DFT computations have been performed on nucleophilic substitutions of phenacyl bromides with pyridines to investigate the mechanism of the reaction. In contrast with earlier suppositions, tetrahedral intermediate is not formed by the addition of pyridine on the C --O group of phenacyl bromide, because the total energy of the reacting species increases continuously, when the distance between the N and C( --O) atoms of reactants is shorter than 2.7 Å . At a greater distance, however, a bridged complex of the reactants is observed, in which the N atom of pyridine is slightly closer to the C atom of the C --O, than to the C atom of the CH 2 Br group of phenacyl bromide, the distances are 2.87 and 3.05 Å , respectively. The attractive forces between the oppositely polarized N and C( --O) atoms in the complex decrease the free energy of activation of the S N 2 attack of pyridine at the CH 2 Br group. The calculated structural parameters of the S N 2 transition states (TS) indicate, that earlier TSs are formed when the pyridine nucleophile bears electron-donating (e-d) groups, while electron-withdrawing (e-w) groups on phenacyl bromide substrate increase the tightness of the TS. Free energies of activation computed for the S N 2 substitution agree well with the data calculated from the results of kinetic experiments and correlate with the s Py substituent constants, derived for pyridines, and with the Hammett s constants, when the substituents (4-MeO-4-NO 2 ) are varied on the pyridine or on the phenacyl bromide reactants. effect; substituent constants for pyridines; free energy of activation; DFT calculations; activation strain analysis enolate form of the C --O group, but the carbanion canonical form of the CH 2 group contributes mainly to the TS of the S N 2 process. S N 2 mechanism was also proposed by Kevill and Kim [28] for the solvolyses of phenacyl derivatives, proceeding with poor nucleophiles in different solvents. Investigating the kinetics of the reaction of substituted phenacyl bromides (1) with pyridines (2) in acetonitrile and methanol solvents, Lee et al. [29][30][31] found evidences for the first addition-substitution (path A), while Forster and Laird [20,21] for the second S N 2 mechanism (path B).Not being convinced of the interpretation of the kinetic experiments and of the proposed strange bridged TS (4), we have performed DFT computations to test both mechanisms of the reaction of substituted phenacyl bromides (4-MeO-4-NO 2 ) and pyridines (4 0 -MeO-4 0 -NO 2 ). The results are presented in this paper.Earlier, we studied the effect of substituents on activation parameters and transition structures of aliphatic nucleophilic (www.interscience.wiley.com)

DFT computations have been performed at different levels of theory to identify the mechanism for the oxidation of sulfides and sulfoxides with periodates. The periodate ion (IO 4 -), periodic acid (HIO 4 ) and their hydrated derivatives all oxidize sulfides to sulfoxides in one-step oxygen-transfer reactions and the relative reactivities are HIO 4 ϾϾ H 5 IO 6 Ͼ IO 4 -Ͼ H 4 IO 6 -ϾϾ H 3 IO 6 2-. The hydration and dissociation equilibria of the periodates are shifted towards IO 4 -in neutral and moderately acidic solutions, and sulfides are oxidized mainly with IO 4 -under normal experimental conditions. The oxygen atoms of the periodates attack the sulfides perpendicularly to the plane of the C-S-C atoms and the S···O···I atoms are in a linear arrangement in the very early transition state (TS). The sulfides are the electron donors and periodates are the

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