17O NMR spectroscopy: Effect of substituents on chemical shifts for p‐substituted benzoic acids, methyl benzoates, cinnamic acids and methyl cinnamates

Abstract The 17O NMR spectra for a series of 17O‐enriched p‐substituted benzoic acids, methyl benzoates, cinnamic acids and methyl cinnamates in acetone at 40°C are reported. The carboxylic acids showed one signal (benzoic 250.5 ppm, SCS range p‐MeO to p‐NO2 = 10.5 ppm; cinnamic 254.1 ppm, SCS range p‐MeO to p‐NO2 = 5.4 ppm). The esters showed two signals [methyl benzoate (CO) 341.3 ppm and (OCH3) 128.0 ppm; methyl cinnamate (CO) 339.9 ppm and (OCH3) 134.2 ppm]. The SCS ranges for the carbonyls of the esters were larger than those for the corresponding acids, while those for the OCH3 groups of the esters were slightly smaller. The carbonyl data gave good correlations with σ+ constants, while the OCH3 data gave at best only a poor correlation with σ constants. Dual substituent parameter treatment improved the correlations for all the data using σR+ constants. The ratios of ρI to ρR+ were similar for all the sets of data.