| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 91 === Weakening of the C(3)-C(2) bonds in the series: H3-n(Me)nC-C≡N has been explained by the well-known hyperconjugation concept which may be classified as the donor-acceptor interaction between □σC(3)H(4) and □σ*C(2)C(3). Natural Bond Orbital (NBO) analysis of the orbital interaction energy, E(2), in this series of molecules reveals that the □σC(3)H(4) and □σ*C(2)C(3) interaction is in general smaller than □σC(3)C(Me) and □σ*C(2)C(3). Orbital interactions involving □σC(2)C(3) as donating bond orbital and □σ*C(2)C(3) as accepting bond orbital play critical roles concerning the strength of C(2)C(3) bonds. The values of E(2) for the former interactions increase in the trend: 13.06, 17.08, 21.34, 25.22 kcal/mol and E(2) for the latter indicates the same trend as well: 23.88, 29.12, 34.08, 40.44 kcal/mol, which primarily arises from the increasing □σC(3)C(Me) and □σ*C(2)C(3) donor-acceptor interactions. Both trends can be employed to account for the weakened C(2)-C(3) bonds since the increasing number of methyl substitution leads to more or higher orbital interactions involving the C(2)-C(3) bonding and antibonding orbitals.
The electric field gradients (EFG) of para-carbon and para-nitrogen atoms in mono-substituted benzene and pyridine were calculated, respectively. The linear relationship between EFG and charge for either carbon or nitrogen atom, which is only influenced by the □π-effect of the substituent, R, has been found. Furthermore, the relative proportionality constants can be related to the value of qo, representing the EFG resulting from one electron filled in the 2p-orbital. Our results can be employed to rationalize the semi-empirical relationship proposed in the literature for evaluation of backbonding electrons in transition-metal carbonyls. In other words, the work presented here provides evidence in support of the Townes-Dailey theory applied to systems in which only □π-effects are relevant.
|
|---|
