Molecular Dynamics Studies of the Ho(III) Aqua-tris(dibenzoylmethane) Complex: Role of Water Dynamics

The seven-coordinate Ho(III) aqua-tris(dibenzoylmethane)(DBM)complex,referred to as Ho-(DBM)(3)& BULL;H2O, was firstreported in the late 1960s. It has a threefold symmetric structure,with Ho at the center of three dibenzoylmethane ligands and hydrogen-bondedwater to ligands. It is considered that the hydrogen bonds betweenthe water molecule and the ligands surrounding Ho play an importantrole in the formation of its symmetrical structure. In this work,we developed new force-field parameters for classical molecular dynamics(CMD) simulations to theoretically elucidate the structure and dynamicsof Ho-(DBM)(3)& BULL;H2O. To develop the forcefield, structural optimization and molecular dynamics were performedon the basis of ab initio calculations using theplane-wave pseudopotential method. The force-field parameters forCMD were then optimized to reproduce the data obtained from ab initio calculations. Validation of the developed forcefield showed good agreement with the experimental crystalline structureand ab initio data. The vibrational properties ofwater in Ho-(DBM)(3)& BULL;H2O were investigatedby comparison with bulk liquid water. The vibrational motion of waterwas found to have a characteristic mode originating from stationaryrotational motion along the c-axis of Ho(III) aqua-tris(dibenzoylmethane).Contrary to expectations, the hydrogen-bond dynamics of water in Ho-(DBM)(3)& BULL;H2O were found to be almost equivalent tothose of bulk liquid water except for librational motion. This developmentroute for force-field parameters for CMD and the establishment ofwater dynamics can advance the understanding of water-coordinatedmetal complexes with high coordination numbers such as Ho-(DBM)(3)& BULL;H2O. Developednew force-field parameters for molecular dynamicssimulations can theoretically elucidate the structure and dynamicsof the Ho aqua-tris(dibenzoylmethane) complex. The rotational motionof water molecules forming hydrogen bonds with neighboring O in ligandswas discontinuously broken and reformed. Interestingly, the hydrogen-bondlifetime is comparable to the bulk state.

Automated summary

New force-field parameters were developed to theoretically elucidate the structure and dynamics of the Ho aqua-tris(dibenzoylmethane) complex. The developed force-field parameters showed good agreement with experimental and ab initio data. Contrary to expectations, the hydrogen-bond dynamics of water in Ho-(DBM)(3)& BULL;H2O were found to be almost equivalent to bulk liquid water.