Interconversion between Planar-Triangle, Trigonal-Pyramid and Tetrahedral Configurations of Boron (B(OH)3 -B(OH)4- ), Carbon (CH3+ -CH3X) and for the Group 15 Elements as Nitrogen (NH3-NH4+ ). A Modelling Description with Ab Initio Results and Pressure-Induced Experimental Evidence

Recently a mechanistic understanding of the pressure-and/or temperature-induced coordination change of boron in a borosilicate glass has been demonstrated by Edwards et al. In situ high-pressure 11B solid-state NMR spectroscopy has been used in combination with ab initio calculations in order to obtain insight in the molecular geometry for the pressure-induced conversion. The results indicate a deformation of the B(OH)3 planar triangle, under isotropic stress, into a trigonal pyramid that serves as a precursor for the formation of a tetrahedral boron configuration. From our point of view, the deformation controlling the out-of-plane transition of boron accompanied with a D3h into C3v geometric change is an interesting transformation because it matches with our molecular description based on Van’t Hoff modelling for the tetrahedral change of carbon in CH3X by substitution of X with nucleophiles via a trigonal bipyramid state in which the transferred carbon is present as a methyl planar triangle “cation”. Van’t Hoff modelling and ab initio calculations have been also applied on the dynamics of the out-of-plane geometry of a transient positively charged carbon in a trigonal pyramidal configuration into a planar trivalent carbon cation. Finally the same model is also used for the C3v trigonal pyramidal configurations as NH3 of the group 15 elements in their nucleophilic abilities.