Sphericities of Double Cosets,Double Coset Representations,and Fujita’s Proligand Method for Combinatorial Enumeration of Stereoisomers

The concepts of double coset representations and sphericities of double cosets are proposed to characterize stereoisomerism, where double cosets are classified into three types, i.e., homospheric double cosets, enantiospheric double cosets, or hemispheric double cosets. They determine modes of substitutions (i.e., chirality fittingness), where homospheric double cosets permit achiral ligands only; enantiospheric ones permit achiral ligands or enantiomeric pairs; and hemispheric ones permit achiral and chiral ligands. The sphericities of double cosets are linked to the sphericities of cycles which are ascribed to right coset representations. Thus, each cycle is assigned to the corresponding sphericity index (a _d , c _d , or b _d ) so as to construct a cycle indices with chirality fittingness (CI-CFs). The resulting CI-CFs are proved to be identical with CI-CFs introduced in Fujita’s proligand method (S. Fujita, Theor. Chem. Acc. 113 (2005) 73–79 and 80–86). The versatility of the CI-CFs in combinatorial enumeration of stereoisomers is demonstrated by using methane derivatives as examples, where the numbers of achiral plus chiral stereoisomers, those of achiral stereoisomers, and those of chiral stereoisomers are calculated separately by means of respective generating functions.     

3—Mercaptopropionic Acid Capped Gold Nanoclusters：Quantized Capacitance in Aquesous Media

3-Mercaptopropionic acid monolayer protected gold nanoclusters (MPA-MPCs) were synthesized and characterized by transmission electorn microscopy,UV-Vis spectroscopy,X-ray photoelectron spectroscopy and Fourler transform infrared spectroscopy.The exact value of quantized double-layer capacitance of MPCs in aqueous media was obtained by differential pulse voltammograms.     

The Role of Intramolecular Hydrogen Bonds vs. Other Weak Interactions on the Conformation of Hyponitrous Acid and Its Mono- and Dithio-Derivatives

High level ab initio and density functional theory calculations have been carried out to investigate the relative stability of the different conformers of hyponitrous acid and its mono- and dithio-derivatives. Geometries and vibrational frequencies were obtained at the B3LYP/6-311+G(d,p) level and final energies through B3LYP/6-311++G(3df,2pd) single point calculations. The reliability of this theoretical scheme has been assessed by comparing these DFT results with those obtained at the G3 level of theory, for some suitable cases. The cis conformers of hyponitrous acid and its mono- and dithio-derivatives are systematically more stable than the trans ones because in the cis conformation a dative interaction between the nitrogen-lone pairs and the σ_NX^* antibonding orbital is significantly favored. Quite interestingly, in general, the conformers presenting an intramolecular hydrogen bond (IHB) are not the global minima of the corresponding potential energy surfaces and only for hyponitrous acid the conformer with a OH ⋅s O IHB is slightly more stable than the cis conformer without IHB. The low stability of the tautomers with IHB is closely related with another weak intramolecular interaction which involves the lone-pairs of the chalcogen atoms and the π_NN^* antibondig orbital, and which is significantly perturbed when the IHB is formed.     

Controlled ring‐opening polymerization of propylene oxide catalyzed by double metal‐cyanide complex

A double metal‐cyanide catalyst based on Zn₃[Co(CN)₆]₂ was prepared. This catalyst is very effective for the ring‐opening polymerization of propylene oxide. Polyether polyols of moderate molecular weight having low unsaturation (<0.015 meq/g) can be prepared under mild conditions. The molecular weight of polymer is entirely controlled by a reacted monomer‐to‐initiator ratio. The polymers prepared with stepwise addition of monomer exhibit a narrower molecular weight distribution as compared with those prepared with one‐step addition of monomer. Various compounds containing active hydrogen, except basic compounds and low‐carbon carboxylic acid, may be used as initiators. The reaction rate increases with increasing catalyst amount and decreases with rising initiator concentration. Polymerization involves a rapid exchange reaction between the active species and the dormant species. It was also proven that, to a certain extent, the chain termination of this catalytic system is reversible or temporary. ¹³C NMR analysis showed that the polymer has a random distribution of the configurational sequences and head‐to‐tail regiosequence. It is assumed that the polymerization proceeds via a cationic coordination mechanism. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1142–1150, 2002     

A Procedure for Transforming Impedance Spectra for the Determination of Mechanism of Electrochemical Reactions

Transformation of impedance spectra into relaxation time spectra (RTS) is used for determining contributions of individual processes of the oxygen electroreduction reaction (OER) to the polarization resistance of the electrochemical cell. The transformation technique involves the solution of the convolution equation found with the aid of a modified Van Cittert iteration algorithm checked on model impedance spectra. The technique, when used to analyze impedance spectra of electrochemical cells air|Pt|YSZ|YSZ + Pt|air, shows that the conversion of a globular structure of the YSZ + Pt cermet layer to a columnar one is accompanied by a change of peak amplitudes in RTS. The revealed RTS dynamics when heated to 750°C is compared with peculiarities of individual processes in OER.     

