The performance of B3-LYP density functional theory in describing SN2 reactions at saturated carbon

The performance of the B3-LYP variant of density functional theory when used in conjunction with the 6-31G(d) and 6-311 + G(3df, 2p) basis sets in describing the prototypical gas-phase SN2 reactions of Cl− + CH3Cl and CH3Br has been examined in detail. Reasonable values of the complexation energies (ΔHcomp) for the ion-molecule complexes formed in these reactions are obtained. However, the overall (ΔHovr#) and central (ΔHcent#) barriers for these reactions calculated using the B3-LYP functional are significantly underestimated when compared with G2(+) or experimental results. This implies that the B3-LYP energies for the Cl(H3C)Cl− (D3h) and Cl(H3C)Br− (C3v) transition structures are relatively too low. The B3-LYP errors appear to be systematic, with similar errors being found for corresponding quantities for the two reactions examined.