Relationship of Bonding to Electronic Spectra

摘要

It is an experimental fact, reproduced by high-quality ab initio calculations,that the singlet excitation energy much lower for the Si-Si bond than for the C-C bond. In an attempt to rationalize it in simple terms, the difference has been attributed at times to the 'weak-bond effect' : the splitting of the orbitals is said to be smaller, since Si-Si is the weaker bond. This is not a valid explanation, since in fact the Silicon-Silicon and C-C bond strengths are quite similar. For instance, the the Si-Si bond strength in (CH3)3Si-Si(CH3)3 is 80 kcal/mol,2 the central C-C bond strength in (CH3)C-C(CH3)3 is 70 kcal/mol,3 and yet the singlet excitation in the former lies at least 30 kcal/mol below the corresponding excitation in the latter, and possibly much more.