J. Phys. Chem. 1994, 98, 6114-6117
6114
Electron Delocalization in Helical Bis(quinone) Anion Radicals Andrew L. Sargent,+Jan Almliif,'*+***~ and Charles A. Liberko* Department of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455, Army High Performance Computing Research Center, Minneapolis, Minnesota 5541 5, and Minnesota Supercomputer Institute, Minneapolis, Minnesota 5541 5 Received: February 16, 1994; In Final Form: April 6, 1994"
Ab initio quantum chemical calculations were employed to determine the extent of charge localization in the anion radicals of helical bis(quinones). Recent experimental evidence has suggested that the electronic charge in these systems is delocalized, in contrast to that in their linear analogs. The optimized molecular geometries of the neutral and radical anion five- and seven-ring bis(quinones) reveal unusual supershort nonbonded contacts between the outermost rings and large positive electron affinities. The regular helicenes, in contrast, lack the short contacts and have no electron affinities at the SCF level of theory. Of central importance to these short contacts is the interaction between the 27r* orbitals of the carbonyls on the inner edge of the helicene, characteristic only of the five-ring complex, or, for the larger systems, between the 7r systems of the overlapping rings at the opposite end of the helicene, of which the inner carbonyl 27r* orbitals are principal components. The theoretical results suggest that the electronic charge in the helical bis(quinone) anion radicals is best described as delocalized.
Introduction Previous studies have shown that the electroniccharge in anion radicals of linear semiquinones 3 and 4 is localized,' whereas that for the anion radicals of the smaller semiquinones, 1 and 2, is
2
1
charge is delocalized,in contrast to its localized linear analogue! (The structure of 5 has been distorted in the diagram in order to show the connectivity clearly, without attempting to display the actual three-dimensional structure.) The spectroscopic properties of the larger six-, seven- and eight-ring helical bis(quinones) also indicatesimilardelocalized electronic str~ctures.3~~ It has been suggested that the proximity of the two quinone groups at the ends of the helix allows electrons to move between them, implying that charge delocalization in the bis(quinone) systems is a through-space, rather than a through-bond, effect. In other words, it is not the number of bonds that separate the quinone functionalities but rather their spatial separation which affects charge delocalization across these groups. The goal of the present theoretical study is to investigate the geometric and electronic structure of the helical bis(quinone) anion radicals.
Methods 3
4
delocalized.* The localized nature of the electronic structure of complexes 3 and 4 is unusual, given the conjugation of the u electrons,and is ascribed to the polarizability of the species which allows small perturbations in the geometry or solvent to trap the electronic charge at one end of the molecule.lS The synthesis of the helical analog of 4 was recently r e p ~ r t e d . ~ Interestingly, the optical and ESR spectra of the anion radical of the helical five-ring system, 9-, indicate that the electronic
LJyJ II
Geometries of the neutral and anion radical bis(quinone) systems were optimized with full gradient' methods at the restricted Hartree-Fock-Roothaan6 and restricted open-shell Hartree-Fock levels of theory. Effects of electron correlation were estimated with MP2 calculations at fixed, SCF-optimized geometries. For the radical anions, the MP2 approach used was the spin-restricted (ROMP2) version. The basis sets for the firstand second-row atoms were the 3s and 6s3p sets, respectively, of van Duijneveldt*with a general split valence contraction. These basis sets would not be considered reliable for even a qualitative assessment of theelectron affinities for a small system. However, the low-lying radical anion states for these polycyclic aromatic systems are all valence-like, and there is substantial evidence that the basis set problem is less severe for these systems. All calculations were performed on the Cray-XMP and Connection Machine-5 computers at the Minnesota Supercomputer Center with the direct SCF code DISC0.9 Results
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5 AHPCRC. University of Minnesota. 8 MSI. e Abstract published in Aduance ACS Abstracts, May 15, 1994. t
0022-3654/94/2098-6114$04.50/0
The optimized molecular geometries of the neutral and radical anion bis(quinones), 5 and 9-, are shown in Figure 1. The calculated distance between the inner carbonyl groups on opposite ends of complex 5 is 2.78 A, while that for 5'- is 2.51 A. Accordingly, the addition of an unpaired electron to the neutral complex apparently serves to draw together the overlapping 0 1994 American Chemical Society
Helical Bis(quinone) Anion Radicals
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6115
Figure 2. Optimized molecular geometry of a hypothetical neutral fivering planar bis(quinone) complex.
side-on, as illustrated in 7, whereas in the helical systems it is Figure 1. Optimized molecular geometries of the (a, top) neutral and (b, bottom) radical anion five-ring bis(quinone) complexes.
