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

The 17 O NMR spectra for a series of 17 O‐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 ‐NO 2 = 10.5 ppm; cinnamic 254.1 ppm, SCS range p ‐MeO to p ‐NO 2 = 5.4 ppm). The esters showed two signals [methyl benzoate (CO) 341.3 ppm and (OCH 3 ) 128.0 ppm; methyl cinnamate (CO) 339.9 ppm and (OCH 3 ) 134.2 ppm]. The SCS ranges for the carbonyls of the esters were larger than those for the corresponding acids, while those for the OCH 3 groups of the esters were slightly smaller. The carbonyl data gave good correlations with σ + constants, while the OCH 3 data gave at best only a poor correlation with σ constants. Dual substituent parameter treatment improved the correlations for all the data using σ R+ constants. The ratios of ρ I to ρ R+ were similar for all the sets of data.