The Journal of
Physical Chemistry
0 Copyright, 1988, by the American Chemical Society
VOLUME 92, NUMBER 14 JULY 14, 1988
LETTERS Collision- Induced Dissociation of Fe,' S. K. h h , f Li Lian,* David A. Hales,+ and P. B. Armentrout*,fs Department of Chemistry, University of California, Berkeley, California 94720, and Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 (Received: April 22, 1988)
In this paper, we present collision-induced-dissociation(CID) studies of gas-phase Fez+. Experiments were performed on a new guided ion beam mass spectrometer designed to produce cold, mass-selected ions. The energy dependence of the cross section for CID with Xe is presented. Interpretation of the cross-section threshold is consistent with theoretical models and gives D0(Fe2+)= 2.72 f 0.07 eV. Combined with the known ionization potentials and electron affinities of Fe and Fe2, this bond energy also provides D0(Fe2)= 1.15 f 0.09 eV and D0(Fe2-) = 1.90 f 0.09 eV. These dimers are discussed with regard to previous work and their respective bonding schemes.
Introduction The iron dimer is one of the most studied transition-metal dimers. In 1969, Lin and Kant used Knudsen cell mass spectrometry to arrive at bond dissociation energies of 0.82 f 0.30 and 1.04 f 0.22 eV by second and third law analyses.' Shim and Gingerich performed similar studies, but used ab initio calculations to aid their third law interpretation. They determined Do(Fe2) = 0.78 f 0.17 eV, but pointed out that a different interpretation of their data can yield a value as high as 1.5 eV.2 Moskovits and Dilella interpreted resonance Raman spectra of Fez in rare gas matrices to yield De = 1.2 eV and later Do = 0.90 f 0.10 eV.4 Such work has also produced spectroscopic constant^.^^^ Complementary theoretical calculations have been -~ photoionization performed on the Fe-Fe b ~ n d i n g . ~ ~ 'Recent studies of Fen have measured a precise ionization potential (IP) of the dimer, IP(Fe2) = 6.30 f 0.01 eV.l0 This value serves to University of California, Berkeley. University of Utah. SNSF Presidential Young Investigator, 1984-1989; Alfred P. Sloan Fellow; Camille and Henry Dreyfus Teacher-Scholar, 1988-1993. f
0022-3654/88/2092-4009$01.50/0
link the ion and neutral bond dissociation energies by the relation Do(Fez+) = Do(Fez) + IP(Fe) - IP(Fe2) (1) where IP(Fe) = 7.87 f 0.06 eV." [Much of the recent literature cites the earlier ionization potential for atomic iron given by C. (1) Lin, S.; Kant, A. J . Phys. Chem. 1969, 73, 2450. (2) Shim, I.; Gingerich, K. A. J . Chem. Phys. 1982, 77, 2490. (3) Moskovits, M.; Dilella, D. P. Meral Bonding and Inreractions in High Temperature Systems; Gole, J. L.,Stwalley, W. C., Eds.; ACS Symposium Series 179; American Chemical Society: Washington, DC, 1982; p 153. (4) Morse, M. D. Chem. Reu. 1986.86, 1049. ( 5 ) Devore, T. C.; Ewing, A.; Franzen, H. F.; Calder, V. Chem. Phys. Lett. 1975, 35, 78. (6) Moskovits, M.; Dilella, D. P. J . Chem. Phys. 1980, 73, 4917. (7) Anderson, A. B. J . Chem. Phys. 1976, 64, 4046. (8) Geunzburger, D.; Baggio Saitovitch, E. M. Phys. Reu. 1981, 24, 2368. (9) Tomonari, M.; Tatewaki, H. J . Chem. Phys. 1988, 88, 1828. (10) Rohlfing, E. A.; Cox, D. M.; Kaldor, A. J . Chem. Phys. 1981, 81, 3846. Cox, D. M.; Whetten, R. L.; Zakin, M. R.; Trevor, D. J.; Reichmann, K. C.; Kaldor, A. Proc. Int. Laser Sci. Conf. 1986,527. These authors assume that the sharp vertical ionization potential measured corresponds to the adiabatic ionization potential. (11) Corliss, C.; Sugar, J. J . Phys. Chem. ReJ Data 1982, I Z , 138.
