An Excited-State Reaction Photoextrusion of Silylene

An interestng attempt was made by Ramsey [20] to determine the nature of the reactive excited state of a cyclic trisilane by means of an orbital symmetry analysis of its photofragmentation. The question addressed was whether the relevant excitation might be to a low-lying 3d orbital rather than to an antibonding valence orbital, as is usually assumed. 

The photolysis is known to occur readily [21] and can therefore be presumed to be allowed by orbital symmetry conservation. 

Ramsey s argument can be recast as follows. Electrons localized on silicon atoms or involved in SiSi bonding are held more loosely than others in the molecule, so the only orbitals of the trisilane that need be considered in the analysis are the bonding and antibonding combinations of o sisi bonds  

Fragmentation is therefore forbidden by orbital symmetry on the ground-state surface and allowed on the first excited surface, in agreement with experiment 

Ramsey went on to consider the possibility that empty d orbitals of silicon are sufficiently low in energy for one of their combinations to be singly occupied in the first excited state of the trisilane. The lowest such combination is the mutually bonding 7Td a2) orbital da y(l) + dxy 2) + dxy 3). If this orbital were indeed occupied in the first excited singlet, the latter would be Bi and fail to correlate with the products on the lowest excited singlet surface. Ramsay s conclusion the lowest combinations of silicon d orbitals, even in a situation as favorable as this, lie sufficiently far above the antibonding valence orbitals that they can be ignored in photochemical and - a fortiori - in thermal reactions. 

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