Molecular Interactions and Structure of H-Bonded Complexes of Bifunctional Nitrogen-Containing Molecules

The results of experimental studies and quantum chemical calculations of vibrational spectra and struc ture of H-bonded complexes formed by N-containing bases 3,5-dimethylpyrazole, diphenyltriazene and diphenylformamidine with carboxylic acids and HHal are discussed. The IR spectra of solutions in CH2Cl2 and CCl4 and low temperature NMR spectra show the existence of equilibrium between monomers and cyclic selfassociates of the mole ules studied. Under the interaction with weak carboxylic acids the complexes have molecular structure with two H-bonds NH…O=C and OH…N, and the interaction with strong acids results information of cyclic H-bonded ionic pairs with proton transfer to the N atom of the base. The proton transfer in solution was observed also for open complexes with HHal. The quantum chemical DFT calulations in harmonic and an harmonic approximations confirm the formation of cyclic complexes, while the proton transfer along the OH…N bridge was supported for the complexes with strongest ac ids. It was shown that the use of variational multidimensional an harmonic approach is the most preferable for calculations of the high-frequency XH stretch in systems where the corresponding normal mode is less characteristic and involves motions of many atoms.