Investigation of Cu²⁺ Hydration and the Jahn−Teller Effect in Solution by QM/MM Monte Carlo Simulations

Abstract

The Cu2+ hydration shell structure has been studied by a combined ab initio quantum mechanical/molecular mechanical (QM/MM) Monte Carlo simulation, in which the ion and its first hydration sphere are treated at the Born−Oppenheimer ab initio quantum mechanical level, while classical pair and three-body potentials are employed for the remaining system. Whereas traditional simulations based on MM potentials are not able to predict higher-body effects such as the Jahn−Teller (JT) distorted octahedral structure of the first hydration shell of Cu2+, the combined QM/MM approach reproduces correctly this experimentally confirmed property and delivers more accurate hydration energy values as well.

New Insights into the Jahn–Teller Effect through ab initio Quantum‐Mechanical/Molecular‐Mechanical Molecular Dynamics Simulations of CuII in Water

“…This effect is more dominant than in a previous QM/ MM MC simulation (see Table 3), [12] which had supplied a similar distance for the first peak (2.08 ä) but a smaller distance for the second one (4.2 ä). The smaller shift of the second hydration shell to larger Cu-O distances in the QM/MM simulation proves the influence of the different ligand arrangement in the QMtreated first hydration shell on the second shell.…”

“…[12,48] It has been shown that nonadditivity plays a significant role for an accurate description of more subtle solvent effects. This is in agreement with our simulation results which clearly demonstrate that the description of the system with classical potentials is not adequate.…”

“…For Cu II , ab initio studies of successive hydration and QM/MM MC simulations have proven that even three-body terms are not sufficient to describe this ion in solution correctly. [12] The method used in this work for the classical approach is the direct ab initio calculation of three-body energy points and subsequent fitting of the resulting energy values to a chosen analytical formula. In contrast to two-body potentials, a complete sampling of a three-body hypersurface is impossible because of the complex shape of the hypersurface, which means a reduction to chemically relevant configurations during the fitting.…”

“…Earlier MC simulations for hydrated Cu II including three-body potentials [48] and QM/MM MC simulations treating the first hydration shell quantum mechanically [12] led to a first hydration shell containing six water molecules. The recently reported preference of a five-fold coordinated complex resulting from ™first-principles∫ MD simulations (i.e., Car ± Parrinello (CP) DFT) [29] is in strong contrast to these simulations and all experimental data [14,16,17,20] , except a single NDIS (NDIS neutron diffraction with isotopic substitution) result that has been published along with these CP simulations.…”

QM/MM: what have we learned, where are we, and where do we go from here?

“…An important issue that arises in simulating liquid-state phenomena and diffusion through solids is the adaptive movement of the quantum mechanical region, which is called the "hot spot" [50,77,116,178]. Algorithms have been reported for liquid-phase simulation that allows water molecules to enter and leave the QM region dynamically.…”