Comparative DFT study on the role of conformers in the ruthenium alkylidene‐catalyzed ROMP of norborn‐2‐ene

Comparative quantum chemical calculations on the reaction pathways for the formation of ruthena(IV)cyclobutanes from both 1^st‐ and 2^nd‐generation Grubbs catalysts of the general formula RuX₂(L)(L′)(?CH₂) (L = PCy₃ or 1,3‐dimesityl‐4,5‐dihydroimidazolin‐2‐ylidene, L′ = PCy₃) and norborn‐2‐ene (NBE) were carried out on the B3LYP/LACVP** level in dependence on the ligand X = I, Br, Cl, and F. The mechanism proposed by Straub for the formation of (one) active and (three) inactive NBE–Ru–carbene complexes for non‐cyclic alkenes was applied to the cyclic alkene NBE. In RuX₂(PCy₃)₂(?CH₂), the inactive NBE–Ru–carbene complex is energetically more stable than the active one; however, in RuX₂(IMesH₂)(PCy₃)(?CH₂), the active NBE–Ru–carbene complex is more stable than the inactive one. In due consequence, the possible rate limiting barrier for the conversion of the NBE–Ru–carbene complex into the corresponding metallocyclobutane (MCB) is systematically larger in the case of 1^st‐generation Grubbs catalysts than of 2^nd‐generation Grubbs catalysts due to an additional re‐arrangement for the formation of an active π‐complex from the more stable (inactive) conformer. This correlates with the observed reactivity of both types of initiators. There is a strong influence of the ligands L and X on the conformational properties and relative stabilities of the 14‐electron intermediates, which has a direct effect on the distribution of the inactive and active conformations of the corresponding Ru–carbene–NBE complexes. A direct correlation between the conformational properties of the 14‐electron intermediates and the relative stabilities of the active Ru–carbene–NBE complexes was observed. Copyright © 2008 John Wiley & Sons, Ltd.     

Diffusion hysteresis in mesoporous materials

The pulsed field gradient NMR method has been used to study molecular diffusion of adsorbate molecules in mesoporous materials with different pore morphologies as a function of the external gas pressure. The obtained experimental results reveal that in line with the adsorption hysteresis, the measured diffusivities as well exhibit a history-dependent behavior, i.e. diffusivities on the adsorption branch do not follow those measured on the desorption branch. Particular details of the diffusion hysteresis loop depend on the structure of the used porous material. New insight into the mechanisms of adsorption hysteresis, namely indication of two-step relaxation process in the hysteresis region, is gained by comparing the dynamical properties of the adsorbate obtained under equilibrium and non-equilibrium conditions.     

Vortex-to-polarization phase transformation path in ferroelectric Pb(ZrTi)O3 nanoparticles

Phase transformation in finite-size ferroelectrics is of fundamental relevance for understanding collective behaviors and balance of competing interactions in low-dimensional systems. We report a first-principles effective Hamiltonian study of vortex-to-polarization transformation in Pb(Zr0.5Ti0.5)O3 nanoparticles, caused by homogeneous electric fields normal to the vortex plane. The transformation is shown to (1) follow an unusual macroscopic path that is symmetry nonconforming and characterized by the occurrence of a previously unknown structure as the bridging phase, and (2) lead to the discovery of a striking collective phenomenon, revealing how ferroelectric vortex is annihilated microscopically. Interactions underlying these behaviors are discussed.     

On combustion enhancement mechanisms in the case of electrical-discharge-excited oxygen molecules

Combustion intensification mechanisms in a supersonic flow of a hydrogen-oxygen mixture behind the oblique shock wave front are investigated for the case when vibrations and the a ¹Δ_g and b ¹Σ _g ⁺ electron states of a O₂ molecule are excited by an electrical discharge. The presence of vibrationally excited and electronically excited O₂ molecules in the oxygen plasma allows intensification of the chain mechanism in the H₂-O₂ mixture even if the energy put into O₂ molecules in the discharge is low. Excitation of O₂ molecules is several tens of times more efficient for acceleration of oxygen-hydrogen mixture combustion than mere heating of the gas by an electrical discharge. In addition, low-temperature inflammation of the mixture with electrical-discharge-excited O₂ molecules makes it possible to raise the efficiency of conversion of the reactant chemical energy to heat compared with the conventional way of combustion initiation by heating.     

