Examining Trends in the Tetravalent Character of Group 14 Elements (C, Si, Ge, Sn, Pb) with Acids and Hydroperoxides

The shift from tetravalency to divalency in going from carbon to lead chemistry is thought to be one of the most striking periodic properties. Although several molecules in the series R−(XO)−OH and R−Ẍ−O−OH (R = H, CH₃; X = C, Si, Ge, Sn, Pb) have been synthesized, most are unknown to date. Using density functional theory and hybrid Hartree−Fock/density functional theory with the BLYP and B3LYP functionals, the relative stability of hydroperoxycarbene, (H−C̈−O−O−H) to its acid isomer, formic acid, is predicted to be +96.4 kcal/mol with BLYP and a triple-ζ plus double polarization (TZ2P) basis set. As other group 14 atoms replace the carbon atom, this energy difference lessens considerably:  Si, 101.0 (TZ2P) and 92.1 (TZ2P-ECP); Ge, 57.0 (TZ2P), and 65.5 (TZ2P-ECP); Sn, 48.4 (TZ2P-ECP); Pb, 22.1 (TZ2P-ECP), where TZ2P-ECP indicates that effective core potentials (ECP) were used for the group 14 atom only. As one descends group 14, the acid form remains lower in energy than the peroxide form, indicating that even for lead tetravalency is preferred. A similar trend occurs for the relative energies between the group 14 congeners of hydroperoxymethylcarbene (CH₃−C̈−O−O−H) and its acid isomer, acetic acid:  C, 88.1 (TZ2P); Si, 101.9 (TZ2P) and 93.1 (TZ2P-ECP); Ge, 58.7 (TZ2P) and 67.2 (TZ2P-ECP); Sn, 50.4 (TZ2P-ECP); Pb, 25.1 (TZ2P-ECP). Results with B3LYP were similar to those of BLYP though the former are somewhat more dubious due to spin contamination in the carbon congeners. Coupled cluster singles and doubles (CCSD) and CCSD with triple excitations perturbatively added (CCSD(T)) single-point energies also show similar results and favor the tetravalent acid congeners by 4−13 kcal/mol more than density functional theory.