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conformation of the methoxy group relative to the aromatic ring. According to our results, in the planar conformation anisole is a stronger base than phenol, and protonation can occur either on the ring or on the substituent. The rotation of the methoxy group increases the basicity of anisole, which behaves as an oxygen base. This fact could be responsible for the absence of deuterium exchange in anisole, pointed out by Beauchamp.6 Our results bring out, once more, the problem of the conformation of anisole and they ratify, to some extent, the INDO results, though no definitive answer can be given on this particular point. The full geometry optimization at the a b initio level, which would settle the question, would require the use of an optimization procedure similar to that employed to obtain our I N D O results, which would be too expensive. Acknowledgments. All calculations were performed in the IBM 360/65 computer a t the UAM/IBM Center (Madrid). We would like to thank our referees for helpful criticism. References and Notes (1) J. L. Beauchamp, Annu. Rev. Phys. Chem., 22, 527 (1971); J. P. Briggs, R. Yamdagni, and P. Kebarle, J. Am. Chem. Soc., 94, 5128 (1972); B. S. Freiser and J. L. Beauchamp, ibid., 98, 265 (1976); 99, 3214 (1977); R. Yamdagni and P. Kebarle, ibid,, 98, 1320 (1976). (2) P. Kebarle, Annu. Rev. Phys. Chem., 28, 445 (1977), and references cited therein. (3) W. J. Hehre, R. T. Mclver, Jr., J. A. Pople, and P. v. R. Schleyer, J. Am. Chem. Soc., 96, 7162 (1974); J. M. McKelvey, S. Alexandratos. A. Streit-
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wieser, Jr., J. L. M. Abboud, and W. J. Hehre, ibid., 98, 244 (1976). (4) D. J. DeFrees, R. T. Mclver, Jr., and W. J. Hehre, J. Am. Chem. Soc., 99, 3853 (1977); S. K. Pollack, J. L. Devlin 111, K. D. Summerhays, R. W. Taft, and W. J. Hehre, ibid., 99, 4583 (1977); K. D. Summerhays, S.K. Pollack, R. W. Taft, and W. J. Hehre, ibid., 99, 4585 (1977). (5) Y . K. Lau and P. Kebarle, J. Am. Chem. Soc., 98, 7452 (1976). (6) B. S. Freiser, R. L. Woodin, and J. L. Beauchamp, J. Am. Chem. SOC..97, 6893 (1975). (7) 0. P. Martinsen and S. E. Butrill, Jr.. Org. Mass Spectrom., 11, 762 (1976). (8) 0. P. Martinsen and S. E. Butrill, Jr., J. Am. Chem. SOC., 100, 6559 (1978). (9) J. Catalan and M. Yanez, J. Chem. SOC.,Perkin Trans. 2, in press. (10) F. M. Benoit and A. G. Harrison, J. Am. Chem. Soc., 99, 3980 (1977). (1 1) J. Catalan and M. Yahez, Chem. Phys. Lett., 60, 499 (1979). (12) W. J. Hehre, R. F. Stewart, and J. A. Pople, J. Chem. Phys., 51, 2657 (1969). (13) H.M.Seip and R. Seip, Acta Chem. Scand., 27, 4024 (1973). (14) T. B. McMahon and P. Kebarle, J. Am. Chem. Soc., 99, 2222 (1977). (15) J. F. Wolf, R. H. Staley, I. Koppel, M. Taagepera, R. T. Mclver, Jr., J. L. Beauchamp, and R. W. Taft. J. Am. Soc., 99, 5417 (1977). (16) S. R. Smith and T. D. Thomas, J. Am. Chem. Soc., 100, 5459 (1978). (17) W. J. Hehre, L. Radom, and J. A. Pople, J. Am. Chem. SOC.,94, 1496 (1972). (18) J. A. Pople and M. S. Gordon, J. Am. Chem. Soc., 89, 1453 (1967). (19) H. Forest and B. P. Dailey, J. Chem. Phys., 45, 1736 (1966). (20) T. Pedersen, N. W. Larsen, and L. Nygaard, J. Mol. Struct., 4, 59 (1969). (21) H. D. Bist and D. R. Williams, Bull. Am. Phys. Soc., 11, 826 (1966). (22) W. G. Fateley, F. A . Miller, and R. E. Witkowski, Technical Report AFMLTR-66-408, Wright Patterson Air Force Base, Ohio. (23) J. A. Pople, D. L. Beveridge, and P. A. Dobosh, J. Chem. Phys., 47, 2026 (1967). (24) D. Rinaldi and J. L. Rivail, C. R. Acad. Sci., Ser. C, 274, 1664 (1972): D. Rinaldi, Thesis, Nancy, 1975. (25) S. Fewster, Ph.D. Thesis, University of Manchester, 1970; G. Allen and S. Fewster, "Internal Rotation in Molecules", W. J. Orville-Thomas, Ed., Wiley, New York. 1974, Chapter 8.
An ab Initio Study of the Cis-Trans Photoisomerizat ion of Stilbene G . Orlandi,*la.bP. Palmieri,*lc and G . PoggiId Contribution f r o m the Laboratorio di Fotochimica e Radiazioni d'Alta Energia del C N R , Bologna, Italy, lstituto di Chimica Fisica e Spettroscopia, Uniiersitir di Bologna, Bologna, Italy? and lstituro Chiniiro "G. Ciamrcian", Unicersitci di Bologna, Bologna, Italy. Receiced June 5 , I978
Abstract: The molecular energies of the ground and lowest excited mr* states for the cis and trans isomers of stilbene and for the twisted geometry o f the molecule have been evaluated by the SCF method followed by configuration mixing. The orbital basis is a minimal basis of S T 0 / 3 G orbitals and the CI includes all relevant second-order interactions of the ground and lowest excited configurations. Two-term Fourier interpolations of the computed energies have provided potential energy curves along the coordinate of internal rotation. The potential curves support the adiabatic mechanism for the trans-cis thermal conversion in the ground state and for the sensitized photoconversion in the lowest triplet state and a nonadiabatic mechanism for the direct photoisomerization in the lowest excited singlet state.
