The overall research goal in our group is to identify and characterize donor-acceptor complexes that undergo major structural changes when their chemical environment is varied (i.e., gas, solid, solution, etc.). Much of our initial effort involved nitrile - BF3 systems, which for one, exhibit major gas-solid structure differences. In addition, frequency shifts observed in low-temperature IR spectra from a series of inert solid hosts (e.g., Ne, Ar, etc.) indicate that even noble gas matrices offer enough “solvent” stabilization to drive the contraction of the B-N coordinate bond. Ultimately, we demonstrated that these medium effects could be optimized via substituents. At this point, we extend these investigations to complexes involving MX4 and MX3R Lewis acids (M=Si, Ge; X=F, Cl; R=CH3, C6H5). The premise is that the geometries of these acceptors, especially MX3R acids due to the lone organic substituent, may offer a means to embed a medium-sensitive structural motif into larger assembly, which would facilitate the exploitation of this “tunable” structural chemistry for various materials applications (e.g., molecular machines, piezoelectrics). The first step, however, is assessing the overall acceptor strength of the MX3R acids towards various donors, as well as the preferred coordination arrangements. As such, we have investigated the ammonia complexes of various MX3R acceptors using both computations and low-temperature IR spectroscopy. For the four NH3-MX3R systems (M=Si, Ge; X=F, Cl), the minimum energy geometries have the ammonia in an axial position, and the methyl group in an equatorial position. Overall, these complexes are weak with relatively long M-N distances, and binding energies of roughly 2 to 8 kcal/mol. The structures with both methyl and NH3 in axial sites are metastable, and lie about 2 to 4 kcal/mol higher in energy. Interestingly, computational scans of the M-N potential curves for the X=Cl systems are quite anharmonic, and exhibit two distinct minima (even at high levels of theory: CCSD/aug-cc-pVTZ). Moreover, the inner, metastable minima are preferentially stabilized in bulk dielectric media, which signals potentially tunable structures. However, matrix-IR data for NH3–SiCl3CH3 and NH3–GeCl3CH3 are consistent with predictions for the equilibrium (gas-phase) structures. Preliminary results for the pyridine complexes, the analogous 2:1 (donor:acceptor) systems, and the ramifications for materials applications will also be discussed.

Structural and energetic properties of H3N–MX3R complexes: Computations and low-temperature IR spectra

256th National Meeting (2018)

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