A New Look on Hyperconjugation of A Methyl Group Provided by NBO Method

• Main Authors: Chih-YenYu, 游旨儼
• Other Authors: Shao-Pin Wang
• Format: Others
• Language: zh-TW
• Published: 2014
• Online Access: http://ndltd.ncl.edu.tw/handle/42927704240542535438

碩士 === 國立成功大學 === 化學系 === 102 === In this thesis, we present the rationalizations of (1) relative basicity regarding molecules containing hydroxyl group and (2) the UV spectra reported for benzene derivatives. This is achieved by ab initio studies, including Hatree-Fock (HF) and density functional theory (DFT) methods employing the Gaussian computational packages. The natural bond orbital (NBO) method has also been adapted to probe the nature of various substituents, especial of methyl group. The fundamental and/or essential wavefunction-analysis has been performed was employed to quantify the extents of substitution. In the course of this investigation, it has been found that the B3LYP/STO-3G calculations provide the best predictions. In both areas of this research, the orbital interaction approach reported recently provides an insight of hyperconjugation associated with the methyl group as it is introduced to unsaturated systems. Within this approach, the CH bond orbital (BO) of a methyl group would destabilize (or push-up) the one-center BO, lp(O), on the methyl oxygen. Consequently, this lone-pair orbital turns more basic. This destabilization effect is more pronounced on the -type HOMO of benzene, which leads to a reduced HOMO-LUMO gap. Key words: density functional theory; natural bond orbital; wavefunction-analysis; hyperconjugation