Determinations of reactivity by molecular orbital theory. Part 52. Theoretical studies of solvent effects on the decarboxylation of 2-cyanoacetic acid

Abstract

Solvent effects on the decarboxylation of 2-cyanoacetic acid in H₂O and CH₃CN have been investigated by means of MO calculations using the AM1 method. Four regions around the substrate molecule were identified as the probable primary solvation shell. Water was found to solvate essentially by hydrogen-bonding while acetonitrile had mainly a polarization effect in solvation. Both solvate molecules stablized the zwitterionic form causing the elevation of activation barriers, the effect being markedly greater for water. Solvation caused shortening of the bond length and an increase in the bond order of the C–C bond which is cleaved in decarboxylation. The entropy term, TΔS^‡, became increasingly favourable with an increase in the temperature for reaction in solution. Lower barriers were found after the main activation barrier along the reaction co-ordinate due to solvent reorganization.