Investigation of the transmission of substituent effects by Si-29 NMR

Long range substituent effects on the Si-29 NMR chemical shifts in a series of alkylene- and arylene-bridged triethoxysilanes were observed over as ma ny as 11 bonds. The hydrolysis reaction of an ethoxide caused the resonance of the silicon on the opposing end of the bridging unit to move downfield. The alkylene bridging units ranged from ethylene to octylene while the ary lene bridging units included phenyl and biphenyl. Resonance assignments wer e confirmed by the absence of these shifts for the triethoxysilyl in 1-trip henylsilyl-2-triethoxysilylethane. The magnitude of the downfield shift dec reased as the length of the bridging unit between silicon atoms increased. Transmission of the substituent effect along a polymethylene chain was succ essfully modeled by a through-bond mechanism with an attenuation factor of 1.88 for each methylene unit.