Density functional study of the intramolecular proton transfer and conformations in hydroxyaminosilanethione

The structure and reactivity of hydroxyaminosilanethione conformers were studied in the gas phase using density functional theory. Eight local minima were located, including two featuring pentacoordinate silicon atoms (2Z, S2) and one with a nitrososilanethiol structure (4Z). The global minimum at the B3LYP/6-31++G(dp)//B3LYP/6-31G(d) levels of theory is S2. Natural bond orbital (NBO) analysis reveals that there is a strong n S 2 → σ N 3 Si1 ∗ interaction in the majority of isomers. The 3d orbitals of silicon are of minor importance in bonding for the isomers with the exception of those where the silicon is penta-coordinate (2Z and S2), where the occupancy of the 3d orbitals is 2.93, 4.34 respectively. The charge density and its Laplacian at the bond critical point (BCP), (ρ(r) and ∇2ρ(r)), indicate that all SiS, SiN and SiH bonds were constructed by ionic or electrostatic interactions, whereas NH and OH bonds are predominantly covalent. In addition, seven transition states connecting the various minima were located. TS7 has the highest energy (75.75 kcal/mol) in the reaction coordinates and is therefore the rate-determining step in the conversion of 1Z–4Z. TS7, TS2 and TS3 have the highest energies (75.75, 66.50 and 57.92 kcal/mol, respectively). The computed harmonic frequencies results were in good agreement with available experimental values. The spectra of S1 and 2Z have the lowest NH frequencies (3117 and 2965 cm−1), this shift towards the lower frequency being attributable to the presence of intramolecular hydrogen bonding in S1 and 2Z.