Directionality of Halogen Bonds: An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Study.

Area of Science:

• Quantum chemistry
• Computational chemistry
• Chemical bonding

Background:

• Halogen bonds are crucial non-covalent interactions.
• Understanding halogen bond directionality is key to predicting molecular interactions.
• Existing models often focus on primary halogen-nitrogen interactions.

Purpose of the Study:

• To elucidate the origin of directionality in halogen bonds (X=Cl, Br).
• To identify the energetic contributions governing halogen bond angular preferences.
• To analyze the role of intra- and interatomic energies in system behavior.

Main Methods:

• Interacting Quantum Atoms (IQA) analysis to calculate intra- and interatomic energies.
• Interacting Quantum Fragments (IQF) analysis for fragment-based energy contributions.
• Relative Energy Gradient (REG) method to rank energy contributions and identify key factors.

Main Results:

• Halogen-nitrogen interaction energy correlates with total system energy but has a small contribution.
• Secondary interactions (e.g., C-H···N, F···N) and atomic self-energies significantly influence angular preferences.
• REG analysis confirmed the dominant role of self-energies and secondary interactions over direct halogen-nitrogen interactions.

Conclusions:

• Halogen bond directionality arises from a complex interplay of factors, including secondary interactions and atomic self-energies.
• IQA and IQF analyses, coupled with REG, provide a detailed energetic picture of halogen bonding.
• The study highlights the limitations of focusing solely on primary interactions for explaining halogen bond geometry.