FTIR spectroscopic studies and AM1 semi‒empirical calculations of some hydrogen‒bonded complexes of 2,5‒dihydroxy‒3,6‒dichlorobenzoquinone and anilines

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 (νcg, cm–1) and the calculated proton affinities of the investigated anilines (PA, kcal/mol) as well as the Hammett substituent constants, (σH) were presented. A deep minimum was located at PA = 210−211 kcal/mol and σH = 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, (q0 and qN, esu), the OH and NH bond distances (ROH and RNH, A) 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.