How Does Aromaticity Rule the Thermodynamic Stability of Hydroporphyrins?

Several measures of aromaticity including energetic, magnetic, and electron density criteria are employed to show how aromatic stabilization can explain the stability sequence of hydroporphyrins, ranging from porphin to octahydroporphin, and their preferred hydrogenation paths. The methods employed involve topological resonance energies and their circuit energy effects, bond resonance energies, multicenter delocalization indices, ring current maps, magnetic susceptibilities, and nuclear-independent chemical shifts. To compare the information obtained by the different methods, the results have been put in the same scale by using recently proposed approaches. It is found that all of them provide essentially the same information and lead to similar conclusions. Also, hydrogenation energies along different hydrogenation paths connecting porphin with octahydroporphin have been calculated with density functional theory. It is shown by using the methods mentioned above that the relative stability of different hydroporphyrin isomers and the observed inaccessibility of octahydroporphin both synthetically and in nature can be perfectly rationalized in terms of aromaticity.     

Stability and growth behavior of transition metal nanoparticles in ionic liquids prepared by thermal evaporation: how stable are they really?

Recently we developed an access to metal- and metal-oxide colloids based on thermal evaporation of metals into ionic liquids (ILs). Here we present systematic studies on the long-time stability of gold and copper nanoparticles (NPs) in different ILs. The influence of metal concentration and temperature on the ripening of the as-prepared gold NPs in different ILs was investigated by transmission electron microscopy (TEM) and UV-vis absorption measurements. Short alkyl chain-length-methyl-imidazolium salts with weakly coordinating perfluorinated counter anions (PF(6), BF(4) or Tf(2)N, bis(trifluoromethanesulfonyl)amide) were found to be better stabilizers compared to ILs with cations bearing long alkyl chains (trihexyltetradecylphosphonium, 1-octyl-3-methylimidazolium) and anions of higher coordination strength (DCA, dicyanamide). In the latter ILs fast particle growth and agglomeration was observed. In the well-stabilizing ILs initially very small NPs form which undergo a similar ripening process at room temperature as at higher temperatures. The final particle size depends largely on the used IL and the metal and to a minor extent on the temperature. The metal concentration seems to be an unimportant factor.     

Thermal Stability of α-Phycoerythrocyanin

The α-subunit of the phycobiliprotein, phycoerythrocyanin (α-PEC), from Mastigocladus laminosus shows photoreversible photochromism that is based on the Z ↔ E isomerization of the phycoviolobilin (PVB) chromophore. Thermal stability of the photochemistry and chromoprotein secondary structure have been studied by absorption and circular dichroism (CD) spectroscopy. Both photoisomers are stable and photoconvertible up to ∼ 70 °C. At T > 75 °C, photochemistry ceases because the E-state reverts rapidly thermally to the Z–state. The chromoprotein melts at 72 °C, the apoprotein already at 55 °C, indicating a considerable stabilization of the protein secondary structure by the chromophore.     

Stability of Oligomeric α-Alkoxyhydroperoxides

The thermal decomposition of oligomeric -alkoxyhydroperoxides (I) is described by a first-order rate law and characterized by an activation energy equal to 19.7 kJ/mol. A chain mechanism and a rate law for this reaction are proposed. The effect of amine and phenolic stabilizers on decomposition is studied, the efficiency of phenolic stabilizers is examined, and the effective rate constant of the acid-catalyzed decomposition of compounds I is determined.     

Reaction of dianions derived from β-ketoesters with epoxides-utility in the preparation of synthetically useful tetrahydrofurans

Presented here are several examples which demonstrate that ether substituents α- or β- to an epoxide ring can be tolerated in the ring-opening reaction with β-keto ester dianions. Subsequent acid-promoted cyclisation of the γ-hydroxy β-ketoesters then leads to synthetically useful tetrahydrofurans, as demonstrated by application of this approach to the preparation of (±)-methyl nonactate and (±)-methyl 8-epi-nonactate.     

Study on Stability of Nateglinide Polymorphism

The stability of three forms of nateglinide, especially S-form and H-form, was determined. S-form was a new crystal structure of nateglinide. Three forms of nateglinide were treated in different conditions, such as in various temperatures, humidity, light and so on.Analysis of their crystal structures was performed by X-ray powder diffraction ( XRD ) and their particle shapes were observed with scanning electron microscope ( SEM ). The results indicated that the stability of S-form of nateglinide is the best among the three forms and their particle shapes are quite difference. S-form is the sheet structure of layer upon layer, H-form looks like a hank of silk lines and B-form is the clubbed shape.     

Thermal Stability of γ-irradiated Tolbutamide

The thermal stability of tolbutamide before and after exposure to various γ-radiation doses was investigated. The data were followed by studying DTA, X-ray diffraction, IR, and UV absorption spectra before and after γ-irradiation. The results obtained were promising, and were explained, and discussed on the basis of γ-enhanced stabilization through recombination of free radicals. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermal Stability of Immobilised α-chymotrypsinogen

The unfolding of α-chymotrypsinogen covalently immobilized on silica beads has been studied by differential scanning calorimetry (DSC). The enzyme undergoes an unfolding transition which, unlike the free protein, cannot be approximated by a single two-state process. After immobilization, the unfolding is characterized by the presence of two partially overlapping transitions, both of them show two-state behavior. The two processes correspond to the separate unfolding of the two domains of the α-chymotrypsinogen molecule. The loss of cooperativity behavior is a consequence of the covalent immobilization. The two domains showed different thermal stability as functions of pH. One of them unfolded with a transition temperature T _m2 higher than T _m of the free enzyme, implying stabilization effect of immobilization. However, below pH 4.5, its native structure is lost. The other transition shows a remarkable pH-independent thermal stability from pH 2.5 to 7.0. This revised version was published online in July 2006 with corrections to the Cover Date.     