0 1988 American Chemical Society
4010
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988
E. Moore, IP(Fe) = 7.90 f 0.01 eV.lZ] Do(Fe2+)can then range from -2.3 to -3.1 eV based on the Fe2 neutral bond energies. There have been three independent studies of Fez+. In the earliest study, Jacobson and Freiser (JF) attempted to use collision-induced-dissociation (CID) studies conducted with ion cyclotron resonance mass spectrometry to obtain a bond energy.I3 They concluded that Do(Fez+) lies below Do(Fe+-OH) = 3.17 f 0.13 eV, above DO(Fe+-benzene), and is about equal to Do(Fe+-CH3). Also based on CID studies, J F assigned Do(Fe+benzene) N Do(Fe+-H), but the accepted value for the latter bond energy has been revised from 2.52 f 0.22 to 2.16 f 0.06 eV,14 and the value for DO(Fe+-benzene) has been measured by photodissociation to be 2.38 f 0.22 eV.I5 The value for Do(Fe+-CH3) has also been reevaluated and reduced from 3.0 f 0.2 to 2.51 f 0.10 eV.I6 Thus, the bracketing studies of J F suggest that 3.17 f 0.13 > Do(Fez+) N 2.51 f 0.10 > 2.38 f 0.22 eV. However, these relative values are questionable since they are derived from comparison of CID fragment intensities and neglect dynamic and kinetic effects. Thus, branching ratios do not necessarily correlate directly with thermodynamic stability."J* A good example of this for CID processes is given in a recent study of Nb4+clusters in our 1aborat0ry.I~ In 1986, Brucat et al. performed photofragmentation studies of jet-cooled Fez+ in which the binding energy was investigated by using laser fluence dependence measurements.20 It is found that to dissociate Fe2+, one 2.92-eV photon or two 2.43-eV photons are required. Therefore, they conclude that the bond dissociation energy of the ion must be between these limits. In 1987, Hettich and Freiser photodissociated Fez+ formed by reaction of Fe+ with Fe(CO)5 followed by removal of the CO ligands by CID.Z' While these ions may not be internally cold, the photodissociation threshold was found to be 2.69 f 0.22 eV, within the limits of the previous studies. In this work, we present direct measurements of the energy dependence of Fez+ CID. With three important caveats, such experiments have been shown to be an accurate means of obtaining bond dissociation energies.2z First, activation barriers or kinetic shifts must not be present. Generally, activation barriers should not be a problem in an ion-molecule reaction without spin or orbit restriction^.^^ Also, kinetic shifts are not expected for a species with few internal modes such as a dimer. Second, to obtain meaningful thermochemistry, the quantum states of the reactants and products should be known. For instance, cold and internally hot dimer ions (Mnz+ and Co2+) have quantitatively different fragmentation and reactive behaviors.24 As a practical matter, this problem can be experimentally addressed by using cold reactants, though some assumption about the state of the products at threshold must still be made. The third concern, discussed below, is how to interpret the energy dependence of CID cross sections. Experimental Section Experiments were conducted on a new guided ion beam mass spectrometer, designed to measure the energy dependence of cold (12) Moore, C. E. Natl. Bur. Stand. Circ. (US'.)1949, 467. (13) Jacobson, D. B.; Freiser, B. S . J . Am. Chem. SOC.1984, 106, 4623. (14) Elkind, J. L.; Armentrout, P. B. J. Am. Chem. SOC.1986,108, 2765. (15) Hettich, R. L.; Jackson, T. C.; Stanko, E. M.; Freiser, B. S. J. Am. Chem. SOC.1986, 108, 5087. (16) Schultz, R. H.; Elkind, J. L.; Armentrout, P. B. J . Am. Chem. SOC. 1988, 110, 411. (17) Elkind, J. L.; Armentrout, P. B. J. Phys. Chem. 1985,89, 5626; Ibid. 1987. ~. 91. 2037. ( 1 8 ) - A ~ s ~ oN.; v , Armentrout, P. B. J . Phys. Chem. 1987, 91, 6178. (19) Loh, S. K.; Lian, L.; Hales, D. A.; Armentrout, P. B. J . Chem. Phys., accepted for publication. (20) Brucat, P. J.: Zheng, L.-S.; Pettiette, C. L.; Yang, S.; Smalley, R. E. J. Chem. Phys. 1986, 84, 3078. (21) Hettich, R. L.; Freiser, B. S. J. Am. Chem. SOC.1987, 109, 3537. (22) Aristov, N.; Armentrout, P. B. J . Phys. Chem. 1986, 90, 5135. (23) Talrose, V. L.; Vinogradov, P. S.; Larin, I. K. Gas Phase Ion Chemistry;Bowers, M. T., Ed.; Academic: New York, 1979; Vol. I , p 305. (24) Ervin, K.; Loh. S. K.; Aristov, N.; Armentrout. P. B. J . Phvs. Chem. 1983,87, 3593. Armentrout, P. B. StructurelReactioity and Thehnochemistry of Ions, Ausloos, P., Lias, S.G., Eds.; Reidel: Dordrecht, 1987; p 97. Hales, D. A.; Armentrout, P. B., work in progress.
