Theoretical Study of Ultraviolet Absorption Spectra of Tetra- and Pentacoordinate Silicon Compounds

Single-excitation configuration interaction (CIS) and time-dependent density functional (TD-DFT) calculations have been carried out on the excited singlet states of tetracoordinate trisilane [(CH) 3 Si] 2 Si(Cl)CH 3 ( 2 ) and pentacoordinate trisilane ( 5 ) to explain the remarkable red shift of the λ max (Δλ max = 10 nm) exhibited by 5 . The calculated λ max of (H 3 Si) 2 Si(Cl)CH 2 NHCHO ( 3 ) was red-shifted by 11.2 nm with respect to that of (H 3 Si) 2 Si(Cl)CH 3 ( 1 ), in agreement with experiment. In both compounds, the strongest absorption originates from the σ SiSi → σ* SiSi transition. Compared with that of 1, the σ SiSi orbital of 3 was substantially destabilized, whereas the σ* SiSi orbital was slightly changed. With the additional calculations on tetracoordinate model compounds having an Si−O bond, we concluded that the red shift of the UV spectrum of 5 is caused by the antibonding interaction of the p π orbital on the oxygen atom and the σ SiSi orbital of the same symmetry.