First solvation shell structure and dynamics of solvated Ca²⁺ in dilute aqueous ammonia by first principle approach: a QMCF MD simulation study

The investigation of solvated ion in nonaqueous or mixed solvent is a challenging task for experimental and theoretical chemistry. One of the promising approaches to elucidate the properties of solvated ion in mixed solvents is quantum mechanical charge field molecular dynamics (QMCF MD) simulation. In this study, we report the first application of QMCF MD simulation to investigate the structural and dynamical properties of solvated Ca²⁺ in 18.4% aqueous ammonia. Radial distribution function analysis showed that the average distances of Ca²⁺–N and Ca²⁺–O are 2.55 and 2.74 Å, respectively. The mean residence times for water and ammonia in the first solvation shell were calculated to be 2.8 and 2.74 ps, respectively. These values indicated a labile first solvation shell of Ca²⁺ in 18.4% aqueous ammonia. Meanwhile, angular distribution function analysis revealed the polyhedral structure of the first solvation shell. The average coordination numbers of 5.1 and 2.7 were obtained for water and ammonia, respectively, during the simulation. The presented simulation data provide detailed information about the properties of solvated Ca²⁺ in aqueous ammonia which will be beneficial to the investigation of the role of the ion in biological processes. Graphic abstract: