Helides of carbon and silicon: an ab initio study of their geometric and electronic structures

Abstract

The electronic and geometric structures of carbon and silicon helide ions (CHe^m+_n and SiHe_n^m+; n = 1−4 and m = 1−4) were examined using ab initio molecular orbital theory. Equilibrium geometries were obtained at the MP2/6−31G* and QCISD(T)/6−31311G(MC)*^* levels. Consistent with previous results, the ground states of the mono- and di-cations are found to have long and weak bonds, whereas the excited states are characterized by shorter and stronger bonds. The more highly charged tri-cations and tetra-cations all exhibit stronger bonds to helium than their singly- and doubly-charged counterparts. These and other interesting structural features of the helide ions have been rationalized using a simple molecular orbital (MO) model. In particular, the trends in C-He and Si-He bond lengths can readily be understood in terms primarily of the number of electrons occupying antibonding orbitals. Electrostatic repulsion and the modification of the nature of the antibonding orbitais with increasing molecular charge also play an important role in determining the equilibrium bond distances in the helide ions. The SiHe_n^m+ systems are found to be better able to accommodate positive charge than the CHe^m+_n ions. Reference values of C-He and Si-He bond lengths, for systems with no occupied antibonding orbitals and in which electrostatic repulsion is not significant, are assigned as approximately 1.05 and 1.49 Å, respectively. The ground states of helide ions with a particular charge show nearly constant bond lengths, independent of the number of helium atoms; this result may be attributed to a constant occupancy of antibonding orbitals. For example, there is a clustering of Si-He distances around the value of 1.57 Å in the quadruply-charged ions SiHe⁴⁺, SiHe₂⁴⁺, SiHe₃⁴⁺ and SiHe₄⁴⁺, in each of which the antibonding orbitals are empty.