We report an extensive computational and spectroscopic study of several fluoropyridine–HCl complexes, and the parent, pyridine-HCl system. Matrix-IR spectra for pentafluoropyridine–HCl, 2,6-difluororpyridine–HCl, and 3,5-difluororpyridine–HCl in solid neon exhibit shifts for...

We have explored the structural and energetic properties of a series of RMX 3-NH 3 (M=Si, Ge; X=F, Cl; R=CH 3 , C 6 H 5) complexes using density functional theory and low-temperature infrared spectroscopy. In the minimum-energy structures, the NH 3 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 RMCl 3-NH 3 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 CH 3 SiCl 3-NH 3 , but analogous results for CH 3 GeCl 3-NH 3 are less conclusive. Computational scans of the M-N distance potentials for CH 3 SiCl 3-NH 3 and CH 3 GeCl 3-NH 3 , both in the gas phase and bulk dielectric media, reveal a great deal of anharmonicity and a propensity for condensed-phase structural change. K E Y W O R D S hydrogen bonding, molecular complexes, molecular machines, sigma holes, tetrel bonding 1 | INTRODUCTION Interest in the structure and bonding of molecular complexes (also called "charge-transfer" or "donor-acceptor" complexes) has persisted for many decades. Most notably perhaps, Odd Hassel centered the lecture celebrating his 1969 Nobel prize on "Structural Aspects of Interatomic Charge-Transfer Bonding." [1] A substantial review, [2] as well as several monographs, [3-6] was published around that time as well, and in those initial works, the foundational ideas regarding the bonding interactions in these systems were outlined. More recently, interest has been spurred, at least in part, by quantum-chemical investigations of molecular complexes, [7,8] which have revealed, more clearly, the underlying nature of the interactions in these systems. In addition, these studies have led to the onset of newly named subcategories, including "halogen" bonding, [9,10] as well as "triel" [11] and "tetrel" bonding, [12] for which the names acknowledge the geometries about the coordination centers, which in turn affect the symmetry properties of the electron-deficient regions and acceptor orbitals. In all of these cases, however, the fundamental acid-base bonding motif (electron-donor to electron-acceptor) prevails.

We have explored the structural and energetic properties of a series of RMX 3 -NH 3 (M=Si, Ge; X=F, Cl; R=CH 3 , C 6 H 5 ) complexes using density functional theory and low-temperature infrared spectroscopy. In the minimum-energy structures, the NH 3 binds axially to a halogen, while the organic group resides in equatorial site about the metal. 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 RMCl 3 -NH 3 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 axial to the organic substituent. Acid fragment charge analysis also provide a clear rationale as to why these configurations are less stable than their R-equatorial counterparts. In matrix-IR experiments, we see clear evidence of the minimum-energy form of CH 3 SiCl 3 -NH 3 , but analogous results for CH 3 GeCl 3 -NH3 are less conclusive. Computational scans of the M-N distance potentials for CH 3 SiCl 3 -NH 3 and CH 3 GeCl 3 -NH 3 , both in the gas phase and bulk dielectric media reveal a great deal of anharmonicity, and a propensity for condensed-phase structural change.