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, dimethylforamide, 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-(o)-zero spectral moment (integrated band intensity B), M-(1)-first spectral moment (band gravity center) and effective half-width (Delta nu(1/2))(eff) which is related to the second central moment M-(2): (Delta nu(1/2))(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=delta Y/delta 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 -Delta H and Delta S 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,gamma(HNH), force constants K(NH), electro optic parameters delta mu/delta q (derivative of the dipole moment by the length of NH-bond) and delta mu/delta 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 DFr-B3LYP/6-31G** and ab initio MP2/6-31G**, the atom charge distribution (according to Mulliken), geometrical and force characteristics and dipole moments mu(0) of 2-aminothiazoles were calculated. The quantum mechanical calculations, valence force field calculations and the experimental results were shown to be in good agreement. (c) 2005 Elsevier B.V. All rights reserved.