portions of the molecule's extremities. From a physical standpoint, the strong overlap of the inner carbonyl 2u* orbitals, 6, in the 7
6
LUMO of the neutral complex (an orbital of B symmetry under the C2 molecular point group) appears to be the driving force for this attraction. The computed, nonadiabatic electron affinity (EA) is 10.5 kcal/mol, at the SCF level of theory, while the adiabatic EA is 16.8 kcal/mol. Given the well-known fact that EA's are systematically underestimated by the Hartree-Fock model, and probably also by the basis set used, both these values are unusually large and underscore the stability of the radical anion species. The short nonbonded contact between the carbonyl groups, 2.51 A, supports the hypothesis that the ends of the quinone interact with each other. However, the calculation was run under the C2 molecular point group which predisposes that its solution will represent the delocalized electronic charge. Relaxing the symmetry constraint and reoptimizing the wave function at the fixed C2 geometry resulted in no significant change in the energy or wave function. Reoptimizing the geometry of 5" without any symmetry constraints yielded a localized electronicstructure with C1geometry, lower in energy than the C2 structure by 7.3 kcal/ mol, and with the singly occupied molecular orbital localized on one end of the helix. In an attempt to confirm these results at a higher level of theory, second-order Msller-Plesset (MP2) energy calculations were performed at the optimized SCF geometries in C1 and C2 symmetries. At this level of theory, the complex of C2 symmetry was lower in energy by 0.9 kcal/mol, suggesting that the electronic structure of 5'- is best described as delocalized but also that this preference is quite marginal and could easily be reversed at an even higher level of theory. The important result from this study is that the two are essentially isoenergetic, and as long as that is the case, it is largely irrelevant which symmetry is preferred to any given level of theory. An asymmetric perturbation, such as one due to intermolecular or solvent interactions, could easily result in the stabilization of the species which has the localized electronic charge, as a consequence of this near-degeneracy. While the delocalized electronic charge of the helical bis(quinone) radical anion supports the through-space interaction theory, it is important to note that the orbital interactions are different in going from the linear to the helical systems. In the linear systems, the assumed through-space orbital interaction is
end-on, as in 6. This difference makes it difficult to draw conclusions regarding the correlation between charge delocalization and the distance separating the carbonyl functionalities, as it relates to the linear systems. In an attempt to shed additional light on this problem, we have investigated the hypothetical bis(quinone) system 8, which retains the original side-on orbital
o@?&ro 8
interaction of the linear systems but also reduces the distance between the quinone groups. The optimized geometry of the neutral five-ringplanar complex is shown in Figure 2. As in the smallest linear bis(quinone), the distance separating theoxygens on the end rings is 3.39 A. Under the Cb molecular point group symmetry, the nonadiabatic and adiabatic EA's are 13.1 and 17.2 kcal/mol for the 2B2 state. Similar to the C2 symmetry specification in the five-ring helical system, the Cb specificationpredisposes that the electronic charge of the radical anion will be delocalized. SCF calculations run under the Cspoint group, where the plane of symmetry coincides with the plane of the molecule, result in the electronic charge localizing on one end of the molecule. However, while this 2Aff state is 5.1 kcal/mol lower in energy than the 2B2state, the MP2 energies at the optimized SCF geometries of the radical anions favor for the delocalized (Cb)structure by 3.2 kcal/mol. These results, along with those from the fivering helical system, support the hypothesis that the extent of charge (de)localization in the bis(quinone) systems is influenced by the through-space, rather than the through-bond, interactions. The overlap of the carbonyl 2u* orbitals at the ends of the helix, illustrated in 6 and characteristic of the five-ring complex, is not a prerequisite for electron delocalization in helicene bis(quinone) systems. In the larger seven-ring system, the carbonyl groups are not oriented directly on top of each other, as can be seen in Figure 3, yet this complex, as well as the six- and eightring complexes, exhibits the visible, near-IR, and ESR spectra characteristic of systems with a delocalized electronic charge. Our studies of the optimized molecular geometries of the sevenring system yield results similar to that of the five-ring system. In the structure of the 2A state of the radical anion, the ends of the helix are approximately 0.2 A closer than they are in the structure of the neutral complex, as shown in Figure 4. The nonadiabatic and adiabatic EA's are 6.2 and 1 1.6 kcal/mol, which
Sargent et al.
6116 The Journal of Physical Chemistry, Vol. 98, No. 24, 1994
Figure 5. Optimized molecular geometry of the neutral seven-ring helicene.
Figure 3. Perspective orientations of the carbonyl groups in the (top) five- and (bottom) seven-ring helical bis(quinone) systems. The views are perpendicular to the principal C2 axis.
interactions of the CO and CH fragments with the rest of the helicene, only the former has low-energy virtual orbitals (the 2 ~ * that ) can combine with the end-ring 'K molecular orbital to yield a very-low-energy virtual molecular orbital. The carbon 2p .K virtual orbital of the CH fragment, on the other hand, is very high in energy and does not yield a low-lying virtual molecular orbita1,whencombined with the corresponding MO of the helicene fragment. The experimental study of the infrared spectroscopy of the helical bis(quinones) raises morequestions than it answers." The spectra of the neutral five-ring complex reveals a broad absorption peak in the carbonyl stretching region of 1680 cm-1. Upon reduction, this peak sharpens considerably but remains centered at about the same frequency. Our calculations of these species suggest that the frequency shift for the inner carbonyls is larger than that for the outer carbonyls by approximately 40 cm-l.12 Considering the trans-annular interaction of which the former groups partake and ofwhich the latter groups do not, this disparity in the frequency shifts is understandable. The broad band observed for the neutral complex is likely to be the superposition of two or more bands, specifically those for the inner and outer carbonyls, the former of which shifts to a lower frequency upon reduction. What remains is the stretching band for the outer carbonyl groups. Conclusions
Figure 4. Optimized molecular geometries of the (a, top) neutral and (b, bottom) radical anion seven-ring bis(quinone) complexes.
