| Summary: | 博士 === 國立成功大學 === 化學系碩博士班 === 96 === This article includes four units and the first unit contains Ab Initio Molecular Orbital (MO) theory. Here it introduces the Hamiltonian and wavefunctions of the Schrödinger equation, and molecular orbitals obtained from the Schrödinger equation. The related software introduction is Gaussian package in chief. The second unit covers Natural Bond Orbital (NBO) analysis. It could acquire quantized Lewis structures on the basis of the density matrix obtained from MO theory. According to second-order perturbation theory, the donor-acceptor interaction between bonding and antibonding orbitals can stabilize a molecule on the whole, and the orbital energy lowing due to this interaction is called second-order energy lowing [E(2)]. The related software introduction is NBO 5.0 program in chief. The fourth unit involves some calculations about the properties of ionic liquid solutions.
The third unit embraces applications for example. Using Linux version of Gaussian 98 and NBO 5.0 program, we can obtain 2pi orbital populations ([2pi]) of transition-metal hexacarbonyl [M(CO)6] and mono-substituted pentacarbonyl [M(CO)5X] complexes. In M(CO)6 (M = Cr, Mo, W), pi-back-donations of the calculated and NMR experiments show the similar trend: 3d ~ 5d > 4d. According to the results of NBO analysis, M(CO)6 contains two donor-acceptor interactions, 3-center, 4-electron hyperbond → pi*CO (3CHB hyperconjugation) and M → pi*CO, for pi-back-donation. The trend of M → pi*CO is 3d < 4d < 5d and that of 3CHB hyperconjugation is 3d > 5d ~ 4d. It could be found that the 3CHB hyperconjugation is the main factor that influences the pi-back-donation. In M(CO)5X complexes (M = Cr, Mo, W), we can arrange the pi-acceptor ability of ligands X in increasing order: X = F– < Cl– < Br– < I– < CN– < Quinuclidine < NMe3 < Pyridine < Pyrazine < N2 < PPh3 < PPh2Me < PPhMe2 < PMe3 < H2 < P(OMe)3 < PI3 < PBr3 < PCl3 < PF3 < CO < SiO < CS < BF < NO+, according to axial [2pi].
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