CH3 center dot is planar due to H-H steric repulsion. Theoretical study of MH(3)center dot and MH(3)Cl (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-zeta STO basis (TZ2P). The CH3. radical is planar (D-3h) whereas the heavier central atom analogs are pyramidal: the H-M-H bond angle beta (=120.00, 112.66, 112.44, 110.56 degrees) decreases, and the inversion barrier Delta E(inv) + Delta ZPE (= 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 + Delta ZPE 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 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 CH3-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.