Stability, metastability, and unstability of three-electron-bonded radical anions. A model ab initio theoretical study

The stability of OthereforeO, NthereforeN, SthereforeS, PthereforeP, and SithereforeSi three-electron bonds in anionic radicals isoelectronic to dihalogen radical anions is studied by means of ab initio calculations on model systems. The difficulty of generating the dissociation energy profiles of such anions and their rearrangement to neutral species is solved by a practical method which consists of calculating the neutral and anionic energy profiles separately and shifting the curves with respect to each other to match the experimental energy gap between the asymptotes. Here the neutral and anionic reaction profiles are calculated at the CASPT2 and MP2 levels, respectively. The calculations predict that the OthereforeO bond is likely to be observed in anions of the type [ROthereforeOR](.-), where R is any alkyl substituent or carbon chain. The anion Si2H6.- is found to be a metastable species, with a fair barrier to electron detachment. The barrier is much smaller for N2H4.- and P2H4.-, thus precluding experimental observation. However, these species can be stabilized by electron-attractor substituents, the effect of which can be quantitatively estimated by means of the parent anion's diagrams and some fast complementary calculations. An example is given with the [CF(3)HNthereforeNHCF(3)](.-) anionic complex

Identity S(N)2 reactions X-+CH3X -> XCH3+X-(X=F,Cl,Br, and I) in vacuum and in aqueous solution: A valence bond study

The recently developed (L. Song, W. Wu, Q. Zhang, S. Shaik, J Phys. Chem. A 2004,108,6017) valence bond method coupled with a polarized continuum model (VBPCM) has been applied to the identity S,2 reaction of halides in the gas phase and in aqueous solution. The barriers computed at the level of the breathing orbital VB method (P C. Hiberty, J. P. Flament, E. Noizet, Chem. Phys. Lett. 1992, 189, 259), BOVB and VBPCM//BOVB, are comparable to CCSD(T) and CCSD(T)//PCM results and to experimentally derived barriers in solution (W. J. Albery, M. M. Kreevoy, Adv. Phys. Org. Chem. 1978, 16, 85). The reactivity parameters needed to apply the valence bond state correlation diagram (VBSCD) method (S. Shaik, J. Am. Chem. Soc. 1984, 106, 1227), were also determined by VB calculations. It has been shown that the reactivity parameters along with their semiempirical derivations provide a satisfactory qualitative and quantitative account of the barriers

An accurate barrier for the hydrogen exchange reaction from valence bond theory: Is this theory coming of age?

One of the landmark achievements of quantum chemistry, specifically of MO-based methods that include electron correlation, was the precise calculation of the barrier for the hydrogen-exchange reaction (B. Liu, J. Chem. Phys. 1973, 58, 1925; P. Siegbahn, B. Liu, J. Chem. Phys. 1978, 68, 2457). This paper reports an accurate calculation of this barrier by two recently developed VB methods that use only the eight classical VB structures. To our knowledge, the present work is the first accurate ab initio, VB barrier that matches an experimental value. Along with the accurate barrier, the VB method provides accurate bond energies and diabatic quantities that enable the barrier height to be analyzed by the VB state correlation diagram approach, VBSCD (S. Shaik, A. Shurki, Angew. Chem. 1999, 111, 616; Angew. Chem. Int. Ed. Engl. 1999, 38, 586). This is a proof of principal that VB theory with appropriate account of dynamic electron correlation can achieve quantitative. accuracy of reaction barriers, and still retain a compact and interpretable wave function. A sample Of S(N)2 barriers and dihalogen bonding energies, which are close to CCSD(T) and G2(+) values, show that the H-3 problem is not an isolated case, and while it is premature to conclude that VB theory has come of age, the occurrence of this event is clearly within sight

Valence bond Modeling of barriers in the nonidentity hydrogen abstraction reactions, X 'center dot+H-X -> X '-H+X center dot (X ' not equal X = CH3, SiH3, GeH3, SnH3, PbH3)

Breathing orbital valence bond (BOVB) computations (Hiberty, P. C.; Humbel, S.; Byrman, C. P.; van Lenthe, J. H. J. Chem. Phys. 1994, 101, 5969) are used to obtain nonidentity barriers for hydrogen transfer reactions between X and X' groups, X not equal X' = CH3, SiH3, GeH3, SnH3, PbH3. Modeling of these barriers by means of VB state correlation diagrams (Shaik, S.; Shurki, A. Angew. Chem., Int. Ed. Engl. 1999, 38, 586) leads to a simple expression for the barrier (eq 29) as an interplay of an intrinsic term and the reaction driving force. The equation predicts barrier heights that are compatible with the BOVB computed barrier heights. Its comparison with the Marcus equation shows similarities and differences

Identity hydrogen abstraction reactions, X-center dot+H-X '-> X-H+X '(center dot) (X = X ' = CH3, SiH3, GeH3, SnH3, PbH3): A valence bond modeling

Breathing orbital valence bond (BOVB) computations (Hiberty, P. C.; Humbel, S.; Archirel, P. J. Phys. Chem. 1994, 98, 11697) are used to obtain identity barriers for hydrogen transfer reactions between X groups, X = H, CH3, SiH3, GeH3, SnH3, and PbH3. Modeling of these barriers by means of VB state correlation diagrams (Shaik, S.; Shurki, A. Angew. Chem. 1999, 38, 586) lead to simple expressions for the barriers (eqs 21 and 22). These expressions show that the organizing quantity of the barriers is the singlet-triplet excitation energy (DeltaE(ST)) or bond energy (D) of the X-H bond that undergoes activation. The larger the DeltaE(ST) or D, the higher the identity barrier. These equations are successfully applied to deduce barriers for hydrogen transfers between electronegative groups, X = X' = F, Cl, Br, and I. The "polar effect" (Russell, G. A. In Free Radicals; Kochi, J. K., Ed.; Wiley: New York, 1973; Vol 1, p 293-298) is shown to be significant but virtually constant in the series. Thus, identity processes mask the polar effect which is more clearly expressed in nonidentity hydrogen transfer reactions. Generalization of the model to other atom transfer reactions is discussed

Bonding Conundrums in the C-2 Molecule: A Valence Bond Study

Natural Science Foundation of China [20873106, 21003101]; Israel Science Foundation [ISF 53/09]The ab initio VB study for the electronic structure of the C-2 molecule in the ground state is presented in this work. VB calculations involving 78 chemically relevant VB structures can predict the bonding energy of C-2 quite well. Sequentially, a VBCIS calculation provides spectroscopic parameters that are very close to full CI calculated values in the same basis set. Furthermore, the analysis of the bonding scheme shows that a triply bonded structure is the major one in terms of weights, and the lowest in energy at the equilibrium distance. The second structure in terms of weights is an ethylene-like structure, displaying a sigma + pi double bond. The structure with two suspended pi bonds but no sigma bond contributes only marginally to the ground state. This ordering of weights for the VB structures describing the C-2 molecule is shown to be consistent with the shape of the molecular orbitals and with the multireference character of the ground state. With the triply bonded bonding scheme, the natures of the pi and sigma bonds are investigated, and then the corresponding "in situ" bond strengths are estimated. The contribution of the covalent-ionic resonance energy to pi and sigma bonding is revealed and discussed