Photoelectron spectra and molecular properties. 30, Pi-interactions in H3Si- and H3C-substituted acetylenes

Abstract

The photoelectron (pe) spectra of mono- and disubstituted silyl- and methylacetylenes are assigned by comparison with the corresponding ionization potentials of acetylene, disilane, and ethane. The observed π splittings can be rationalized within a parameterized hyperconjugation model. Calculations using a modified CND0/2 procedure with and without inclusion of 3d orbitals in the basis set demonstrate that the usual parameters tend to overemphasize p,d, back-bonding. To rationalize available experimental data, 3d si orbitals need not to be considered explicitly-neither for the different dipole moments of methyl- and silylacetylenes nor for the general pe spectroscopic pattern of silicon compounds, i.e., high first ionization potential, reduced spin-orbit coupling, or band broadening.