Negative Linear Compressibility in [NH 3 NH 2 ]Co(HCOO) 3 and Its Structural Origin Revealed from First Principles

Ghosh, P. S.; Ponomareva, I.

doi:10.1021/acs.jpclett.1c02156

Abstract We 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 NH₃binds 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‐NH₃complexes, 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–NH₃are less conclusive. Computational scans of the M‐N distance potentials for CH₃SiCl₃–NH₃and 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.

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