Electronic states and barriers to internal rotation in silaallenes

Abstract

AbstractThe electronic states and barriers to internal rotation in allene (1a), 1‐silaallene (2a), and 2‐silaallene (3a) are investigated computationally using ab‐initio molecular orbital methods. Planar geometries with two‐, three‐, and four‐π‐electron configurations have been considered as possible transition states (1b–3d). Structures have been optimized at the Hartree–Fock level with a small split valence basis set (3‐21G) and higher level calculations with basis sets of split valence (6‐31G) and split valence plus polarization function (6‐31G*) quality include correlation energy estimates from Møller–Plesset second‐and third‐order perturbation theory. The electronic barrier to internal rotation in allene is estimated near 53 kcal/mol whereas the corresponding barriers in 1‐silaallene and 2‐silaallene are considerably smaller, ca. 35 and 20 kcal/mol, respectively. The transition states are predicted to possess bent geometries in all three molecules with open‐shell singlet, three‐π‐electron configurations in 1 and 2 (1c, 2c) but a closed‐shell singlet, two‐π‐electron configuration in 3 (3d).