Complexation of CaX₂ With nNH₃ (X = H, F; n = 1-3): Insight From Quantum Chemical and Vibrational Mode Analysis

Area of Science:

• Computational Chemistry
• Quantum Chemistry
• Materials Science

Background:

• Calcium dihalides (CaX2) are important compounds with diverse applications.
• Ammonia (NH3) is a fundamental molecule involved in numerous chemical processes.
• Understanding noncovalent interactions is crucial for predicting molecular behavior and designing new materials.

Purpose of the Study:

• To investigate the complexation of CaX2 (X=H, F) with varying numbers of NH3 molecules (n=1-3).
• To analyze the types and contributions of noncovalent interactions (hydrogen bonding, calcium bonding, dihydrogen bonding) in these complexes.
• To explore the impact of complexation on the structural, energetic, and vibrational properties of NH3.

Main Methods:

• Quantum chemical calculations were employed to model CaX2:nNH3 complexes.
• Analysis of noncovalent interactions, including hydrogen bonding (HB), calcium bonding (CB), and dihydrogen bonding (DHB).
• Fragment-wise interaction energy analysis and Vibrational Mode Automatic Relevance Determination (VMARD) were utilized.

Main Results:

• CaX2:nNH3 complexes are primarily stabilized by Ca···N interactions.
• Closed ring conformers exhibit cooperativity, with Ca···N bond length increasing and H···H/F bond length decreasing as NH3 count rises.
• Two-body ammonia-metal interactions dominate, with nonadditive terms diminishing as NH3 increases.
• Complexation leads to altered intermolecular force constants, loss of NH3 symmetry, and a redshift in symmetric N-H stretching modes.

Conclusions:

• The study elucidates the intricate noncovalent interactions governing CaX2-ammonia complexation.
• Complexation significantly influences the structural and vibrational characteristics of ammonia molecules.
• These findings contribute to a deeper understanding of molecular interactions in chemical systems involving metal halides and ammonia.