J . Phys. Chem. 1988, 92, 6188-6197
6188
surface reactions on well-defined surfaces of organophosphorus, -sulfur, and -nitrogen compounds of sufficient complexity to model environmentally hazardous molecules, (2) the behavior, particularly the catalytic behavior, of compounds such as metal sulfides, phosphides, and nitrides formed during decomposition of target molecules, and (3) the search for ways to continuously generate fresh reaction centers. With regard to materials, the activation of metal oxides for dealkylation, hydrolysis, elimination, and addition reactions is well documented, giving them considerable potential as reagents for the destruction of organic heteroatom compounds. Their potential as catalysts remains to be established. A number of attractive fundamental research areas exist including (1) in situ spectroscopic investigation of adsorbed species, particularly by solid state NMR, (2) preparation and characterization of superacids and superbases, (3) influence of water (liquid and vapor), (4) detailed study of known compound oxidation catalysts for their activity in decomposing organic heteroatom molecules, (5) development of new methods of synthesizing and activating metal oxides, and (6) development of methods to continuously generate reactive defect sites in metal oxides.
Organometallic cluster research can serve to illuminate surface chemistry and heterogeneous reactions in two ways: (1) by providing structural models for adsorbate bonding modes, and (2) by providing qualitative reactivity models. There is now some work in the literature on the bonding modes of sulfates, phosphates, phosphonates, cyanide, and fluoride ligands in organometallic clusters. Building on what is known about organometallics and what is being learned about the surface structure of organic fragments containing phosphorus, sulfur, and nitrogen, excellent opportunities exist for the synthesis of model organometallic clusters relevant to understanding the chemistry involved in the degradation of organic heteroatom compounds.
Acknowledgment. We thank Dr. Robert Shaw for initiating the idea of this review. We acknowledge support by the Army Research Office for the preparation of this review and the hospitality of the Chemistry Department at the University of North Carolina at Chapel Hill, particularly Professors Slayton Evans and Thomas Meyer, who hosted a small conference on this topic. We also thank Dr. George Lester and Dr. Chen Hsu for providing overviews of relevant work.
ARTICLES Electronic Structure and Spectra of Various Spin States of (Porphinato)iron(I I I ) Chloride W. Daniel Edwards,* Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Brian Weiner, Department of Physics, Pennsylvania State University, Dubois, Pennsylvania 15801
and M. C. Zerner* Quantum Theory Project, Department of Chemistry, University of Florida, Gainesville, Florida 3261 1 (Received: July 20, 1987; In Final Form: May 28, 1988)
(Porphinato)iron(III) complexes can exist as low-spin doublets, high-spin sextets, and intermediate-spin quartets. While the experimental ground state depends on the nature and number of axial ligands, INDO calculations on the five-coordinate (porphinato)iron(III) chloride of this study suggests a high-spin sextet and an intermediate-spin quartet to be nearly degenerate in energy. The lowest doublet is calculated some 8000 cm-I above the sextet. The calculated UV/visible spectra of each of these states are reported and compared to experimental spectra and ab initio calculated spectra.
Introduction The electronic absorption spectrum of metalloporphyrin systems has long been understood in terms of the highly successful “four orbital” model first applied to these systems by Gouterman.] This picture, although oversimplified, well reproduces the major features of these systems, namely the weakly allowed Q band in the visible region and the strongly allowed B band in the UV region. The model is based on near-equal-intensity transitions from the two highest occupied H orbitals (al, and a2J to the two lowest unoccupied H* orbitals (e,) of the closed-shell porphine dianion. The B and the Q bands arise from linear combinations of these (1) Gouterman. M . J . Mol. Spectrosc. 1961, 6, 138.
0022-3654/88/2092-6188$01.50/0
one electron transitions. For the lower energy Q band, the transition dipoles nearly cancel giving rise to the relatively weak absorption in the visible. For the higher energy B, or Soret, band, the transition dipoles reenforce, resulting in the very intense absorption in the UV region. In the case of the metal-substituted porphyrins, the metal will act as perturbing field causing the relative energies of these transitions to shift. If the metal is closed shell such as Mg2+ or Zn2+ the ground state of the system will be a singlet and the calculated spectrum will consist of allowed H H * singlets (among them, the B and Q bands), and forbidden x T* triplet^.^,^ If the metal is open shell, a number of spin
--
( 2 ) Edwards, W . D.;Zerner, M . C. Can. J. Chem. 1985, 63, 1763.
0 1988 American Chemical Society
Structure and Spectra of (Porphinato)iron(III) Chloride states become candidates for the ground state. For these open-shell compounds, the spin coupling between the ground-state metal and the excited porphyrin states results in a large number of allowed transitions from the ground state.4 For some of these multiplicities, the formerly forbidden triplet excitations become allowed. This will result in an increase in the number of allowed transitions, greatly increasing the complexity of the resulting spectra. We have recently examined the ground and excited states of ferrous porphine ((p~rphinato)iron(II)).~ In this paper we examine the ground and excited states of ferric porphine ((porphinato)iron(III) chloride). In the case of iron(II1) porphines, the metal ion can be a doublet, a sextet, or an intermediate-spin quartet. These iron spin states can couple with the closed-shell porphinato ion to give an overall ground state that is doublet, quartet, or sextet. Inorganic compounds such as the high-spin Fe(TPP)CIS (TPP = tetraphenylporphyrin), the low-spin Fe(TPP)(CN),,6 and the intermediate-spin Fe(TPP)C(CN)37 have been synthesized and characterized. While these compounds are interesting in their own right, they also serve as models for some important biochemical ferric porphyrins, such as the high-spin cytochrome P-450, or the low-spin metHb(CN).8 Intermediate-spin iron porphines have been postulated for ferricytochrome c' found in bacterial heme and more recently in conjunction with nitrogenase s y ~ t e m s . In ~ this paper we will report the results of spectroscopic I N D O calculations on these spin systems as well as examine the consequences of the spin coupling that occurs between the metal and the porphine excited states. We will also calculate the electronic spectrum of each of these spin states for a model compound in order to catalog these states for future studies on these compounds as well as other models for biologically active systems.
