Nature and strength of group-14 A-A' bonds.

We have quantum chemically investigated the nature and stability of C-C and Si-Si bonds in R3A-AR3 (A = C, Si; R3 = H3, Me3, Me2Ph, MePh2, Ph3, t-Bu3) using density functional theory (DFT). Systematic increase of steric bulk of the substituents R has opposite effects on C-C and Si-Si bonds: the former becomes weaker whereas the latter becomes stronger. Only upon going further, from R = Ph to the bulkiest R = t-Bu, the R3Si-SiR3 bond begins to weaken. Our bonding analyses show how different behavior upon increasing the steric bulk of the substituents stems from the interplay of (Pauli) repulsive and (dispersion) attractive steric mechanisms. Extension of our analyses to other model systems shows that C-Si bonds display behavior that is in between that of C-C and Si-Si bonds. Further increasing the size of the group-14 atoms from C-C and Si-Si to Ge-Ge, Sn-Sn and Pb-Pb leads to a further decrease in the sensitivity of the bond strength with respect to the substituents' bulkiness. Our findings can be used as design principles for tuning A-A and A-A' bond strengths.

Methyl Substitution Destabilizes Alkyl Radicals.

We have quantum chemically investigated how methyl substituents affect the stability of alkyl radicals Mem H3-m C⋅ and the corresponding Mem H3-m C-X bonds (X = H, CH3 , OH; m = 0 - 3) using density functional theory at M06-2X/TZ2P. The state-of-the-art in physical organic chemistry is that alkyl radicals are stabilized upon an increase in their degree of substitution from methyl

The Chemical Bond: When Atom Size Instead of Electronegativity Difference Determines Trend in Bond Strength.

We have quantum chemically analyzed element-element bonds of archetypal Hn X-YHn molecules (X, Y=C, N, O, F, Si, P, S, Cl, Br, I), using density functional theory. One purpose is to obtain a set of consistent homolytic bond dissociation energies (BDE) for establishing accurate trends across the periodic table. The main objective is to elucidate the underlying physical factors behind these chemical bonding trends. On one hand, we confirm that, along a period (e. g., from C-C to C-F), bonds strengthen because the electronegativity difference across the bond increases. But, down a period, our findings constitute a paradigm shift. From C-F to C-I, for example, bonds do become weaker, however, not because of the decreasing electronegativity difference. Instead, we show that the effective atom size (via steric Pauli repulsion) is the causal factor behind bond weakening in this series, and behind the weakening in orbital interactions at the equilibrium distance. We discuss the actual bonding mechanism and the importance of analyzing this mechanism as a function of the bond distance.