Formation of small clusters of NaCl dihydrate in the gas phase†
Isomers of (NaCl)n(H2O)2n clusters, with n = 1, 2 and 4, were investigated using an implementation of the simulated annealing method that imposes a virtual confinement on each isomer to stay within a sphere with an arbitrary radius. By using several confinement radii, we reported 2, 56 and 38 isomers for n = 1, 2 and 4, respectively. For n = 1, the CCSD(T)/aug-cc-pVTZ method was used as a reference by optimizing the corresponding isomers. These results were contrasted with those obtained by the MP2 method and eight exchange–correlation functionals: PBE, PBE0, B3LYP, PBE-D3, PBE0-D3, B3LYP-D3, LC-ωPBE and M06-2X. From this analysis we conclude that the M06-2X method gives reliable results and then, isomers with n = 2 and 4 were analyzed with this exchange–correlation functional. From the analysis of these two systems, the resulting isomers with the lowest energy showed cuboid structures and no breaking bond was present for NaCl molecules. Thus, in the nucleation process of the NaCl dihydrate our results suggest that the Na–Cl bond is present. Besides, bond length and electron density evaluated at bond critical points for the biggest system delivered values close to those found in the crystal structure.
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