Hard-soft acid-base interactions of silylenes and germylenes

A detailed investigation of the electrophilic and nucleophilic character of singlet silylenes and germylenes, divalent compounds of silicon and germanium, respectively, substituted by first- and second-row elements is presented. In a first part, the Lewis acid properties of these compounds were studied through their complexation reaction with the Lewis bases NH3, PH3, and AsH3. The results indicate that this complexation is most favorable with the hardest base NH3, classifying these compounds as hard Lewis acids. This is confirmed by the linear correlation between the interaction energies and the value of the electrostatic potential, used as an approximation to the local hardness, near the empty p orbital of these compounds, indicating a charge-controlled interaction in the complex. Also the electrophilicity index, proposed by Parr et al., computed both at the global and the local level, correlates linearly with the complexation energies of the compounds with NH3. The Lewis base character of these silylenes has been investigated, through their interaction with the acids BH3 and AlH3. Also in this case, the electrostatic potential can be used to probe the reactivity of the compounds. It will finally be demonstrated that an increasing stability of the silylenes and germylenes is accompanied by an increase in their nucleophilicity and a decrease of the electrophilicity.     

Water soluble cyclodextrin polymers: Their interaction with drugs

The complex forming ability of a water-soluble -cyclodextrin epichlorohydrin, polymer (CDPS) and its different molecular weight fractions was studied and compared with the complexing properties of -cyclodextrin (CD) and dimethyl-CD (DM-CD). CDPS was separated into two main fractions. CDPS and its fractions formed well soluble inclusion compounds with the studied drugs. The low molecular weight fraction formed rather stable complexes with small guest molecules, the high molecular weight fraction was found to be more efficient in binding larger substrates. Structural studies of furosemide-CD complexes were attempted by NMR spectroscopy.Presented at the Fourth International Symposium on Inclusion Phenomena and the Third International Symposium on Cyclodextrins, Lancaster, U.K., 20–25 July 1986.     

The Mechanism of Biochemical NO-Sensing: Insights from Computational Chemistry

The binding of small gas molecules such as NO and CO plays a major role in the signaling routes of the human body. The sole NO-receptor in humans is soluble guanylyl cyclase (sGC) – a histidine-ligated heme protein, which, upon NO binding, activates a downstream signaling cascade. Impairment of NO-signaling is linked, among others, to cardiovascular and inflammatory diseases. In the present work, we use a combination of theoretical tools such as MD simulations, high-level quantum chemical calculations and hybrid QM/MM methods to address various aspects of NO binding and to elucidate the most likely reaction paths and the potential intermediates of the reaction. As a model system, the H-NOX protein from Shewanella oneidensis (So H-NOX) homologous to the NO-binding domain of sGC is used. The signaling route is predicted to involve NO binding to form a six-coordinate intermediate heme-NO complex, followed by relatively facile His decoordination yielding a five-coordinate adduct with NO on the distal side with possible isomerization to the proximal side through binding of a second NO and release of the first one. MD simulations show that the His sidechain can quite easily rotate outward into solvent, with this motion being accompanied in our simulations by shifts in helix positions that are consistent with this decoordination leading to significant conformational change in the protein.     

Quantile process for left truncated and right censored data

In this paper, we consider the product-limit quantile estimator of an unknown quantile function when the data are subject to random left truncation and right censorship. This is a parallel problem to the estimation of the unknown distribution function by the product-limit estimator under the same model. Simultaneous strong Gaussian approximations of the product-limit process and product-limit quantile process are constructed with rate . A functional law of the iterated logarithm for the maximal deviation of the estimator from the estimand is derived from the construction. Work partially supported by NSC Grant 89-2118-M-259-011.     

Paving Hessenberg varieties by affines

Regular nilpotent Hessenberg varieties form a family of subvarieties of the flag variety arising in the study of quantum cohomology, geometric representation theory, and numerical analysis. In this paper we construct a paving by affines of regular nilpotent Hessenberg varieties for all classical types, generalizing results of De Concini–Lusztig–Procesi and Kostant. This paving is in fact the intersection of a particular Bruhat decomposition with the Hessenberg variety. The nonempty cells of the paving and their dimensions are identified by combinatorial conditions on roots. We use the paving to prove these Hessenberg varieties have no odd-dimensional homology.      

