Ab initio calculations of minimum-energy pathways of the nucleophilic addition of the H − anion, LiH molecule and Li +/H − ion pair to acetylene and methylacetylene

Abstract

Ab initio calculations were performed for special points of the minimal energy pathways (MEP) of the nucleophilic addition reactions of the isolated H − anion, LiH molecule and Li +/H − ion pair to acetylene (A) and methylacetylene (MA) molecules, proceeding in accordance ( M) and against ( aM) the Markovnikov's rule. All structural parameters were optimized using the restricted Hartree–Fock (RHF) method. For the addition of H −, the 6-31++G ∗ basis set was used and for the reactions of LiH and Li +/H − the 6-31G ∗ basis set with the subsequent recalculation of single point energies, taking into account of electron correlation energy by means of the second-order Möller–Plesset perturbation theory at the MP2/6-31++G ∗∗ level. The results of calculations demonstrate, that the energy characteristics of both M- and aM-additions with H − do not differ sufficiently (0.1–1.2 kcal/mol for the activation energies (Δ E a) and the reaction heats (Δ Q)). The substitution of the H atom by the CH 3 group in A molecule results in practically the same values of Δ Q and Δ E a. On the contrary, for the LiH molecule and Li +/H − ionic pair, the M-addition is favorable (charge control). It is found that the presence of electrophile decreases the activation energy by 3–5 kcal/mol as compared with the addition of the isolated hydride ion H −.