Ethylene dimerization, theory

Fig. 25 Potential energy surfaces for the most feasible two-state reaction pathways for ethylene dimerization catalyzed by Cr(II)OH (If), via either a Cr-carbene mechanism or a metallacycle mechanism determined at the M06 level of theory. Also shown are the crossing points optimized at CASSCF level. The triplet metallacycle reaction pathway is depicted in blue, and the triplet Cr-carbene reaction pathway is shown in dark red. The quintet parts are in black. Energies are in kcal moP and relative to il. Bond lengths are in angstroms. Angles are in degrees...

R.A. King, On the accuracy of spin-component-scaled perturbation theory (SCS-MP2) for the potential energy surface of the ethylene dimer. MoL Phys. 107(8-12), 789-795 (2009)... 

Thermal dimerization of ethylene to cyclobutane is forbidden by orbital symmetry (Sect 3.5 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume). The activation barrier is high E =44 kcal mof ) [9]. Cyclobutane cannot be prepared on a preparative scale by the dimerization of ethylenes despite a favorable reaction enthalpy (AH = -19 kcal mol" ). Thermal reactions between alkenes usually proceed via diradical intermediates [10-12]. The process of the diradical formation is the most favored by the HOMO-LUMO interaction (Scheme 25b in chapter Elements of a Chemical Orbital Theory ). The intervention of the diradical intermediates impfies loss of stereochemical integrity. This is a characteric feature of the thermal reactions between alkenes in the delocalization band of the mechanistic spectrum. 

Application of CM theory to explain pericyclic reactions was first attempted by Epiotis and coworkers (Epiotis, 1972, 1973, 1974 Epiotis and Shaik, 1978b Epiotis et al 1980). The following analysis is a much-simplified treatment of that approach. Let us compare, therefore, the CM analysis for the [4 + 2] allowed cycloaddition of ethylene to butadiene to give cyclohexene with the [2 + 2] forbidden dimerization of two ethylenes to give cyclobutane. For simplicity only the suprafacial-suprafacial approach is considered, although this simplification in no way weakens the argument. 

In agreement with Balandin s theory it was assumed by Eidus (79,80) that methylene radicals adsorbed on two adjacent centers of the catalyst are dimerized to ethylene which remains adsorbed on a doublet subsequently a new methylene group is added to one of the carbon atoms with a hydrogen atom migrating to the carbon atoms of the ethylene which is then desorbed further growth with formation of 1-olefins proceeds similarly a shift of the double bond inside of the molecule may occur. The view of Craxford and co-workers that all polymerizing methylene radicals remain adsorbed to the surface was contradicted by Eidus as inconsistent with experimental evidence (84b). 

Triphenylenes provided with nonionic di(ethylene oxide) side-chains (25f)132 134 or with ionic alkyl chains (25g)135 form supramolecular polymers in water.136 The arene—arene interactions of the aromatic cores allow for the formation of columnar micelles . At low concentrations the columns are relatively short, and the solutions are isotropic. At higher concentrations the longer columns interact and lyotropic mesophases are formed.133 Computer simulations showed that in the isotropic solution the polymerization of the discotics is driven by solute-solute attraction and follows the theory of isodesmic linear aggregation the association constants for dimerization, trimerization, and etc., are equal and the DP of the column thus can easily be tuned by concentration and temperature.137 138 At higher concentrations the sizes of the columns are influenced by their neighbors, the columns align, and the DP rises rapidly. 

K complex cation intermediate and the infinite system (ethylene + ethylene( +)) [22]. In contrast, the cycloreversion reaction of the thymine dimer radical cation is exothermic at the CASSCF level of theory as shown in Table 6. Compared to the cB + case, the n conjugation in thymine is responsible for the greater stabilization of the monomer cation, thus making the reaction exothermic. To estimate the dynamic electron correlation effects, we carried out preliminary MP2 calculations for TTp-1 and T( +) T. The exothermicity was reduced to 5.0 kcal/mol. Since the number of the bonds in TTp-1 is larger than that in T( +) T, TTp-1 is much more stabilized by the dynamical electron correlation effects than T( +) T is. 

Fig. 4 Energy splitting of the HOMO and LUMO levels of ethylene from dimer computation compared by Huang and Ketsetz at different levels of theory (from [93])...

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