The Sudden-Polarization Effect and its Role in the Ultrafast Photochemistry of Ethene

摘要

The term "sudden polarization", which describes the charge separation in excited singlet states of twisted polyenes, especially of ethene, was introduced in the mid-1970s by Salem. The ionic character and the easy polarizability of these valence excited states of alkenes upon a small perturbation was pointed out as early as 1971 by Wulfman and Kumei. Bonacic-Koutecky et al., in the mid-1970s, were the first to perform an ab initio study of the charge separation in 90°-twisted ethene as a function of an internal perturbation: the one-sided pyramidalization of a CH_2 group (D_(2d) geometry). To our knowledge, all the following studies of the sudden-polarization effect of ethene considered only one-sided pyramidalization. We discuss here the effects of one-sided scissoring as another internal perturbation and show that the transfer of electronic charge between the CH_2 groups is first order, whereas the effect is second order in pyramidalization. Moreover, we show that the one-sided scissoring mode plays an essential role in the nonadiabatic photochemistry of ethene.