Na. Richardson et al., Examining trends in the tetravalent character of group 14 elements (C, Si,Ge, Sn, Pb) with acids and hydroperoxides, J AM CHEM S, 121(46), 1999, pp. 10813-10819
The shift from tetravalency to divalency in going from carbon to lead chemi
stry is thought to be one of the most striking periodic properties. Althoug
h several molecules in the series R--(X=O)-OH and R-X-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 hydroperoxy
carbene, (H-C-O-O-H) to its acid isomer, formic acid, is predicted to be +9
6.4 kcal/mol with BLYP and a triple-g plus double polarization (TZ2P) basis
set. As other group 14 atoms replace the carbon atom, this energy differen
ce lessens considerably: Si, 101.0 (TZ2P) acid 92.1 (TZ2P-ECP); Ge, 57.0 (T
Z2P), 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 g
roup 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-C-O-O-H) and its acid i
somer, acetic acid: C, 88.1 (TZ2P); Si, 101.9 (TZ2P) and 93.1 (TZ2P-ECP); G
e, 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 s
omewhat more dubious due to spin contamination in the carbon congeners, Cou
pled cluster singles and doubles (CCSD) and CCSD with triple excitations pe
rturbatively added (CCSD(T)) single-point energies also show similar result
s and favor the tetravalent acid congeners by 4-13 kcal/mol more than densi
ty functional theory.