Isomerization of propargyl cation to cyclopropenyl cation: Mechanistic elucidations and effects of lone pair donors

Abstract

This ab initio study examines two pathways (one concerted and the other two-step) for isomerization of the linear propargyl cation to the aromatic cyclopropenyl cation, also probing the phenomenon of solvation of this reaction by simple lone pair donors (NH3, H2O, H2S and HF) which bind to the substrate at two sites. Fully optimized geometries at the B3LYP/6-31G(d) level were used, along with single point QCISD(T)/6-311+G(d,p) and accurate G3 level calculations upon the DFT optimized geometries. For the unsolvated reaction, the two-step second pathway is energetically favoured over the one-step first pathway. Lone pair donor affinity for the various C3H\(_{3}^{+}\) species follows the uniform order NH3 > H2S > H2O > HF. The activation barriers for the solvated isomerizations decrease in the order HF > H2O > H2S > NH3 for both pathways. The number of lone pairs on the donor heteroatom as well as the heteroatom electronegativity are factors related to both these trends. Compared to the unsolvated cases, the solvated reactions have transition states which are usually ‘later’ in position along the reaction coordinate, validating the Hammond postulate. Two routes for isomerization of propargyl cation to cyclopropenyl cation are studied by three ab initio methods, where the influence of lone pair donors is also examined. A two-step route is favoured over the concerted pathway, and the effect of lone pair donors upon reaction facility follows the general order NH3 > H2S > H2O > HF.