Ab initio MO calculations and 17 O NMR at natural abundance of para ‐substituted acetophenones

17 O NMR chemical shifts and calculated ( ab initio MO theory) electron densities are reported for a series of para ‐substituted acetophenones, XC 4 H 6 COCH 3 , where X = NH 2 , OCH 3 , F, Cl, CH 3 , H, COCH 3 , CN, NO 2 . The 17 O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ I and σ R + using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ I and ρ R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π‐electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ   msx n→π*values are also reported, and the role of the average excitation energy, Δ E , in determining 17 O SCS values is discussed. It is concluded that variations in Δ E are minor and that the local Δ‐electron density is the dominant feature controlling 17 O SCS values.