Comparison of the Accurate Kohn−Sham Solution with the Generalized Gradient Approximations (GGAs) for the SN2 Reaction F- + CH3F → FCH3 + F-: A Qualitative Rule To Predict Success or Failure of GGAs

The impact of the self-interaction error on the density functional theory description of dissociating radical cations: Ionic and covalent dissociation limits

Self-interaction corrected density functional theory was used to determine the self-interaction error for dissociating one-electron bonds. The self-interaction error of the unpaired electron mimics nondynamic correlation effects that have no physical basis where these effects increase for increasing separation distance. For short distances the magnitude of the self-interaction error takes a minimum and increases then again for decreasing R. The position of the minimum of the magnitude of the self-interaction error influences the equilibrium properties of the one-electron bond in the radical cations H2+ (1), B2H4+ (2), and C2H6+ (3), which differ significantly. These differences are explained by hyperconjugative interactions in 2 and 3 that are directly reflected by the self-interaction error and its orbital contributions. The density functional theory description of the dissociating radical cations suffers not only from the self-interaction error but also from the simplified description of interelectronic exchange. The calculated differences between ionic and covalent dissociation for 1, 2, and 3 provide an excellent criterion for determining the basic failures of density functional theory, self-interaction corrected density functional theory, and other methods. Pure electronic, orbital relaxation, and geometric relaxation contributions to the self-interaction error are discussed. The relevance of these effects for the description of transition states and charge transfer complexes is shown. Suggestions for the construction of new exchange-correlation functionals are given. In this connection, the disadvantages of recently suggested self-interaction error-free density functional theory methods are emphasized.

Base-induced 1,4-elimination: insights from theory and mass spectrometry

Experimental and theoretical studies on gas-phase base-induced 1,4-elimination reactions are summarized and discussed. The emphasis is on the synergy that is achieved by combining the complementary data from mass spectrometry and theoretical chemistry. The scope and applications of 1,4-eliminations are discussed and compared with other elementary reactions; e.g., 1,2-elimination and aliphatic (S(N)2) and allylic (S(N)2') nucleophilic substitution. Furthermore, the syn versus anti stereochemistry of 1,4-elimination reactions and the effect of E versus Z stereochemistry of the substrate are examined. Particular attention is paid to the mechanistic nature of 1,4-elimination, i.e., E2 or E1cb, as well as special features such as the single-well E2 and E1cb mechanism. Also, new results from density functional theory computations (BP86/TZ2P) are presented.