The elucidation of the molecular structure-property relationships requires the rational estimation of steric and electronic effects which govern the molecular structures. To this end, we proposed a simple ab initio method called orbital deletion procedure (ODP) which can quantitatively probe the intramolecular electron transfer effect in carbocations and boranes. This method can not only explore the impact of electron transfer on molecular structures and stabilities, but probe the electron transfer efficiency in some electron donor-bridge-electron acceptor (DBA) complexes. In this Review, we will demonstrate the significance of the intramolecular steric effect by comparing the planar and staggered structures of tetrahydrodiboron (B2H4) and halogenated diboranes (B2X4, X = F, Cl, Br). By deactivating the electronic effect, we found that in these systems the staggered structures are consistently stabilized by 4-11 kcal/mol compared with the planar structures, which supports our very recent conclusion that the ethane rotation barrier is dominated by the steric hindrance rather than the hyperconjugative interactions. We further analyzed a few carbocations and demonstrated how conjugation or hyperconjugation can remarkably influence the molecular structures and properties. Based on the ODP method, we proposed a two-state model which can straightforwardly compute the electron-transfer matrix element which is directly related to the electron transfer efficiency and kinetics in H2C-bridge-CH2 + DBA complexes. We employed this ODPbased two-state model to investigate the electron transport across strain-free linear alkyl chains (CH2)n (n=1-8) and linear π-conjugated bridges (CH=CH)n (n=1-5).
Keywords: donor-bridge-electron acceptor (DBA), intramolecular orbital interactions, hyperconjugation, Tetrahydrodiboron, electronic coupling energy
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