dc.creator Haas, Y. en_US dc.creator Zilberg, S. en_US dc.date.accessioned 2007-11-20T17:13:13Z dc.date.available 2007-11-20T17:13:13Z dc.date.issued 1995 en_US dc.identifier 1995-TE-09 en_US dc.identifier.uri http://hdl.handle.net/1811/29788 dc.description Y. Haas and S. Zilberg, J. Am. Chem. Soc., submitted. en_US dc.description Author Institution: The Hebrew University of Jerusalem, Jerusalem, Israel 91904 en_US dc.description.abstract The $\nu_{14}(b_{2u})$ mode of benzene undergoes a significant frequency increase upon excitation of the molecule from the ground electronic state. $S_{0}(^{1}A_{1g})$ to the first electronically excited state $S_{1}(^{1}B_{2q})$. It is the only mode that exhibits this pronounced increase, most other modes show a modest frequency decrease, as expected for the weakend bonding in the excited state. An ab-initio calculation shows that this change involves a considerable increase in the force constant and in the reduced mass of the mode, which becomes essentially a pure carbon atom mode in $S_{1}$. This change is in line with the notion [Shaik, Hiberty. Ohanessian and Lefour, J. Phys. Chem., 92, 5086 (1988)] that the $D_{2u}$ symmetry of the molecule is due to $\sigma$-bonding, and that the $\pi$-electrons actually tend to distort the molecule to a Kekul\'{e} form. According to the theory of Shaik and Hiberty, promotion of a $\pi$ electron to an antibonding orbital is expected to reduce the $\pi$-distortivity power and thus helps to reveal more clearly the influence of the $\sigma$-potential on the Kekule type $b_{2u}$ mode. A similar effect is found to take place for the lowest lying $B_{2u}$ state of other aromatic hydrocarbons, even if they are not $S_{1}$, as shown for anthracene by an ab-initio calculation. The $A_{g}$ and $B_{2u}$ states may be considered as arising primarily from the two Kekule valence bond structures by a positive and negative combinations, respectively. en_US dc.format.extent 68992 bytes dc.format.mimetype image/jpeg dc.language.iso English en_US dc.publisher Ohio State University en_US dc.title THE $\nu 14(b2u)$ MODE OF BENZENE IN SO AND SI: DO THE II ELECTRONS TEND TO DISTORT THE MOLECULE TO THE KEKULE FORM? en_US dc.type article en_US
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