Structural and energetic properties of RMX3‐NH3 complexes

Abstract

AbstractWe have explored the structural and energetic properties of a series of RMX3‐NH3 (M=Si, Ge; X=F, Cl; R=CH3, C6H5) complexes using density functional theory and low‐temperature infrared spectroscopy. In the minimum‐energy structures, the NH3 binds axially to the metal, opposite a halogen, while the organic group resides in an equatorial site. Remarkably, the primary mode of interaction in several of these systems seems to be hydrogen bonding (C‐H‐‐N) rather than a tetrel (N→M) interaction. This is particularly clear for the RMCl3‐NH3 complexes, and analyses of the charge distributions of the acid fragment corroborate this assessment. We also identified a set of metastable geometries in which the ammonia binds opposite the organic substituent in an axial orientation. Acid fragment charge analyses also provide a clear rationale as to why these configurations are less stable than the minimum‐energy structures. Matrix‐isolation infrared spectra provide clear evidence for the occurrence of the minimum‐energy form of CH3SiCl3–NH3, but analogous results for CH3GeCl3–NH3 are less conclusive. Computational scans of the M‐N distance potentials for CH3SiCl3–NH3 and CH3GeCl3–NH3, both in the gas phase and bulk dielectric media, reveal a great deal of anharmonicity and a propensity for condensed‐phase structural change.