However, the presence of methyl groups introduces complications into the interpretation of the photoelectron (PE) spectra, since ionization potentials ( I p s ) associated with the CH3group tend to cluster in a narrow spectral range. The accurate calculation of vertical I P S (VIP’s) can go a considerable way toward solving this problem. Such results can be obtained by the method of Green’s functions using large basis setslo or, in a more cost-effective fashion, by using ordinary Rayleigh-Schrodinger perturbation theory (RSPT).” Recently, (1) D. Colbourne, Ph.D. Thesis, University of British Columbia, 1979. (2) D. Colbourne, D. C. Frost, C. A. McDowell, and N. P. C. Westwood, J. Chem. SOC.,Chem. Commun., 250 (1980). (3) D. Colbourne et al., to be submitted for publication. (4) H. Bock, T. Hirabayashi, and S. Mohmand, Chem. Ber., 114, 2595 (1981). ‘ ( 5 j M . C. L. Gerry, W. Lewis-Bevan, and N. P.C. Westwood, J. Chem. Phvs. 79. 4655 (1983). 16) B . ~Bak, D. Chiistensen, J. Christiansen, L. Hansen-Nygaard, and J. Rastrup-Andersen, Spectrochim. Acta, 18, 1421 (1962). (7) B. Bak, J. J. Christiansen, K. Kuntsmann, L. Nygaard, and J. Rastrup-Andersen, J. Chem. Phys., 45, 883 (1966). (8) W. H. Fletcher and W. B. Barish, Spectrochim. Acta, 21, 1647 (1965). (9) K. P. R. Nair, H. D. Rudolph, and H. Dreizler, J. Mol. Spectrosc., 48, 571 (1973). (10) W. von Niessen, G. H. F. Diercksen, and L. S . Cederbaum, J. Chem. Phys., 67,4124 (1977), and references therein.
0022-3654/84/2088- 1479$01.50/0
the use of RSPT coupled with the effective core potential has Provided a SUWeSsfUl and economical method of computing VIP’s for the monochloro- and dich10roketenes*’2 In this work we employ ordinary third-order RSPT to provide an analysis of the complicated PE spectra of the monomethyland
Experimental Section Methyl- and dimethylketene were prepared by low-pressure gas-phase pyrolysis (700-750 “C) of respectively propanoyl chloride and 2-methylpropanoyl chloride in a 120 mm X 7 mm i.d. quartz tube. The reaction, which involves a dehydrochlorination, can be represented for dimethylketene by (CH3)ZCHCOCl-
A
(CH3)2C=C=O
+ HCl
HCl was removed by gas-phase titration with NH3 and the effluent from the reaction was led directly into the ionization chamber of a photoelectron ~pectrometer.’~As illustrated by the PE spectra, this is an efficient route, giving virtually pure ketenes. Spectra were calibrated with the known IPSof CH31, CO, N,, and Ar.
Computational Details The geometry of methylketene and dimethylketene was taken from ref 7 and 9, respectively. For basis functions, a standard double-{ (DZ) set was used. It consists of Huzinaga’s (9s5p;4s) Cartesian Gaussian f ~ n c t i o n s ’contracted ~ to (4s2p;2s) with Dunning’s coefficients.lS The hydrogen orbitals were scaled by the usual factor of 1.2. ~~
(11) D. P. Chong and S. R. Langhoff, Chem. Phys., 67, 153 (1982). (12) S.R. Langhoff and D. P. Chong, Chem. Phys. Lett., 100,259 (1983). (13) D. C. Frost, S. T. Lee, C. A. McDowell, and N. P. C. Westwood, J . Electron Spectrosc. Relat. Phenom., 12, 95 (1977). (14) S. Huzinaga, J. Chem. Phys., 42, 1293 (1965). (15) T. H. Dunning, Jr., J. Chem. Phys., 53, 2823 (1970).
0 1984 American Chemical Society
