Rotational barriers of disilane, hexafluorodisilane, and hexamethyldisilane: Ab initio, density functional, and molecular mechanics (MM3) studies

Abstract

ABSTRACT:We investigated structures, vibrational frequencies, and rotational Ž . Ž

Disilane Internal Rotation

“…Internal coordinate values of the S and E conformers essentially agree with those found in extensive studies by Leszczynski et al . and others, as well as with the experimental ones . The 1.0 kcal/mol barrier is also consistent with previous studies. ,− This value is ∼1/3 ethane's 3.0 kcal/mol barrier and this difference does not provide, by itself, any discrimination between the various interactions postulated to rationalize the ethane barrier.…”

“… and others, as well as with the experimental ones . The 1.0 kcal/mol barrier is also consistent with previous studies. ,− This value is ∼1/3 ethane's 3.0 kcal/mol barrier and this difference does not provide, by itself, any discrimination between the various interactions postulated to rationalize the ethane barrier. In fact, the large barrier decrease in going from ethane to disilane is simplistically in agreement with both distance attenuated pairwise repulsions and vicinal hyperconjugative interactions between Si−H bonds (of the two methyl/silyl groups).…”

“…Disilane is also important in its own right as the simplest example involving internal rotation of the SiH 3 group. Although electron diffraction experiments do not establish whether the equilibrium conformation is staggered ( S ) or eclipsed ( E ) (because H atoms contribute little to the diffraction pattern), all reported calculations find, as in ethane, that the lowest energy (equilibrium) state is staggered (Figure ). Both sources conclude longer Si−H bonds, and a somewhat larger HSiH angle than the corresponding ones in ethane (Table ).…”

“…Rotating one silyl group by 60° then produces an eclipsed, higher energy conformation (Figure b). The barrier to this process has been both calculated by a variety of methods and experimentally determined from gas-phase Raman frequencies to be near 1 kcal/mol (experiment: 1.26 kcal/mol; theory: 0.82−1.09 kcal/mol ,− ).…”

SrSi6N8—A Reduced Nitridosilicate with a SiSi Bond

“…In contrast, it contains additional SiSi single bonds (235.2(2) pm), the length of which precisely matches (within less than 0.1 %) that of the archetypical covalent SiSi single bond in diamond‐type silicon. It also resembles SiSi bond lengths of the molecular compounds Si 2 H 6 , Si 2 F 6 , or Si 2 Me 6 , which range between 213 and 234 pm 15…”