A theoretical investigation of the mechanism of the CpCoL2 (L = CO, PR3, olefin) catalyzed acetylene cyclotrimerization reaction has been carried out at the ab initio and density functional theory (DFT) levels. The mechanism begins with a stepwise pair of ligand substitution reactions in which precatalyst CpCo(PH3)2 (1) is converted, via CpCo(PH3)(η2-C2H2) (2), to CpCo(η2-C2H2)2 (3) with the liberation of 11.3 kcal/mol at the B3LYP level. Oxidative coupling of the alkyne ligands in 3 to give a cobaltacyclopentadiene complex (4) is exothermic by 13.1 kcal/mol and is predicted to occur in a facile manner (ΔH ⧧ = 12.8 kcal/mol). Reductive cyclization of the bidentate C4H4 ligand in 4 to generate CpCo(η4-cyclobutadiene) (8) is considerably exothermic (ΔH = −34.0 kcal/mol). However, the least motion pathway that transforms 4 directly into 8 which conserves a mirror plane is found to be symmetry forbidden, implying the presence of a large barrier. Coordination of a third acetylene to 4 results in the formation of CpCo(C4H4)(η2-C2H2) (5). Energetically, this third acetylene is weakly bound (12.4 kcal/mol). This is attributed to the parallel orientation the acetylenic C−C bond vector occupies with respect to the Co−Cp bond axis. Collapse of 5 to CpCo(η4-C6H6) (7) occurs in a kinetically very facile process (ΔH ⧧ = 0.5 kcal/mol) reflecting the extremely exothermic nature of this transformation (ΔH = −81.4 kcal/mol). An alternate path converting 5 to 7 via the intermediacy of a cobaltacycloheptatriene complex (6) was found to be energetically prohibitive due to the symmetry-forbidden nature of the reductive elimination converting 6 to 7. In addition, a stationary point corresponding to 6 on the B3LYP potential energy surface could not be located. Completion of the catalytic cycle is achieved via a stepwise ligand substitution process in which two acetylene molecules displace the arene in 7 to regenerate 3 with the release of 7.4 kcal/mol at the B3LYP level. Two alternative pathways leading to arene formation in which a phosphine intercepts 4 and remains attached to the Co atom throughout the arene construction process were found to be unlikely mechanistic candidates.

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The structures of the four lowest alanine conformers, along with their radical cations and the effect of ionization on the intramolecular proton transfer process, are studied using the density functional theory and MP2 method. The energy order of the radical cations of alanine differs from that of the corresponding neutral conformers due to changes in the basicity of the NH2 group upon ionization. Ionization favors the intramolecular proton transfer process, leading to a proton-transferred radical-cation structure, [NH3 + -CHCH3-COO•], which contrasts with the fact that a proton-transferred zwitterionic conformer is not stable for a neutral alanine in the gas phase. The energy barrier during the proton transfer process is calculated to be about 6 kcal/mol.

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