Molecular orbital calculations have been carried out of the basicities of substituted pyridines in the gas phase at the HF/6-31G*, MP2/6-31G*, and B3LYP/6-31G* levels and in solution based on the isodensity surfacepolarized continuum model (IPCM). The correlated gas-phase MO basicities, especially at the MP2 level, agree well with the experimental gas-phase results. The IPCM model with MP2/6-31G* and B3LYP/6-31G* geometries also reproduces well the solvation free energy difference, ∆G°s, between the pyridinium ion and neutral pyridine in water, although it leads to a somewhat greater negative slope for the pK a (aq) vs σ plot (F aq ) -8.6 and -9.0 at the MP2/6-31G* and B3LYP/6-31G* levels, respectively, in contrast to the experimental slope of F aq ) -5.8). The model predicts a linear correlation between the theoretical (IPCM) F values in solution and the Onsager dielectric function ( -1)/(2 + 1). The estimate of ion solvation free energy for H + in acetonitrile (G°s ) -250.4 kcal mol -1 ) indicates that there is a constant pK a difference of ∆pK a ()pK a -(AN) -pK a (W)) ) 7.7. This is due solely to the H + ion solvation free energy difference of 10.5 kcal mol -1 between acetonitrile and water with near unity (1.02) slope for the δpK a (AN) vs δpK a (W) plot. The π donor effect of strong para π acceptors (p-CN, p-NO 2 , etc.) on the pK a values of pyridinium ions was found in the gas phase as well as in solution.

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The gas-phase identity methyl transfer reactions, X- + CH3X ⇌ XCH3 + X-, have been investigated with X = H, F, Cl and Br at the MP2, B3LYP, QCISD and QCISD(T) levels by geometry and energy optimizations using the 6-311++G(3df,2p) basis sets at each level. Energy barriers, Δ , Δ , ΔH ⧧ and ΔG ⧧, are reported relative to both the reactants (ΔG ⧧) and ion−dipole complex levels (Δ ). The electron correlation energy (−E corr) decreases in the MP2, QCISD and QCISD(T) results as the size (number of electron) of the system becomes larger (X = F → Cl → Br). The MP2 and QCISD methods underestimate the electron correlation effects relative to the highest level QCISD(T) results, which are, in general, in good agreement with the available experimental values. The lowest and highest activation barriers obtained with X = F and H, respectively, are found to be the consequences of the strong electrostatic interaction energies in the TS (ΔE es ≪ 0 and ΔE es ≫ 0, respectively), in contrast to small differences between nucleophiles, X, in the proximate σ−σ* charge transfer and deformation energies. The gas-phase barrier heights are in the order X = F < Br < Cl < H, and hence the reactivity and the gas-phase nucleophile strength are in the reverse order. Moreover, the extent of bond formation in the transition state, as expressed by the percentage bond order change, %Δn ⧧, is also in the order of intrinsic nucleophilicity. Thus the stronger the nucleophile, the greater is the bond formation in the transition state for the intrinsic barrier controlled reactions.

The gas-phase acyl transfer reactions X -+ RCOY T RCOX + Ywith R ) H, CH 3 and X, Y ) Cl, Br have been investigated with MO theory at the G2(+)MP2 level. Attempts have been made to locate two types of adducts, a tetrahedral adduct formed by an out-of-plane π-attack (π-adduct) and an adduct formed by an in-plane σ-attack (σ-adduct). In all cases, the σ-type adducts are nonexistent. Careful examination of the energies (∆E) at the MP2/6-311+G** level shows that the transition structure region is very flat with a very low barrier for decomposition of the intermediate. This small barrier becomes inverted (i.e., the intermediate level is higher than the transition state) as the zero-point and thermal energy corrections are applied, which is eventually restored to the normal barrier (δ∆G ) ∆G TS -∆G Int = -0.9 kcal mol -1 ) when entropy effect is accounted for. The π-adducts are stabilized mainly by the proximate second-order σ-σ* type charge transfer interactions. The solvent effect evaluated in acetonitrile by the IPCM (isodensity polarizable continuum model) method raises activation energies, ∆G * , by 9 ∼ 13 kcal mol -1 , but the relative reactivity order in the gas phase, -∆G * (X,Y):(Cl, Br) > (Cl, Cl) > (Br, Br) > (Br, Cl), is maintained in solution. The stepwise mechanism predicted in the gas phase and in solution is consistent with the experimental result.

