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NMR偶合常数在判别1,2-双取代乙烯分子构型方面十分有用,但目前尚无简单有效的计算方法.我们在以往计算烃类分子偶合常数~3J_(H-H)的基础上,提出计算1,2-双取代乙烯偶合常数~3J_(H-H)的简单方法,并可以作为确定乙烯型分子构型的判据. 乙烯分子中的氢原子被某些基团取代后,与取代基相连接的碳原子的电子密度ρc会发生改变,分子的几何构型以及电子云分布也可能受到影响,从而影响烯质子的偶合常数~3J_(H-H).如果我们只考虑取代基对ρc的影响,并略去数值很小的空间偶合项~3J_(H-H)~(2),根据前文提出的公式不难得到:
NMR coupling constants are very useful in judging the molecular configuration of 1,2-disubstituted ethylene. However, there is no simple and effective method for calculating the molecular structure of 1,2-disubstituted ethylene. In the past, based on the calculation of the coupling constant of hydrocarbon molecules ~ 3J_ (HH) , 2-disubstituted ethylene coupling constant ~ 3J_ (HH) of the simple method, and can be used as a criterion to determine the molecular structure of ethylene vinyl molecules ethylene atoms are replaced by certain groups, connected with the substituent The electron density ρc of carbon atoms will change, the geometry of the molecule and the electron cloud distribution may also be affected, thus affecting the coupling constant of the olefinic protons. If we only consider the influence of substituents on ρc, To a small value of the space coupling term ~ 3J_ (HH) ~ (2), according to the formula presented above is not difficult to get: