Extremely narrow SiON angles in siloxy-substituted nitrogen-containing rings: a computational investigation

Quantum chemical calculations have been performed for a number of siloxy derivatives of pyrrolidine and piperidine in an attempt to gauge the effect of the saturated ring on the SiON angle. Most derivatives of pyrrolidine had SiON angles comparable to those previously observed for substituted N,N-dimethylhydroxylamines. However, piperidine derivatives were predicted to have particularly narrow SiON angles. Anti, anti, axial-F₃SiO-piperidine was found to have an angle of 82.9°, smaller than the narrowest angle observed by experiment in the gas phase, for (F₃C)F₂SiONMe₂. The size of the ring and, in particular, the CNC angle appears to play a crucial role in determining the SiON angle. Further calculations including other heterocyclic rings were therefore performed to help explain this finding. Using the (F₃C)F₂SiO- group as a substituent on nitrogen allowed SiON angles of less than 80° to be calculated for several five-, six- and seven-membered rings.