Stereochemical effects in the mass spectra of some cyclopropylsilanes explained by the MNDO molecular orbital method. I. Silicon-oxygen interactions

Large differences have been found between the mass spectra of cis- and trans-1-carbomethoxy-2-trimethylsilycyclopropane. To help in explaining these differences, quantum chemical calculations were performed by the MNDO method on the neutral molecules, on the vertical and adiabatic molecular ions and also the [M - methyl]+ ions. To evaluate the results, heats of formation, bond orders, molecular geometry, spin and charge densities and orbital coefficients were used. These indicate the formation of a very strong bonding interaction between silicon and oxygen in the molecular ion of the cis but not the trans isomer, leading to spontaneous methyl loss and a stabilization energy of approximately 2.6 eV.