Structures of Tetrasilylmethane Derivatives C(SiXMe₂)₄ (X=H, F, Cl, Br) in the Gas Phase and their Dynamic Structures in Solution

Abstract

The structures of the molecules C(SiXMe2)4 (X=H, F, Cl, Br) have been determined by gas electron diffraction (GED). Ab initio calculations revealed nine potential minima for each species, with significant ranges of energies. For the H, F, Cl, and Br derivatives nine, seven, two, and two conformers were modelled, respectively, as they were quantum-chemically predicted to be present in measurable quantities. Variable-temperature 1H and 29Si solution-phase NMR studies and, where applicable, 13C NMR, 1H/29Si NMR shift-correlation, and 1H NMR saturation-transfer experiments are reported for C(SiXMe2)4 (X=H, Cl, Br, and also I). At low temperature in solution two conformers (one C1-symmetric and one C2-symmetric) are observed for each of C(SiXMe2)4 (X=Cl, Br, I), in agreement with the isolated molecule ab initio calculations carried out as part of this work for X=Cl, Br. C(SiHMe2)4 is present as a single C1-symmetric conformer in solution at the temperatures at which the NMR experiments were performed.

Structures of Tetrasilylmethane Derivatives (XMe2Si)2C(SiMe3)2 (X = H, Cl, Br) in the Gas Phase, and their Dynamic Structures in Solution

“…By independently setting the two XMe 2 Si to all possible combinations of these angles, it can be seen that there are nine (=3 2 ) possible minimum-energy conformers for each of 1 – 3 . Experience of studying a similar set of species (XMe 2 Si) 4 C, where X = H, F, Cl, Br, has shown that the opposite sense of each angle (i.e., −80, +40, and +160°) should also be considered when looking to identify all possible conformers …”

“…Experience of studying a similar set of species (XMe 2 Si) 4 C, where X = H, F, Cl, Br, has shown that the opposite sense of each angle (i.e., −80, +40, and +160°) should also be considered when looking to identify all possible conformers. 40 All calculations used Gaussian 09 41 on either the University of Edinburgh's ECDF cluster 42 or the UK's National Service for Computational Chemistry Software clusters. 43 Geometry optimizations and frequency calculations were carried out to determine which ground-state conformers had the lowest energies.…”