Thermodynamic properties of the hydrogen bonded complexes between N-substituted anilines and proton acceptors

Abstract

The influence of temperature, within the range 285–340 K, on spectral characteristics of ν(NH) absorption bands in ‘free’ N-methyl aniline, N-ethyl aniline, diphenylamine and N-methyl-4-nitroaniline in CCl 4 as well as in their hydrogen bonded complexes with acetonitrile, tetrahydrofurane, dimethylformamide (DMF), dimethylsulfoxide (DMSO) and hexamethylphosphoramide (HMPA) was studied. Spectral moments of ν(NH) absorption bands were determined: M (0)—the zero spectral moment (integrated intensity), M (1)—the first spectral moment (the centre of band gravity), M (2)—the second central moment as well as ‘effective’ half width of absorption band (Δν 1/2) eff=2( M (2)) 1/2. The coefficients of the linear correlation of these parameters with a temperature variation Y= aT+ b ( Y= M (0), M (1), 2( M (2)) 1/2) were calculated for ‘free’ and hydrogen-bonded molecules. It was demonstrated that these spectral characteristics considerably depend on the character of the N-substitute. The difference in the position of absorption bands ν m(NH) in the spectra for non-bonded molecules of N-substituted anilines in CCl 4 is caused by σ–π conjugation of alkyl radicals with the N atom and substitute polarization influence. Thermodynamic parameters −Δ H and Δ S of the complex formation process between amines and proton acceptors were determined from the temperature dependence of the equilibrium constants on the basis of van't Hoff equation. The increase of −Δ H enthalpy was observed in the rows of proton donors: N-alkyl-substituted anilines, diphenylamine and N-methyl-4-nitroaniline; and proton acceptors: acetonitrile, tetrahydrofurane, DMF, DMSO and HMPA. The correlations between −Δ H values and the shift of the first spectral moment Δ M c (1)= M m (1)− M c (1), on the one hand, and −Δ H values and the increment of the square root of integrated intensity Δ B 1/2= B c 1/2− B m 1/2, on the other hand, were found on passing from free to bonded molecules. However, the proportionality factor α in the equation −Δ H=αΔ B 1/2 depends on individual characteristics of proton donors and remains the same for each proton donor in the row of proton acceptors.