Hydrogen bond types which do not fit accepted definitions.

Area of Science:

• Chemical Physics
• Quantum Chemistry
• Molecular Interactions

Background:

• Current hydrogen bond definitions are imprecise, leading to ambiguity in classifying certain interactions.
• Typical hydrogen bonds involve single-atom, electronegative centers (A-H⋯B).
• Non-typical interactions feature uncommon proton donors/acceptors or multi-center systems.

Purpose of the Study:

• To review and analyze interactions that partially or fully deviate from accepted hydrogen bond definitions.
• To discuss the characteristics and classification challenges of non-typical hydrogen bonds.
• To highlight specific examples like A-H⋯π, A-H⋯σ, and π⋯H⁺⋯π systems.

Main Methods:

• Literature review of diverse chemical interactions.
• Analysis of structural and electronic characteristics of hydrogen bonds.
• Comparison of observed interactions with established hydrogen bond criteria.

Main Results:

• Identified interactions with multi-center proton acceptors (e.g., π-electron systems) and donors.
• Discussed systems like the π⋯H⁺⋯π in acetylene dimers, classifiable under the 2sHB definition.
• Highlighted unclassified interactions such as hydride bonds and charge-inverted hydrogen bonds (CIHBs).

Conclusions:

• Existing hydrogen bond definitions require refinement to encompass a broader range of molecular interactions.
• Further research is needed to precisely categorize novel interactions like proton sponges and specific ionic systems ([FHF]⁻, [NgHNg]⁺).
• The study emphasizes the dynamic and evolving nature of understanding chemical bonding.