Heterocyclic aromatic amines in H-bonded complexes (1:1 and 1:2 composition) in solutions with various proton acceptors

The influence of the position and number of the nitrogen atoms in the aromatic ring on the proton donation ability in intermolecular H-bond, spectral, geometrical, force, electrooptic and energetic characteristics of the amino group was investigated for free and H-bonded (1:1 and 1:2, with various proton acceptors) molecules of 2-aminopyrazine (I) and 3-amino-1,2,4-triazine (II). Acetonitrile, dioxane, tetrahydrofourane, dimethylformamide, dimethylsulfoxide and hexamethylphosphoramide were used as proton acceptors. Their proton accepting properties vary in a wide range. In the range of the amino group stretching and deformation vibrations, within the temperature interval 285–330 K, IR spectra of free and H-bonded (1:1) molecules of compounds I, II in CCl4 were studied. Complexes of 1:2 composition were studied in the undiluted aprotic solvents. The following absorption band spectral characteristics were determined: M(0) – zero spectral moment (integrated intensity B), M(1) – first spectral moment (the centre of gravity of the band), M(2) – second central moment related to the effective half width (Δν1/2)eff = 2(M(2))1/2. It was shown that within the explored temperature interval the absorption band spectral characteristics of the monomers and complexes are well approximated by the linear functions Y = aT + b (Y = B, M(1), 2(M(2))1/2). The linear regression parameters for these equations were determined. The vibrational and electro-optic problems for free and H-bonded molecules of compounds I, II were solved in approximation of the R-NH2 valence force field model. Valence angles γ (HNH), force constants K (NH), electro-optic parameters ∂μ/∂q (derivative of the dipole moment by the length of the corresponding bond) and ∂μ/∂q′ (derivative of the dipole moment by the length of the neighboring bond) were determined. It was shown that the absorption band spectral characteristics of the amino group stretching vibrations of the examined compounds in CCl4 are most sensitive to the o-position of nitrogen atoms in the aromatic ring relatively to the amino group. Increasing the number of the nitrogen atoms in the ring leads to increasing the force constants K (NH), valence angle γ (HNH), and electrooptic parameters ∂μ/∂q and ∂μ/∂q′. The linear correlations were obtained between spectral, geometrical, force and electrooptic characteristics of amino group of free and H-bonded (1:1 and 1:2) molecules. The comparative analysis was performed on the dependence of the correlation relations on the position and number of the nitrogen atoms in the aromatic ring. The dependence of the monomer-complex (1:1) equilibrium constants on the temperature was examined. The thermodynamical characteristics of the complex: entropy and enthalpy of the complex formation were obtained on the basis of the Vant–Hoff equation. The H-bond enthalpy in complexes of 1:2 composition was calculated using A.V. Iogansen’s “intensities rule”. The basic regularities observed in the spectra of free molecules of compounds I, II are confirmed by the quantum mechanical calculations in the DFT-B3LYP/6-31G** approximation and ab initio MP2/6-31G**.