Ligand-exchange processes on solvated beryllium cations - Part III. Which model reaction is preferable for quantum-chemical investigations of a water-exchange mechanism?

On the basis of DFT calculations (B3LYP/6-311 + G**), the possibility to include solvent effects is considered in the investigation of the H2O-exchange mechanism on [Be(H2O)(4)](2+) within the widely used cluster approach. The smallest system in the gas phase, [Be(H2O)(4)(H2O)(2+), shows the highest activation barrier of + 15.6 kcal/mol, whereas the explicit addition of five H-bonded H2O molecules in [{Be(H2O)(4)(H2O)}(H2O)(5)](2+) reduces the barrier to + 13.5 kcal/mol. Single-point calculations applying CPCM (B3LYP(CPCM:H2O)/6-311 + G**//B3LYP/6-311 + G**) on [Be(H2O)(4)(H2O)](2+) lower the barrier to + 9.6 kcal/mol. Optimization of the precursor and transition state of [Be(H2O)(4)(H2O)](2+) within an implicit model (B3LYP(CPCM:H2O)/6-311 + G** or B3LYP(PCM:H2O)/6-311 + G**) reduces the activation energy further to + 8.3 kcal/mol but does not lead to any local minimum for the precursor and is, therefore, unfavorable.