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We describe herein the hydrogen-atom transfer (HAT)/proton-coupled electron-transfer (PCET) reactivity for Fe(IV)-oxo and Fe(III)-oxo complexes (1-4) that activate C-H, N-H, and O-H bonds in 9,10-dihydroanthracene (S1), dimethylformamide (S2), 1,2-diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1-3, the iron is pentacoordinated by tris[N'-tert-butylureaylato)-N-ethylene]aminato ([H3buea](3-)) or its derivatives. These complexes are basic, in the order 3 ≫ 1 > 2. Oxidant 4, [Fe(IV)N4Py(O)](2+) (N4Py: N,N-bis(2-pyridylmethyl)bis(2-pyridyl)methylamine), is the least basic oxidant. The DFT results match experimental trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT)/electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O···H···X (X = C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred separately from the proton. The Bell-Evans-Polanyi principle does not account for the observed reactivity pattern, as evidenced by the scatter in the plot of calculated barrier vs reactions driving forces. However, a plot of the deformation energy in the TS vs the respective barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C-H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N-H/O-H bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of a stabilizing interaction between the TS fragments. A valence bond model is used to link the observed results with the basicity/acidity of the reactants.

This commentary summarizes the authors' basic disagreements with the paper, "Rabbit-Ears, VSEPR Sterics, and Other Orbital Anachronisms," which criticizes the authors' usage of the hybrid orbitals for H[subscript 2]O in their book, "A Chemist's Guide to Valence Bond Theory" (Shaik and Hiberty, 2008). The current article shows that the lone pairs ("rabbit ears") are not superior or anachronistic to the equivalent hybrid orbitals. The authors briefly summarize the frequently used sets of orbitals that can meaningfully describe a molecular electronic state, and subsequently, examine the critiques of the paper in detail.

Density functional theoretic calculations are performed for the high-spin copper clusters ((super n!)[Cu.sub.n]; n = 2-14), which are devoid of electron pairs but possess strong bonding. The model has shown how a weak interaction in the diatomic triplet molecule can become a strong binding force that binds together mono-valent atoms without even a single electron pair.

QM/MM calculations are used to elucidate the Poulos-Kraut mechanism of O-O bond activation and compound I formation in horseradish peroxidase, in conditions corresponding to neutral to basic pH. Attempts to generate compound I directly from the [Fe[H.sub.2][O.sub.2]] complex by migrating the proton from the proximal oxygen to the distal one result in high barriers.

The density functional theory calculations are used to investigate the structural and electronic properties of metalloporphyrin (MP) pi-cation radicals in order to provide a resolution of the long-standing problem regarding the nature of the mixed A(sub 1u)-A(sub 2u) states of porphyrin radical cations. Orbital projection technique or complementary spin density decomposition are used to estimate the degree of mixing, showing that the frontier orbital has played a vital role in determining the electronic properties.

The reaction of formaldehyde radical anion with methyl chloride, CH2O(super -)+CH3Cl is an example in which a single transition state leads to two products, substitution at carbon and electron transfer. The branching ratio is studied and the energies of transition states and intermediates are computed.

