Investigation of the transmission of substituent effects by 29Si NMR

Long range substituent effects on the 29Si NMR chemical shifts in a series of alkylene- and arylene-bridged triethoxysilanes were observed over as many 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 arylene bridging units included phenyl and biphenyl. Resonance assignments were confirmed by the absence of these shifts for the triethoxysilyl in 1-triphenylsilyl-2-triethoxysilylethane. The magnitude of the downfield shift decreased as the length of the bridging unit between silicon atoms increased. Transmission of the substituent effect along a polymethylene chain was successfully modeled by a through-bond mechanism with an attenuation factor of 1.88 for each methylene unit.

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