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Introductory chemistry textbooks often present valence bond (VB) theory as useful, but incorrect and inferior to molecular orbital (MO) theory, citing the electronic structure of O2and electron delocalization as evidence. Even texts that initially present the two theories on equal footing use language that biases students toward the MO approach. However, these “failures” of VB are really just misconceptions and/or misapplications of the theory. At their theoretical limits, both VB and MO are equivalent; they simply approach that limit from different sides. Certain concepts may be easier to grasp with one theory or the other so that having a commanding knowledge of both is extremely beneficial. However, presenting one theory as superior to the other suppresses the ability to look at a problem from both sides and is therefore detrimental to students and the whole of chemistry. It is time for VB and MO to be taught on equal footing like the complementary theories they are.

The flexibility of valence bond (VB) theory provides a new method of calculating π‐bond energies in the double‐bonded species HmA=B Hn, where A, B=C, N, O, Si, P, S. This new method circumvents the problems usually associated with obtaining π‐bond strengths by targeting only the π bond, while all other factors remain constant.In this manner, a clean separation between σ‐ and π effects can be achieved which highlights some expected trends in bond strength upon moving from left to right and up and down the Periodic Table. Intra‐row π bonds conform to the classic statement by Pauling [ L. Pauling, The Nature of the Chemical Bond, Cornell University Press, Ithaca, 1960, 3rd edition] regarding the relationship of heteronuclear bond strengths to their homonuclear constituents whereas inter‐row π bonds do not. This variance with Pauling's statement is shown to be due to the constraining effect of the underlying σ bonds which prevents optimal pπ‐pπoverlap. While Pauling's statement was based on the assumption that the resonance energy (RE) would be large for heteronuclear and small for homonuclear bonds, we have found large REs for allbonds studied herein; this leads to the conclusion that REs are dependent not only on the electronegativity difference but also the electronegativity sum of the constituent atoms. This situation where the bond is neither covalent nor ionic but originates in the covalent‐ionic mixing has been termed charge shift (CS) bonding [ S. Shaik, P. Maitre, G. Sini, P. C. Hiberty, J. Am. Chem. Soc. 1992, 114, 7861]. We have shown that CS bonding extends beyond single σ bonds in first row molecules, thus supporting the idea that CS‐bonding is a ubiquitous bonding form.

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(6×6)} computations, while the quality of VB‐DTF utilizing the same three configurations improves upon CASSCF(6×6) analogous to CASPT2. The dominant VB configuration in 1contributes little to 2, while the most important biradical configuration in 2plays 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 TSis geometrically ≈80 % product‐like and electronically ≈70 % reactant‐like. While the π resonance in the TSis 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 σ electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional σ resonance in 2results in unusually high values for the nucleus‐independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TSis due mostly to σ resonance to which the NICS procedure is relatively sensitive.

The five energetically lowest minima on the potential energy surface of N‐acetyl‐N′‐methylalaninamide were optimized at the Becke3LYP/DZdlevel of theory to compare these density functional theory results with the literature findings at restricted Hartree‐Fock/3‐21G. While the relative energies are very similar, the amide moiety is predicted to be much more flexible at Becke3LYP/DZd. As a consequence, the three minima that favor a nonplanar amide group differ by up to 14° in their ϕ and ψ values between the two levels. To compare the change in the density functional NMR chemical shifts with respect to ϕ and ψ with experimental results, Becke3LYP/DZdwas employed to optimize a structure for N‐acetyl‐N′‐methylalaninamide at each 30° interval on the (ϕ, ψ) surface in the regions that correspond to the α helix and the β‐pleated sheet and at each 60° interval elsewhere. The corresponding NMR chemical shielding surface was computed with the density functional program deMon. The resultant NMR chemical shielding surfaces for Nand Cβare in good agreement with the experiment, while the change in the NMR chemical shielding of C′ and Cαcannot be described only in terms of ϕ and ψ. The chemical shifts for those atoms also depend on the nonplanarity of the amide moiety. We evaluated this dependence for N‐methylacetamide as a model system. Estimates of the parameters derived from N‐methyl‐acetamide allowed the NMR‐shielding surfaces of C′ and Cαto be corrected for the nonplanar nitrogen influence. Although the effect is less pronounced with lower level theoretical geometries, due to the smaller degree of pyramidalization of the amide nitrogen, the (ϕ, ψ) NMR chemical shielding surfaces will need to be corrected. The agreement with the experiment was much better for the corrected surface of C′ when the nitrogen in the α helix had a nonplanar environment. © 1997 by John Wiley & Sons, Inc.