IR spectra of cyclic hydrogen-bonded complexes of bifunctional nitrogen compounds in solution

We measured the IR absorption spectra of self-associates and complexes with carboxylic acids of 3,5-dimethylpyrazole (DMP), diphenylformamidine (DPFA), diphenyltriazene (DPT), and diphenylguanidine (DPG) in solutions in a wide range of concentrations and temperatures and calculated spectroscopic, geometric, and energy characteristics of complexes in the quantum-mechanical harmonic and anharmonic 1D and 2D approximations. Spectroscopic data show that, in the case of DMP, cyclic trimers are predominantly formed; DPFA and DPG form cyclic dimers with two NH...N bonds in inert solvents, whereas, upon the complexation of DPT, cyclic structures do not occur, and only open dimers are formed. Upon the interaction of DMP, DPFA, and DPT with weak carboxylic acids (HCOOH, CH3COOH, CH2ClCOOH) in CCl4 or in CH2Cl2, molecular cyclic structures with NH...O=C and OH...N H-bonds are formed, whereas cyclic dimer complexes with stronger acids (CHCl2COOH, CCl3COOH, CF3COOH) predominantly have the structure of hydrogen-bonded ion pairs with proton transfer from the hydroxyl group to the proton-acceptor nitrogen atom. The calculations of the structure and vibrational frequencies using various basis sets of atomic functions confirm the formation of cyclic complexes in accordance with experimental results and, in the case of interaction with strong carboxylic acids, the proton transfer along the OH...N hydrogen bridge.