The similarity of r(2s) ≈ r(2p) leads to pronounced sp-hybrid bonding of the light p-block elements, while the heavier p-elements with n≥3 exhibit r(ns) « r(np) of ca. The radially nodeless 1s, 2p, 3d, 4f valence AOs are part-icu-larly compact. Half a century ago the unique chemistry of the light homologs was corre-lated to the then available atomic orbital (AO) radii. T The Periodic Table, and the unique chemical behavior of the first element of a group, were discovered simultaneously one and a half centuries ago. In other words, as strange as it may sound, VB can be extended to the study of atoms and, therefore, is a much more general model than MO. Moreover, when examined from the permutation group perspective, it becomes clear that the concepts introduced by Pauling to deal with molecules can be equally applied to the study of the atomic structure. Practically all the conflicts among the practitioners of the two models can be traced down to the lack of permutation symmetry in the MO wave functions. Nevertheless, there is a much more fundamental difference between these two models which is only revealed when the symmetries of the many-electron Hamiltonian are fully taken into account: while the VB and MO wave functions exhibit the point-group symmetry, whenever present in the many-electron Hamiltonian, only VB wave functions exhibit the permutation symmetry, which is always present in the many-electron Hamiltonian.
VB and molecular orbital (MO) models are normally distinguished by the fact the first looks at molecules as a collection of atoms held together by chemical bonds while the latter adopts the view that each molecule should be regarded as an independent entity built up of electrons and nuclei and characterized by its molecular structure. The intra-atomic energy changes that occur in these systems are related to the hybridization of the heavy atoms in an analogous manner to the hybridization of C in CH4 from (2s)²(2p)² to sp³ hybrid orbitals. In contrast, for C2H2, the interatomic interactions that create bonds prevail over the intra-atomic energy changes that occur when the dibridged molecule reconstructs into the trans-bent and linear structures. For Si2H2, the antibonding intra-atomic energy changes that occur when the dibridged molecule reconstructs into the trans-bent and linear structures prevail over the interatomic interactions that induce bond formation. By contrast, the inter-atomic bonding contributions become energetically more favorable in that order for both C2H2 and Si2H2. The analysis shows that the intra-atomic contributions to the molecular energy become less favorable in the order dibridged→trans-bent→linear for both C2H2 and Si2H2.
C2H2 MOLECULAR GEOMETRY FULL
In this study, the intra-atomic (antibonding) and bonding contributions to the total molecular energy of these valence isoelectronic molecules are computed by expressing the density matrices of the full valence space multi-configuration self-consistent field wave function in terms of quasi-atomic orbitals.
The molecular energy of Si2H2 geometric structures increases in the order dibridged
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