Disilane Internal Rotation

Abstract

Energetics of hindered rotation in disilane are analyzed and compared to ethane. In disilane weakened (rotationally governed) hyperconjugative interactions, paramount in controlling the ethane barrier, leave the nonrotational part of the torsional coordinate as the primary contribution to disilane barrier energetics. In this regard, the 0.012 Å Si-Si bond lengthening that accompanies rotation is found to account for most of the 1.0 kcal/mol barrier. Although both the Si-Si bond expansion in disilane and the 0.014 Å C-C bond expansion in ethane are nearly the same, the mechanisms for these expansions are found to be different. Unlike in ethane, where the expansion is largely due to the hyperconjugation decrease in the eclipsed conformer, in disilane it is almost entirely due to electrostatic repulsion between Si-H bonds in the two silyl groups.

Electronic Properties of Disilane: An ab initio Calculation

“…Our results agree favorably with similar methods reported in the accurate work by Cho et al [6]. From the optimized geometries we conclude that the magnitude of the rotational energy red-shift is 0.043 eV (1.0 kcal/mol), which is comparable to the experimental result of 1.26 kcal/mol [15] and to the theoretical calculations of Pophristic et al [5] and of Cho et al [6]. This energy corresponds to a temperature of around 500 K, which is much higher than the typical experimental temperatures (as in CVD [16]), and implies that at low temperatures a mixture of both conformations will be found.…”

“…For the staggered conformation the vibrational frequencies agree successfully with experimental values [15] and the results computed by Cho et al [6]. The major difference is noticed for the A 1g rotation symmetry, whose blue-shift is an indication 392 108.3 108.5 [5], 108.08 [6] that the molecule in the eclipsed conformation requires more oscillation energy. Instead, for the Si-Si stretching mode we obtain a frequency red-shift of only 10-20 cm À1 relative to the staggered conformation, indicating an increase of Si-Si bond length (less rigid bond).…”

“…In particular, the fundamental processes in the disilane decomposition on silicon surfaces is relevant to the understanding and optimization of the growth of epitaxial Si on Si substrates. The morphological parameters of the eclipsed and staggered silane conformations were recently calculated by Pophristic et al [5]. They concluded that the origin of the eclipsed to staggered relaxation can be attributed to preferential hyperconjugative stabilization (i.e.…”

“…Recently, in addition to ethane, disilane (Si 2 H 6 ) has also attracted attention [5,6] due to its importance in the production of silicon based semiconductor devices. In particular, the fundamental processes in the disilane decomposition on silicon surfaces is relevant to the understanding and optimization of the growth of epitaxial Si on Si substrates.…”

Internal rotation of disilane and related molecules: a density functional study

“…The morphological parameters of the eclipsed and staggered silane conformations were recently calculated by Pophristic and co-workers (1). They concluded that the origin of the eclipsed to staggered relaxation is related to the preferential hyperconjugative stabilization (meaning energy stabilization through electron excitation to a delocalized state).…”