First Row Anomaly

Xu, L. T.; Thompson, J. V. K.; Dunning, T. H., Jr. Spin-Coupled Generalized Valence Bond Description of the Group 14 Species: The Carbon, Silicon and Germanium Hydrides, XHn (n = 1–4). J. Phys. Chem. A 2019, 123, 2401-2419.

Although elements in the same group in the Periodic Table tend to behave similarly, the differences in the simplest Group 14 hydrides—XHn (X = C, Si, Ge; n = 1−4) are as pronounced as their similarities. Spin-coupled generalized valence bond (SCGVB) as well as coupled cluster [CCSD(T)] calculations are reported for all of the molecules in the CHn/SiHn/GeHn series to gain insights into the factors underlying these differences. It is found that the relative weakness of the recoupled pair bonds of SiH and GeH gives rise to the observed differences in the ground state multiplicities, molecular structures, and bond energies of SiHn and GeHn. A number of factors that influence the strength of the recoupled pair bonds in CH, SiH, and GeH were examined. Two factors were identified as potential contributors to the decrease in the strengths of these bonds from CH to SiH and GeH: (i) a decrease in the overlap between the orbitals involved in the bond and (ii) an increase in Pauli repulsion between the electrons in the two lobe orbitals centered on the X atoms. Finally, an analysis of the hybridization of the SCGVB orbitals in XH4 indicates that they are closer to sp hybrids than sp3 hybrids, which implies that the underlying cause of the tetrahedral structure of the XH4 molecules is not a direct result of the hybridization of the X atom orbitals.

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