Stabilities And Formation Mechanisms For DA Covalence Dimers Of Pyridine: A Theoretical Study

The stabilities and formation mechanisms for Diels-Alder (DA)covalence dimers of pyridine were studied by means of high levelquantum chemical methods (MP2, B3LYP and MPW1K) with6-311G(d,p) basis set. According to the different relative position oftwo N atoms in forming pyridine dimers, total eighteen dimers canbe formed in the first DA , and they may finally transform to totaltwelve final dimmers(DII). For all DA dimers and transition states,optimized geometries by the three methods are similar but MP2gives lower relative energies. All the dimerization paths need highactivation energies, and path 5 and path 6 are most favorable oneswith the most smaller barrier heights of 54.1kcal/mol and54.2kcal/mol at the MP2/6-311G(d,p) level. The factors influencingthe reaction barriers of DA dimerization are as follows: 1) bindingpattern of two pyridine monomers respectively as diene anddienophile ,which is main ; 2) the distance between the two N atomsof the structure. Stablity of eighting DIobtained in the first step ofdimerization are mainly determined by the two factor mentionedpreviously, and the DIwith the binding pattern ofα-b&γ-a orα-b&γ-band the greater distance of N atoms is more stable.However, thestablity of twelve DIIobtained in the second step of dimerization aremainly determined by the positions where two N atoms locate, and it is also influenced by the distance between the two N atoms of thestructure, the DIIwith the greater distance of N atoms is morestable.