The reactions of aryl benzenesulfonates (YC6H4SO2OC6H4Z) with benzylamines (XC6H4CH2NH2) in acetonitrile at 65.0 degrees C have been studied. The reactions proceed competitively by S-O (kS-O) and C-O (kC-O) bond scission, but the former provides the major reaction pathway. On the basis of analyses of the Hammett and Brönsted coefficients together with the cross-interaction constants rho(XY), rho(YZ), and rho(XZ), stepwise mechanisms are proposed in which the S-O bond cleavage proceeds by rate-limiting formation of a trigonal-bipyramidal pentacoordinate (TBP-5C) intermediate, whereas the C-O bond scission takes place by rate-limiting expulsion of the sulfonate anion (YC6H4SO3-) from a Meisenheimer-type complex.

The kinetics and mechanism of the anilinolysis (XC 6 H 4 NH 2 ) of dithio esters, RC(᎐ ᎐ S)SC 6 H 4 Z with R = C 2 H 5 and C 6 H 5 CH 2 are investigated in acetonitrile at 45.0 ЊC. By application of various structure-reactivity correlations, selectivity parameters ρ X , β X , ρ Z , β Z and ρ XZ are determined. The reactions are predicted to proceed stepwise with rate-limiting expulsion of the ArS Ϫ group. The dithio ester with R = C 2 H 5 exhibits the fastest rate and the largest positive ρ XZ value; this is interpreted to result from the strongest electron donating ability of the ethyl group in the intermediate and a crowded tetrahedral intermediate and transition state in which the nucleophile (X) and leaving group (Z) are in close proximity due to the bulky C 2 H 5 group. Much faster rates are observed for the thiocarbonyl (C᎐ ᎐ S) rather than carbonyl (C᎐ ᎐ O) esters in the stepwise nucleophilic substitution reactions, which may be ascribed to the lower π* C᎐ ᎐ S and σ* C-LG levels than those of the corresponding antibonding levels in the carbonyl esters. The normal kinetic isotope effects, k H /k D > 1.0, involving deuterated anilines suggest concurrent proton transfer with the expulsion of the ArS Ϫ leaving group in a four-center hydrogen bonded transition state.

Kinetics and mechanism of the aminolysis of Z-thiophenyl acetates with X-benzylamines are investigated in acetonitrile at 45.0 o C. The magnitudes of Brönsted coefficients βX (=1.3~-1.6) and βZ (= -2.1~-2.4) are all large and cross-interaction constant ρXZ is relatively large and positive (0.90). These trends are consistent with the rate-limiting breakdown of a tetrahedral intermediate, T ± . The proposed mechanism is also supported by adherence of the rate data to the reactivity-selectivity principle (RSP). The kinetic isotope effects, kH/kD, are greater than unity (1.3-1.4) suggesting a possibility of hydrogen-bonded four-centered transition state. The activation parameters, ∆H ≠ and ∆S ≠ , are consistent with this transition-state structure.

The rates of the aminolysis of S-phenyl substituted-acetate series (RC(=O)SC 6 H 4 Z, with R=Me, Et, i-Pr, t-Bu and Bn) with benzylamines (XC 6 H 4 CH 2 NH 2) are not correlated simply with the Taft's polar (σ *) and/or steric effect constants (E s) of the substituents due to abnormally enhanced rate of the substrate with R=Et. Furthermore, the cross-interaction constant, ρ XZ , is the largest with R=Et. These anomalous behaviors can only be explained by invoking the vicinal bond (σ)-antibond (σ *) charge transfer interaction between CC α and C-S bonds. In the tetrahedral zwitterionic intermediate, T ± , formed with R=Et the vicinal σ CC -σ * C-S delocalization is the strongest with an optimum antiperiplanar arrangement and a narrow energy gap, ∆ε = ε σ*-ε σ. Due to this charge transfer interaction, the stability of the intermediate increases (with the concomitant increase in the equilibrium constant K (= k a /k −a)) and also the leaving ability of the thiophenolate leaving group increases (and hence k b increases) so that the overall rate, k N = Kk b , is strongly enhanced. Theoretical support is provided by the natural bond orbital (NBO) analyses at the B3LYP/6-31+G* level. The anomaly exhibited by R=Et attests to the stepwise reaction mechanism in which the leaving group departure is rate limiting.