The goal of our research is to explore the structural and energetic properties of a series of donor-acceptor complexes of the form H3N–SiX3R (X= Cl, F; R= C6H5, CH3) and investigate their capacity for condensed-phase structural changes. Using density functional theory, we obtained equilibrium geometries, binding energies, and N-Si potential energy curves. These curves were mapped in the gas phase as well as in bulk dielectric media using the polarized continuum model with dielectric constants ranging from 1.0 to 10.0. Each complex has four possible geometries in which the NH3 and R-group can reside in one of two distinct bonding sites (axial or equatorial with respect to a trigonal bipyramidal geometry). In most cases, only two of these orientations are stable; specifically, the axial-equatorial (NH3, R) forms tend to be the lowest in energy (e.g., H3N-SiCl3CH3 ΔE=4.19 kcal/mol and R(N-Si)=3.641 Å). For H3N–SiCl3CH3, the potential energy curves for each stable orientation show two distinct minimum-energy structures, and in dielectric media, these two distinct minima are nearly equal in energy. In addition, we found that a stable reaction product forms immediately when H3N and SiCl3CH3 are mixed. To aid in the characterization of this product, guided in part by previous results for nitrogen donor-MX4 complexes, we also investigated the 2:1 (NH3:SiCl3CH3) complex via density functional theory. We considered three general geometries for this system, with Cl’s in “fac” or “mer” arrangements. All were stable, but the mer form with two NH3 units in a trans orientation was found to be lowest in energy (ΔE=10.2 kcal/mol; R(N-Si)=1.985 Å, 1.985 Å). We have obtained IR spectra of the solid product, but the interpretation is difficult because the key peaks (Si-Cl stretching) are located below the standard IR window. In addition, we obtained low temperature IR spectra of argon matrices seeded with NH3 and SiCl3CH3 (at 20 K) as well NH3/SiCl3CH3 thin-films (at 60 K). The matrix-IR results are largely consistent with the predicted frequencies for the 1:1 NH3-SiCl3CH3 complex.

Ammonia-SiX3R complexes: Exploring properties via computational modeling and low-temperature IR spectroscopy

2019 Spring National Meeting

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