Methodology The S C F calculations we performed were of the intermediate neglect of differential overlap (INDO) type'*I4 and were done on four low-lying states of the five-coordinate compound, (porphinato)iron(III) chloride. A single frozen geometry was used for all calculations and was based on the geometry used for the planar four-coordinate iron(I1) compound reported in ref 3. The five-coordinate iron(II1) compounds are nonplanar, so the iron atom was moved out of plane by 0.43 A. This displacement out of plane corresponds to the average displacement in high-spin five-coordinate (porphinato)iron(III) c o m p l e ~ e s and ' ~ results in an increase of the F e N distance to 2.054 A. A chloride ion was chosen as a fifth ligand and placed at a distance of 2.32 A from the iron atom. Iron Pd was set to -23.0 eV on the basis of calculations of model ferric cytochrome comp1exesI6and studies of (porphinato)iron(II) c o m p l e x e ~ . ~ The four reference states examined in detail are ZBzg,,Eg, 4A2g, and 6A,,. Since each of these states has unpaired electrons,
(3) Edwards, W. D.; Weiner, B.; Zerner, M. C. J . Am. Chem. SOC.1986, 108, 2196. (4) Ake, R.; Gouterman, M. Theor. Chim. Acta 1969, 15, 20. (5) Hoard, J. L.; Cohen, G. H.; Glick, M. D. J . Am. Chem. SOC.1967,89, 1992. (6) Scheidt, W. R.; Hailer, K. J.; Hatano, K. J . Am. Chem. SOC.1980, 102, 3017. (7) Summerville, D. A,; Cohen, I . A,; Hatano, K.; Scheidt, W. R. Inorg. Chem. 1978, 17, 2906. f8) Eaton. W. A.: Hochstrasser. R. M. J . Chem. Phvs. 1968. 49. 985. (9j Munck, R.; Zimmerman, R.'Abstr of Papers, l o t i Mossbauer'Symposium, New York, Feb. 1976. (IO) Pople, J. A.; Beveridge, D. L.; Dobosch, P. J . Chem. Phys. 1967, 47, 2026. (11) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1973, 32, 111. (12) Ridley, J. E.; Zerner, M. C. Theor. Chim. Acta 1976, 42, 223. (13) Bacon, A. D.; Zerner, M. C. Theor. Chim. Acta 1979, 53, 21. (14) Zerner, M. C.; Loew, G. H.; Kirchner, R. F.; Mueller-Westerhoff, U . T.J . Am. Chem. SOC.1980, 102, 589. (15) Scheidt, W. R.; Reed, C. A. Chem Reu. 1981, 81, 543. (16) Waleh, A,; Collins, J. R.; Loew, G. H.; Zerner, M. C. Int. J . Quantum Chem. 1986, 29, 1575.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6189 TABLE I: Reduction in Symmetry
TABLE II: SCF and CI Energies (in bartrees) CI from state
2B2,
orbital occum d,2dy,2d,'
2E, d,,2dyz1d,2
SCF enerev -188.972090
CI enerev -, -188.973299 (8300)" (8 000) -1 88.974043 -1 88.98 16 14 (7900) (6200) I -
2A2g d x r l d y r l d ~ ~ d , z l 4A2, dxr1dyr1dxy2dz,1 4E,
-189.009732 (20)
-189.009766 (0)
dxz1dyi1dXy1dlz1
4A9. SCF -e -188.971765 (8300) -1 88.979874 (6600) -1 88.970460 (8600) -189.009766 (0) -188.976901 (7200)
6AI, dxrldyrldxyldi~ldxx-yyyyl -1 89.00982 1 -1 89.001 547 (0)
(1800)
"Numbers in parentheses are excitation energies in cm-I relative to the
6Al,. *Numbers in parentheses are excitation energies in cm-I relative to the 4A2,.
open-shell R H F S C F calculations were performed using a generalized open-shell operator.]' Spectroscopic parameters were used throughout, and consistent with this scheme, a single excitation configuration interaction (CI) was performed on each of these states in order to obtain the electronic spectra. These CI were generated by using a Rumer diagram techniquei8 and consisted of approximately 200 configurations divided into the four irreducible representations of C, symmetry. Because the Rumer diagram technique does not generate orthonormal configurations, the diagonal elements of the one matrix (in the basis of MO's) were calculated and the resulting orbital occupation numbers were used to interpret the origins of the various transitions. The molecular orbitals and states are labeled by using the D4h symmetry labels appropriate for planar magnesium porphine as is becoming conventional in porphyrin chemistry. A D4,,/Chcorrelation table is included in Table I for convenience.
SCF Results At the S C F level, the lowest energy state was found to be the high-spin 6A1g (dxy',dxrl,dyr',dZrl,dxx-yyl) with the 4A2r state (dx~,dzr',dxr',dy,'),only 20 cm-I above it, Table 11. Since the separation is so small, the multiplicity of the ground state will certainly depend on such effects as detailed molecular geometry, ligand strength, and solvent effects. This energy separation is comparable to the spin-orbit coupling constant (about 300 cm-' in ferric heme comple~es'~) and suggests the possibility of quantum mechanical admixing of spin states resulting in a single state that is a mixture of high-spin and intermediate-spin states. This should be distinguished from thermal mixtures of spin states which consist of an equilibrium mixture of distinguishable pure spin states which could also occur. This admixed state has been analyzed in detail by MaltempoZ0and the results obtained here are consistent with his analysis. It should be noted that the lowest energy sextet calculated at the S C F level had "cracked" symmetry; Le., the molecular orbitals were not of pure C,, symmetry. This symmetry cracking represents a Hartree-Fock instability and is currently being investigated. Such symmetry breaking generally indicates the presence of very near lying states of different spacial or spin symmetry. In order to calculate the sextet excited states, it was thus necessary to impose C,, symmetry as a constraint. This resulted in a raising of the sextet S C F energy by 1800 cm-'. As discussed subsequently, (17) Edwards, W. D.; Zerner, M. C. Theor. Chim. Acta 1987, 7 2 , 347. (18) Reeves, C. Commun. A . C. M . 1966, 9, 276. (19) Salmeen, 1.; Palmer, G. J . Chem. Phys. 1968, 48, 2049. (20) Maltempo, M. M. J . Chem. Phys. 1974, 61, 2540.
6190
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
CI does not change the relationship between these nearly degenerate 6Algand 4A,, states. Ligand field theory predicts that five-coordinate weak-field systems should favor the high-spin case while strong-field ligands should favor six-coordinate low-spin complexes. The former would have all iron d orbitals singly occupied while the latter should have unoccupied d, and d, orbitals. For a pure state of intermediate spin, only the d,,-, orbital is empty. Classical crystal field theory predicts a decreasing ,d orbital energy as the d, orbital interacts more strongly with fifth and sixth position ligands. Further complicating this analysis is the fact that increased axial ligation may pull the iron atom out of plane, adding a z component to the iron porphine interaction and altering the d,,~,,,,/d, orbital splitting. Shelly, Bartczak, Scheidt, and Reed2' have suggested that an iron(II1) porphine without any axial ligand should be a pure intermediate-spin system. In order to achieve this, one needs an anionic counterion that is noncoordinating in nondonor solvent. A number of systems have been tried (BF4-, PF6-, SbF6-22)but none of these systems show all the characteristics of a pure intermediate-spin system, such as a spin only value of 3.9 p for the magnetic moment. One system that does show no detectable admixture of high-spin component is the vinylidene insertion compound;23however, vinylidene cannot be considered a normal ligand. Previous two-electron corrected extended Huckel calculations have suggested that increased ligand strength in the z direction will force the spin state from sextet directly to doublet if the iron atom remains out of the plane of the porphyrin ring.24 Only in-plane movement of the iron atom for a fixed z-axis coordination allows the formation of the intermediate-spin quartetzs At the S C F level, the 2E, state is calculated to be 7900 cm-I above the 6A1, while the *Bzgis calculated to be an additional 400 cm-' higher. That these low-spin doublet states are considerably higher in energy is not surprising in light of the fact that the calculations were done on the nonplanar five-coordinate (porphinato)iron(III) chloride. While a few five-coordinate low-spin complexes of this type are known, it is widely accepted that low spin is diagnostic of a coordination number of six. These sixcoordinate compounds would have the iron atom more nearly in the porphine plane greatly altering the orbital interaction pattern. Thus the energy of these states relative to the other spin states could easily be altered by changes in geometry and axial ligands.
