| Summary: | 碩士 === 國立成功大學 === 化學系碩博士班 === 90 === In the current research we have proposed that the betaine structure is the most appropriate single hybrid structure for sydnone representative. Whereas the conventional mesoionic concept is ruled out through analysis of values of electric field gradients (EFG) and screen constants, both obtained from ab initio calculation. In other words, the sydnone compounds cannot be classified as ketones.
Studies of shielding constants for substituted benzenes have also been undertaken. It is found the calculated substitution effect on carbon-13 chemical shifts are in good agreement with the empirical substitution parameters reported in literature. More significantly, the well-known additivity of substitution effect is also evidenced by our calculated results. Negative hyperconjugation can be employed to account for the EFG values calculated for the CFnH3-n substituted benzenes and ethylenes. This conclusion is derived from (1) the effects of conformation on EFGs in CF3 –substituted compounds and (2) the trend of EFGs in both series, which are coorelated with the number of F-atom(n) present in CFnH3-n.
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