The Hammett parameter is interpreted as a measure of electron withdrawing strength (for positive values) and is defined for both para-(σ p ) and meta-(σ m ) positions relative to the carboxylic acid group. The electron withdrawing or donating effect can be decomposed into inductive (σ i ) and resonance (σ r ) contributions, such that σ p ≈ σ i + σ r and σ m ≈ σ i , where meta-substituents lack a resonance component since no probable resonance structure exists for this configuration. Thus, comparing para-and meta-substituted species makes it possible to determine whether inductive or resonance effects are responsible for an observed linear trend. These parameters are tabulated in Table .