Doublet Spectra The electronic spectrum of each of the previously discussed spin states has been calculated by using the corresponding S C F wave function as the reference state. For 2B2g,configurations were generated by using single excitations from the doubly occupied eg(dxz,dy,)orbitals (which mix with e, combination of nitrogen P, orbitals) and the doubly occupied a2,(a) and al,(a) orbitals. The one singly occupied orbital is b2,(dXy). Transitions included in the CI involved excitations from these occupied orbitals into the virtual orbitals which can be labeled as e,(a*), alg(d,,), big(dxx-yy),and b2,(a*). Transitions from selected doubly occupied orbitals into the singly occupied d, were also included in the CI. The total number of the configurations included in the CI was 260 divided among the four irreducible representations of the subgroup C2". It should be noted that, for a doublet with one unpaired electron, single excitations from a doubly occupied orbital to empty orbitals results in three unpaired electrons. Three unpaired electrons, doublet coupled, give rise to two nondegenerate doublet states. For example al,(a) e,(x*) transitions give two nondegenerate states called trip-doublets and sing- doublet^.^ The former can be thought of as a triplet porphine excited-state coupling with the doublet iron to give a doublet state. The latter would be the more
-
(21) Shelly, K.; Bartczak, T.; Scheidt, W. R.; Reed, C. I. Inorg. Chem. 1985, 24, 4325. (22) Reed, C. A,; Mashiko, T.; Bentley, S. P.; Kastner, M. E.; Scheidt, w. R.; Spartalian, K.; Lang, G.J . Am. Chem. SOC.1979, 101, 2948. (23) Mansuy, D.;Morgenstern-Badarau, I.; Lange, M.; Gans, P. Inorg. Chem. 1982, 21, 4143. (24) Zerner, M. c.;Gouterman, M. P. Theor. Chim.Acta 1966,6,363. (251 Griffith, J. S. Proc. R. SOC.1956, ,4235, 23; Nature 1957, 180, 29.
Edwards et al. familiar singlet porphine excited-state coupling with the doublet iron to give a doublet state. In these calculations, the trip-doublets are nearly pure al,(a) e,(**) or a2,(a) e,(a*) transitions and transitions to these states from the 2B2sground state are allowed. For this molecule, the sing-doublets are roughly 60/40 mixtures of alu/azU e,(**) and can be labeled B and Q corresponding to the same type of combinations in the four-orbital model. The trip-doublets are calculated to lie at 10200 and 14700 cm-l while the sing-doublets are calculated at 16 500 and 30 200 cm-'. In general, we note that these calculations accurately predict the Q band usually observed between 16000 and 18 000 cm-I, but generally overestimate the transition energy of the B band, observed at 23 00026 000 cm-' by some 6000 cm-1.2,3*16 These results as well as all other transitions below 30 200 cm-l are listed in Table 111. The labels attached to these reflect the major components of the initial and final states as determined from the diagonal of the one-electron density. Type I11 also shows low-lying charge-transfer transitions which show the same type of spin coupling. The transition a,,,(a) alg(d,,) gives two nondegenerate doublets at 16 800 and 25 300 bl, (d,,-,,J lies at 18 200 and cm-I while the transition al,(a) 21 200 cm-I. Some d d transitions also show this spin coupling with the e (d,) alg(dzr)transitions calculated at 12 900 and 17000 cm-f and the e,(d,) blg(dxpy,,)at 21 600 and 23 600 c d . Transitions involving the partially occupied d, orbital however will result in only one state since only one electron will remain alg(dzz)transition is predicted at 20 100 unpaired. The b,,(d,) cm-' and bzg(dxy) b,,(d7x-yJ at 23 300 cm-I. The e,(d,) b,,(d,) transition is of special importance since the resulting 2E, state is the usual description of a low-spin ground state. In this CI, the 'E, root is 8000 cm-I higher than the 2B2, reference state. However, because the CI is based on a ,B2, S C F wave function, this ordering reflects a certain prejudice favoring B2, states. It is important therefore to do comparable S C F and CI calculations on both the ,E, and the ,B2, states to correctly establish their ordering. We have also done calculations on the ,E, state and find that at the S C F level the 2E, state is lower than the 2B2, state by 429 cm-I. Singles only CI on each of these wave functions (which include the Brillouin theorem violating interactions3) increases the separation to 1825 cm-l. In the ,Eg CI the ,B2, root is 3600 cm-' higher than the ,E, reference state. Our best estimate from these calculations is that the 2E, state lies some 1800 cm-' lower in energy than the 2B2gstate, in agreement with the ordering suggested by Griffith some 30 years ago.25 Interpreting the spectrum of the 2E, state becomes much more complicated. The doubly occupied orbitals of interest include the metal d, and the porphine a orbitals, a2, and a,". The open-shell orbitals are the degenerate d,, and dyzcontaining three electrons. Since the CI allows only integer occupation numbers, it is necessary to include two reference states; (d,: d,,,') and (dxzld,,'). The resulting 2E, CI includes a total of 303 configurations and the low-lying states are reported in Table IV. The intensities from each of these two (degenerate) roots of the CI are also reported in the table. As before, transitions from a', e,(*) and a2, e&a) lead to trip-doublets and sing-doublets with the trip-doublet states corresponding to simple transitions and the sing-doublets a 60/40 mixture typical of the B and Q transitions. These excited states have occupations of alU(a)l,eg(a*)l, and e,(dJ3. The excited-state symmetries are al, X e, X eg = Ai, A,, + Bl, + B2,. Thus e, transition should give rise to four the usual porphyrin al, distinct states, one in each irreducible representation. Each of these four nearly degenerate transitions is given the same label in Table IV. Compared to the spectrum of the 2B2pground state, the Q bands are shifted some 2000 cm-' to highe; energy. The B band is broadened and is again calculated about 6000 cm-' too high in energy when compared to typical porphyrin spectra, see below. Low-lying d, d,, transitions are predicted at 3000, 6700, 7500, and 14400 The degenerate d, orbitals can have an or (d,>,dy,2)-both are one-electron occupation of (d,2.dy:)
-
-
-
-
- -
+
+
--
-
-
-
-.
-
+
Structure and Spectra of (Porphinato)iron(III) Chloride
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6191
N
8
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6192
Edwards et al.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
8 m
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ssssss s s s ss=== s s s szssss
Structure and Spectra of (Porphinato)iron(III) Chloride
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6193
TABLE V: A Comparison of the Predicted INDO/CI Doublet Spectra with Experimental Values (See Also Text and Tables I11 and IV)
exptl
2B2*
2E,
ref 34
ref 36
6.4-10.5
5.8 6.8 9.5 (?)
ref 39"
3.0 8.0
(2E,)
16.7 10.2 12.8 14.9 16.5 23.6 28.4 28.9 30.2
trip-doublet
Q band
B band a
10.6, 13.9 10.9 10.9-12.9
(0.11) (0.10) (0.42) (0.04) (4.01)
14.4 15.4
(2)
8.0 (?)
