O 2P-PI, SI 3P, AND C 2P INTERACTION IN THE HIGHEST ENERGY OCCUPIED MOLECULAR-ORBITALS OF METHYLATED SILOXANES

Momentum distribution, p(q), for electrons in the predominantly O 2p-pi HOMO's of hexamethyldisiloxane, Si2O(CH3)6, and the methylated cyclosiloxanes [SiO)(CH3)3]n, n = 3 and 4, have been measured by (e,2e) spectroscopy and calculated at the RHF/3-21G* level. In contrast to the simple one-peak p(q) observed for the 1bl O 2p-pi HOMO of H2O, the HOMO's of the methylated siloxanes show experimental p(q) with multiple maxima, a result of significant Si and C contributions. Variations in p(q) for the various HOMO's have been analyzed by taking the Fourier transform of p(q) to obtain position space autocorrelation functions, B(r), and by plotting orbital position distributions, psi(r). For each siloxane the HOMO contains a small contribution from a p orbital on Si and a large contribution from a p orbital on C. Both the p orbital lobes on Si and those on C generate features discernible in the autocorrelation differences, DELTA-B(r), between the siloxanes and H2O. The p(q) and DELTA-B(r) distributions calculated for the methylated species agree reasonably well with the experimental data. Substitution of H for CH3 in the calculations causes a significant change in both p(q) and DELTA-B(r). There are only small differences between the p(q) distributions for Si2O(CH3)6, Si3O3(CH3)6, and Si4O4(CH3)8, but they all differ substantially from that calculated for SiH3OH. Thus, (e,2e) spectroscopy is potentially a probe for the coordination of the O atom in siloxane molecules or on the surfaces of solid silicates.