Ck. Sohn et al., Transmission of substituent effects through five-membered heteroaromatic rings, II. Deprotonation equilibria of phenol analogues, B KOR CHEM, 21(9), 2000, pp. 891-895
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
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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.