ENERGY (RV. Lob)
1
*
. a
" c r w. LYU Figure 1. Collision-induced dissociation of FezCby Xe. T h e CID cross section is plotted as a function of kinetic energy in the center-of-mass frame (lower x axis) and the laboratory energy (upper x axis).
metal cluster reactions. The cluster ion source has been discussed.25 A more complete description of the apparatus and source modifications will be published shortly.26 Fez+ is formed from laser vaporization and condensation in a continuous flow of He. An external ionizer, which can energize internal modes of the dimer ion, is not used. Ions undergo > l o 5 thermalizing collisions and a mild supersonic expansion, further cooling internal and translational modes. After being skimmed, ions are accelerated through a 7.6-cm-long, differentially pumped region (1 X Torr). To prevent any collisional reheating, voltages are generally kept below -8 V in this region; however, voltages up to 300 V are not observed to change the data quantitatively. In contrast, extraction potentials applied during the expansion do appear to cause extra collisions. As a result, the high-pressure source region is kept field-free. Fez+ is mass-selected by a magnetic sector and then injected into an octopole ion beam guide, which passes through a gas cell. The octopole allows accurate energy measurement and ensures efficient product collection.z6 Fez+ interacts with Xe at pressures (-0.1 mTorr) that are low enough to ensure single collision conditions. Of the rare gases, Xe has been found to be the most effective for CID.z2 Product ions are then identified by a quadrupole mass filter, which is set to a low resolution ( m / A m = 2.1) to minimize mass discrimination effects. A Daly detector, consisting of a high-voltage conversion dynode, scintillator, and photomultiplier, serves to count individual ions. Raw ion intensities are converted to absolute reaction cross sections,26with a reproducibility of 25%. Absolute uncertainties are f 2 0 % assuming complete ion collection. Using the octopole as a retarding energy analyzer, ion translational energies are measured to an accuracy of f0.05 eV, lab frame (f0.035 eV, center-of-mass frame). Results
The energy dependence of Fe2+ + Xe is shown in Figure 1. From an apparent threshold of >2 eV, the cross section rises and levels off by -10 eV with a maximum of -3 A2. At higher energies, the cross section remains constant, indicating efficient product collection. These results have been replicated over a period of several months and six independent data sets. (25) Loh, S . K.; Hales, D. A,; Armentrout, P. B. Chem. Phys. Lett. 1986, 129, 527. (26) With the exception of the source, a similar apparatus and the means of data interpretation are discussed in Ervin, K. M.; Armentrout, P. B. J . Chem. Phys. 1985.83, 166.