A valence bond self-consistent field (VBSCF) calculation has been carried out on the bimolecular elimination (E2) reaction of fluoride ion with ethyl fluoride. The transition state was first optimized at HF/6-31+G and the resulting orbitals were used in the VBSCF calculation. A transition-state hybrid consisting of four structures ([F.sup.-] HC[H.sub.2]C[H.sub.2]-F, F-HC[H.sub.2]=C[H.sub.2][F.sup.-], F-H[sup.-]C[H.sub.2]-C[H.sub.2]-F, and [F.sup.-]H-C[H.sub.2]-C [H.sub.2][sup.+][F.sup.-]) failed to give a reasonable energy. Incorporation of four charge-separated structures ([F.sup.-][H.sub.+][sup.-]C[H.sub.2]-C[H.sub.2]-F, [F.sup.-][H.sup.+][sup.-] C[H.sub.2]-C[H.sub.2.sup.+][F.sup.-], [F.sub.-][H.sup.+]C[H.sub.2]=C[H.sub.2]F, and F-H[sup.-]C[H.sub.2]- C[H.sub.2.sup.+][F.sub.-]) for a total of eight resulted in an energy slightly below Hartree-Fock. A full-space VBSCF calculation utilizing 50 structures showed that the eight structures give a sufficiently accurate wave function. The transition state is dominated by charge-separated structures, leading to a greater degree of charge separation than in either the reactant or product complex. Therefore, the transition state does not evolve synchronously from the reactants, and explicit allowance should be made for this fact when applying descriptions of the reaction path, such as that in the More O'Ferrall--Jencks diagram. Key words: elimination reaction, E2, transition state, VBSCF calculation, XMVB programs. Resume: On a effectue des calculs de liaison de valence en champ autocoherent (VBSCF) sur la reaction d'elimination bimoleculaire (E2) de l'ion fluorure a partir du fluoroethane. Dans une premiere etape, on a optimise l'etat de transition au niveau HF/6-31+G et on a utilise les orbitales qui en ont resulte darts les calculs VBSCF. Un etat de transition hybride forme des quatre structures ([F.sup.-] HC[H.sub.2]C[H.sub.2]-F, F-HC[H.sub.2]=C[H.sub.2][F.sup.-], F-H[sup.-]C[H.sub.2]-C[H.sub.2]-F, et [F.sup.-]H-C[H.sub.2]-C[H.sub.2][sup.+][F.sup.-]) ne permet pas d'obtenir une energie raisonnable. L'incorporation des quatre structures a charges separees ([F.sup.- ][H.sub.+][sup.-]C[H.sub.2]-C[H.sub.2]-F, [F.sup.-][H.sup.+][sup.-]C[H.sub.2]-C[H.sub.2.sup.+][F.sup.-], [F.sub.- ][H.sup.+]C[H.sub.2]=C[H.sub.2][F.sup.-], F-H[sup.-]C[H.sub.2]-C[H.sub.2.sup.+][F.sup.-] pour un total de huit structures conduit a une energie legerement inferieure a celle prevue par des calculs de Hartree-Fock. Un calcul VBSCF a plein espace, faisant appel a 50 structures a permis de montrer que les huit structures conduisent a une fonction d'onde suffisamment exacte. L'etat de transition est domine par les structures a charges separees et ceci conduit hun degre plus eleve de separation de charge qui est plus eleve que dans le reactif ou dans le complexe du produit. De cette facon, l'etat de transition n'evolue pas d'une facon synchrone avec les reactifs et on doit en consequence faire des previsions explicites pour ce fait lorsqu'on applique les descriptions de la voie reactionnelle, par exemple darts le diagramme de More O'Ferrall--Jencks. Mots cles : reaction d'elimination, E2, etat de transition, calcul de liaison de valence en champ autocoherent, programmes XMVB. [Traduit par la Redaction], Introduction The bimolecular elimination (E2) reaction [1] [B.sup.-] + H-C[R.sub.2]-C[R.sub.2]-X → B-H + C[R.sub.2]=C[R.sub.2] + [X.sup.-] has long been of interest to mechanistic chemists both for its own sake and [...]

The reaction of a formaldehyde radical anion with methyl chloride is an example of a reaction in which a single transition state serves two mechanisms namely, substitution at carbon (Sub(C)) and electron transfer (ET). The reaction is studied using ab initio molecular dynamics at the HF/6-31G(d) level of theory.

The valence bond self-consistent field (VBSCF) calculations explain the qualitative properties of the complexes and transition structures for proton transfer processes. The VBSCF calculations also provide satisfactory analyses of the elements contributing to the barrier heights.

Density functional calculations are used to generate a family of the high spin lithium clusters (super n+1)Lin (n = 2-12), which do not possess any electron pairs but which are nevertheless strongly bond. Ferromagnetic bonding is a delocalized covalent- ionic fluctuation that spreads over the entire cluster.

A valence bond (VB) is presented and applied to calculate the hidden excited states, 2(super 1)A(sub g) and other covalent excited states of polyenes from C4H6 to C28H30. The VBDFTs method presented enables to gain an insight into a variety of properties of various covalent states of polyene, such as excitation energies and their convergence.

A Kohn--Sham-type computational scheme capable of treating systems with strong nondynamic correlation is examined. The experiment results of REKS calculation are compared with the available Kohn--Sham solutions for cases in which the exact density is known, as well as with results of conventional multireference ab initio methods.

Valence bond theory has been used to investigate the transition metal hydride cations TMH+. It was established that modern valence bond methods are valuable for studies involving transition metals.

Research is presented applying REKS-type (Filatov, M.; Shaik, S. Chem. Phys. Lett. 1999) density functional calculations to study the molecular and electronic structure of the triplet and the lowest singlet state of tetramethyleneethane diradical.

Valence bond state correlation diagrams (VBSCD) has shown that a simple proton transfer such as that in the E1cB irreversible reaction is shown to have a lower barrier than a synchronous concerted bimolecular (E2) reaction. The E2 transition state has avoided the energetic disadvantage by becoming localized and nonsynchronous, although with electronic and geometric changes at all of the reacting centers.