Geometrical Isotope Effects on Chemical Bonding in Hydrogen Bonded Systems: Combining Nuclear-Electronic Orbital DFT and Energy Decomposition Analysis

Abstract

We investigated primary and secondary geometric isotope effects (H, D, T) on charge-inverted hydrogen bonds (CIHB) and dihydrogen bonds (DHB) using nuclear-electronic orbital density functional theory (NEO-DFT). The dianionic but electrophilic boron cluster [B₁₂H₁₂]²⁻ served as a bonding partner, exhibiting a negatively polarized hydrogen atom in the BH bond. CIHB systems included interactions with Lewis acids (AlH₃, BH₃, GaH₃) and carbenes (CF₂, CCl₂, CBr₂), while DHBs were analyzed with NH₃, HF, HCl, and HBr. Isotope substitution systematically decreased intermolecular and intramolecular bond lengths (H > D > T). Energy decomposition analysis (EDA) combined with Hirshfeld partial charge analysis confirmed the bonding interpretation but revealed significant variations in bonding contributions across different complexes. While some systems exhibited increased electrostatic attraction, others showed enhanced orbital interactions or shifts in Pauli repulsion, which could stabilize or destabilize the interaction. Natural orbital for chemical valence (NOCV) analysis highlighted charge depletion from the partially negative hydrogen towards the vacant orbital of the bonding partner in CIHB systems, further supporting the bonding model. This study demonstrates how isotope substitution influences electronic structure and lays the groundwork for extending such analyses to more strongly bound systems, where isotope effects may be more pronounced.