The nmr study and cndo/2 molecular orbital calculation of silacyclobutanes

Abstract

29Si, 13C and 1H chemical shifts have been determined for twenty-four substituted silacyclobutanes. Coupling constants, 1 J(SiC), for directly-bonded silicon and carbon atoms have been measured. Six substituted silacyclopentanes have been also studied for comparison with the strained 4-membered cyclic compounds. Molecular orbital calculations for cyclic silanes have been carried out using the CNDO/2 method. Net atomic charges, bond-order and other quantum chemical parameters have been obtained from SP- and SPD-basis set computations. Only for the SPD calculation was generally good agreement obtained between experimental data and computed values. The effect of substitution in silacyclobutanes on the chemical shifts and coupling constants is compared to that for silacyclopentanes and acyclic silanes. The unusual trend in the screening of Si nuclei and the sensitivity of C3 to substitution on the Si atom in the silacyclobutanes are explained by the existence of a strong 1-3 transannular interaction, which is confirmed by the measurements of 2 J(SiC 3) and by the computed resonance energies- E R AB.