B3LYP calculations water/methyl chlorid

B3LYP/6-311+- -G -level calculations have been used to predict the structure of the encounter complex (68) for the. S n2 reaction between F and methyl chloride in the presence of one molecule of water.106 Structure (68) was chosen as the encounter complex even though it does not have the lowest energy, but because it has the greatest negative charge on fluorine. Structure (69) is the encounter complex for the reverse reaction. 

The Sn2 reaction in solution. We saw above the application of microsolvation to Sn2 reactions ([14, 15]). Let us now look at the chloride ion-chloromethane Sn2 reaction in water, as studied by a continuum method. Figure 8.2 shows a calculated reaction profile (potential energy surface) from a continuum solvent study of the Sn2 attack of chloride ion on chloromethane (methyl chloride) in water. Calculations were by the author using B3LYP/6-31+G (plus or diffuse functions in the basis set are considered to be very important where anions are involved Section 5.3.3) with the continuum solvent method SM8 [22] as implemented in Spartan [31] some of the data for Fig. 8.2 are given in Table 8.1. Using as the reaction coordinate r the deviation from the transition state C-Cl... 

The transition structures for the hydrolysis reactions of methyl, /-butyl, and ada-mantyl chlorides in the gas phase and in water were calculated using the B3LYP/6-31-G(d) level of theory and the PCM solvation model.82 In the gas phase, backside attack is strongly favoured for the methyl chloride reaction and slightly favoured for the t -butyl chloride reaction. Frontside attack is favoured for the adamantyl chloride... 

We applied ONIOM-PCM to the SN2 reaction between chloride and methyl chloride, with one explicit water molecule (Figure 4.9) [42], Chloride-methyl chloride was calculated at the B3LYP level of theory, and the water molecule with HF. For both methods we used the 6-31G(d,p) basis set. The absolute results did not reproduce the target B3LYP results exactly, but this was due to the poor description of the solute-water hydrogen bond at the HF level. When we considered only the solvent effect on the reaction, we saw that ONIOM-PCM/A reproduces the full B3LYP results nearly exactly. Of the approximations, ONIOM-PCM/X performs remarkably well. 

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