Quantification of substituent effects using molecular electrostatic potentials: additive nature and proximity effects

Abstract

Several ortho, meta, and para substituted benzoic acids have been studied to quantify the substituent effects by analysing subtle variations in the molecular electrostatic potential minimum (V_min) at the response site of the carboxylic acid moiety using density functional theory. For the first time, the ortho substituent effect is separated into contributions from electronic and proximity effects. A molecular fragment approach in conjunction with a rotation experiment on the COOH moiety of benzoic acid was used to quantify the proximity effects. The quantified proximity effect is in accord with previously proposed steric parameters. The proximity effect-corrected V_min of ortho systems showed excellent linear correlations to both V_min of para and meta systems which enabled the computation of the meta : para, ortho : para and meta : ortho electronic effect ratios yielding respective values of 1 : 1.108, 1 : 1.042 and 1 : 1.047. The additive nature of the substituent effects was also tested using the V_min computation on multiply-substituted benzoic acids. It is found that the total substituent effect is approximately 86.3% of the sum of the individual contributions which was in contrast to a value of 98.5% observed in aliphatic systems (Phys. Chem. Chem. Phys., 2008, 10, 6492–6499).