18.5-18.7 (0.29) 23.9 (0.13)
16.9
17.7-18.9
30.1-30.7 (4.62)
24.2
24.0 24.4
For ferricytochrome c.
doublets-or an occupation of (dx,',dYz1)-a three-electron doublet representing two states; thus one transition represents a total of four states. All are low lying but probably depend strongly on nature of the fifth ligand. This same structure holds for d, dxx-yytransitions which are found at somewhat higher energies: 22 700, 23 600, 26 400, and 33 200 cm-'. A d, d,, excitation is predicted to lie at low energy, 10 600 cm-I. Excitations to this resulting state, of 2E symmetry, is z allowed in C4, symmetry. This state has a calculated oscillator strength of 0.0002. Other z-polarized low-lying transitions (all 2E 2E) include a,,(r) d,, and aZu(r) d,, charge transfers calculated at 13 900 and 16 000 cm-I, respectively. These three z-polarized excitations all involve the d,, orbital and should be quite sensitive to fifth and sixth position ligation. Nevertheless, it is irresistible to compare our findings with other calculations, and with experiment, even when the systems are not identical. Extended Huckel (EHT) calculations on Fe(III)PCN, (porphinato)iron(III) cyanide, predict the 2E state lower in energy d,, than the 2B2gstate by about 1000 cm-'." Transitions d, are calculated at about 15300, 17700, 18600, 29000 cm-I in reasonable accord with the values obtained here for the much weaker C1- ligand, 3000, 6700, 7540, and 14400 cm-'. Note especially the spread in values. Charge-transfer excitation al,(r) d, and a2,(r) d,, are calculated by the EHT at 15 000 and 23 000 cm-I, again in reasonable accord with the values obtained in this study of 13 900 and 16 000 cm-'. The order here, however, has been reversed in keeping with the observation that E H T calculations predict orbital energies t(a2,(r)) > t(alu(r)), in contrast to the order obtained by S C F methods such as these calculations. Computationally30~31 calculations, or a b and e ~ p e r i m e n t a l l ythe ~ ~cations ~ ~ ~ found by removing an electron from porphyrins produce nearly degenerate 2A2uand 2Alustates. The E H T calculations, however, predict a l u ( r ) , a2,,(r) d, transitions at only 2000 and 4000 cm-', over 20000 cm-I lower in energy than we predict here. We have no convincing explanation for this discrepancy. Some experimental findings on low-spin systems are summa~ ~ ferricytochrome ~~~ c.39 rized in Table V for m e t r n y o g l ~ b i nand
-
-
-
-
-
-
-
12.9 ( z )
-
-
~~
~
(26) Rawlings, D. C.; Gouterman, M.; Davidson, E. R.; Feller, D. Int. J . Quantum Chem. 1985, 28, 797. (27) Obara, S.;Kaskiwaga, H. J . Chem. Phys. 1982, 77, 3155. (28) Kashiwaga, H.; Takada, T.; O k r a , S.;Migoshi, E.; Ohno, K. fnt. J . Quantum Chem. 1978, 14, 13. (29) Dedieu, A.; Rohmer, M.-M.; Veillard, A. In Aduances in Quantum Chemistry; Lowdin, P. O., Ed.;Academic: New York, 1982; Vol. 16. (30) Edwards, W. D.; Zerner, M. C. Int. J . Quantum Chem. 1983, 23, 1407. (31) Edwards, W. D.; Zerner, M. C. Can. J. Chem. 1985, 63, 1863. (32) Browett, W. R.; Stillman, M. J. Znorg. Chem. Acta 1981, 49, 69. (33) Felton, R. H.; Owen, G. S.;Dolphin, D.; Forman, A,; Borg, D. C.; Fajer, J . Ann. N . Y . Acad. Sci. 1973, 206, 504. (34) Makinen, M. W.; Churg, A. K. Iron Porphyrins; Lever and Gray, Eds.; Addison-Wesley: Boston, MA, 1983; Part I. (35) Cheng, J. C.; Osborne, G. A.; Stephens, P. J.; Eaton, W. A. Nature 1973, 241, 193. (36) Schejter, A,; Eaton, W. A. Biochemistry 1984, 23, 1081.
Makinen and Churg report an ( x or y ) polarized band at about 8000 cm-I. Cheng, Osbourne, Stephens, and Eaton who have carefully examined the near-red MCD spectra of low-spin cyanomethemoglobin and cyanornetmyogl~bin~~ find three peaks at about 6400, 7900, and 9400 cm-', also in-plane polarized. The even spacing and decreasing intensity suggests a vibrational progression with allowed electronic origin, although this is not completely clear from the reported spectra, or from subsequent work by Schejter and E a t ~ nwho , ~ ~examine this region for a series of ferricytochrome c complexes. On the basis of extended Huckel calculation^,^^-^^ the absence of any detectable natural CD, and an analysis of the MCD calculated from the g values obtained from the EPR, Schejter and Eaton assign this region with two eg(d,) and al,(r) e,(d,), as did clear bands to a2"(r) Makinen and Churgs4 for structure at slightly higher energy. Makinen and Churg also report a z-polarized band at 12 500 cm-', whereas a z-polarized band is observed by Eaton and Hoc h s t r a s ~ e r ~at* -14 ~ ~400 cm-', and a higher energy z-polarized transition at 22700 cm-'. An examination of Makinen and Churg's published paper, however, does not confirm a z-polarized band at 12 900 cm-I, but an argument can be made for a z-polarized peak at about 15 000 crn-', near that reported by Eaton and Hochstrasser at 14 400 cm-I. The spectrum calculated assuming a 2B2gground state presents only low-lying 2E states, ( x y ) polarized, and strong ligands will only increase the excitation energies involving the d, orbital. This presents yet one more argument against assigning 2B2, symmetry to these systems. Assuming the 2Egreference state is much richer in possibilities, no fewer than 12 transitions are calculated to lie below the onset of the Q band. The lowest lying ( x or y ) polarized band reported by Makinen and Churg at 8000 cm-I 34 and the three bands examined by d,, exSchejter and Eaton might be associated with the d, citations (2E 2Al and 2E 2A2in Cb).Other bands that might be found in this region are the a l u ( r ) d,,, calculated at 13 800 cm-I, or the d, d,, excitation, calculated at 10600 cm-I. Both of these excitation are 2E 2E and are z polarized in C4". The first trip-doublet is calculated at 10900 cm-I and is (x,y) allowed. A z-polarized transition with reasonable oscillator strength, 2.4 X a2,,(r) d,,, is calculated at 22700 cm-I and it is tempting to assign this to the observed band at 21 000 cm-'. Schejter and Eaton have assigned the three bands observed at 6400-9400 cm-I to a,,,(r) d, and/or aZu d, charge transfers. This argument is made especially appealing as they note that the transition frequency increases as the ligand field strength decreases. Our calculated excitation energies for these two states, however, are at 26 800 and 31 500 cm-I. We further note that these excitations, 2E 2A2and 2E 2Al are ( x , y ) ,not z , polarized for these complexes. It is not obvious, however, how our postulated assignments for this region, al,(r) d,, or a2,(7r) d,, or d,
-
-
-
- - -
-
-
-
-
-
(37) Eaton, W. A.; Hofrichter, J. Methods Enzymol. 1981, 76, 175. (38) Eaton, W. A.; Hochstrasser, R. M. J . Chem. Phys. 1969, 49, 985. (39) Eaton, W. A.; Hochstrasser, R. N. J . Chem. Phys. 1967, 46, 2533.
6194
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Edwards et al.
