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 (nu(cg), cm(-1)) and the calculated proton affinities of the investigated anilines (PA, kcal/mol) as well as the Hammett substituent constants, (sigma(H)) were presented. A deep minimum was located at PA=210-211 kcal/mol and sigma(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, 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.