Hydrogen bonding is a fundamental non-covalent interaction that plays a crucial role in determining the structure and properties of molecular systems, from small clusters to complex biomacromolecules. While the IUPAC definition of hydrogen bonding has long been considered comprehensive, recent studies have uncovered intriguing complexities, particularly in the case of hydrogen bonds involving hydridic hydrogen atoms. This groundbreaking research sheds light on the remarkable similarities between protonic and hydridic hydrogen bonds, despite their seemingly opposite electronic characteristics.

Unraveling the Intricacies of Hydrogen Bonding

Exploring the Dual Nature of Hydrogen

The periodic table of elements reveals a fascinating duality in the nature of hydrogen. While hydrogen is typically covalently bound to more electronegative atoms, forming what are known as protonic hydrogen bonds, it can also be bound to less electronegative atoms, resulting in hydridic hydrogen bonds. This distinction has profound implications for the electronic properties and charge transfer dynamics within these hydrogen-bonded systems.

Protonic vs. Hydridic Hydrogen Bonds: Opposite Directions, Surprising Similarities

In protonic hydrogen bonds, the hydrogen atom bears a partial positive charge, acting as an electron acceptor, with the electron density flowing from the electron-rich donor to the hydrogen. Conversely, in hydridic hydrogen bonds, the hydrogen atom carries a partial negative charge, behaving as an electron donor, with the electron density flowing in the opposite direction.Despite these fundamental differences in the direction of charge transfer, the experimental and computational analyses of the C₆F₅H⋯NH₃ and Et₃GeH⋯ICF₃ complexes, representing protonic and hydridic hydrogen bonds, respectively, reveal a remarkable similarity in their spectral characteristics. Both systems exhibit a significant red shift in the X-H stretching frequency and a substantial increase in the intensity of the corresponding spectral band upon complex formation.

Unifying Protonic and Hydridic Hydrogen Bonds: A New Perspective

The consistent observation of these spectral changes, regardless of the hydrogen's electronic character, suggests a deeper underlying principle that unifies protonic and hydridic hydrogen bonds. Computational analyses of a broader set of 30 protonic and 30 hydridic hydrogen-bonded complexes further corroborate this finding.The key to understanding this unification lies in the changes in electron density within the X-H bond upon complex formation. In protonic hydrogen bonds, the increase in electron density in the X-H antibonding orbital weakens and elongates the bond, leading to the characteristic red shift and intensity increase. Conversely, in hydridic hydrogen bonds, the decrease in electron density in the X-H bonding orbital also results in bond weakening and elongation, manifesting in similar spectral changes.

Redefining Hydrogen Bonding: Towards a Unified Perspective

The IUPAC definition of hydrogen bonding, while comprehensive in its recognition of both red-shifted and blue-shifted hydrogen bonds, does not explicitly encompass the hydridic hydrogen bonds. This has led to the introduction of alternative terminology, such as "hydride bonds" or "charge-inverted hydrogen bonds," which are fundamentally at odds with the existing definition.The findings of this study suggest that a revision of the IUPAC definition may be warranted to accommodate both protonic and hydridic hydrogen bonds under a unified framework. This would not only resolve the nomenclature issues but also provide a more comprehensive understanding of the diverse range of hydrogen-bonding phenomena.As the scientific community continues to explore the intricacies of hydrogen bonding, this research serves as a crucial step towards a more inclusive and nuanced understanding of this fundamental non-covalent interaction. By recognizing the remarkable similarities between protonic and hydridic hydrogen bonds, we can unlock new avenues for the design and manipulation of molecular systems, with far-reaching implications across various fields of science.