Hypervalent Systems Research Articles

Understanding the C-F Bond Activation Mediated by Frustrated Lewis Pairs: Crucial Role of Non-covalent Interactions.

The activation of a single C-F bond in di- and trifluoromethyl groups by frustrated Lewis pairs (FLPs) has been computationally explored by means of Density Functional Theory calculations. It is found that in this activation reaction the FLP partners exhibit a peculiar cooperative action, which is markedly different from related FLP-mediated processes, and where non-covalent interactions established between the Lewis base and the substrate play a decisive role. In addition, the process proceeds through the intermediacy of a hypervalent species featuring a pentacoordinate carbon atom, which is rare in the chemistry of FLPs. The physical factors controlling this process as well as the bonding situation of these hypervalent intermediates have been quantitatively analyzed in detail by using state-of-the-art computational methods to not only rationalize the mechanism of the transformation but also to guide experimentalists towards the realization of these so far elusive hypervalent systems.

Superconductivity in the Hypervalent Compound Ba2Bi(Sb1−xBix)2 with a Square-Honeycomb Lattice

We studied the structural and physical properties of the hypervalent system Ba2Bi(Sb1−xBix)2 (0 ≤ x ≤ 1) with an anisotropic “square–honeycomb” layer. We found that the orthorhombic Ba2BiSb2 (x = 0) shows a charge density wave (CDW) transition at approximately 230 K accompanied by a significant elongation of the b-axis, indicating the quasi-one-dimensional nature along the b-axis in its electronic state, as supported by first-principles calculations. This CDW transition is rapidly suppressed with increasing x, leading to the appearance of superconductivity for 0.375 ≤ x ≤ 1. The superconducting transition temperature Tc increases slightly with x and the maximum Tc was 4.4 K for Ba2Bi3 (x = 1).

The shapes of three hypervalent systems of first-row atoms FH3, H4o, and F3H

Musher's theory of hypervalent molecules (2) accounts for the shape of numerous molecules containing second- and third-row atoms of groups V, VI, and VII. In an attempt to examine the hypervalent bonding mechanism in a system free of competing d orbital effects, we consider three hypervalent systems of first-row atoms, H4O, FH3, and HF3. Gaussian computations employing very small lobe-representations of Slater-type aos indicate that H4O assumes a typical hypervalent geometry, with two colinear weak OH bonds and two covalent OH bonds in a normal plane. This prediction survives an improvement in basis. Our crudest computation indicates a D3h geometry for FH3, but an improved basis leads to a T-shaped molecule consistent with the theory of hypervalent molecules. F3H does not possess a stable hypervalent geometry; it is computed to be unstable relative to rare gas structure F+3 + H-. The reliability of our small basis in a system with three fluorines is questionable.

Gaussian-4 theory using reduced order perturbation theory

Two modifications of Gaussian-4 (G4) theory [L. A. Curtiss et al., J. Chem. Phys. 126, 084108 (2007)] are presented in which second- and third-order perturbation theories are used in place of fourth-order perturbation theory. These two new methods are referred to as G4(MP2) and G4(MP3), respectively. Both methods have been assessed on the G3/05 test set of accurate experimental data. The average absolute deviation from experiment for the 454 energies in this test set is 1.04 kcalmol for G4(MP2) theory and 1.03 kcalmol for G4(MP3) theory compared to 0.83 kcalmol for G4 theory. G4(MP2) is slightly more accurate for enthalpies of formation than G4(MP3) (0.99 versus 1.04 kcalmol), while G4(MP3) is more accurate for ionization potentials and electron affinities. Overall, the G4(MP2) method provides an accurate and economical method for thermochemical predictions. It has an overall accuracy for the G3/05 test set that is much better than G3(MP2) theory (1.04 versus 1.39 kcalmol) and even better than G3 theory (1.04 versus 1.13 kcalmol). In addition, G4(MP2) does better for challenging hypervalent systems such as H(2)SO(4) and for nonhydrogen species than G3(MP2) theory.

Ab initio investigations on the HOSO2+O2→SO3+HO2 reaction

HOSO 2 radical is the key intermediate for the oxidation SO2 to SO3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO2+O2→SO3+HO2 have been studied using Mo/ller–Plesset (MP2) and density functional (DFT) techniques with 6-31G** and triple-ζ, quadruple-ζ, and quintuple-ζ quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.

Sulfuranes Lacking Benzoannelation. Sulfuranes and Other Hypervalent Molecules Studied by 17O-NMR

The rates of hydrolysis of benzoannelated vs nonbenzoannelated sulfuranes, viz. 5 vs 3 or 5 vs 4, were compared. Benzoannelation was found to provide very modest kinetic stabilization. Crystal structures of sulfuranes 3 and 4 were obtained and compared with each other, with a dibenzoannelated sulfurane, 17, and with a non-sulfurane analogue of 4. Bond length variations could be understood in the context of simple resonance arguments. (17)O NMR studies of 3-5 showed that this technique was indeed sensitive to sulfurane structure. For example, the chemical shifts of the two carbonyl oxygens of 4 differed by over 20 ppm. Other hypervalent systems, mainly iodinanes, were studied by (17)O NMR. A variety of theoretical methods were surveyed to test how well they could reproduce the geometry of 3. Density functional theory calculation outperformed ab initio geometry optimization at the MP2/3-21G() level. Finally, a cis-trans isomerization of the double bond of 11 and one of the two double bonds of 10 was studied.