Infrared and ab initio studies of hydrogen bonding and proton transfer in the complexes formed by pyrazoles

Abstract The results of experimental studies and quantum mechanical calculations of vibrational spectra and structure of hydrogen bonded complexes formed by pyrazole (P) and 3,5-dimethylpyrazole (DMP) are presented. IR spectra of pyrazoles in solutions, gas phase, and solid state have been investigated in wide range of concentrations and temperatures. It has been found that in the gas phase both P and DMP reveal the equilibrium between monomers, dimers, and trimers. In solutions the equilibrium between monomers and trimers dominates, no bands, which can be attributed to dimers were detected. DMP retains the trimer structure in solid state, while in the case of pyrazole P, formation of the crystal provides another type of association. Geometrical and spectral characteristics of dimers and trimers, obtained by ab initio calculations, are presented and compared with experimental data. IR spectra of solutions containing P and DMP with a number of acids (acetic and trifluoroacetic acids, pentachlorophenol, HBr) have been studied in parallel with ab initio calculations. It has been found that pentachlorophenol forms with pyrazoles complexes with one strong hydrogen bond O–H⋯N, while NH pyrazole group remains unbonded. With carboxylic acids DMP forms 1:1 cyclic complexes with two hydrogen bonds. In the case of acetic acid, the complex in CH 2 Cl 2 solution reveals molecular structure with OH⋯N and CO⋯HN bonds, in accordance with results of the calculations. For trifluoroacetic acid, the calculations predict the molecular structure to be energetically more stable in the case of the isolated binary complex (in gas phase), while the experimental spectrum of CH 2 Cl 2 solution gives an evidence of the proton transfer with formation of the cyclic ionic pair with two NH + ⋯O − bonds. The agreement with experimental results can be improved by taking into account the influence of environment in the framework of Onsager or Tomasi models. The shape of proton potential function of the complexes and medium effect on its parameters, resulted from experimental data and calculations, are discussed. It has been found that the number of potential minima and their relative depth depend strongly on the method of calculations and the basic set. Under excess of trifluoroacetic acid, the formation of 2:1 acid–DMP complex has been detected. Spectral characteristics and results of calculations point to the cyclic structure of this complex, which includes homoconjugated bis-trifluoroacetate anion and DMPH + cation. With HBr both studied pyrazoles were found to form ionic complexes including one or two pyrazole molecules per one acid molecule and correspondingly monocation or homoconjugated cation BHB + .