[Be(NH ₃ ) ₁₆ ] ^[2] ⁺ Microsolvation: Structure, Energetics, and Temperature Effects

Gas-phase structural, energetic, and thermal properties of the [Be(NH₃)₁₆]² cluster are investigated by using the MP2/6-311++G** level of theory. The relative stability of isomers is explained evidencing the long-range electrostatic interactions and the spatial arrangement of NH₃ ligands around Be² ⁺ cation. The computed isomers binding strength and energies values are compared with that with beryllium cation coordinated from n = 1-6 ligands. A fitting approach yields an asymptotic binding energy of -32.2 kcal mol^-1. Clustering energies suggest a compact and strongly bound first solvation shell, with weaker, secondary interactions beyond four to five ligands. The cluster thermal behavior is probed through temperature-dependent solvation enthalpies (ΔH) and free energies (ΔG) in gas phase. Results show that ΔG slightly decreases with temperature, while ΔH increases, emphasizing the role of entropy in thermal stabilization. Finally, Quantum Theory of Atoms in Molecules analysis reveals the coexistence of Be² ⁺N coordination bonds and a network of NH…N hydrogen bonds. These cooperative noncovalent interactions significantly enhance both structural and energetic stability.