TY - JOUR
T1 - A simple electrostatic model for trisilylamine
T2 - Theoretical examinations of the n→σ* negative hyperconjugation, p(π)→d(π) bonding, and stereoelectronic interaction
AU - Mo, Yirong
AU - Zhang, Yongqing
AU - Gao, Jiali
PY - 1999/6/23
Y1 - 1999/6/23
N2 - A block-localized wave function method was used to examine the stereoelectronic effects on the origin of the structural difference between trisilylamine and trimethylamine. The pyramidal geometry of trimethylamine along with its high basicity is consistent with the traditional VSEPR (valence shell electron-pair repulsion) model for σ bonding. On the other hand, in trisilylamine, the silicon d orbitals make modest contribution to the electronic delocalization, although the key factor in charge delocalization is still n(N)→σ(SiH)* negative hyperconjugation. Interestingly, the gain in p(π)→d(π) bonding stabilization is offset by a weaker negative hyperconjugation effect in trisilylamine, resulting in an overall smaller delocalization energy (-18.5 kcal/tool) than that in trimethylamine (-23.9 kcal/mol), which contains little p(π)→d(π) bonding character. Significantly, because of the relatively low electronegativity of silicon, the N-Si bond is much more polar than the N-C bond. Weinhold's natural population analyses of the BLW and HF wave functions for these compounds reveal that the origin of the planar geometry of trisilylamine is due to the polar σ-effect that yields significant long-range electrostatic repulsion between the silyl groups. In addition, it was found that only the most electronegative substituents such as F and OH can result in a pyramidal geometry at the nitrogen center for silylamines. This is in good accord with the recent X-ray structure of a pyramidal silylamine, N(CH3)(OCH3)(SiH3).
AB - A block-localized wave function method was used to examine the stereoelectronic effects on the origin of the structural difference between trisilylamine and trimethylamine. The pyramidal geometry of trimethylamine along with its high basicity is consistent with the traditional VSEPR (valence shell electron-pair repulsion) model for σ bonding. On the other hand, in trisilylamine, the silicon d orbitals make modest contribution to the electronic delocalization, although the key factor in charge delocalization is still n(N)→σ(SiH)* negative hyperconjugation. Interestingly, the gain in p(π)→d(π) bonding stabilization is offset by a weaker negative hyperconjugation effect in trisilylamine, resulting in an overall smaller delocalization energy (-18.5 kcal/tool) than that in trimethylamine (-23.9 kcal/mol), which contains little p(π)→d(π) bonding character. Significantly, because of the relatively low electronegativity of silicon, the N-Si bond is much more polar than the N-C bond. Weinhold's natural population analyses of the BLW and HF wave functions for these compounds reveal that the origin of the planar geometry of trisilylamine is due to the polar σ-effect that yields significant long-range electrostatic repulsion between the silyl groups. In addition, it was found that only the most electronegative substituents such as F and OH can result in a pyramidal geometry at the nitrogen center for silylamines. This is in good accord with the recent X-ray structure of a pyramidal silylamine, N(CH3)(OCH3)(SiH3).
UR - http://www.scopus.com/inward/record.url?scp=0033597630&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=0033597630&partnerID=8YFLogxK
U2 - 10.1021/ja9904742
DO - 10.1021/ja9904742
M3 - Article
AN - SCOPUS:0033597630
SN - 0002-7863
VL - 121
SP - 5737
EP - 5742
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 24
ER -