环己二烯与丙烯加成反应的正则速率常数

Theoretical Calculation on the Canonical Rate Constants for the Addition Reaction of 1,3-cyclohexa-diene with Propylene

采用从头算UHF方法在6-31G基组上研究了1，3-环己二烯与丙烯的加成反应所有可能的反应通道.优化得到了反应途径的所有驻点的几何构型.该反应可按分步和协同过程进行.分步过程的通道是能量最为有利的反应途径.对于分步过程形成exo和endo产物各存在两类分步加成机理，即“由丙烯双键端位碳先进攻环己二烯”机理和“由丙烯中心碳原子先进攻环己二烯”机理，共四条反应通道。它们均为途经一双自由基中间体的分步加成过程.反应的第一步为反应的速控步骤，计算的活化位垒在102-114kJ·mol-1之间.在从头算计算基础上，采用过渡态的微正则理论计算了分步过程四条通道的正则速率常数.计算结果表明：无论按照哪种反应机理进行，形成endo产物的通道的反应速率常数均大于形成exo产物的通道的值;无论形成哪类产物，均是“由丙烯双键端位碳先进攻环己二烯”机理的通道的速率常数大于“由丙烯中心碳原子先进攻环己二烯”机理的通道的值.但四条通道的速率常数相差不大，反应为竞争机制.

The mechanisms of all the addition reaction paths of 1,3-cyclohexa-diene with propylene have been studied by using ab initio UHF/6-31G* method. All geometries of the stationary points have been optindzed- Stepwise and concerted processes are both possible for the reaction.The stepwise processes are most favorable in all reaction paths- The exo and endo products of the reaction can be formed through two mechanisms, i.e. the terminal double-bonded carbon of propylene attacks 1,3- cyclohexa-diene at first step or so do the center double-bonded carbon. The stepwise processes contain four reartion paths involving biradical intermediates. The calculated activation barriers of the rate deterndning steps of these paths are about 102 ~ 114 kJmol-1. Based on microcanonical transition state theory the canonical rate constants for the four stepwise paths have been computed by using data calculated at UHF/6-31G* level. The computational results show that the canonical rate constants for the reaction paths of forming endo product are larger than that for exo ones, and that the canonical rate constants for the reaction paths with the terndnal double-bonded carbon of propylene attasking 1,3- cyclohexa-diene at first step arc larger than that with the center carbon atom- The canonical rate constants of four stepwise paths are close to each other, so they are competing reactions.