Hydrogen bonding and delocalization in the ELF analysis approach

Area of Science:

• Quantum Chemistry
• Materials Science
• Solid-State Physics

Background:

• Understanding electron density delocalization is crucial for characterizing hydrogen bond strength.
• Existing methods for fragment partitioning in hydrogen bonds have limitations.

Purpose of the Study:

• To investigate electron density delocalization in proton donor fragments for a wide range of hydrogen bond strengths.
• To correlate electron density variance with hydrogen bond strength using the Electron Localization Function (ELF) partition.
• To apply the ELF partition method to study hydrogen-bonded crystals.

Main Methods:

• Studied 21 molecular complexes (A-HB) with varying hydrogen bond strengths.
• Defined proton donor and acceptor fragments using the minimum variance principle via ELF partition.
• Compared ELF partition with the Quantum Theory of Atoms in Molecules (QTAIM) framework.
• Applied ELF to analyze electron density in KHF2, KDP, and ice VIII (including pressure-induced phase transitions).

Main Results:

• Electron density variance and charge transfer are correlated with hydrogen bond strength for both ELF and QTAIM.
• The ELF partition method provides a more accurate measure of variance and charge transfer compared to QTAIM.
• Variance scales with the square root of the ELF value at the interaction point, validating its use as an indicator of hydrogen bond strength.
• Analysis of crystals revealed very strong hydrogen bonds in KHF2 and KDP, and medium-weak bonds in ice VIII, which strengthen with pressure leading to a phase transition to ice X.

Conclusions:

• The choice of partition scheme significantly impacts the analysis of proton donor fragments.
• Electron density delocalization, often mislabeled as covalence, plays a key role in hydrogen bonding.
• The ELF partition method offers a robust approach for quantifying hydrogen bond strength and electron density delocalization in molecular complexes and crystals.