Hydrogen Bonds of Hydrides and Specific Interactions of Alkyl Compounds of Main Group Elements

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Borane molecules and alkyl boron have an equilateral planar trigonal structure, with an angle of 120° between ligands. Based on the comparative analysis by Cvitas et al. of the electron structure of isostructural molecules of diborane and ethylene molecules based on the photoelectron spectra and nonempirical calculations [1], Nefedov and Vovna [2] remark that significant differences are present in the nature of the МО for the 1b 2u orbital, the orbital of π-type ethylene, and bridgehead B–H → B in the diborane molecule. Observed changes in the binding type of the 1b 2u orbital are accompanied by its stabilization by 4.2 eV in relation to the 1(π) molecule С2H4. Shifts of the photoelectron band at the level of s-type 1a g and 1b 2u at values of 3.0 and 2.1 eV, respectively, as well as the levels of p-type 1b 2g , 2a g , and 1b 1u at 1.0, 1.5, and 2.1 eV, respectively, are correlated with the energy change due to ionization of the 2s and 2p electrons of carbon and boron atoms. As described by Bieri et al. [3], the electron population of atomic orbitals of s-3.137e, 2p x , 2p y -0.902, 2p z -0e, and hydrogen 1s-1.20e in molecules of BH3, points to the insignificant negative charge at the hydrogen atom and its low donor properties in the formation of intermolecular hydrogen bonds in crystalline and liquid borane (Fig. 1.1). The planar structure of the borane molecule with six coordinated boron atoms forms a volumetric chain structure wherein, under the influence of the formed hydrogen bonds, the boron atoms are out of the plane and located at the apex of the mild trigonal pyramid. The chains are crosslinked by the hydrogen and boron atoms of the contacting chains due to hydrogen bond formation. Breakage of the seven hydrogen bonds in the vaporization process, with the transition to the vapor state of diborane, is accompanied by a reduction in the coordinated number of boron atoms from six to four, which is reflected by the stabilization of the two remaining intermolecular hydrogen bonds DB–H → B in the structure of the diborane molecule.