29Si and 13C NMR spectra of 4-substituted 2-methoxytrimethylsiloxybenzenes. Factors determining the chemical shifts in models of lignin constituents

29Si and 13C NMR chemical shifts are reported for a series of twenty 4‐substituted 2‐methoxytrimethyl‐siloxybenzenes; the set of substituents incorporates a basic set of ten substituents differing in their relative polar and resonance effects and ten other substituents which model substituents encountered in lignins. The factors affecting the chemical shifts are discussed using a comparison with the NMR data for a similar series of compounds, correlations with substituent parameters and electronic charges calculated by a quantum chemical method (MNDO calculations on force‐field optimized molecular geometries) for the basic set. It is concluded that the methoxy group in the ortho position to the trimethylsiloxy group takes, for most of its time, the conformation in which the methyl group lies in the plane of the benzene ring and is turned away from the trimethylsiloxy group which assumes a perpendicular conformation. Proximity deshielding effects observed on both the methoxy and trimethylsiloxy groups are influenced by the substituents in position 4, which is in agreement with the notion that these effects are due to an interaction of the vicinal oxygen atoms or their unshared electrons. The dependence of the 29Si chemical shifts on σpis not substantially affected by the presence of the ortho methoxy group.