| Sumario: | The S2 identity exchange reactions of the fluoride ion with benzyl fluoride and 10 para-substituted derivatives (RCHCHF, R = CH, OH, OCH, NH, F, Cl, CCH, CN, COF, and NO) have been investigated by both rigorous ab initio methods and carefully calibrated density functional theory. Groundbreaking focal- point computations were executed for the CHCHF + F and CHCHCI + Cl S2 reactions at the highest possible levels of electronic structure theory, employing complete basis set (CBS) extrapolations of aug-cc-pVXZ (X= 2-5) Hartree—Fock and MP2 energies, and including higher-order electron correlation via CCSD/aug-cc-pVQZ and CCSD(T)/aug-cc-pVTZ coupled cluster wave functions. Strong linear dependences are found between the computed electrostatic potential at the reaction-center carbon atom and the effective S2 activation energies within the series of para-substituted benzyl fluorides. An activation strain energy decomposition indicates that the S2 reactivity of these benzylic compounds is governed by the intrinsic electrostatic interaction between the reacting fragments. The delocalization of nucleophilic charge into the aromatic ring in the S2 transition states is quite limited and should not be considered the origin of benzylic acceleration of S2 reactions. Our rigorous focal-point computations validate the benzylic effect by establishing S2 barriers for (F, Cl) identity exchange in (CHCHF, CHCHCI) that are lower than those of (CHF, CHCI) by (3.8, 1.6) kcal mol, in order.
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