Molecular structure of H-bonded complexes of N,N-diphenylformamidine studied by IR and NMR spectroscopy and quantum chemical calculations

The results of experimental studies and theoretical calculations of vibrational frequencies and structure of H-bonded associates of N,N-diphenylformamidine and its complexes with carboxylic acids and hydrogen chloride in solution are discussed. The IR and low temperature NMR spectra show the existence of equilibrium between s-trans-monomers and cyclic dimers, such as cyclic H-bonded complexes with carboxylic acids in solution. The complexes with weak acids have molecular structure. The interaction with strong acids and hydrogen chloride results in formation of H-bonded ionic pairs with proton transfer to the N atom of the base. The results of quantum chemical calculations confirm the formation of cyclic molecular complexes with two H-bonds NH center dot center dot center dot O = C and OH center dot center dot center dot N at the interaction with weak carboxylic acids; the proton transfer along the OH center dot center dot center dot N bridge was found for the complexes with stronger proton donors. The vibrational frequencies and intensities were obtained also by a version of variational multidimensional anharmonic calculations of vibrational electrooptic parameters in space of normal coordinates. It was shown that this approach is more preferable for calculating the high-frequency XH stretch in systems where the corresponding normal mode is less characteristic and involves motions of many atoms. (C) 2007 Elsevier B.V. All rights reserved.