The Journal of Physical Chemistry, Vol. 92, No. 14, 1988 4011
Letters M R G Y (eV. Lab)
is toward the upper end of these broad limits is evidence that our source produces Fe2+ without excessive internal excitation. To unambiguously identify this CID threshold with the bond energy for Fe2+,we need to exclude the possibility of activation barriers. Barriers could arise if the ground-state dimer dissociates to excited-state atomic species. To consider this, we need to know the electronic structure of Fe2+. The likely ground-state configurations for Fe2+ are ( 4 ~ u , ) ~ 3 dand ' ~ possibly (4su,)'3dI4. The lowest energy diabatic dissociation pathways for these states are Fe+*(4F,3d7) Fe(5D,4s23d6)and Fe+*(4F,3d7)+ Fe*(5F,4s3d7), respectively. These dissociation limits (for the lowest energy J levels) lie a t 0.23 and 1.09 eV," respectively, compared to ground-state products, Fe+(6D,4s3d6) + Fe(SD,4s23d6). Thus, if dissociation occurs diabatically, the true value of D0(Fe2+)would be 2.49 or 1.63 eV, respectively. However, these diabatic dissociation surfaces must cross those evolving from ground-state Fe+ + Fe. These surfaces will probably undergo avoided crossings and other couplings such that dissociation of Fe2+should occur adiabatically. Since all these surfaces should be attractive at long range due to the ion-induced-dipole potential, we anticipate no barrier to dissociation along this adiabatic surface. Thus, we identify Do(Fe2+) = 2.72 f 0.07 eV. With this value and eq 1, we derive D0(Fe2) = 1.15 f 0.09 eV, toward the upper end of the broad range of values previously determined for Fez. These relative bond strengths have been rationalized by Rohlfing et al.1° by using promotion energy arguments. Specifically, they note that, in order to form Fez with a calculated ground-state configuration of ( 4 ~ ~ , ) ~ 3 d both '~,~*'~ atoms must be promoted from ground-state Fe(5D,4s23d6)to Fe*(5F,4s3d7),0.86 eV higher per atom." In contrast, Morse4 has noted that Fez+ with a (4suJ23d13 configuration can be formed from Fe+(6D,4s3d6)+ Fe*(5F,4s3d7),such that the promotion energy is half that of the neutral. The situation is actually more favorable than this since, as noted above, this ionic configuration can also be formed from Fe+*(4F,3d7) Fe(5D,4s23d6),with a promotion energy of only 0.23 eV. Thus, the diabatic dissociation energy (DDE) of Fe2+ is 2.72 0.23 = 2.95 eV, similar to that for Fe2, 1.15 + 0.86 + 0.86 = 2.87 eV. This agreement suggests that the bonding in the cation and neutral dimer is similar, consistent with the sharp ionization threshold observed by Rohlfing et a1.I0 We can take this promotion energy model one step further by considering the anion. Based on our neutral bond energy and the electron affinities of the monomer and dimer of Fe,30 we find D0(Fe2-) = 1.90 f 0.09 eV. If the electron configuration of Fey is ( 4 s ~ , ) ~ ( 4 s u , ) ' 3 d ~then ~ , the anion diabatically dissociates to Fe-(4F,4s23d7)+ Fe*(5F,4s3d7)with a promotion energy of 0.86 eV. Thus, DDE(Fel) = 1.90 0.86 = 2.76 eV, in good agreement with those for Fez and Fe2+. Again this means that the bonding in Fe2- is similar to that in Fe2 and Fe2+,suggesting that the 4su, orbital is only mildly antibonding. This can achieved by mixing substantial 4p character into the 4su, orbital, as indicated by calculations of Leopold et aL31 If the 3d electrons in the ( 4 ~ a , ) ~ 3 dconfiguration '~ of Fez interact weakly, then calculations indicate a large number of lowlying electronic states? This seems inconsistent with the extremely simple photoelectron spectrum observed for To explain this observation, Leopold et al. suggest that the bonding in Fez is ( 4 s ~ , ) ~ ( 4 s u , * ) ' 3 dand ~ ~ in Fez- is (4sa )2(4su,*)23d'3, where the 4su, orbital is only mildly antibonding.K These configurations correspond to diabatic dissociation to Fe(5D,4s23d6) Fe*(5F,4s3d7) and Fe(5D,4s23d6) Fe-(4F,4s23d7),with promotion energies of 0.86 and 0.0 eV, respectively. Therefore, DDE(Fe,) = 1.15 0.86 = 2.01 eV and DDE(Fel) = 1.90 eV, considerably less than DDE(Fe2+) = 2.95 eV. Removal of the 4su, electron cannot account for this large increase in the cation DDE since this orbital is only mildly antibonding. Neither can this increase be explained by an ion-induced-dipole attraction, since this effect
+
o+.&, 1.0
2.0
3.0
4.0
5.0
, I ,
I
6.0
7.0
, B.0
,I 9.0
EhEKGv fev, CUI
Figure 2. Threshold energy dependence of Fez+collision-induced dissociation. Points represent an average of six independent data sets. The dashed line is a model discussed in the text. The solid line represents the model convoluted with the experimental energy distribution. The threshold is indicated by the arrow.
