Energetics and Stoichiometric Mechanism of the Gas-Phase SN2 Reactions

This unexpected situation, when the energy level of the transition state structure with D3h symmetry turns out to be lower than that of the initial system of the reactants, is predicted for a number of other 8 2 reactions even with the correlation effects taken into account [22]  

More complete lists of calculated energies of the gas-phase Sn2 reactions of Y -h CH3X general type are available in Refs. [30,37] (the i-INDO method) and [26,34] (4-3IG). The data obtained make it possible to check how the well-known relationships among the structural, kinetic and thermodynamic characteristics, such as the Bell-Evans-Polanyi principle, the Hammond postulate, etc., are obeyed in the case of the reactions which proceed without a solvent. 

Calculations [26] of the heats AH of the reactions Y -I-CH3F yield a sequence of exothermicity which represents very well the experimental trend H H2N HO HCr CH30 F NC H00 CN- F0 . The length of the bond C—F in II [Eq. (5.2)] increases in this order from 1.725 A to 1.986 A while the angle 6 (see Fig. 5.1) is reduced from 96.3 to 84.2. Thus, the results of the calculations are in agreement with the above-noted relationships, according to which the more exothermic a reaction, the closer to the starting reactants the structure of a transition state will be. Furthermore, they 

Complex character of the energy profile of the gas-phase reactions 8 2 (see Fig. 5.2) explains the lack of direct correlations between the experimentally assessed activation barriers AE (when they are positive) and the exothermicity of the reaction AH. A better-grounded correlation should be sought between the values of the intrinsic barriers AE and the quantity AE = AH -I- (AEx — AEyx). Wolfe et al. [26] treated, using the 4-3IG basis set, the results of their calculations on over twenty 8 2 reactions and found that the data obey quite satisfactorily the well-known Marcus equation [48] which describes reactions of electron and proton transfer as well as those of the alkyl groups transfer [49] in solution. 

In place of the values of free energies (for solutions), potential energies are used, and the energy barrier of the reactions Y + CH3Y and X -I- CH3X  

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