Theoretical approach to the conformational analyses of dithiazinane, thiadiazinane and triazinane, their N-borane adducts and N-H cations

Computational methods were applied to the study of the different nitrogen and chair conformers of [1,3,5]-dithiazinanes, [1,3,5]-thiadiazinanes and [1,3,5]-triazinanes. Optimisation of heterocycles bearing N-H and N-Me were performed by B3LYP density functional and the 6-311++G∗∗basis set methods, and their free energies were correlated. N-Borane adducts and the protonated cations of these heterocycles were also optimized. The relative free energy differences (ΔG) in gas phase of the chair conformers of eighteen heterocycles were calculated. Calculations agree with experimental data and allow to explain the steric and electronic factors that determine the preferred conformations. The distribution of the electronic density and the NBO atomic charges of the optimized structures support the experimental conformational analyses. The bonding and intramolecular interactions were analysed by using critical point search of the electronic density. They were also evaluated by exchange reduced density gradients associated to overlapping, using the PBE functional with the DZUP basis set. Proton-hydride, proton-proton, hydride-hydride, hydride-sulphur, proton-sulphur and hydride-nitrogen stabilising weak interactions were analysed.