The Long and Short Radii for Hydrogen in Hydrogen Bonded Complexes

Area of Science:

• Chemical Physics
• Crystallography
• Computational Chemistry

Background:

• Traditional hydrogen bond identification relies on comparing distances to van der Waals (vdW) radii or defined hydrogen bond radii (HBR).
• These methods unrealistically assume atoms behave as perfect spheres, potentially leading to inaccuracies in characterizing hydrogen bonds.

Purpose of the Study:

• To investigate the atomic structure of hydrogen in hydrogen bond complexes using electron density profiles.
• To challenge the spherical atom assumption in hydrogen bond distance calculations.
• To propose a more accurate, directional model for hydrogen bond radii (HBR).

Main Methods:

• Analysis of electron density profiles to determine the shape of hydrogen atoms in isolated HD molecules and A•••HulletulletulletD complexes.
• Quantum chemical calculations and Atoms in Molecules (AIM) theory to validate findings.
• Examination of hydrogen bonds in the CCDC database (OulletulletulletH•••N and NulletulletulletH•••N).

Main Results:

• Hydrogen atoms exhibit an elliptical shape, with the minor axis along the HulletulletulletD bond.
• Hydrogen bond radii (HBR) show angle dependence due to the anisotropic structure of hydrogen atoms in non-linear bonds.
• The study validates the utility of directional, environment-dependent short and long radii over spherical models.

Conclusions:

• The assumption of spherical atoms is inadequate for precise hydrogen bond analysis.
• Directional and environment-dependent hydrogen bond radii (HBR) are necessary for accurate identification.
• This research provides a more refined approach to understanding and quantifying hydrogen bonding interactions.