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

Ab initio calculations were performed for special points of the minimal ene rgy pathways (MEP) of the nucleophilic addition reactions of the isolated H - anion, LM molecule and Li+/H- ion pair to acetylene (A) and methylacetyle ne (MA) molecules, proceeding in accordance (M) and against (aM) the Markov nikov'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 Moller-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 w ith H- do not differ sufficiently (0.1-1.2 kcal/mol for the activation ener gies (Delta E-a) and the reaction heats (Delta Q)). The substitution of the H atom by the CH3 group in A molecule results in practically the same valu es of Delta Q and Delta E-a. On the contrary, for the LiH molecule and Li+/ H- ionic pair, the M-addition is favorable (charge control). It is found th at the presence of electrophile decreases the activation energy by 3-5 kcal /mol as compared with the addition of the isolated hydride ion H-. (C) 1999 Elsevier Science B.V. All rights reserved.