TABLE VI: Ouartet A,. CI Occmancies‘
SCF orbital occupn E(2) = 7214 E(3) = 10479 A l ( 2 ) = 1 1 738 A2(2) = 14638 E(4) = 14655 A l ( 3 ) = 14819 E(5) = 16350 E(6) = 19283 A l ( 4 ) = 19357 A l ( 5 ) = 19658 A l ( 6 ) = 21 490 E(7) = 22508 A2(3) = 23436 E(8) = 23469 A2(4) = 24062 AI (7) = 25 562 E(9) = 26 370 A2(5) = 26386 A l ( 8 ) = 26386 A l ( 9 ) = 26503 E(10) = 26691 Al(10) = 26791 E(11) = 27153 E( 12) = 27 947 Al(11) = 29159 E(13) = 29685 A2(6) = 30445 Al(12) = 30539 A2(7) = 31 510 E(14) = 31 614 A2(8) = 31 882 Al(13) = 32404 E( IS) = 33 255
xy
xy
2.00
2.00
-0.48
-0.51
P(x) 2.00
2.00
a2,,
al,
2.00
2.00 -1.00
(0.0002)
yz
xz
1.00 1.00 0.98 0.04
-0.49 -0.51
-0.37
(0.1056) -0.48 -0.49
-1.00 -0.62 0.03 -0.99 0.02 -1 .oo -0.99
(0.1226) (0.0022) -0.02
(0.3920) -0.52
(0.0078) -0.49
-0.68 -0.10 -0.88 -0.08 -0.77 -0.53 -0.88
0.02
0.96
0.98 0.77
(1.8166)
-0.15
(3.1628)
-0.10 -0.10 -0.86
(1.1678)
-0.78 -0.68 -0.73 -0.03 -0.09
0.32 -0.52
0.13
0.17
0.07 0.41 0.84 0.02 0.09 -0.07 0.20 0.06 0.12 0.56 -0.03
0.24 0.02
1 .oo
0.15 0.43
0.08 0.08 0.04 0.08
--
--
0.02 1.oo 1 .oo
--
0.98 0.02 0.95 0.35 1.oo 0.87 0.51 0.99
- 4s)
trip-quartet d(xy) d ( 4 d(zz) d(xx-yy) trip-doublet a,, -- d(xx-w) sing-quartetQ trip-quartet a,, 422) d(xy) d(xx-w) d(XY) d(xx-yy) trip-doublet a2u d(xx-w) trip-doublet a2u d(xx-yy) a,, d(xx-yy)
0.59 0.36 0.96
0.21
-0.21 0.13 -0.03 -0.04 -0.13 -0.45 -1.00 -0.17 -0.15 -0.24 -0.77 0.02 -0.04
dby)
0.03 0.99
-0.51
(0.1350) (0.1800)
b,.
0.02
-0.72 -0.27 -0.90 -0.04 0.06 -0.06 0.03 0.90 -1 .oo 0.98 -1.00 -0.29 -0.35 0.22 -0.22 0.13 0.17 -0.99 -0.99 -0.46
b,h
0.95
-0.51 -0.51
(0.0088)
-0.48
e ( . ’ )xx-yy
0.97 -0.70 0.03 -0.08
-0.30 -0.90
(0.0002)
zz 1.00
0.99
alu
--
d(xy) 0.12
-
blb
d(xx-yy)
trip-doublet
0.10
TX 0.03 0.03 0.46 0.97 0.75 0.13 0.71 0.12 0.86
0.95
trip-doublet 0.45 0.07 0.17 0.06
sing-quartet B 0.24 0.03 0.04 0.64 TX
OEnergy is in cm-l. Numbers in parentheses are computed oscillator strengths. For states labeled E (the degenerate B, and B2 transitions), the oscillator strengths reported are the sum of the individual components.
d,, follow the observed trend of decreasing excitation energy with increasing ligand field strength.
-.+
Quartet Spectrum
The intermediate-spin quartet spectra was calculated by using single excitations out of the 4A2g.referencestate. Our results are summarized in Table VI. Active orbitals included the doubly occupied b,,(d,) which mixes strongly with the porphine bz,(a) and the four-orbital model aZu(a)and al,(r) porphine orbitals. The open-shell e,(d,) (with two electrons) and al,(d,) (with one electron) were also Included. The active virtual orbitals were the porphine e,(a), the iron blg(dxx-yy)and the ligand bZg(a). The C1 included a total of 171 configurations. As before, a single excitation can give rise to a number of quartet states (as well as a number of doublet and sextet states). For example, if the metal has three unpaired electrons coupled as a quartet, this can mix with porphine excited states which have two unpaired electrons coupled as a singlet or triplet giving a total of five unpaired electrons which can couple to form four quartet states: a tripquartet, a sing-quartet, and two different trip-doublets. Thus the a,, e,(r*) and azu e,(x*) will give a total of eight pairs of states, with allowed transitions from the ground state. Two of these are the mixed, alu/a2u e?(a*) sing-quartets calculated at I6400 cm-’ (Q) and 31 600 cm- (B). Three that are principally a l u e, are calculated at 10 500 cm-’ (trip-quartet), 19 300 cm-’ (trip-doublet), and 22 500 cm-’ (trip-doublet). Four transitions (in addition to the B and Q bands) are found to be mostly aLU eg and they are calculated at 14 700 cm-l (trip-quartet), 23 500 cm-I (trip-doublet), 27 200 cm-’ (trip-doublet), and 29600 cm-’ (trip-doublet). The “extra” transition in this latter group is a result of the strong mixing of the azu(a) e,(d,) with the B band, and those calculated at 29 600 cm-I. The d d transitions fall into two categories: those that maintain three unpaired electrons and those that contain five
-
-
-
-
-
-
-
--
- -
unpaired electrons. The former are calculated at 7 200 cm-I (d, d,), 14600 cm-l (dZz dxx-yy)11 700 cm-I (dxy d,,), and dxx-yy). The latter are the d, dxx-yy 26 300 cm-I (d, transitions and are calculated to lie at 19 700, 21 500, 26400, and 38 800 cm-I. Chlorine to iron charge-transfer excitations are calculated at 27 900 and 33 300 cm-I. As menioned previously, no clear-cut examples of intermediate iron porphyrin complexes are known. Sextet Spectrum
At the SCF level, the sextet state was found to be lowest in energy. However, the resulting molecular orbitals were not of pure symmetry, even in Czo. Because of this cracked symmetry solution, we were unable to run the open-shell CI starting from these vectors. The following discussion is based on a sextet SCF calculation where the orbitals were restricted to have C,, symmetry. Imposing this restriction resulted in a raising of the S C F energy by nearly 1800 cm-*, or 5.1 kcal/mol. The spectrum of the sextet, given in Table VII, is complicated since seven unpaired electrons give rise to six different sextet states: one trip-sextet, one sing-sextet, and four trip-quartets. For the 6Al,, the doubly occupied orbitals are the four-orbital model azu(r) and alu(a)while the singly occupied orbitals are all the iron d’s. Empty orbitals included in the CI are e,(a*), bZu(r*), and a,, (Fe 4s and 4p,). The CI consists of a total of 273 configurations. d spectrum. The d, and e,(r) There is no spin-allowed d porphyrin molecular orbitals in the calculation mix more than usual. This mixing remains throughout the CI. The result is the prediction of a normal Q band calculated at 16 400 cm-’, similar to that found when the ground state was of 2B2, symmetry. The al, d, and azu d, transitions have gained intensity through this mixing and the latter, calculated at 24 300 cm-I, leads to the prediction of a “split Soret”. The intensity of the “normal” Soret is spread between four 6E, transitions separated by some 4000
