Complexation of CaX₂ With nNH₃ (X = H, F; n = 1-3): Insight From Quantum Chemical and Vibrational Mode Analysis

Quantum chemical methods are employed to investigate the complexation of CaX₂ with nNH₃ (X = H, F; n = 1-3). In the presence of CaX₂, NH₃ molecules engage in several types of noncovalent interactions, namely, hydrogen bonding (HB), calcium bonding (CB), and dihydrogen bonding (DHB). The CaX₂:nNH₃ complexes are primarily stabilized by Ca···N interactions, though other noncovalent contacts also contribute in stabilizing or destabilizing different conformers. Closed ring conformers in CaX₂:nNH₃ complexes show high degree of cooperativity together with a consistent increase in Ca···N bond length and decease in H⁺···H^-/ H⁺···F^- bond length with increasing number of NH₃ molecules. Fragment-wise interaction energy analysis indicates that two-body ammonia-metal hydride/fluoride (A_i-CaX₂) interactions dominate the total interaction energy, while nonadditive terms contribute up to ∼10% in certain heterotrimer conformers but diminish as the number of ammonia molecules increases. Vibrational mode automatic relevance determination analysis (VMARD) of CaX₂:nNH₃ complexes shows unequal contributions from atomic motions within the three different bonds in NH₃ molecule, revealing that complexation induces different intermolecular force constants, leading to loss in symmetry of NH₃ molecules. Pronounced redshift of the symmetric NH stretching mode is consistently observed, accompanied by symmetry lowering of the degenerate asymmetric NH stretching mode.