Several models for the threshold energy dependences of atomdiatom CID cross sections have been developed. These generally predict a form u(E) = ao(E - Eo)"/E
+
(2)
where E = translational internal energy and Eo = threshold energy or reaction endoergicity. By using a statistical approach, Rebick and Levine find that n should be 1.5 and -2 for indirect and direct CID processes, r e s p e c t i ~ e l y . ~In~ a simplistic limit, their formulation reduces to that of Levine and Bernstein, who use a simple optical analysis to find n = 2.5 for a direct process.28 Chesnavich and Bowers29have also proposed a model that leads to n = 1.5 for direct reactions. This is based on transition-state theory with the proviso that only translational energy is considered to contribute to the reaction coordinate. We model the cross section with eq 2, where E is the relative translational energy and uo, n, and Eo are independent variables. Essentially, a,, acts as a scaling factor. A trial model is convoluted with the energy distribution of the ion beam (-0.27 eV, center-of-mass frame) and with the thermal motion of the Xe, then fit to the data.26 By use of a least-squares analysis, the model is optimized. We find that the best fit to the data is produced by a model with uo = 2.0 f 0.4 A2, n = 1.42 f 0.09, and Eo = 2.72 f 0.07 eV. Unconvoluted and convoluted fits of this form are shown in Figure 2 as the dashed and solid lines, respectively. The uncertainties (one standard deviation) reflect the uncertainty in the energy measurement and variations among the closely related forms of the model that fit the various data sets. Note that the value of n is in good agreement with theoretical predictions.
-
Discussion The endoergicity (Eo) represents a rigorous upper limit to D0(Fe2+), since there should be little internal excitation in the Fe2+which would serve to lower the threshold. In the absence of activation barriers, the threshold equals the bond dissociation energy, D0(Fe2+) = 2.72 f 0.07 eV. This value is in good agreement with Hettich and Freiser's photodissociation result, 2.69 f 0.22 eV,21and is within the limits set by Brucat et al. and JF and those derived from the Fez results. The fact that our value (27) Rebick, C.; Levine, R. D. J. Chem. Phys. 1973, 58, 3942. (28) Levine, R. D.; Bernstein, R. B. Chem. Phys. L e f f .1971, Zl, 552. (29) Chesnavich, W . J.; Bowers, M. T. J . Phys. Chem. 1979, 83, 900.
+
+
+
+
+
+
(30)Leopold, D. G.; Lineberger, W. C. J . Chem. Phys. 1986, 85, 51. (31) Leopold, D. G.;Almlof, J.; Lineberger, W. C.; Taylor, P.R.J . Chem. Phys. 1988, 88, 3780.
J . Phys. Chem. 1988, 92, 4012-4015
4012
should have a similar magnitude in the anion, inconsistent with the similar DDEs of Fe2 and Fez-. Thus, this bonding model appears inconsistent with simple promotion energy arguments, while such arguments can provide a consistent explanation of the relative bond energies of Fe2+,Fez, and Fez- in the first bonding model presented above. This model also can explain the sparse photoelectron spectrum of Fez-, if it incorporates bonding interactions between the 3d electrons, as originally suggested by Leopold and Linebergere30 Further evidence for such bonding comes from a comparison of the iron DDEs with those of Do(Ni2) = 2.07 f 0.01 eV and Do(Cu2)= 2.01 f 0.08 eV.4 These molecules are generally agreed
to have ( 4 s ~ bonds ~ ) ~ with weakly interacting 3d cores. Their diabatic dissociations lead to promotion energies of 0.06 and 0.0 eV, respectively, such that 2 eV is a reasonable estimate for the DDE of a ( 4 s ~ bond. ~ ) ~ Since the iron DDEs are -40% stronger than this, the iron dimers appear to involve more than a simple ( 4 s ~ bond. ~ ) ~ They presumably must include extensive d-d bonding as well. Acknowledgment. This work is supported by the Army Research Office, DAAL03-87-2211, and a National Science Foundation Graduate Fellowship (D.A.H.). We thank Profs. Michael Morse and Jack Simons for several useful discussions.
Structured Hole-Burned Spectra of Reaction Centers of Rhodopseudomonas virids D. Tang, R. Jankowiak, J. K. Gillie,+G . J. Small,* Ames Laboratory-USDOE
and Department of Chemistry, Iowa State University, Ames, Iowa 5001 1
and D. M. Tiede Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received: April 5, 1988)
Structured hole-burned spectra for P960 of Rps. viridis are reported which, for appropriate burn wavelengths, exhibit four holes (including a zero-phonon hole). The data indicate that two electronic states contribute significantly to P960 and suggest that the primary electron-transfer step should be modeled in terms of coupled adiabatic trimer states.
