WHY IS THE SI-X STRETCHING FREQUENCY OF X(8)SI(8)O(12) (X=H, CH3, CL)MUCH HIGHER THAN THAT OF XSI(OSIME(3))(3)

The totally symmetric stretching frequencies v(s)(Si-X) of the octasil ases-quioxanes X(8)Si(8)O(12) are distinctively larger than those of X Si(OSiMe(3))(3) for X = H, CH3, Cl whereas v(s)(Si-H) of H8Si8O12 and of H10Si10O15 differ very little. Inspection of the experimental data and molecular orbital calculations of the EHMO-ASED type show that thi s striking discrepancy is caused by the differences in the X-Si-O-Si c onformations. The cage structure of X(8)Si(8)O(12) requires the anti X -Si-O-Si conformation whereas the syn conformation is the stable one i n XSi(OSiMe(3))(3). The SI-H bond order decreases from anti to syn cau sed by a decreasing interaction of the H-ls orbital with the Si-O-p pi type orbitals.