Hydrogen bond networks about solvated ions can form many alternative structures, requiring extensive conformational searches with accurate but affordable energy computation. Combining Monte Carlo searches with the OPLS-AA force field plus geometry optimizations with computational efficient density functional-based tight-binding (DFTB) method, we compute the stepwise hydration energy of the ammonium ion in NH4 +(H2O)n clusters (n = 1-8), for which experimental data are available. The resulting structures from the conformational search for each n was then re-optimized with 2 quantum chemistry methods including HF, MP2 and DFT. The binding energies were further calculated with the CBS-Q, CBS-QB3, and CBS-APNO methods. Calculated geometries and charge densities for the clusters are also presented. The results show that the calculated DFTB binding energies reproduce the experimental values as well as the energies predicted by highly correlated (and significantly more expensive from the computational point of view) ab initio quantum chemical methods. These encouraging results suggest that the computational procedure presented in this work should be useful for simulating relatively large ion/solvent systems.
Citation: Journal of Physical Chemistry A
Pub Type: Journals
Hydrogen bond, ammonium, water, tight-binding theory