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·H2O, was first
reported in the late 1960s. It has a threefold symmetric structure,
with Ho at the center of three dibenzoylmethane ligands and hydrogen-bonded
water to ligands. It is considered that the hydrogen bonds between
the water molecule and the ligands surrounding Ho play an important
role 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 dynamics
of Ho-(DBM)3·H2O. To develop the force
field, structural optimization and molecular dynamics were performed
on the basis of ab initio calculations using the
plane-wave pseudopotential method. The force-field parameters for
CMD were then optimized to reproduce the data obtained from ab initio calculations. Validation of the developed force
field showed good agreement with the experimental crystalline structure
and ab initio data. The vibrational properties of
water in Ho-(DBM)3·H2O were investigated
by comparison with bulk liquid water. The vibrational motion of water
was found to have a characteristic mode originating from stationary
rotational motion along the c-axis of Ho(III) aqua-tris(dibenzoylmethane).
Contrary to expectations, the hydrogen-bond dynamics of water in Ho-(DBM)3·H2O were found to be almost equivalent to
those of bulk liquid water except for librational motion. This development
route for force-field parameters for CMD and the establishment of
water dynamics can advance the understanding of water-coordinated
metal complexes with high coordination numbers such as Ho-(DBM)3·H2O.