Hyperconjugative Interactions in Permethylated Siloxanes and Ethers: The Nature of the SiO Bond

The paradoxically low basicity (despite high anionicity) of oxygen in the characteristic Si-O-Si linkages of silicone polymers is investigated with hybrid density functional and natural bond orbital (NBO) computational methods, extending a previous study of idealized disiloxane and dimethyl ether parent species to fully methylated derivatives that more faithfully model the silicone polymers of industrial and environmental importance. Despite the complicating distortions of the sterically crowded di-t-butyl ether analog, the physical picture of enhanced hyperconjugative (resonance-type) delocalization in Si-O vs C-O bonding is essentially preserved (and indeed accentuated) in permethylated species. NBO-based orbital overlap diagrams are employed in conjunction with structural, hybridization, and polarity descriptors to illustrate the subtle phase-matching relationships that confer superior enthalpic and entropic stability (and low basicity) on permethylated Si-O-Si linkages. Our results challenge both ionic models of Si-O bonding and conventional electrostatic-type models of H-bonding and acid-base reactivity.