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(not equal model)) for the model heterolysis of the ions at 25 degrees C. Very good correlations between G(not equal model) values and corresponding measured G(not equal) 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 (N-f(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.