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computational models are validated against various experimental data in order to assess overall accuracy and identify the method most apt for a given situation. In my research group, we have often validated quantum-chemical structure predictions against experimental gas-phase geometries. However, these structures are formally different; the experimental results reflect average distances that result upon ground-state vibrational averaging, whereas the theory predicts equilibrium geometries. Almost always, this distinction is trivial, and beyond the precision afforded by either experiment or theory. In one rather noteworthy instance, however, the CH3CN–BF3 complex, we demonstrated that there was a genuine, and rather significant difference between the experimental and theoretical geometries; the experimental B-N distance that was nearly 0.2 Å longer than that in the theoretically-determined equilibrium geometry. This resulted from an extreme anharmonicity in the donor-acceptor (B-N) potential, which manifested a significant asymmetry in ground state vibrational wavefunction. In our recent work, we have a similar, albeit more subtle trend in several other donor-acceptor systems including H2O–SO3, H3N–SiF4, C6H5N–SO2, and H3N–SO3. What we have noted specifically, is that the predicted donor-accptor bond lengths among several DFT and post-HF methods (with the aug-cc-pVTZ and aug-cc-pVQZ basis sets) are incredibly consistent, yet they differ from the experimental bond lengths to an extent that exceeds the quoted uncertainty. For pyridine-SO2, these differences rival CH3CN–BF3 (about 0.2) and in fact, the shape of the donor-acceptor potentials are quite similar. The other cases are more subtle, with experiment-theory differences of several hundredths of an angstrom. Yet the donor-acceptor potential curves in these systems are significantly anharmonic. The extent of the asymmetry in the ground vibrational wavefunction for the donor-acceptor stretching mode will be explored explicitly, in order to quantitatively assess the differences between the equilibrium and vibrationally averaged structures. 

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 the H–Cl stretching band that parallel the effects of fluorination on hydrogen-bond strength. Analogous spectral shifts observed across various host environments (solid neon, argon, and nitrogen) for pentafluoropyridine–HCl and 2,6-difluororpyridine–HCl convey a systematically varying degree of matrix stabilization on the hydrogen bonds in these complexes. An extended quantum-chemical study of pyridine–HCl and eight fluorinated analogs, including 2-, 3-, and 4-fluoropyridine–HCl, 2,6- and 3,5-difluororpyridine–HCl, 2,4,6- and 3,4,5-trifluropyridine–HCl, as well as pentafluoropyridine–HCl, was also performed. Equilibrium structures and binding energies for the gas-phase complexes illustrate two clear trends in how fluorine substitution affects hydrogen bond strength; increasing fluorination weakens these interactions, yet substitution at the 2- and 6-positions has the most pronounced effect. Bonding analyses for a select subset of these systems reveal shifts in electron density that accompany hydrogen bonding, and most notably, the values of the electron density at the N–H bond critical points among the stronger systems in this subset significantly exceed those typical for moderately strong hydrogen-bonds. We also explored the effects of dielectric media on the structural and bonding properties of these systems. For pyridine–HCl, 3-fluoropyridine–HCl, and 3,5-difluororpyridine–HCl, a transition to proton transfer-type structures is observed at ε -values of 1.2, 1.5, and 2.0, respectively. This is signaled by key structural changes, as well as an increase in the negative charge on the chorine, and dramatic shifts in topological properties of the H–Cl and N–H bonds. In the case of pentafluoropyridine–HCl, and 2,6-difluororpyridine–HCl, we do not predict proton transfer in dielectric media up to ε = 20.0. However, there are clear indications that the media enhance hydrogen-bond strength, and moreover, these observations are completely consistent with the experimental IR spectra. 

We have explored the structural and energetic properties of OC–BX 3 (X = F, Cl, or Br) complexes using computations and low-temperature infrared spectroscopy. Quantum-chemical calculations have provided equilibrium structures, binding energies, vibrational frequencies, and B–C potential energy curves. The OC–BF 3 system is a weak, long-bonded complex with a single minimum on the B–C potential ( R (B–C) = 2.865 Å). For the remaining two complexes, OC–BCl 3 and OC–BBr 3 , computations predict two stable minima on their B–C potential curves. The BCl 3 system is a weak complex with a long bond ( R (B–C) = 3.358 Å), but it exhibits a secondary, meta-stable minimum with a short bond length of 1.659 Å. For OC–BBr 3 , the system is a weak complex with a relatively short bond of 1.604 Å (according to wB97X-D/aug-cc-pVTZ), but also has a secondary minimum at R (B–C) = 3.483 Å. This long-bond structure is the global minimum according to CCSD/aug-cc-pVTZ. In addition, the long-bond forms of both OC–BCl 3 and OC–BBr 3 were observed in matrix-isolation IR experiments. The measured CO stretching frequencies were 2145 cm −1 and 2143 cm −1 , respectively. No signals due to the short-bond forms of OC–BCl 3 and OC–BBr 3 were observed. 

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.