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 σI and σ0R 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.