Geometric Parameters and Shape of the Tetra-n-Butylammonium Ion and the Tetrabutyl Methane Molecule in a Vacuum and in Water According to the Data of AB Initio Calculations

Abstract

The shape of ions of quaternary ammonium bases should be known to correctly interpret the properties of their salts; however, there is almost no available numerical data on geometric parameters of their molecules. We report ab initio (HF/6-31G) calculation data on the structure of an all-atom model of the cluster containing one ion pair of tetrabutylammonium bromide TBA⁺Br^– in a vacuum and in a water environment. The same calculations are performed for its neutral structural analogue tetrabutylmethane (TBM). The following geometric parameters are reported: distances from the central atom to the terminal carbon and hydrogen atoms; distances between terminal atoms; excess charges on hydrogen atoms and angles between atoms characterizing the structure; volumes of structures approximating real species; average numbers and lengths of hydrogen bonds in water molecules depending on the distance to the central atom of the species. It is shown that the determined shapes correspond to irregular tetrahedra with densely packed hydrocarbon radicals. The shape and size of TBA⁺ and TBM are almost identical and do not depend on the environment or the electric charge on the central atom.