Angewandte 0.01 -0.01 C-DTB -88 0.02 00 0.02 004 -0020 Tothe extent that this is maintainedin ct-DTB (s)in (300 nm EPA might appear emark le if it were not for a conta ations in ct-DTE that the m H the star,consistent with direct cDTB-DTB two-bond -with the n-bondDTB st to t situation in EPA. Pubished colmer either c ing from eithe stru redDTB pec rum.The medium change to IP hi n dominant conformer a1993.97.251 663967891a6-1037b15aie and OBT pathways (Scheme 2).H pathways are eliminated 马风风起 to The ompeting photoomef :e)R.S H.Liu G.S. e formation of unstable confo The-raerystastructure of c-TB reveals a single (B3LYPith the 6-31G(d.p)basis set) .A.M.Turek..Chem 四8KmR1ak1咖aamA2m 08 Wiley-VCH Verlag CmbH Co.KGaA.Weinhein ndte.org These are not the final page numbers the start, consistent with direct cc-DTBA!tt-DTBA two-bond isomerization in competition with the one-bond cc-DTBA! ct-DTBA process. In contrast to the situation in EPA, there is no significant spectral change on thawing and recooling the solution after irradiation of either cc- or ct-DTB. Starting from either cis isomer, final photoproduct spectra coincide with the highly structured tt-DTBA spectrum. The medium change from EPA to IP has a dramatic effect on the equilibrium conformer distributions of cc- and ct-DTB, shifting each to a single dominant conformer. This serendipitous medium effect allows us to establish that in IP the photoisomerizations of cc- and ct-DTB involve no unstable conformer products. Cis– trans photoisomerizations that convert stable reactant con￾formers to stable product conformers are accounted for by BP and OBT pathways (Scheme 2). HT pathways are eliminated in IP, because they require formation of unstable conformers. The inhibition of the BP process aside, we see no reason to expect a mechanism change in EPA.[10] The X-ray crystal structure of cc-DTB reveals a single conformer, cc-DTBA, with the o-tolyl groups in parallel planes oriented at 568 to the plane of the diene moiety.[16] Calculated conformer structures (B3LYP with the 6-31G(d,p) basis set) for ct-DTB similarly show large o-tolyl–diene dihedral angles (42.1 and 19.78 for ct-DTBA and 62.0 and 20.88 for ct-DTBB on the cis and trans sides of the molecule, respectively, Figure 6).[16] ct-DTBB is predicted to be 1.95 kcalmol1 higher in energy in the gas phase.[16] To the extent that this energy difference is maintained in IP, the predominance of the ct-DTBA conformer at 77 K in that medium is not surprising. The presence of both con￾formers in EPA might appear remarkable if it were not for a highly significant precedent. Strong CD spectral evidence shows that the previtamin D and tachysterol conformer distributions change significantly on changing the medium from a hydrocarbon glass to EPA at 92 K.[19] The claim of HT products from previtamin D in EPA[5] is questionable, because it did not consider that medium effect. Received: September 27, 2007 Published online: && &&, &&&& .Keywords: dienes · isomerization · one-bond twist · photochemistry [1] a) J. Saltiel, A. S. Waller, D. F. Sears, Jr., J. Am. Chem. Soc. 1993, 115, 2453 – 2465; b) J. Saltiel, A. S. Waller, D. F. Sears, Jr., C. Z. Garrett, J. Phys. Chem. 1993, 97, 2516 – 2522. [2] a) J. Saltiel, J. Am. Chem. Soc. 1967, 89, 1036 – 1037; b) J. Saltiel, J. Am. Chem. Soc. 1968, 90, 6394 – 6400. [3] A. Warshel, Nature 1976, 260, 679 – 683. [4] a) R. S. H. Liu, A. E. Asato, Proc. Natl. Acad. Sci. USA 1985, 82, 259 – 263; b) R. S. H. Liu, D. Mead, A. E. Asato, J. Am. Chem. Soc. 1985, 107, 6609 – 6614. [5] A. M. MIller, S. Lochbrunner, W. E. Schmid, W. Fuß, Angew. Chem. 1998, 110, 520 – 522; Angew. Chem. Int. Ed. 1998, 37, 505 – 507. [6] a) R. S. H. Liu, G. S. Hammond, Proc. Natl. Acad. Sci. USA 2000, 97, 11153 – 11158; b) R. S. H. Liu, Acc. Chem. Res. 2001, 34, 555 – 562; c) R. S. H. Liu, G. S. Hammond in Handbook of Organic Photochemistry and Photobiology, 2nd ed. (Eds.: W. M. Horspool, F. Lenci), CRC, London, 2004, pp. 26/1 – 26/11; d) R. S. H. Liu, G. S. Hammond, Acc. Chem. Res. 2005, 38, 396 – 403; e) L.-Y. Yang, M. Harigai, Y. Imamoto, M. Kataoka, T.-I. Ho, E. Andrioukhina, O. Federova, S, Shevyakov, R. S. H. Liu, Photochem. Photobiol. Sci. 2006, 5, 874 – 882; f) R. S. H. Liu, L. Y. Yang, J. Liu, Photochem. Photobiol. 2007, 83, 2 – 10. [7] L. Yang, R. S. H. Liu, K. J. Boarman, N. L. Wendt, J. Liu, J. Am. Chem. Soc. 2005, 127, 2404 – 2405. [8] M. V. Alfimov, V. F. Razumov, A. G. Rachinski, V. N. Listvan, Yu. B. Scheck, Chem. Phys. Lett. 1983, 101, 593 – 597. [9] N. Castel, E. Fischer, J. Mol. Struct. 1985, 127, 159 – 166. [10] J. Saltiel, T. S. R. Krishna, A. M. Turek, R. J. Clark, Chem. Commun. 2006, 1506 – 1508. [11] J. Saltiel, T. S. R. Krishna, R. J. Clark, J. Phys. Chem. A 2006, 110, 1694 – 1697. [12] For organic glass viscosities, see: a) J. R. Lombardi, J. W. Raymonda, A. C. Albrecht, J. Chem. Phys. 1964, 40, 1148; Figure 5. Combination coefficients for the spectral matrix obtained on irradiation of cc-DTB (*) and ct-DTB (~) in IP at 77 K (lrad>300 nm). Points on the ct-DTB/tt-DTB side are for lexc=345 nm. Spectra for lexc=300 and 315 nm define the lower line, revealing some cc-DTB contaminations in ct-DTB. Scheme 2. The competing photoisomerization pathways of cc-DTB in IP at 77 K. Figure 6. Stationary-point geometries on the ct-DTB So surface; ct￾DTBA on the left corresponds to the global energy minimum.[16] Angewandte Chemie Angew. Chem. Int. Ed. 2008, 47, 1 – 5  2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.angewandte.org 3 These are not the final page numbers!