The Anomalously High Boiling Point of Water Revisited

Typically, boiling points increase with molecular weight because dispersion forces strengthen. However, H₂O deviates from this trend, exhibiting a higher boiling point than its heavier group-16 analogs. This anomaly is traditionally attributed to water's larger dipole moment, which is thought to promote stronger electrostatic intermolecular hydrogen bonds. Yet, this textbook explanation lacks quantitative support. Here, alongside thermodynamic and structural effects, we identify a key microscopic, quantum chemical factor that explains trends in interaction strength, which in turn are associated with water's abnormally high boiling point. Within the series of binary hydrides studied, Kohn-Sham molecular orbital theory reveals that the Pauli exclusion principle is an important factor behind water's high boiling point, as it modulates intermolecular distances and, thereby, influences the strength of all attractive interactions, including Coulombic attraction. The compact H₂O molecule experiences significantly less intermolecular Pauli repulsion, allowing for shorter intermolecular distances, enhancing all attractive forces, and resulting in elevated boiling points. These findings not only provide a rigorous, quantitative framework for understanding a fundamental property of water but also call for a critical re-evaluation of conventional teaching and interpretation.