CH₃^• Is Planar Due to H−H Steric Repulsion. Theoretical Study of MH₃^• and MH₃Cl (M = C, Si, Ge, Sn)

Abstract

The molecular structure and bonding mechanisms of MH3 • radicals and MH3Cl (M = C, Si, Ge, Sn) have been studied with the use of local (LDA) as well as nonlocal (NL-SCF) density-functional theory (DFT) and a large, doubly polarized triple-ζ STO basis (TZ2P). The CH3 • radical is planar (D 3h ) whereas the heavier central atom analogs are pyramidal: the H−M−H bond angle β (=120.00, 112.66, 112.44, 110.56°) decreases, and the inversion barrier ΔE inv + ΔZPE (=0.0, 3.7, 3.8, 7.0 kcal/mol) increases along the series CH3 •, SiH3 •, GeH3 •, and SnH3 • (NL-SCF/TZ2P). The homolytic M−Cl bond dissociation energy D homo + ΔZPE is 81.7, 105.6, 96.2, and 93.6 kcal/mol for CH3−Cl, SiH3−Cl, GeH3−Cl, and SnH3−Cl, respectively (NL-SCF/TZ2P). A detailed analysis of the bonding mechanisms shows that the CH3 • radical is planar because of the steric repulsion between the hydrogen ligands. This steric H−H repulsion is much weaker for SiH3 •, GeH3 •, and SnH3 • in which the ligands are farther removed from each other. Electronic effects (i.e. electron pair bonding between the central atom and hydrogen ligands) always favor a pyramidal structure, although only slightly so for the methyl radical. The analysis of the MH3−Cl bond reveals that initially the bond strength increases with the increasing M−Cl electronegativity difference (from M = C to Si) and then decreases together with the bond overlap between the MH3 • and Cl• SOMOs (from Si to Sn). The results are discussed in the context of those previously obtained for the complementary series of the CH3−X bond (X = F, Cl, Br, I) to provide a more complete insight into the electronic structure and bonding of the archetype MH3X molecule.

Chemistry with ADF

“…This is nicely confirmed in a recent BP86/TZ2P study, 193 which shows that the H-A-H angle β in AH 3 · decreases (β = 120, 112.7, 112.4, and 110.6 • ), whereas the inversion barrier increases along A = C, Si, Ge, and Sn (0.0, 3.7, 3.8, and 7.0 kcal/mol).…”

“…21) that has C-H and C-F antibonding character (see also the Role of the Pauli Steric Repulsion section). 193 Therefore, the antibonding character of the LUMO is reduced and it decreases in energy, as the C β -H and C α -F bonds elongate. This is the key to the answer of our question.…”

Hypervalent Silicon versus Carbon: Ball‐in‐a‐Box Model

“…Note that this behavior is counteracted, but not overruled, by the destabilization in the interaction with the equatorial H 3 CCC substituents (consistent with the findings in reference [28]). …”

“…This is due to the fact that the AH 3 moiety goes from a pyramidal to a planar configuration in which the hydrogen atoms are slightly further away from each other and, therefore, experience less mutual steric (Pauli) repulsion. [28] This effect is much more pronounced for ClCH 3 Cl À than for ClSiH 3 Cl À because the hydrogen atoms in the former are in closer proximity due to the shorter C À H as compared to Si À H bonds. [28] Thus, the origin of 1 a being a transition state and 2 a a stable, hypervalent species is located somewhere in the other interaction steps, in the two steps (a) and (b) defined in each of the Equations 1-3 (see blue and red curves, respectively, in Figure 3).…”

“…[28] This effect is much more pronounced for ClCH 3 Cl À than for ClSiH 3 Cl À because the hydrogen atoms in the former are in closer proximity due to the shorter C À H as compared to Si À H bonds. [28] Thus, the origin of 1 a being a transition state and 2 a a stable, hypervalent species is located somewhere in the other interaction steps, in the two steps (a) and (b) defined in each of the Equations 1-3 (see blue and red curves, respectively, in Figure 3). A closer inspection shows that in each of the three fragmentation modes, the convex (1) or concave (2) nature of the total energy curve is determined by the interaction of the central moiety (either A or AH 3 ) with the axial substituents (or all substituents simultaneously).…”