Complexation mechanism in LiH-ammonia borane complexes: insights from chemical shielding and vibrational analysis

The influence of LiH on nNH₃BH₃ (n = 1, 2) complexes is investigated using quantum chemical analysis, employing various density functional theory (DFT), along with MP2 calculations, complemented by DLPNO-CCSD(T) benchmark calculations, to describe their potential energy surfaces and intermolecular interactions. This provides a computationally efficient and reliable framework for an acceptable description of diffuse electron density and intermolecular polarization. Analyses based on quantum theory of atoms in molecules (QTAIM) and natural bond orbital (NBO) reveal that stabilization is primarily governed by dihydrogen bonding (H⋯H interactions), accompanied by strengthening of the B-N bond. Many-body interaction energy decomposition shows that stabilization is dominated by pairwise interactions, while cooperative many-body effects contribute to only ∼5%. Among the investigated DFT functionals, CAM-B3LYP-D3(BJ) and ωB97X-D3 provide an accurate description of the non-additive contributions. Vibrational analysis shows significant mode coupling and blue shifts in N-H and B-N stretching frequencies. Chemical shielding calculations suggest saturation of the electronic environment beyond hetero-dimer formation and highlight the anisotropic character of the H⋯H interactions. These results highlight the role of LiH in modulating intermolecular interactions, and interplay between dihydrogen bonding, bond strengthening, and weak many-body effects, providing fundamental insight into interaction mechanisms relevant to hydrogen-rich molecular systems.