Computational study of nitroguanidine (NQ) tautomeric properties in aqueous solution

This study focused on investigation of the tautomeric properties of nitroguanidine (NQ), an energetic material, using theoretical approaches for NQ in both the gas phase and in aqueous solution. Density functional method (M062X) was used to estimate physical and chemical properties of NQ tautomers. Two different solvent models (CPCM and PCM) and two basis sets (6-31G(d,p) and 6-31 + G(d,p)) were utilized. A ranking of the stability order of the tautomers was predicted based on changes in Gibbs free energy values and were generally found to be in good agreement with previous theoretical and experimental studies. Electronic properties including dipole moment, electron affinity, ionization potential and charges of NQ were predicted in both gas and aqueous phases. In order to analyze NQ behavior in aqueous solution, several properties were calculated. Deprotonating as well as protonating of NQ revealed positive energies indicating that those processes are rather unfavorable and further characterized NQ as a compound with weak acid–base properties. We have calculated pKa and pKb values using data obtained from quantum chemical calculations using CPCM and COSMO-RS approaches. The COSMO-RS approach was able to reproduce experimental values with high accuracy, while the results obtained with CPCM method significantly differ from values obtained experimentally.