Crystal structure and vibrational spectra of salts of 1H-pyrazole-1-carboxamidine and its protonation route

Abstract

Crystal structures of five salts of 1H-pyrazole-1-carboxamidine, PyCA, with various inorganic acids were determined, (HPyCA)Cl, (HPyCA)Cl·H2O, (HPyCA)Br, (HPyCA)2(I)I3, and (HPyCA)HSO4. Theoretical calculations of the protonation route of PyCA showed that the cationic form present in the studied crystals is energetically privileged. Tautomeric equilibrium constants indicated two isomers as the most stable neutral forms. Calculations for two other tautomers failed resulting in pyrazole and carbodiimid tautomer of cyanamide. Such decomposition is important in a view of guanylation reaction. Hydrogen bonding patterns were studied by means of the graph-set approach. Similarities of the patterns in different crystal structures were demonstrated by the algebraic relations between descriptors of the patterns. The strength of hydrogen bonding network in the crystals was assessed analyzing vibrational spectra. The bands were assigned on the basis of theoretical calculations for the complex [(HPyCA)2Cl4]2– ion and potential energy distribution analysis. The strength of hydrogen bonds was set in the following ascending series (HPyCA)2(I)I3 (4) < (HPyCA)Br (3) < (HPyCA)Cl (1) < (HPyCA)Cl·H2O (2) < (HPyCA)HSO4 (5).