Molecular and crystal structure of 4′,4″-dimethyl-6′,6″-di-t-butyl-2-diphenylamino-4,5;7,8-dibenzo-6H-1,3,2-dioxaphosphocine

A. E. Arbuzov Institute of Organic and Physical Chemistry, Kazan' Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 3, pp 166–170, May–June, 1989.     

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.     

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.