emphasize the stability of the radical anion species, although to a slightly lesser extent than in the five-ring system. A substantial overlap between the carbon 2p .K orbitals of the outermost rings in the SOMO of the radical anion serves as the driving force for the attractive interaction. Experimental studies of the regular helicenes have suggested that the overlap between the outermost rings, referred to as transannular overlap, is purely repulsive.*O Our theoretical analysis of the neutral seven-ring helicene corroborates this conclusion. In the optimized molecular structure, shown in Figure 5 , the distance between carbon atoms on the outer edge of the overlapping rings is 0.66 A longer than the corresponding distance in the neutral seven-ring bis(quinone) complex. Furthermore, no electron affinity was obtained for this system at the SCF level of theory. The open-shell eigenvector, for both the 2B and 2A states under the C, point group, contains almost no carbon 2p u character on the overlapping rings. Combined, these results underscore the importance of the presence of carbonyl groups on the end rings of the helicene complexes to the trans-annular electron delocalization. From the standpoint of molecular orbital
The electronic charge in helical bis(quinone) anion radicals is best described as delocalized, although easily localizable, in contrast to that of their linear analogues, which are localized even in the absence of external perturbations. Our theoretical investigation has revealed that theoptimized molecular geometries of the five- and seven-ring systems exhibit unusually short contacts between the outermost rings of the helicene. In addition, these complexes have large positive electron affinities. The regular helicenes, which lack the carbonyl groups, do not exhibit the short contacts between the outermost rings and have no electron affinities at the SCF level of theory. The low-energy virtual orbitals of the carbonyl fragments manifest themselves as the principal componentsof the low-energy virtual molecular orbitals of the neutral helical bis(quinone) system, thereby leading to the positive electron affinities. Acknowledgment. This project grew out of discussions with Prof. L. Miller, and helpful suggestions from him are gratefully acknowledged. The work was supported in part by a contract between the Army Research Office and the University of Minnesota for the Army High Performance Computing Research Center. Additional support was furnished by the National Science Foundation, Grants CHE-8915629 and CHE-9223782, and by the Minnesota Supercomputer Institute.
References and Notes (1) (a) Jozefiak, T.H.;AlmlBf, J. E.;Feyereisen, M. W.; Miller, L. L.
J . Am. Chem. Soc. 1989, 111,4105. (b) AlmlBf, J. E.;Feyereisen, M. W.; Jozefiak, T. H.;Miller, L. L. J . Am. Chem. Soc. 1990, 112, 1206. (2) Jozefiak, T. H.;Miller, L. L. J . Am. Chem. Soc. 1987,109,6560.
Helical Bis(quinone) Anion Radicals (3) Yang, B.; Liu, L.; Katz, T.J.; Liberko, C. A.; Miller, L. L. J. Am. Chem. SOC.1991, 113, 8993. (4) Liberko, C. A.; Miller, L. L.; Katz, T.J.; Liu, L. J. Am. Chem. Soc. 1993, 115, 2478. (5) Pulay, P. Mol. Phys. 1969, 17, 191. (6) Roothaan, C. C. J. Rev. Mod. Phys. 1951,23, 69. (7) McWeeny, R.; Dierksen, G. J. Chem. Phys. 1968,49, 4852. (8) Van Duijneveldt, F. B. IBM Res. Rep. 1971, RJ 945. (9) DISCO is a direct ab initio electronic structure code written by J. AlmlBf, K. Faegri, Jr., M.W. Feyereisen, T.Fischer, K. Korsell, and H.P.
The Journal of Physical Chemistry, Vol. 98, No. 24, 1994 6117 Liithi. See also: Almltif, J.; Faegri, K.; Konell, K. J. Comput. Chem. 1983, 3, 3003. Almltif, J.; Sargent, A.; Feyereisen, M. W. SIAM News 1993, 26 (I), 14. (10) (a) Fey, H.J.; Kurreck, H.;Lubitz, W. Tetrahedron 1979,35,905. (b) Alcndorfer, R. D.; Chang, R. 1.M a p . Reson. 1971,5,273. (c) Weissman, S. L;Chang,R. J.Am.Chem.Soc. 1972,94,8683. (d)Obenland,S.;Schmidt, W. J. Am. Chem. Soc. 1975, 97,6633. (11) Liberko, C. A.; Miller, L. L. Unpublished results. (12) The calculated frequency shifts were roughly 90 and 130 cm-* for the outer and inner carbonyl groups, respectively.
