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本文提出了S_N2′反应过渡态的量子化学模型,指出S_N2′反应过渡态离域区由三个碳原子实、亲核试剂的进攻原子及亲去基团共五个原子实组成.运用HMO法和近似处理证明其分子轨道的久期行列式与五碳环的Hückel和Móbius轮烯的久期行列式完全相似.离域电子效应为4加有效孤对电子.如果亲核试剂直接起进攻作用的原子是N.O.Cl等,有效弧对电子接近于2个电子,此时Hückel环有较低能量,S_N2′反应按顺式取代.如果亲核试剂直接起进攻作用的原子是C.S等,有效孤对电子远小于2个电子,约为1左右,此时Hückel环和Móbius环能量接近,顺反两种取代产物占一定比例,有时是以反式产物为主.本文提出的量子化学模型及相应的分子图与反应物实体是拓朴等价的.按照Hammond假说,它是较为合理的也是接近真实的,因而较园满地解释了S_N2′反应的立体化学选择性和实验事实,最后对S_E2′反应过渡态作了类似的处理和立体化学推测.
In this paper, a quantum chemical model of transition state of S_N2 ’is proposed, which indicates that the delocalized region of S_N2’ transition state is composed of three carbon atoms, the attack atom of nucleophile and the affinity group. And the approximate treatment shows that the secular determinants of their molecular orbitals are exactly similar to those of the Hückel and Móbius olefins of the five-carbon ring. The delocalization electron effect is 4 plus effective lone pair electrons. If the nucleophile acts directly offensive Of the atoms are NOCl, effective arc pairs of electrons close to two electrons, then Hückel ring has lower energy, S_N2 ’reaction by cis replacement. If the nucleophile directly attack the atom is CS, effective The electron is far less than two electrons, about 1, then Hückel ring and Móbius ring energy close to the cis-and-anti-substitution products accounted for a certain percentage, sometimes in the trans-based products.The proposed quantum chemical model and the corresponding Is the topological equivalent of the reactant entity, which is more reasonable and near real according to the Hammond hypothesis, thus explaining the stereochemical selectivity and experimental facts of the S_N2 ’reaction. Finally, ’ It should be made similar to the transition state and stereochemical estimation process.