Molecular surface electrostatic potentials as guides to Si-O-N angle contraction: tunable σ-holes.

We have demonstrated that the variation in the experimentally-determined Si-O-N angles in XYZSi-O-N(CH(3))(2) molecules, which depends upon the positions and natures of the substituents X, Y and Z, can be explained in terms of computed electrostatic potentials on the molecular surfaces of the corresponding XYZSi-H molecules. The latter framework has been used as a model for what the nitrogen lone pair in the XYZSi-O-N(CH(3))(2) molecules sees. Both optimized geometries and electrostatic potentials of our model XYZSi-H systems have been obtained at the B3PW91/6-31G(d,p) level. We propose that the driving force for the observed Si-O-N angle contraction in XYZSi-O-N(CH(3))(2) molecules is largely the electrostatic attraction between a positive σ-hole on the silicon and the lone pair of the nitrogen. Negative regions that may be near the silicon σ-hole, arising from substituents with negative potentials, also play an important role, as they impede the approach of the nitrogen lone pair. These two factors work in synergy and attest to the electrostatically-driven nature of the Si---N intramolecular interactions, highlighting their tunability.