Cascade synthesis of polyoxygenated 6H,11H-[2]benzopyrano-[4,3-c][1]benzopyran-11-ones

2-(methoxymethoxymethyl)aryllead triacetates, obtained in situ from the corresponding arylboronic acids, reacted with 4-hydroxycoumarins, leading to 3-(2-methoxymethoxymethyl)aryl-4-hydroxycoumarin derivatives in good to high yields. These compounds underwent a cascade sequence of reactions, deprotection-halogenation-annulation, to afford polyoxygenated tetracyclic 6H,11H-[2]benzopyrano-[4,3-c] [1]benzopyran-11-ones in good yields. Some compounds showed a moderate cytotoxicity against human epithelial mammary HBL100 cells.     

Reactions of halogenated hydroperoxides and peroxyl and alkoxyl radicals from isoflurane in aqueous solution

Model systems, based on aqueous solutions containing isoflurane (CHF(2)OCHClCF(3)) as an example, have been studied in the presence and absence of methionine (MetS) to evaluate reactive fates of halogenated hydroperoxides and peroxyl and alkoxyl radicals. Primary peroxyl radicals, CHF(2)OCH(OO*)CF(3), generated upon 1-e-reduction of isoflurane react quantitatively with MetS via an overall two-electron oxidation mechanism to the corresponding sulfoxide (MetSO). This reaction is accompanied by the formation of oxyl radicals CHF(2)OCH(O*)CF(3) that quantitatively rearrange by a 1,2-hydrogen shift to CHF(2)OC*(OH)CF(3). According to quantum-chemical calculations, this reaction is exothermic (DeltaH = -5.1 kcal/mol) in contrast to other potentially possible pathways. These rearranged CHF(2)OC*(OH)CF(3) radicals react further via either of two pathways: (i) direct addition of oxygen or (ii) deprotonation followed by fluoride elimination resulting in CHF(2)OC(O)CF(2)*. Route i yields the corresponding CHF(2)OC(OO*)(OH)CF(3) peroxyl radicals, which eliminate H+/O(2)*-. The resulting ester, CHF(2)OC(O)CF(3), hydrolyzes further, accounting for the formation of HF, trifluoroacetic acid, and formic acid with a contribution of 45% and 80% in air- and oxygen-saturated solutions, respectively. A competitive pathway (ii) involves the reactions of the secondary peroxyl radicals, CHF(2)OC(O)CF(2)OO*. The two more stable of the three above mentioned peroxyl radicals can be distinguished through their reaction with MetS. Although the primary CHF(2)OCH(OO*)CF(3) oxidizes MetS to MetSO in a 2-e step, the majority of the secondarily formed CHF(2)OC(O)CF(2)OO* reacts with MetS via a 1-e transfer mechanism, yielding CHF(2)OC(O)CF(2)OO-, which eventually suffers a total breakup into CHF(2)O- + CO(2) + CF(2)O. Quantum-chemical calculations show that this reaction is highly exothermic (DeltaH = -81 kcal/mol). In air-saturated solution this pathway accounts for about 35% of the overall isoflurane degradation. Minor products (10% each), namely, oxalic acid and carbon monoxide originate from oxyl radicals, CHF(2)OC(O)CF(2)O* and CHF(2)OCH(O*)CF(3). An isoflurane-derived hydroperoxide CHF(2)OCH(OOH)CF(3) in high yield was generated in radiolysis of air-saturated solutions containing isoflurane and formate either via a H-atom abstraction from formate by the isoflurane-derived peroxyl radicals or by their cross-termination reaction with superoxide O(2)*-. CHF(2)OCH(OOH)CF(3), is an unstable intermediate whose multistep hydrolysis is giving H(2)O(2) + 2HF + HC(O)OH + CF(3)CH(OH)(2). In the absence of MetS, about 55% of CHF(2)OCH(OO*)CF(3) undergo termination via the Russell mechanism and 27% are involved in cross-termination with superoxide (O(2)*-) and peroxyl radicals derived from t-BuOH (used to scavenge *OH radicals). The remaining 18% of the primary peroxyl radicals undergo termination via formation of alkoxyl radicals, CHF(2)OCH(O*)CF(3).     

