Mechanism For Covalence Bond Benzene Dimmers Trimers And Tetramers Formation: A Theoretical Investigation

A large number of experiments prove that aromatic compoundscan polymerize under high pressure, and Diels-Alder was identifiedas the most favorable reaction. Due to the DA reaction haswidespread application prospect in chemical area, therefore, theresearch on benzene covalent dimerization and polymeriation isvery meaningful. In this article, the reaction mechanism forcovalence bond benzene dimmers, trimers and tetramers formationwere comprehensively investigated by means of threedensity functional theory (B3LYP, BB1K, MPW1K) and MP2methodat6-31G(d)/6-311G(d,p) level.1) Mechanism for covalence bond benzene dimers formation: ADFT and MP2investigation.For the mechanism for covalence bond benzene dimersformation, all the theoretical methods (MP2, B3LYP, BB1K andMPW1K) predicted similar meta-and para-cycloadditionmechanisms. For each stable point other than reactant, B3LYPmethod predicts higher relative enthalpy than the other two DFTmethods, while MP2method gives the smallest relative enthalpy.According to the results of Quenneville and Germann the relativeactivation enthalpy calculated by MP2/6-31G(d) is close to that calculated by CCSD(T)/6-31G(d) for Diels–Alder reaction of benzene.So, we can see that MP2/6-311G(d,p) may give more reliable relativeenthalpy than the three DFT methods.Benzene dimerization starts by three cycloaddition pathways:(1)para-cycloaddition of benzene leads to Diels–Alder adduct pAD;(2)endo-meta-cycloaddition of benzene gives mAD1;(3)exo-meta-cycloaddition generates mAD2. Further intramolecularcycloadditions of pAD and mAD1afford D2h-(CH)12and D3d-(CH)12,respectively. Due to the steric limitation, mAD2is unable to undergofurther intramolecular meta-cycloaddition.MP2results showed that (1) the activation enthalpies of boththe endo-and exo-meta-cycloadditions are about90kcal/mol, twiceas high as that of para-cycloaddition and (2) further transformationof para-cycloaddition adduct to D2h-(CH)12andendo-meta-cycloaddition adduct to D3d-(CH)12need to surmount highenthalpy barriers of65.02and90.78kcal/mol (relative to twomolecules of benzene), respectively. Molecular orbital analysisrevealed that meta-cycloaddition of benzene is ground stateallowed reaction.2) Mechanism for covalence bond benzene trimers andtetramers formation: A MP2investigation.Used the calculation method and results of the mechanism forcovalence bond benzene drimers formation for reference, we havemade a simple discussion about the mechanism for covalence bondbenzene trimers and tetramers formation. The studied onmechanism for covalence bond benzene dimers formation showedthat MP2method gives more reliable relative enthalpy. So, the theoretical study on mechanism for covalence bond benzenetrimers and tetramers formation is calculated at the MP2/6-311G(d,p)level.Theoretical calculation shows that (1) benzene and drimerthrough four intermolecular DA cycloaddition paths to afford3AIM1、3BIM1. The enthalpies barriers of them are23.18、23.95、22.41、19.51kcal/mol. The enthalpies barriers from3AIM1、3BIM1to afford4AIM1、4BIM1、4CIM1、4DIM1are17.49、18.31、16.22、13.18kcal/mol,13.26、16.12、17.45、17.11kcal/mol through eight paths. These lowenthalpy barriers showed that once the benzene dimerizationoccurs, the trimerization and tetramerization of benzene willbecome very easy to happen.(2) Further intramolecularcycloaddition of trimer3AIM1、3BIM1by [4+2]、[2+2],[2+2]cycloaddition mechanism to generate the final product3AP1、3AP2,3BP.