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GROUND AND EXCITED PROPERTIES OF THE PREVITAMIN D CONFORMERS Olga Dmitrenko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, 252022 Kiev, Ukraine Wolfgang Reischl Institute of Organic Chemistry, University of Vienna, A-1090 Vienna, Austria James T. Vivian and John H. Frederick Department of Chemistry, University of Nevada, Reno, NV 89557, USA Abstract. Ground and excited states properties of previtamin D individual conformers are examined by use of the MMX (PCMODEL) empirical force field, and AM1 and QCFF/pi semi-empirical calculations. Ground state geometries obtained by these methods indicate a highly twisted triene system with similar degrees of non-planarityfor all conformers. The results for origin transitions support the suggestion that cZc conformers absorb at longer wavelength than tZc ones, thereby contributing to the understanding of wavelength dependence of previtamin D photochemistry. Keywords. Previtamin D conformers, ground and excited states, force-field calculations, AM1 conformational analysis, QCFF/PI calculations, absorption properties of conformers