Nonadditive carbon-13 nuclear magnetic resonance substituent shifts in 1,4-disubstituted benzenes. Application of the DSP-NLR equation to the OCOCH₃, CONH₂ and CSNH₂ series

Abstract

Carbon-13 Substituent Chemical Shifts (SCS) for all the ring carbons for a series of para-substituted phenyl acetates, benzamides, and thiobenzamides are reported. The measurements were made under standard conditions at low concentration (in deuterochloroform for the acetates, and (CD₃)₂SO for the amides) and data are reported for an extensive range of para substituents (NMe₂, NH₂, OCH₃, OCOCH₃, F, Cl, Br, Me, H, CF₃, CN, COOR, COCH₃, NO₂, CHO). The observed non-additive substituent chemical shifts have been interpreted in terms of variation of the Shift Charge Ratio and mutual substituent interaction. The substituent constants σ_I and σ⁰_R for OCOCH₃, CONH₂, CSNH₂ substituents are determined and compared with previously published data. Electronic properties of OCOCH₃, OCH₃ substituents and CONH₂, CSNH₂ substituents are discussed. The self-consistency and advantages of the Dual Substituent Parameter equation (DSP) and the recently developed Non-Linear-Resonance equation (DSP-NLR) treatments of ¹³C SCS data are discussed, and contrasted with an alternative single parameter approach.