Abstract
Fourier transform infrared (FTIR) spectra of a series of 1 : 1 hydrogen-bonded complexes between 2,5-dihydroxy-3,6-dichlorobenzoquinone (chloranilic acid, CA) and anilines in the crystalline form were investigated. The correlation between the center of gravity of the protonic vibration bands (
ν
cg
, cm
–1
) and the calculated proton affinities of the investigated anilines (PA, kcal/mol) as well as the Hammett substituent constants, (σ
H
) were presented. A deep minimum was located at PA = 210−211 kcal/mol and σ
H
= 0.1 suggesting the existence of a critical behavior. The semi‒empiricalsemi‒empirical quantum mechanical method, AM1 was utilized to correlate the experimental FTIR data. The obtained data showed a reasonable consistency with the experimental results. An intersection point was recorded at PA = 210–211 kcal/mol in the correlations between the oxygen and nitrogen atomic charges, (q
0
and q
N
, esu), the OH and NH bond distances (
R
OH
and
R
NH
, Å) and PA values of the substituted anilines. The strict correlation between the experimental and the calculated results confirmed the validity of the AM1 semi-empirical method for studying chloranilic acid – anilines hydrogen-bonded complexes.