The disappearance of the stable slightly-bent isomer of germasilaallenes and the appearance of its cyclic isomer

Abstract

Systematic density functional theory (DFT) calculations and natural bond orbital (NBO) analyses were carried out to understand the reason behind the disappearance of linear and slightly-bent isomer of germasilaallenes and the bond-stretch phenomenon of their cyclic isomer, unusual electronic properties of highly-bent isomers of germasilaallenes. Our results show that in contrast to trisilaallene and trigermaallene, 1,3-digermasilaallene and 1-germadisilaallene do not have stable cyclic isomers, due to increased ring strains. DFT calculations and NBO analyses indicate that the stability of highly-bent isomers of germasilaallenes arise from three parts: p(terminal) → p(central) donor–acceptor interaction, p(terminal)-p(terminal) partial overlapping and the aromatic delocalization along the ring. Different from mono- and di-germasilaallene, DFT and CCSD calculations indicate that there is no stable minimum along the first half, from 180° to 90.0°, of the potential energy surface of trigermasilaallene. For trigermasilaallene, for bending angle α > 120.0°, π-π* orbital delocalization effect dominates the stability and when α < 120.0°, aromatic delocalization of the conformer becomes the main factor. With the bending angle lighter, the stable energy becomes larger. Thus, the germasilaallenes gradually become more stable along the bending PES from 180° to 90.0° and no barrier stabilizes the slightly-bent isomer. DFT calculations indicate that strong donor effect from substituents attached to terminal Si (Ge) to central Si (Ge) can stabilize the slightly-bent isomer of germasilaallenes.