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

The molecular structure and bonding mechanisms of MH 3 • radicals and MH 3 Cl (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 CH 3 • radical is planar ( D 3 h ) 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 Δ E inv + ΔZPE (=0.0, 3.7, 3.8, 7.0 kcal/mol) increases along the series CH 3 •, SiH 3 •, GeH 3 •, and SnH 3 • (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 CH 3 −Cl, SiH 3 −Cl, GeH 3 −Cl, and SnH 3 −Cl, respectively (NL-SCF/TZ2P). A detailed analysis of the bonding mechanisms shows that the CH 3 • radical is planar because of the steric repulsion between the hydrogen ligands. This steric H−H repulsion is much weaker for SiH 3 •, GeH 3 •, and SnH 3 • 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 MH 3 −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 MH 3 • and Cl • SOMOs (from Si to Sn). The results are discussed in the context of those previously obtained for the complementary series of the CH 3 −X bond (X = F, Cl, Br, I) to provide a more complete insight into the electronic structure and bonding of the archetype MH 3 X molecule.