DFT-PCM Study on Solvolytic Behaviour of N-alkyl-X-pyridinium Ions

摘要

Using the M06-2X method and IEFPCM for ethanol, geometries of various N-benzhydryl-X-pyridinium ions and corresponding heterolytic transition structures have been optimized to calculate free energies of activation (~G?model ) for the model heterolysis of the ions at 25°C. Very good correlations between ~G?model values and corresponding measured ~G? values have made it possible to estimate reliable reactivities of N-(4,4’- dimethoxybenzhydryl)-X-pyridinium ions in protic solvents as well as nucleofugality parameters (Nf calc ) for 18 X-substituted pyridines. Computational results further reveal that geometric progression along the heterolytic reaction coordinate is synchronized with ionic charge transfer. A Leffler-Hammond coefficient of 0.65 (x100%) corresponds to both the amount of the ionic charge transferred from the X-pyridine moiety to the alkyl electrofuge (65-67%) and the extent of geometric progression (64%) on proceeding from the ionic substrate to TS in the thermoneutral heterolysis. Consequently, exergonic heterolyses of very reactive N-alkyl-X-pyridinium ions proceed through the geometrically late transition states.