Transmission of substituent effects through five-membered heteroaromatic rings, II. Deprotonation equilibria of phenol analogues

Transmission of substituent effects through 5-membered heteroaromatic rings is investigated theoretically at the RHF/6-31+G* and B3LYP/6-31+G* levels using the deprotonation equilibria of phenol analogues with heteroatoms Y = NH, O, PH and S. The increase in the resonance delocalization of the pi lo ne-pair on the phenolic [GRAPHICS] oxygen atom, n(pi)(O), accompanied with the deprotonation depends on the he teroatom Y, in the order NH < O < PH < S. This represents the pi electron a ccepting ability, or conversely reverse order of the pi electron donating a bility of the pi lone-pair on Y, n(pi)(Y). The transmission efficiency of s ubstituent effects is, however, in the reverse order NH > O > S, which repr esents the order of delocalizability of n(pi)(Y). A better correlation is o btained with sigma(p)(-) than with sigma(p) for the Hammett type plots with the positive slope, rho(-)>0, of the magnitude in the same order as that f or the delocalizability of n(pi)(Y). The deprotonation energy, Delta G = [G (PA) + G(H+)] - G(P), decreases with the increase in the extent of resonanc e delocalization in the order NH > O > PH > S.