Hydrogen bonds of 2-aminothiazoles in intermolecular complexes (1:1 and 1:2) with proton acceptors in solutions

IR spectra of the free molecules of 2-aminothiazole and 2-aminobenzothiazole and their H-bonded complexes of 1:1 composition with acetonitrile, dioxane, tetrahydrofourane, dimethylformamide, dimethylsulfoxide and hexamethyl-phosphoramide in CCl4 were studied in the range of stretching and deformational vibrations of the amino group. The complexes of 1:2 compositions were studied in undiluted aprotic solvents.The absorption band spectral characteristics of monomers and complexes were determined: M(0)—zero spectral moment (integrated band intensity B), M(1)—first spectral moment (band gravity center) and effective half-width (Δν1/2)eff which is related to the second central moment M(2): (Δν1/2)eff=2(M(2))1/2.The temperature influence on the absorption band spectral characteristics of the amino group stretching vibrations for monomers and 1:1 complexes were studied in the interval 283–323 K. It was shown that within the range mentioned above, spectral characteristics have practically linear dependence on temperature. Parameters of the linear regression equation Y=aT+b (Y=B, M(1), 2(M(2))1/2) were determined. It was stated that the temperature sensitivity a=∂Y/∂T of spectral characteristics for 1:1 complexes of 2-aminothiazoles with proton acceptors is approximately 10 times higher than for monomers.The monomer-complex equilibrium constants K(T) (1:1) were calculated and the thermodynamical characteristics –ΔH and ΔS were determined, based on Vant-Hoff equation.For free (R–NH2, R–NHD, R–ND2) and H-bonded (1:1 and 1:2, with various proton acceptors) molecules of 2-aminothiazoles, the vibrational and electro optic problems were solved in the approximation of the six-coordinate valence force field model. The valence angles γ(HNH), force constants K(NH), electro optic parameters ∂μ/∂q (derivative of the dipole moment by the length of NH-bond) and ∂μ/∂q′ (derivative of the dipole moment by the length of the neighboring NH-bond) were determined. The correlations were stated between spectral, geometrical, force and electro optic parameters of free and bonded molecules.In the approximation of DFT-B3LYP/6-31G** and ab initio MP2/6-31G**, the atom charge distribution (according to Mulliken), geometrical and force characteristics and dipole moments μ0 of 2-aminothiazoles were calculated.The quantum mechanical calculations, valence force field calculations and the experimental results were shown to be in good agreement.