Introduction Several mechanisms have been proposed for the direct cistrans photoisomerization of stilbene and of the related a,wdiphenyl polyene^.^,^ Given the correlation diagram of the orbital levels in cis- and trans-stilbene (Figure l ) , the isomerization reaction, after W o o d w a r d - H ~ f f m a n n ,is~ allowed in the lowest excited B (SI,T I )and A (Sz) states. The excitations to the B and A states are essentially described as (4a) (4b) and (4a)' (4b)2, respectively. An alternative description of the S I ,Ti, and Sz states can be given in terms of biradicaloid states5 Experimentally it has been possible to prove that the direct photoreaction does not follow the triplet p a t h ~ a yAmong .~ the mechanisms involving states in the singlet manifold, it was first proposed that the optical excitation to the S I state is followed
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0002-78631791 I501-3492$01 .OO/O
by adiabatic motion on the S I energy curve to a minimum located a t the twisted geometry (p = 90') and by deactivation onto the ground potential energy surface. This mechanism contrasts with the results of a spectroscopic investigationh where it was deduced that the S I state has in fact a barrier at p = 90' high enough to make the state inactive in the photoreaction. For this reason a second mechanism was postulated,' where the molecule, before deactivation, undergoes a nonadiabatic crossing from the SI to the Sz potential surface, which has a minimum a t p = 90'. The S2 state, owing to its forbidden character, has not been observed by conventional one-photon spectroscopy. The overall experimental knowledge of the S I potential curve is also rather I i m i ted. The validity of the one-electron model, on which the
0 1979 American Chemical Society
Orlandi, Palmieri, Poggi
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/ Cis- Trans Photoisomerization of Stilbene
Woodward-Hoffmann correlation diagrams are based, for inferring the qualitative shape of the SI and Sz potential curves may in principle be established by actual computations within the framework of molecular orbital theories. In addition, theoretical computations may provide estimates of the relative energies of the SI and Sz states and of the potential barrier hindering the internal rotation in the S I state. The results of the theoretical treatments reported so f a r Xl o~ are, in some respects, conflicting since the shape of the SI and S2 potential curves and the relative energies of the two states proved to be sensitive to the empirical model Hamiltonian adopted in the computations (in most cases of Pariser-ParrPople type) and to the amount of configuration mixing included in the treatment. Following a similar calculation on styrene,IlJ to which we refer for the details of the computational procedure, we have reinvestigated the problem by using an a b initio approach. A more efficient coding of all computer programs needed in the CI computations has allowed the explicit inclusion of all 96 electrons of the molecule in the molecular wave functions. Atomic Orbitals and Electronic Configurations Any theoretical description of a conjugated molecule as large as stilbene inevitably suffers from limitations in the choice of the orbital basis and of the electronic configurations. The results hereafter presented have been obtained with a minimal basis set of ST0/3G orbitalsl’by using a CI procedurelj which includes all important excitations within the subspace of the TT orbitals of the molecule. The assumption inherent in the choice of the orbital basis, which does not allow an adequate description of the Rydberg states, is that the lowest excited states of the molecule have a definite valence character. O n physical grounds, this assumption appears to be justified since the lowest X T * configurations are, in this molecule, well below the ionization limit and, therefore, unlikely to interact strongly with Rydberg configurations.l4 In order to determine the amount of Rydberg character of the lowest mr* states of stilbene, a preliminary investigation has been undertaken on the closely related molecule styrene, with orbital bases of increasing size and flexibility.IlbThe results have indicated that the electronic states relevant to the present discussion (SI,Sz, Sj) have in fact a definite valence character. The truncation of the C1 expansion amounts to neglecting entirely the correlation between 0 and x electrons. Actual computations of potential energy curves for the cis-trans isomerization processes of ethylene derivatives have been performed, in a few cases,11b.’5by a b initio CI procedures, which include the excitations from the highest occupied to the lowest virtual g MOs, mostly distributed in the C-H bond region, and the relevant energy terms were not found important for a qualitative description of the potential energy curves. The inclusion of the g-7r correlation in the C-C bond region, on the other hand, requires the inclusion of K(T* and UT* configurations with higher energy. A detailed analysis is available for the Bl, state of ethylene, which clearly shows that the cr-x correlation in this molecular region is mainly of left-right type and is related to the ionic character of the state.I4 Since the ionic character of S I does not depend on the T T X overlap and on the degree of torsion around the double bond,’ we expect that these energy terms are nearly constant along the torsional coordinate i n S I .On similar grounds, the (T-T correlation is expected to have similar values in SIand Sz at the twisted geometry, since both states are zwitterionic states a t this geometry.’ Results and Discussion Ordering of the Electronic States. The configuration interaction energies, which have been obtained for the trans isomer and the twisted geometry (p = 90°), are listed in detail in Table
e.
7b.
14
12 6b.
10 8 6
/a
4 2
0
2 4
E
f
3 b ~ 3a. ,
b.a a z
1C
b
li 11
d
1
90
I 180
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Figure 1. Correlation diagram o f the orbital levels of t r a m - ( q = 0’) and