-
-
-
Structure and Spectra of (Porphinato)iron(III) Chloride
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6195
TABLE VII: Sextet A,, CI Occupancies’
e SCF occupation E(2) = 10459 A2(2) = 14066 E(3) = 15385 E(4) = 16383 Al(2) = 19645 Al(3) = 20904 E(5) = 21 078 A2(3) = 22043 E(6) = 24273 A2(4) = 24652 E(7) = 25097 E(8) = 25881 Al(4) = 26296 Al(5) = 26959 E(9) = 28476 A2(5) = 29320 E(10) = 30 130 Al(6) = 30974 E(11) = 31 205 A2(6) = 31 215 E(12) = 31 433 E(13) = 31 698 A2(7) = 31 745 E(14) = 32 197 E(14) = 32277 E(15) = 32542 A2(8) = 32669 Al(7) = 32940 Al(8) = 33 198 A2(9) = 33848 A2(10) = 34923 E(16) = 35 191 Al(9) = 35445 A2(11) = 36460 A2( 12) = 36 537 Al(10) = 36550 E(17) = 37331 Al(11) = 37629 Al(12) = 37917 E( 18) = 37 937 E(19) = 38 168 Al(13) = 38302
2.00
2.00
bl,
2.00
(0.0020) (0.1120) (0.0050) (0.0160)
a2u 2.00
alu XY YZ xz zz 2.00 1.00 1.00 1.00 1.00 -1.00 0.06 -1.00 1 .oo -0.88 -0.02 0.05 -0.08 -0.35 -0.63 0.07 0.04 -0.99 0.99 -1.00 1.00 -0.07 -0.93 0.79 0.02 -1
(0.4630)
.oo
-0.86 -0.11 -1.00
(0.0050) -0.45 -0.28 -0.46 -0.41 (0.3360) -0.25 -0.31 -0.23 (0.0030) -0.02
-0.46 -0.25 -0.39 -0.51 -0.02 -0.26 -0.25 -0.19 -0.02
-0.08 -0.44
0.99 0.97 0.05
0.05 0.04 0.05
0.61 -0.05 0.02
0.06 0.02
.oo
0.89 0.13
0.25 0.02 0.06 0.88 0.03 0.02 0.78 0.40 0.95
0.02
0.21 -0.08
0.02 0.10 0.35 0.02 0.98 0.02
-0.24 -0.27
-0.13
-----
0.17 0.96 0.03 0.94 0.58 0.14 0.02
b2g
d(xx-yy)
0.10 a2,, b2, trip-quartet 0.07 a,, b2g sing-sextet B
-
0.84 0.02 0.14
0.07 0.37 0.38 0.05 0.43 0.22 0.20 0.02 0.12 0.27 0.32 0.07 0.05 0.45 0.36 0.07 0.07 -0.28 0.88 0.02 0.14 0.15 0.42 0.10 0.11 0.02 -0.02 0.24 0.29 0.19 0.19 0.23 0.30 0.02 0.02 -0.26 -0.02 0.98 0.02 0.03 -0.19 0.33 0.27 0.18 0.16 0.02 0.26 0.31 -0.05 0.98 0.02 -0.43 0.82 -0.66 0.10 0.10 0.03 0.02
-0.02
a,,
0.05 a2”
0.06 0.05 0.76
0.06 0.05 0.14 0.05
-0.02 -0.06 -0.08
-0.07 -0.66 -0.43 -0.04 -0.17 -0.29 -0.70 -0.03 -0.94 -0.36 -0.29 -0.05 -0.07 (0.0180) -0.42 -0.49 -0.04 -0.03 -0.05 -0.89 (0.0110) -0.28 -0.14 -0.12 -0.13 -0.02 -0.06
0.09 0.10
0.09 0.82
bl,
trip-sextet AI, d(zz) trip-sextet sing-sextet Q azu d(zz) a,, WY) a,, dCvz) 0.1 1 a,, d(xx-yy) aZu 4 x z ) a2” d(xv) trip-quartet trip-quartet
0.05 0.13
0.02
-0.10 -0.87 (0.8910) -0.63 -0.02 -0.26 -0.05 -0.07 -0.71 (0.0920) -0.09 -0.10 -0.71 -0.03 -0.03 -0.10 -0.84 (1.9270) -0.04 -0.02 -0.35 -0.49 -0.10 -0.40 -0.45 -0.14 (0.5290) -0.06 -0.90 -0.02 (0.0030) -0.99 -0.02 -0.16 -0.79 -0.99 (0.0250) -0.02 -0.96 (2.2500) -0.23 -0.02 -0.30 -0.22 -0.14
a2u
0.93 0.02 0.32 0.56
0.78
1 .oo
e(.*) b2, 0.93
0.89
-0.99 -1.00 -0.89 -0.13 -0.06 -0.12
-1
XX-YY
trip-quartet trip-quartet 0.73 alu bl, trip-quartet
-
1.oo
0.07 0.08 0.05 0.23 0.26
0.95 0.20 0.09 0.04 0.02 0.67 0.06
‘Energy is in cm-I. Numbers in parentheses ae computed oscillator strengths. For states labeled E (the degenerate BI and B2 transitions), the oscillator strengths reported are the sum of the individual components.
-
cm-l and centered at about 3 1 500 cm-I. Other ligand-to-metal charge-transfer excitations are less mixed. These are a,, d,, at 14 000 cm-I, a2” d,, 19 600 cm-I, alu d, at 20 900 cm-I, a,, dxx-yyl22 000 cm-I, a2, d, 24 700 cm-’, and a2” d, 27 000 cm- . We summarize our calculated spectrum of the 6A1, reference state in Table VIII. An observed band at 9700 cm-’ in aquome t m y ~ g l o b i nwas ~ ~ assign to the trip-sextet calculated at 10 400 cm-’ in agreement with the assignment from the calculations of Rawlings, Gouterman, Davidson, and Feller on Fe(III)P(NH3)2.26 Cheng et al. report a similar band centered at 10000 cm-I for high-spin derivatives of methemoglobin and m e t m y ~ l o b i n .This ~~ band is “quite different, however, from that in the low spin cyanide derivatives”, consistent with our assignment of these bands in the d type. low-spin 2Eg complex to d A second trip-sextet at 15 400 cm-’ has calculated oscillator strength, but might be buried under the Q band in metmyoglobin, but could also correspond to the first reported feature of the spectra of Fe(III)(TPP)CL6 We calculate the Q band at 16400 cm-I and the B band distributed among four 6E bands between 28 500 and 21 400 cm-’. Again the center of the B band is calculated some 6000 cm-I to high in energy. Of interest is the prediction of a low-lying a,,(r) d,, charge-transfer band at 14 066 cm-l. Somewhat surprising is that this excitation lies lower in energy than a l u ( r ) d,, a l u ( r ) d, and a l u ( r ) dxx-yyrsuggesting the ligand field ordering d,, < d, d, < dxx-yy.
- -
-
-
-
-
-
-
-
TABLE VIII: A Comparison of the Predicted INDO/CI 6Alr Spectra with ab Initio and Experimental Values (See Also Table VII)“
symmetry INDO/CI ab initiob 6E 10.4 20.2 (0.00) 6B 14.1 6E 15.4 (0.02) 22.7 (0.02) 6E (Q) 16.4 (0.11) 28.2 (0.01) 6A 19.6 (0.01) 6A 20.9 (0.00) 6E (B’) 21.1 (0.02) 36.0 (0.91) 6E (B’) 24.3 (0.46) 36.9 (0.01) 6E 25.1 (0.00) 39:s (0.01) 6E 25.9 (0.00) 40.5 (0.06) 6E (B) 28.5 (0.89) 43.8 (0.10) 6E (B) 30.1 (0.09) 46.6 (0.00) 6E (B) 31.0 (0.00) 6E (B) 31.2 (1.93) 47.6 (5.78) 6E (B) 31.4 (0.53) 6E 32.5 (2.25)
expt 9.7‘ 14.6d 15.8 (x,y)‘ 15.3 18.5 17.2 ( x , y ) 17.4 15.3 (z) 18.7 (x,y) 19.5
20.0 (X,Y) 21.7 (x,yV)
24.1 25.8 27.4
“Energiesare in 1000 cm-’. Numbers in parentheses are computed oscillator strength. bFrom ref 26. ‘From ref 34. dFrom ref 22. e From ref 39, for sperm whale ferrimyoglobin formate.
