There is concern within theoretical chemistry that density functional methods are fundamentally in error for negative ions. We have tested this hypothesis by evaluating electron affinities for F and F2 with a variety of density functionals and extremely large, diffuse basis sets. In addition, we have observed the behavior of a known unbound system Ne−. We have found no convincing evidence to support the claims of negative ion instability with density functional methods.

Charge‐shift bonds (CSBs) constitute a new class of bonds different than covalent/polar‐covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures. This Essay describes the reasons why the CSB family was overlooked by valence‐bond pioneers and then demonstrates that the unique status of CSBs is not theory‐dependent. Thus, valence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as a variety of electron density theories all show the distinction of CSBs vis‐à‐vis covalent and ionic bonds. Furthermore, the covalent–ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules, e.g., benzene. The Essay ends by arguing that CSBs are a distinct family of bonding, with a potential to bring about a Renaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.

The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space [CASSCF(6 x 6)] computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6 x 6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically approximately 80% product-like and electronically approximately 70% reactant-like. While the pi resonance in the TS is very small, it is large (64.4 kcal mol(-1)) for 2 (cf. benzene=61.5 kcal mol(-1)). As a consequence, substituents operating on the sigma electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional sigma resonance in 2 results in unusually high values for the nucleus-independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to sigma resonance to which the NICS procedure is relatively sensitive.

The flexibility of valence bond (VB) theory provides a new method of calculating p-bond energies in the double-bonded species H m ABH n

Stichwçrter: bond theory ·charge-shift bonds · covalent bonds ·ionic bonds ·v alence bonds Dedicated to Roald HoffmannAbstract: Charge-shift bonds (CSBs) constitute anew class of bonds different than covalent/polar-covalent and ionic bonds. Bonding in CSBs does not arise from either the covalent or the ionic structures of the bond, but rather from the resonance interaction between the structures.T his Essay describes the reasons why the CSB family was overlooked by valence-bond pioneers and then demonstrates that the unique status of CSBs is not theory-dependent. Thus,v alence bond (VB), molecular orbital (MO), and energy decomposition analysis (EDA), as well as av ariety of electron density theories all show the distinction of CSBs vis-à-vis covalent and ionic bonds. Furthermore,t he covalent-ionic resonance energy can be quantified from experiment, and hence has the same essential status as resonance energies of organic molecules,e .g., benzene. The Essaye nds by arguing that CSBs are ad istinct family of bonding,w ith ap otential to bring about aR enaissance in the mental map of the chemical bond, and to contribute to productive chemical diversity.