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

Abstract

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 (Nfcalc) 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 (×100 %) 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.

Prediction of the kinetic stability of N‐alkyl‐X‐pyridinium ions in dichloromethane

“…[ 9 ] We have previously shown that free energies of activation for the model heterolysis of some N ‐[4,4′‐bis (dimethylamino)benzhydryl]‐X‐pyridinium ions (DMABh‐X‐pyridinium ions), calculated by using the M06‐2X method [ 10 ] in combination with the IEFPCM solvation model [ 11 ] for ethanol, correlate very well with measured free energies of activation for corresponding solvolyses in several hydroxylic solvents and mixtures. [ 12 ] Besides, the inclusion of the reactive 4,4′‐bis (dimethylamino)benzhydryl electrofuge (DMABh electrofuge) in the model reaction enabled optimization of heterolytic transition structures in a very wide range of reactivity. Accordingly, we have employed the same model reaction and computation methods in this study.…”

“…We have shown in the previous study that the slight increase in the α‐C–N bond length toward more reactive DMABh‐X‐pyridinium ions is indicative of increasing instability of the ions. [ 12 ] The increase of the electron‐withdrawing ability of substituents on the X‐pyridine moiety of the ionic substrates decreases the amount of the ionic charge in the moiety, which weakens the α‐C–N bond, destabilizes the ionic substrates and, consequently, decreases the free energy barrier to heterolysis. [ 10 ] In addition, the α‐C–N distances for the two transition structures presented in Figure 1 indicate that the heterolytic TS more resembles a reactant as the reactivity, that is, the instability, in the series of the DMABh‐X‐pyridinium ions increases.…”

“…Nevertheless, it has been shown that even highly exergonic heterolyses in the series proceed through the transition states whose features are closer to those of products than to those of reactants. [ 12 ]…”

“…Accordingly, errors associated to the low harmonic frequencies cancel out when calculating activation free energies for the heterolysis of these substrates. [ 12 ]…”

“…Recently, we succeeded in optimizing transition structures for the heterolysis of various N ‐benzhydryl‐X‐pyridinium ions, by using the M06‐2X method [ 10 ] in combination with the IEFPCM solvation model [ 11 ] for ethanol. [ 12 ] Very good correlations between computed and measured free energies of activation were used to obtain nucleofugality parameters ( N f calc ) for 18 X‐substituted pyridine leaving groups in five hydroxylic solvents and mixtures. A series of N f calc parameters for each solvent spans a reactivity range of 11 orders of magnitude.…”