-
Both the INDO/CI calculations and the ab initio calculations r* band with considerable oscillator strength suggest a A between the Q and B bands, a consequence of the porphyrin*/metal-d, mixing. As the distribution of oscillator strengths among the 6E states is a sensitive function of the details of the
6196 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Edwards et al.
TABLE IX: Calculated Properties for Different States of rPorDhinatoliron(II1)Chloride doublet B2& doublet E, quartet A,, population total spin total spin total spin Fe S 0.279 0.000 0.272 0.000 0.019 0.319 Fe P(x) 0.212 0.000 0.210 0.201 0.012 0.007 0.201 0.012 0.210 0.007 0.212 0.000 Fe P b ) 0.000 0.076 0.000 Fe P(z) 0.08 1 0.1 14 0.018 0.000 0.543 0.000 Fe D(z2) 0.420 1.228 0.730 Fe D(x2-y2) 0.629 0.000 0.517 0.000 0.000 0.619 0.988 1.995 0.000 0.000 1.995 Fe D(xy) 1.008 Fe D ( x z ) 1.986 0.000 1.514 1.419 0.951 1.039 0.951 1.986 0.000 1.514 1.419 1.039 Fe D b z )
Fe total N” S N P(X) N Pb) N P(z) N total
Cl
s
CI P(x)
c1 P b )
c1 P(z)
CI total
6.813 1.649 1.171 1.055 1.527 5.402 2.134 2.01 1 2.01 1 1.633 7.790
0.000 0.000 0.000 0.000 0.000 0.000
0.000 0.000
6.832 1.657 1.186 1.053 1.509 5.405 2.135 2.007 2.007 1.595 7.746
0.002 0.004 0.000
0.006 0.000
0.013 0.013 0.000
6.773 1.646 1.167 1.054 1.542 5.409 2.132 2.000 2.000 1.691 7.826
0.002 0.003 0.000
0.018 0.002 0.010 0.010 0.142
sextet Alg total
0.348 0.231 0.234 0.136 1.313 1.298 1.009 1.049 1.048 6.666 1.670 1.179 1.054 1.530 5.433 2.131 2.000 2.000 1.651 7.784
spin
0.013 0.007 0.007 0.014 0.658 0.684 0.988 0.943 0.944 0.029 0.033 0.000 0.030
0.002 0.010 0.010
0.170
AEq, mm/s
calcd obsdb
3.24
1.19 1.7-2.3
1.27 3.0-3.6
0.1 1 0.4-1.0
‘The nitrogen P(x) orbital is directed toward the iron atom. From ref 40. two calculations, the resulting predictions of intensities for so many near-lying states is somewhat uncertain. Nevertheless, the spectrum of Fe(I1I)TPP does show “split Soret”, with a low-energy component at about 19 500 cm-’, perhaps to be compared with the band calculated by us at 24300 cm-’ and calculated by Rawlings et al. at 36 900 cm-l. We note that the a b initio calculations are in good accord with experiment and our own results if the transition energies of the former are scaled by a factor of 0.5. The calculations of Rawlings et al. used the 6A1, as a reference state and seem to reproduce the 6A1, spectra most accurately. Unfortunately, it is difficult to compare our results with the a b initio results for the other states we calculated.
The Ground State of (Porphinato)iron(III)Chloride Given the geometry and the ligation that we have chosen for our model calculations it is not possible through these calculations to select the lowest energy state from among 6A1, and 4A2,. At the SCF level the 6Al, is calcuated 20 cm-I lower in energy, but this state shows cracked spacial symmetry, suggesting admixtures of other states of near energy. Restoring the symmetry of this state raises the energy 1800 cm-’ above that of the 4A2g,and subsequent CI does not change this relationship, as presented in Table 11. The last column in this table summarizes CI results for all spin states using the molecular orbitals obtained from the 4A20S C F reference function. Although giving a natural bias to the 4A2gstate, the fact that the state of higher multiplicity lies higher in energy strongly suggests that the 4A2gin fact has the lower energy of the two states given the geometric structure of our model compound. This finding is similar to that suggested by Maltempo in his magnetic studies on Chromatiurn ferricytochrome c ’ . ~ O For these studies a spin admixture of 4A,, and 6Al, is proposed to describe the temperature-dependent magnetic moment. A best fit is obtained with A/( = 0.8, suggesting that the 4A2,lies about 250 cm-’ below the 6A1gfor Chromatiurn. This result however is very sensitive to the specific ligation and an analysis on the very similar R . rubrum ferricytochrome c’suggests that the 6Al, lies lower than the 4A2, by about 170 cm-I. Again spin mixing through spin-orbit coupling appears important. The low-spin doublet states are characteristic of stronger ligands and fifth and sixth position ligands. Two states, 2E, and 2B2,, are (40) Maltempo, M. M.; Moss, T. H.; Cusanovich, M. A. Biochim. Biophys. Acta 1974, 342, 290. (41) Sams, J. R.; Tsin, T. B. The Porphyrins; Dolphin, Ed.; Academic: New York. 1976; Vol. I V .
calculated to lie very near in energy. For our assumed structure, the 2E, state is predicted to lie about 1600 cm-I lower than the 2B2,. This result should be relatively insensitive to z-axis coordination, although strong r-bonding ligands might further stabilize the 2E,. Table IX, in addition to presenting a detailed Mulliken population analysis of the lowest lying states, also presents calculated Moessbauer quadrupole splittings, AEq. Only one-center terms are included for this calculation, and a Sternheimer correction (1-R) = 0.92 and a nuclear quadrupole moment of Q = 0.153 are assumed. The calculated value for the 2E, state of 1.9 mm/s is in good accord with the observed values that always lie between 1.7 and 2.3 mm/s$2 suggesting that the 2E, state is lowest doublet under a variety of conditions. The addition of two-center terms may raise the calculated y value by 0.3-0.4 mm/s. The observed value is considerably below the 3.2 mm/s value predicted for the 2B2, ground state. The 6Al, state has a calculated value for AE, of 0.1 mm/s, which when corrected for two-center terms, is in good agreement with experimental values observed, 0.4-0.5 m m / s vs 0.4-1.0 mm/s. Observed quadrupole splittings for reported quartet states of 3.0-3.6 mm/s are difficult to reconcile with our calculated value of 1.3 mm/s for the 4A2,. The 4E, state is estimated to have a negative value for AE,. A similar difficulty in explaining the quartet quadruple observation was also noted by Sams and T ~ i n , ~ ’ in which shortened Fe-N bonds in Fe(TPP)C104 of 1.997 A42were suggested as leading to greater covalency. In all of our calculations we have used an Fe-N bond distance of 2.054 A. In the above discussion we have chosen not to include directly the contribution of the two-center terms to the quadrupole splittings, estimated at 0.3-0.4 mm/s for all cases, since this contribution also contains uncertain empirical parameters. By doing so, the values reported in Table IX for AEq have only the proportionality constant (1 - A)Q = 0.96. An examination of the Mulliken populations in Table IX, obtained by deorthogonalizing the INDO wave function assuming a symmetrically orthogonalized relationship between the ZDO basis and Slater type orbitals, suggests that the net charge on the iron atom increases with increasing spin multiplicity. This seems reasonable considering that higher multiplicities require the occupation of 3d orbitals with greater covalency. In passing from the doublets to the quartet much of this charge flow is absorbed (42) Kastner, M. E.; Scheidt, W . R.; Mashiko, T.; Reed, C. A . J . A m . Chem. SOC.1978, 100, 666.