Supramolecular approach to crystallization of polynuclear chromium(III) aqua hydroxo complexes: synthesis and crystal structures of complexes [Cr2(OH)2(H2O)8]4+ and [Cr4(OH)6(H2O)12]6+ with cucurbit[n]uril (n = 7, 8)

Supramolecular compounds of the compositions {[Cr₂(OH)₂(H₂O)₈](C₄₂H₄₂N₂₈O₁₄)₂}-(NO₃)₄·18.75H₂O (1) and {[Cr₄(OH)₆(H₂O)₁₂](C₄₈H₄₈N₃₂O₁₆)₃(NO₃)₆·55H₂O (2) were synthesized from aqueous solutions of chromium(III) nitrate and the macrocyclic cavitand cucurbit[n]uril (C_6n H_6n N_4n O_2n , where n = 7 or 8, respectively). According to the X-ray diffraction study, the polynuclear chromium aqua complexes are disposed in cavities formed by the cucurbit[n]uril molecules and are linked to these molecules through hydrogen bonds between the hydroxo and aqua ligands of the polycations and the portal oxygen atoms of the macrocycles. Compound 1 is the first example of supramolecular compounds of cucurbit[7]uril with metal aqua complexes. The isolation of the supramolecular adduct with cucurbit[8]uril 2 in the single-crystalline state allows the determination of the structure of the tetranuclear chromium aqua complex having an adamantane-like structure, [Cr₄(μ₂-OH)₆(H₂O)₁₂]⁶⁺, which has been previously unknown in the solid state.     

Direct atomic scale observation of linkage isomerization of As4S4 clusters during the photoinduced transition of realgar to pararealgar

The reaction mechanism underlying the photoinduced linkage isomerization of discrete arsenic-sulfur clusters in the realgar form of tetraarsenic tetrasulfide (alpha-As4S4) to its pararealgar form was studied on a natural specimen of the mineral with a combination of in situ single-crystal X-ray photodiffraction and Fourier transform infrared spectroscopy. The photodiffraction technique provided direct atomic resolution evidence of formation of intermediate As4S5 phase in which half of the realgar molecule is retained in its envelope-type conformation, while the other half is transformed by effective switching of positions of one sulfur and one arsenic atom. The initiation and propagation stages of the process are studied under light and dark conditions, during and after photoexcitation with polychromatic visible light. In the "light" reaction stage, the interatomic and cell parameters averaged over the crystal volume and photoexcitation time remain almost unchanged. The residual electron density features are indicative for formation of a small amount of As4S5 clusters, which at this stage do not affect the overall crystalline order. In the "dark" reaction stage, a set of self-sustainable autocatalytic reactions results in strong and nearly isotropic expansion of the unit cell. The structure in the dark stage represents direct evidence of formation of pararealgar which was obtained in yield of about 5% in the single crystal of realgar. The cell expansion is due to increased mole ratio of clusters of pararealgar relative to realgar and to increased intercluster separation. Due to lattice incompatibility, a higher content of the product results in progressive decrease of crystal quality. Creation of small amount of arsenolite (As2O3) which appears as byproduct in the light stage and remains unreacted in the product mixture was confirmed by far-IR spectroscopy.