Studies of πBonding Characteristics of Nitrile Compound by NMR Spectroscopic Techniques-Evidence of Negative Hyperconjugation

• Main Authors: Li Yuan Tzu, 李苑慈
• Other Authors: Hui-Lien Tsai
• Format: Others
• Language: zh-TW
• Published: 1999
• Online Access: http://ndltd.ncl.edu.tw/handle/61992440235150960396

碩士 === 國立成功大學 === 化學系 === 88 === We have measured the values of nitrogen-14 quadrupole coupling constants (QCC), employing NMR relaxation techniques, for a designed series of para-substituted benzonitrile derivatives. It is found that for substituents bearing lone-paired electrons result in lower magnitudes of QCCs(14N) than the parent compound, benzonitrile. This observation provides solid evidence in support of the existence of “negative hyperconjugation” (NHC). For the methyl-substituted benzonitrile, the same observation is also present and can be rationalized by the well-known hyperconjugation effect. Although both effects may be viewed as the no-bond double-bond resonance based on valence bond theory, only NHC can be well demonstrated by “orbital interaction,” reported by Schleyer, within the molecular orbital theory. In cyanohalogens, the trend of QCCs(14N) cannot be accounted for in terms of the inductive effect reported for BrCN. On the other hand, it can be well understood on the NHC basis. Moreover, this trend of QCCs indicates that the -effect is shielded by the carbon atom and thus only pi-effect is the dominant factor affecting variations of QCC(14N) values. This is in accord with the “shielding effect” proposed much earlier in literature. More significantly, the NHC argument can be applied to account for the chain-conjugated structure proposed for sydnone compounds in the manner that the lone-paired electrons on the exocyclic oxygen, based on the Kartritzky hybrid structure, proceeds NHC into the pi-system of the N(2)-N(3)-C(4)-C(5) framework. Combined with the “ionic covalent bond” utilized for interpretation of the substantial stabilization of fluoro-substituted methanes, we conclude that the Kartritzky structure may also explain the IR and X-ray data observed for the highly polar C(5)-O(6) bond. It is then reasonable to exclude the aromaticity of the sydnone ring, whereas the semi-aromatic resonance structure is still favored from the viewpoint of NHC.