Self-assembled metallocycles with two interactive binding domains

Five metallocycles 1 a-e have been self-assembled from S-shaped bispyridyl ligands 2 a-e and a palladium complex, [Pd(dppp)(OTf)(2)] (dppp=1,3-bis(diphenylphosphanyl)propane), and have been characterized by elemental analysis and various spectroscopic methods including (1)H NMR spectroscopy and electrospray ionization (ESI) mass spectrometry. These metallocycles all are monocyclic compounds, but can fold to generate two binding domains bearing hydrogen-bonding sites based on pyridine-2,6-dicarboxamide units. The binding properties of the metallocycles with N,N,N',N'-tetramethylterephthalamide (G) have been probed by means of ESI mass spectrometry and (1)H NMR spectroscopy. The results both in the gas phase and in solution are consistent with the fact that the metallocycles accommodate two molecules of the guest G. Thus, the ESI mass spectra clearly show fragments corresponding to the 1:2 complexes in all cases. (1)H NMR studies on 1 a and G support the formation of a 1:2 complex in solution; the titration curves are nicely fitted to a 1:2 binding isotherm, but not to a 1:1 binding isotherm. In addition, a Job plot also suggests a 1:2 binding mode between 1 a and G, showing maximum complexation at approximately 0.33 mol fraction of the metallocycle 1 a in CDCl(3). The binding constants K(1) and K(2) are calculated to be 1600 and 1400 M(-1) (+/-10 %), respectively, at 25 degrees C in CDCl(3), indicative of positively cooperative binding. This positive cooperativity was confirmed by the Hill equation, affording a Hill coefficient of n = 1.6. Owing to insufficient solubility in CDCl(3), for comparison purposes the binding properties of the metallocycles 1 b-e were investigated in a more polar medium, 3 % CD(3)CN/CDCl(3). (1)H NMR titrations revealed that the metallocycles all bind two molecules of the guest G with Hill coefficients ranging from 1.4 to 1.8. This positive cooperativity may be attributed to a structural reorganization of the second binding cavity when the first guest binds to either one of the subcavities present in the metallocycles.     

Protein dynamics tightly connected to the dynamics of surrounding and internal water molecules.

Proteins are key components of biological cells. For example, enzymes catalyze biochemical reactions, membrane transporters are responsible for uptake and release of critical and superfluous components from the cell environment, and structural proteins are responsible for the stability of the cell wall and cytoskeleton. Many of the diverse protein functions involve dynamic transitions ranging from small local atomic displacements up to large allosteric conformational changes. In any conformation, proteins are in contact with the universal solvent medium of cells, water. Water not only surrounds proteins but is often an integral part of proteins and also is involved in key mechanistic steps. This Minireview discusses recent experimental and theoretical results on the role of water for protein dynamics and function.     

N-Bromosuccinimide-dibenzoyl peroxide/azabisisobutyronitrile: a reagent for Z- to E-alkene isomerization

N-Bromosuccinimide-dibenzoyl peroxide/azobisisobutyronitrile is used to carry out several types of Z- to E-alkene isomerizations. The NBS-bromination conditions are sufficient for both allylic bromination and alkene isomerization. When the allylic hydrogens are not available in substrates, only the isomerization of the alkene takes place. The present conditions for isomerization of carbon-carbon double bonds are mild and efficient.     

Chirality transfer from guest chiral metal complexes to inorganic framework: the role of hydrogen bonding

The structure elucidation of a new zinc phosphate [Co(II)(en)(3)][Zn(4)(H(2)PO(4))(3)(HPO(4))(2)(PO(4))(2 H(2)O)(2)] (1) reveals that the racemic cobalt complex templates the zinc phosphate framework in such a way that the local C(2) point symmetry of the structural motif of the inorganic framework conforms with that of the cobalt complex pairing with it, in essence transferring its chirality to the inorganic host. An analysis of hydrogen bonding between the guest molecules and the inorganic host framework reveals that hydrogen bonding is responsible for the stereospecific structural arrangement. Upon examining previously reported chiral metal-complex-templated structures of metal phosphates, it is revealed that such hydrogen bonding is the common origin for inducing chirality transfer in metal-phosphate frameworks templated with chiral metal complexes. Crystal data of 1: orthorhombic, Pbcn (no. 60), a=10.4787(8) A, b=20.0091(14) A, c=14.9594(10) A, and Z=2.     

Zur Reaktion von Makrozyklen mit Lanthanoiden. I. Die Struktur von [Li(thf)][(C22H22N4)2Ce] · THF

On the Reaction of Macrocycles with Lanthanoids. I. The Crystal Structure of [Li(thf)][(C₂₂H₂₂N₄)₂Ce] · THF In THF CeBr₃ forms with [(TMTAA)Li₂] the paramagnetic doubledecker complex [Li(thf)][(TMTAA)₂Ce]. The complex crystallizes with 1 Mol THF per formula unit. The structure was characterized by X-ray single crystal structure analysis (space group C2 (No. 5), z = 6, a = 1741.8(2) pm, b = 1622.1(2) pm, c = 2540.4(3) pm, β = 104.72(1)°). The sandwich-like arrangement of the heterocyclic ligands leads to a quadratic-prismatic coordination of the Ce³⁺ ion. One macrocyclic ligand is additionally coordinated by a [Li(thf)]⁺ fragment. The coordination of the Li ion is square pyramidal.