IONIZATION ENERGIES OF LINEAR AND CYCLIC POLYSILANES - APPLICATION OFTHE GREENS-FUNCTION METHOD COUPLED WITH SEMIEMPIRICAL MOLECULAR-ORBITAL CALCULATIONS

The ionization energies of the parent and permethylated linear [R(3)Si (SiR(2))(n)SiR(3)] (n = 1-3; R = H, CH3) and cyclic [(c-SiR(2))(n)] (n = 3-6; R = H, CH3) polysilanes have been calculated with the outer va lence Green's function (OVGF) technique, using as the zeroth order app roximation the wave functions obtained with semiempirical AM1, PM3, an d MNDO methods. It is found that the OVGF(AM1) method gives Significan tly better agreement with the experimental ionization potentials than semiempirical calculations using Koopmans' theorem. Furthermore, the O VGF(AM1) results are of comparable quality to those of OVA and EPT ab initio calculations. The mean deviation, for all the studied molecules , between the experimental and the OVGF(AM1) values is only 0.21 eV. T he ionization energies of the studied polysilanes behave as follows: ( 1) The first ionization energies of the permethylated polysilanes are significantly (by 1-1.7 eV) lower than those of the corresponding pare nt polysilanes. (2) In the cyclic silanes the first IP is higher when the number of silicon atoms in the polysilane increases, while in the open-chain silanes the first IP decreases with the number of silicon a toms. The crossover between the two series occurs between the Si-4 and Si-5 polysilanes.