The crystal structure determination of the reaction centers (RC) from the photosynthetic bacteria Rps. uiridisl-' and R b . sphaeroides4s5have stimulated an even greater activity directed toward understanding the primary charge separation step of the RC. Excellent reviews of our current understanding of structure-primary photochemistry relationships have recently been given by Kirmaier and Holten6 and Budil and co-workers.' In the structures, two pigment branches (related by an approximate C, rotation axis) extend from the special bacteriochlorophyll pair or dimer (P), each containing a BChl monomer (B) followed by a bacteriopheophytin monomer (H) and a quinone molecule (Q). The excited singlet state that gives rise to the 870- and 960-nm absorption bands of Rb. sphaeroides and Rps. viridis, respectively, is the primary electron donor state, which is generally assigned as P*. Two questions of current interest are, what is the electronic structure of P*6-8and why does electron transfer from P* occur . ~is* convenient ~ to despredominantly along the L b r a n ~ h ? ~ It ignate the ground state of L branch as IPBH), while realizing that all pigments must be taken into account for a proper description of the ground and excited states of the RC. Another important question pertains to the role of intermediation for B in the formation of the charge-separated state IP'BH-) *. Two possibilities, which have been considered and debated, are that IP+B-H)*'O serves as a real intermediate statel' or as a virtual state in a superexchange mechanism.12 Recent 100-fs-resolution experiments have produced no evidence for the bleaching of the Qq transition of B in either Rps. viridis or Rb. sphaer~ides.'~-''Thus, it was concluded that if IP+B-H)* is a real intermediate state, i t must undergo decay in a time substantially shorter than 100 'William E. Catron Research Fellow.
0022-3654/88/2092-4012$01.50/0
fs.'"17 The same conclusion would hold for IPB+H-)*, which has also been suggested as a real intermediate state.I8 The time (1) Deisenhofer, J.; Epp, 0.;Miki, K.; Huber, R.; Michel, H. J . Mol B i d . 1984, 180, 385.
(2) Deisenhofer, J.; Epp, 0.; Miki, K.; Huber, R.; Michel, H . Nature (London) 1985, 318, 618. (3) Michel, H.; Epp, 0.;Deisenhofer, J. EMBO J . 1986, 5, 2445. (4) Allen, J. P.; Feher, G.; Yeates, T. 0.; Rees, D. C.; Deisenhofer, J.; Michel, H.; Huber, R. Proc. Natl. Acad. Sci. U.S.A. 1986, 83, 8589. (5) Chang, C. H.; Tiede, D.; Tang, J.; Smith, U.; Norris, J.; Schiffer, M. FEBS Lett. 1986, 205, 82. (6) Kirmaier, C.; Holten, D. Photosynth. Res. 1987, 13, 225. (7)Budil, D. E.; Gast, P.; Chang, Ch. H.; Schiffer, M.; Norris, J . Annu. Reo. Phys. Chem. 1987, 38, 561. (8) See: Warshel, A,; Parson, W. W. J . Am. Chem. SOC.1987,109,6143. Scherer, P. 0. J.; Fisher, S. F. Biochim. Biophys. Acta 1987, 891, 157, and references therein. (9) See: Michel-Beyerle, M. E.: Plato, M.; Deisenhofer, J.; Michel. H.; Bixon, M.; Jortner, J. Biochim. Biophys. Acta 1988, 932, 52, and references therein. (10) We use the customary diabatic description for electron transfer until later in text. (11) Marcus, R. A. Chem. Phys. Lett. 1987, 133, 471. (12) Bixon, M.; Jortner, J.; Michel-Beyerle, M. E.; Ogrodnik, A,; Lersch, W. Chem. Phys. Lett. 1987, 140, 626. (13) Martin, J. L.; Breton, J.; Hoff, A. J.; Migus, A,; Antonetti, A. Proc. Natl. Acad. Sei. W.S.A. 1986, 839 957. (14) Breton, J.; Martin, J. L.; Antonetti, A,; Orszag, A. Proc. Natl. Acad. Sci. W.S.A. 1986, 83, 5121. (15) Wasielewski, M. R.; Tiede, D. M. FEBS Lett. 1986, 204, 368. (16) Breton, J.; Martin, J. L.; Fleming, G. R.; Lambry, J. C. Biochemistry, submitted. (17) Fleming, G. R.; Martin, J . L.; Breton, J. In Photosynthetic Bacterial Reaction Centers, Structure and Dynamics; Breton, J., Berneglio, A,, Eds.; NATO AS1 Series; Plenum: New York, in press. Fleming, G. R.; Martin, J. L.: Breton, J. Nature (London) 1988, 333, 190.
0 1988 American Chemical Society
