A. Imamura, R. Hoffmann
Sep 1, 1968
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Influential Citations
115
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Journal
Journal of the American Chemical Society
Abstract
The electronic structure of biphenyl, fulvalene, and related molecules in ground and excited states is studied. To a r-electron SCF and SCF-CI calculation we couple an evaluation of the H-H repulsion to estimate torsional potential energy curves in ground and excited states. The changes in conformational preferences in excited states are easily predicted from a simple correlation diagram connecting planar and twisted molecules. Thus for two coupled q 7r-electron systems one expects planar ground states, possibly twisted excited states, for q = 4n + 1 or 4n + 3 ; planar ground or excited states, possibly triplet or quintet ground states, for q = 4n; possibly twisted ground states and planar excited states for q = 4n + 2. he molecule of biphenyl is planar or nearly so in T the solid state,’V2 twisted some 40” around the central single bond in the vapor phasei3 The groundstate torsion is thus clearly a delicate balance of nonbonded repulsion and conjugation, and the groundstate rotational potential has attracted some theoretical attention. 4-6 There have been a number of calculations directed toward explicating the spectrum of biphenyl.’-I3 These calculations probe only the ground-state geometry, and generally good agreement with experiment is obtained for twist angles correlating well with the vaporphase equilibrium geometry. That the inter-ring bond acquires some double-bond character in the lowest excited state of the molecule is an obvious conclusion from either a ~a lence-bond’~ or molecular-orbitalI5 viewpoint. The influence of this potential energy change on the position and intensity of electronic transitions has been ably discussed by Jaffd and Orchin. Recently, some stimulating experiments were reported by Wagner.I6 From a study of the singlet-triplet absorption, its quenching, and the phosphorescence of biphenyl, it was concluded that in its lowest excited triplet biphenyl was planar, in contrast to its twisted ground-state equilibrium conformation. (1) J. Trotter, Acta Cryst . , 14, 1135 (1961); A. Hargreaves and S. H. (2) G. B. Robertson, Nature, 191, 593 (1961). (3) A. Almenningen and 0. Bastiansen, Kgl. Danske Norske Videnskab Selskab Skrifter, No. 4 (1958); 0. Bastiansen, Acta Chem. Scand. 4, 926 (1950). (4) S. Samoilov and M. Dyatkina, Zh. Fiz. Khim., 22, 1294 (1948). (5) F. J. Adrian, J . Chem. Phys. , 28, 608 (1958). (6) I. Fischer-Hjalmars, Tetrahedron, 19, 1805 (1963). (7) A. London, J . Chem. Phys. , 13, 393 (1945); E. T. Stewart, J . (8) H . C. Longuet-Higgins and J. N. Murrell, Proc. Phys. SOC. (Lon(9) K. Iguchi, J . Phys. SOC. Japan, 12, 1250 (1957). (10) H . Suzuki, Bull. Chem. SOC. Japan, 32, 1340 (1959); H. Suzuki, “Electronic Absorption Spectra and Geometry of Organic Molecules,” Academic Press, New York, N. Y., 1967, p 262. (11) Y. Gondo, J . Chem. Phys., 41, 3928 (1964). (12) R. Grinter, Mol. Phys., 11, 7 (1966). (13) A. Golebiewski and A. Parczewski, Theoret. Chim. Acta, 7, 171 (14) G. N. Lewis and M. Kasha, J . Am. Chem. SOC., 66, 2100 (1944). (15),(a) H. H. Jaffe an; M. Orchin, “Theory and Applications of Ultraviolet Spectroscopy, John Wiley and Sons, Inc., New York, N. Y., 1962, p 389; (b) H. H. Jaffk and M. Orchin, J . Chem. Soc., 1078 (1960). Rizvi, ibid., 15, 365 (1962). Chem. SOC., 4016 (1958). don), A68, 601 (1955).