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Download AB INITIO Quantum molecular dynamics by Ben-nun M., Martinez T.J. PDF

By Ben-nun M., Martinez T.J.

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Salem and co-workers [161–163] noted that mono-pyramidalization of ethylene (keeping the molecule in Cs symmetry) results in a large dipole moment. The onset of this phenomenon is quite sudden; that is, small distortions result in a large change in the dipole moment (which is zero by symmetry at both the planar and twisted geometries). This arises due to an avoided crossing between the valence and ionic states (V and Z in Mulliken’s notation) near the twisted (nonpyramidalized) geometry. The original speculation that the electrical signal thus generated might trigger conformational changes in the visual pigment proteins [162] was abandoned when it was shown that proton transfer was required for vision [164].

Using the normal mode assignment of previous workers [186,190], they concluded that this mode is a non-totally symmetric CH2 twist, projecting directly onto the disrotatory twist of the CH2 groups. In 1995, this interpretation was challenged by Negri et al. [191]. They ab initio quantum molecular dynamics 491 revisited the normal mode analysis of CB and concluded, in agreement with a previous assignment by Wiberg and Rosenberg [187], that the 1075 cmÀ1 mode is mainly a CH2 deformation (and not twist), while the lower-frequency 848 cmÀ1 mode is a CH2 twist mode with a dominant disrotatory component.

In particular, the minimal S0 –S1 energy gap that results is approximately 60 kcal/mol, implying a long excited-state lifetime and significant fluorescence. Yet there is no detectable fluorescence from the excited state of ethylene. The absence of fluorescence suggests a short excited-state lifetime, which has recently been investigated using femtosecond pump-probe spectroscopy [152]. The broad and diffuse absorption spectrum of ethylene has been assigned by Wilkinson and Mulliken [153] to the p !

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