Interaction of triols with formaldehyde and acetone: Experimental and theoretical study

Experimental and theoretical aspects of the condensation of glycerol and its homologs (1,2,3- and 1,2,4-butanetriols) with formaldehyde and acetone are studied under conditions of acid catalysis. Calculation of the thermodynamic parameters of the resulting products by the composite method CBS-QB3 shows that the six-membered heterocycles, the products of the interaction of triols with formaldehyde, are thermodynamically more stable than the five-membered acetals, while the reaction of the same triols with acetone is preferable for the formation of the five-membered acetals. This is due to the fact that the regioselectivity of the studied reactions is determined by the structural features and reactivity of the carbocations formed in a condensed medium during the course of the reaction. According to the theoretical data obtained experimentally, during the condensation of glycerol and 1,2,4-butanetriol with formaldehyde in the most stable form of the six-membered cyclic carbocation, intramolecular hydrogen bonding and anomeric stabilization due to the axially oriented hydroxyl group take place. As a result, cation 1b–1 is 1.2–1.6 kJ/mol more stable than its five-membered isomers ( 1a–1 and 1b–2 ). It leads to the predominant formation of 1,3-dioxane ( 3b ). However, upon condensation of butanetriol-1,2,3 with formaldehyde, the intermediate cation 4a–1 turns out to be significantly more stable than the other isomers due to the strong intramolecular hydrogen bond in the six-membered ring with the participation of the hydroxyl group of the substituent and the hydroxyl group of the cationic center, leading to the predominant formation of the dioxolane 6a .     

Molecular structure and internal rotation potential of perfluoro (2,4-dimethyl-3-oxa-2,4-diazapentane), (CF3)2N–O–N(CF3)2

Ab initio quantum chemical calculation were performed on R₂N–O–NR₂ type (R=H, F, CH₃ and CF₃) molecules, using the HF, B3LYP and MP2/6-31G* levels of theory. The equilibrium structures and the internal rotation potentials have been determined. Three stable conformers were found for R=H, F and CH₃ while only two in case of R=CF₃. The rotation potential energy curves do not change significantly upon fluorination. The calculations suggests that in the ED measurement of the title compound the NC and NO bond length might have been interchanged.     

Interaction of Hydroxy Acids with β-Cyclodextrin

The solubility of -cyclodextrin (-CD) was studied in aqueous solutions of various organic acids. The hydroxy acids, especially citric and tartaric acid were found to increase the solubility of -CD, while some other carboxylic acids reduced it. From solubility data the apparent complex association constants were calculated.     

Multiple pathways for the oxygenation of a ruthenium(II) dithiocarbamate complex: S-oxygenation and S-extrusion

The reactions of Ru(bpy)(2)(N,N-dimethyldithiocarbamate)(+), 1, with O-atom-transfer reagents such as hydrogen peroxide, m-chloroperoxybenzoic acid, and oxone have been studied and several resulting derivatives isolated and structurally characterized. Both S-oxygenation and S-extrusion may occur depending upon reagent and conditions. Excess peroxygenation leads to a stable dioxygenate, Ru(bpy)(2)(N,N-dimethylthiocarbamatesulfinate-S,S)(+), 3. Stoichiometric oxygenation leads to mixtures of products from which two forms of monooxygenated species Ru(bpy)(2)(N,N-dimethylperoxydithiocarbamate-S,S), 2a, and Ru(bpy)(2)(N,N-dimethylperoxydithiocarbamate-O,S), 2b, and an S-extruded product, Ru(bpy)(2)(N,N-dimethylmonothiocarbamate)(+), 4, have been isolated as PF(6)(-) salts. The S,S-bound monooxygenate is unstable over time toward either O-atom-transfer reactions via disproportionation or reaction with phosphines or S-extrusion yielding complex 4 in which the thiocarbamate is bonded solely through the remaining S atom. All the complexes have been characterized by (1)H NMR, UV-vis, and mass spectroscopies, and all but the highly reactive 2a structurally determined by X-ray crystallography.