J . Phys. Chem. 1988, 92, 6197-6202 by the C1 atom through occupation of the d,, orbital, while in passing from the quartet to sextet, the additional charge loss is absorbed by the four porphyrin nitrogen atoms via covalent mixing with the dxx-yyorbital. In passing from the doublets through the quartets to the sextets there is also a systematic increase in 4s population. This increase seems to be reflected in the observed Mossbauer isomer shifts 6 that increases roughly from about 0.17 f 0.07 mm/s for the doublets to about 0.40 f 10 mm/s for the sextet.
Conclusions In this study we examine the ground and excited states properties of (porphinato)iron(III) chloride as a model for ferric porphyrin complexes. For the geometry and chelation we assume, 6A,gand 4A2gstates are predicted to lie very near in energy. As suggested by many others, and as discussed in the text, two states so close in energy could both be thermally populated or they might mix directly through spin-orbit coupling to form Kramers doublets in which only J , is a good quantum number. The nature of these two states, and indeed their mixing, will be a sensitive function of the details of the model assumed. Four spin states of *E, and ZB2gsymmetry are predicted to lie lowest under stronger ligand fields. For our model structure the ZE,state is predicted about 1800 cm-I lower in energy than the ZB2s. The calculated Mossbauer quadrupole splitting for the 2E, state of 1.91 mm/s is in reasonable agreement with the range of values observed for low spin complexes, 1.7-2.3 mm/s. The electronic spectra of (porphinato)iron(III) chloride is calculated assuming ground-state symmetries 6A1,, 4A2g,2E,, and
6197
'B2,. The low-lying excitations are in good agreement with experimental values when they are known. The Q band in the visible is well reproduced. Porphyrin triplet states are predicted to gain intensity through spin coupling through the central metal, and we suggest their role in the spectra observed below the Q band. The 6Al,has considerable mixing between e,(T) and metal d, (d, and dy,) orbitals leading to a predicted intensity pattern among the many allowed E,, excited states that is quite sensitive to details of geometry and calculation. In our calculation, as well as in the a b initio calculations of Rawlings, et a1.26a "split Soret" is predicted for the 6A1,. The B or Soret band is predicted some 6000 cm-' higher in energy than observed. No reliable spectra for a spin 3/2 complex is available but we are not certain these calculations could be used to distinguish the 4A2 and 6Al, spectra. Both suggest low-lying bands about 10400 c m j , weakly allowed bands at about 15 000 cm-', a Q band about 16 500 cm-I and even one component of a split Soret at about 24000 cm-' (see Tables VI and VII). The calculated spectra from the 2E, state are in reasonable agreement with that observed and again suggest the 2E, state is the lowest doublet. Acknowledgment. This work was supported in part through grants from the CRDEC Chemical Systems Technology Research Center at the University of Florida, DAAAl5-85-C-0034, and Eastman Kodak Co. Computer resources were made available through the Office of Naval Research. We would like to thank William Eaton (NIH, Bethesda) for his very incisive comments on this manuscript. Registry No. (Porphinato)iron(III)chloride, 13221-12-0.
Emission of Pyridine and Polypyridine Chromium( I I I ) Complexes in Rigid and Fluid Media Abdulatif M. Ghaith, Leslie S. Forster,* and John V. Rund Department of Chemistry, University of Arizona, Tucson, Arizona 85721 (Received: September 9, 1987; I n Final Form: February 26, 1988)
The emission spectra and lifetimes of C r ( b ~ y ) ~(bpy ~ + = 2,2'-bipyridine), trans-Cr(py),F2+ (py = pyridine), cis-Cr(phen),F2+ (phen = 1,lo-phenanthroline),trans-Cr(py),FBr+,and cis-Cr(bpy),C12+have been recorded in hydroxylic solvents as a function of temperature from 77 K to a point where the solvents become very fluid. The 2E energy is only slightly dependent upon solvent rigidity, but the energy of 'EQ,one of the 2T, components, is depressed when the solventsolute interactions are decreased by solvent motions. The excited-state changes that provide the driving force for the dynamical processes are not alterations in the dipole moment. In favorable cases, it is possible to separate the effects of solvent motions and thermal energy on the decay rates.
Introduction The effect of solvent mobility on emission spectra and lifetimes has been of continuing The emission characteristics of electronically excited species have been measured in fluid solutions and rigid environments. The spectral and lifetime changes associated with the marked reduction in solvent viscosity that characterizes the "melting" of a rigid glass belong to the more general class of emission dependence upon solvent relaxation times. The term rigidochromic has been applied to the abrupt emission spectral changes that occur in the narrow temperature interval where the microviscosity change is very large.2 The microviscosity (1) Barigelletti, F.; Belser, P.; von Zelewsky, A,; Juris, A.; Balzani, V. J . Phys. Chem. 1985, 89, 3680. (2) Wrighton, M.; Morse, D. L. J . Am. Chem. SOC.1978, 96, 998. (3) Gudgin-Templeton, E.; Ware, W. R. J . Phys. Chem. 1984,88, 4626. (4) Strambini, G. B.; Gonnelli, M. Chem. Phys. Lett. 1985, Z1.7, 196. (5) Bagchi, B.; Oxtoby, D. W.; Fleming, G. R. Chem. Phys. 1984.86, 257. (6) Van der Zwan, G.; Hynes, J. T. Chem. Phys. Lett. 1983, 101, 367. (7) Su,S.-G.; Simon, J. D. J . Phys. Chem. 1986, 90, 6475.
0022-3654/88/2092-6197$01.50/0
can vary by 4 orders of magnitude over a 20 OC interval in alcohol/H20 solutions: and any emission property that is sensitive to viscosity will exhibit a sharp change with temperature under these conditions. The dependence of the emission spectra and lifetimes of Cr(II1) complexes upon environment and temperature varies widely. trans-Cr(~yclam)(CN)~+ (cyclam = 1,4,8,1 l-tetraazacyclotetradecane) stands at one extreme-the spectrum and lifetime are nearly the same in rigid and fluid hydroxylic solvents.s In contrast, many Cr(II1) complexes exhibit a precipitous drop in lifetime after the temperature reaches a threshold that depends upon intramolecular and solvent proper tie^.^ Smaller lifetime decreases have been ascribed to increased vibrational anharmonicity in the ground state as the solvent mobility is increased.1° (8) Kane-Maguire, N. A. P.; Crippen, W. S . Inorg. Chem. 1983, 22, 696. (9) Forster, L. S.; Rund, J. V.; Castelli, F.; Adams, P. J. Phys. Chem. 1982, 86, 2395. (10) Fucaloro, A. F. Forster, L. S. Inorg. Chim. Acta 1987, 132, 253.
0 1988 American Chemical Society