The two faces of chemistry: can they be reconciled?

Shortly before his death, Richard Bader commented in this Journal on the dichotomy that exists within chemistry and between chemists. We believe that the dichotomy results from different goals and objectives inherent in the chemical disciplines. At one extreme are designers who synthesize new molecules with interesting properties. For these chemists, the rationale underpinning molecular synthesis is far less important than the end product—the molecules themselves. At the other extreme are the chemists who seek a fundamental understanding of molecular properties. We suggest that the Quantum Theory of Atoms in Molecules, by virtue of the rich hierarchical structure inherent in the theory, offers a bridge through which to unite these two groups. However, if there is to be reconciliation, it falls to the theorists to develop “quantum mechanically” correct tools and concepts useful to the synthetic and applied chemist.     

Biomedical applications of shape-memory polymers: howpractically useful are they?

Shape-memory effect (SME) is the ability of a material to change its dimension in a predefined way in response to an external stimulus. Polymers that exhibit SME are an important class of materials in medicine, especially for minimally invasive deployment of devices. However, the rate of translation of the concept to approved products is extremely low, with mostly nitinolbased devices being approved. In this review, the general aspects of the different types of stimuli that can be used to activate SME are reviewed and sterilization issues of shape-memory polymer (SMP)-based medical devices are addressed. In addition, the general usefulness as well as the limitations of the shape-memory effect for biomedical applications are described.     

Stability of Polymer–Monomer Particles of Synthetic Latexes

The presence of “living” macroradicals in the volume of a polymer–monomer particle may be one of the factors responsible for the loss of the aggregative stability of latexes. If the adsorption protection of a polymer–monomer particle is insufficient, high content of “living” macroradicals leads to gelation in the latex in the course of storage. Correlation between the latex life time, hydration of nonionic surfactant molecules in the adsorption layer of a polymer–monomer particle, and extent of the action of the macroradicals was determined. Naphthalenesulfonic dispersing agents enhance the stability of latex systems in the step of polymerization and distillation of the monomers owing to a decrease in the critical micelle concentration of the emulsifier, to extension of the micellar period of the polymerization, to an increase in the degree of saturation of polymer–monomer particles, and to an increase in the probability of macroradical recombination in the volume of a polymer–monomer particle. With an increase in the degree of polycondensation of naphthalene-containing dispersing agents, their surface activity increases, whereas the ability to support the aggregative stability of latexes decreases.     

Stability of Au 2 2+ : metastability through an excited state?

We discuss the stability of doubly charged Auclusters. From a calculation using semi-empirical linear combination of atomic-orbitals (LCAO) method, we conclude that Au ₂ ²⁺ in an excited state (³Σ _u ⁺ or¹Σ _u ⁺ ) is metastable with an energy barrier of about 0.3 eV. In the ground state (¹Σ _g ⁺ ) Au ₂ ²⁺ is unstable, except for small values of Δ(=ε _s ?ε _d ) (difference between thes andd atomic energy levels of Au), when a very shallow minimum appears in the binding energy curve. These results are critically discussed and compared with those obtained by different calculations.     

Distribution of N-glycosylation sequons in proteins: how apart are they?

N-glycosylation is a common protein modification process, which affects a number of properties of proteins. Little is known about the distribution of N-glycosylation sequons, for example, the distance between glycosylated sites and their position in the protein primary sequence. Using a large set of experimentally confirmed eukaryotic N-glycoproteins we analyzed the relative position and distribution of sequons. N-Glycosylation probability was found to be lower in the termini of protein sequences compared to the mid region. N-glycosylated sequons were found much farther from C terminus compared to the N-terminus of the protein sequence and this effect was more pronounced for NXS sequons. The distribution of sequons, modeled based on balls-in-boxes classical occupancy, showed a near-maximum probability. Considerable proportion of sequons was found within a distance of ten amino acids, indicating that the steric hindrance was not a key factor in protein N-glycosylation. Interestingly, the distribution of all sequons present in N-glycoproteins showed a pattern very similar to that of glycosylated sequons. The results indicate that protein N-glycosylation chiefly follows a random design.     

Structures and Stability of NHS2 Isomers

Potential energy surface of HNS2 is investigated by means of second-order Moller-Plesset Perturbation theory(MP2) and QCISD(T) (single-point)methods,At final QCISD (T)/6-311 G(3df,2p)//MP2/6-311 G(d,p),level with zero-point vibrational energies included,cis-NHSS is found to be global minimum on the potential energy sufrace,followed by low-lying trans-HNSS ,HN(S)S(C2v).cis-HSNS,cis-NSSN,trans-HSNS,trans-NSSN,and HN(S)S(Cs) by 13.46,66.92,78.25,80.38,81.22,81.38 and 86.40 kJ/mol,respectively.A new high-lying HS(N)S isomer with Cs symmetry is located on the potential energy surface ,The kinetic stabilities of all isomers are predicted.Comprisons are made for HNS2 with its analogues,NHO2,HPS2 and HPO2,The causes that lead to the differences between HNS2 and its analogues are hypervalent capacity of phosphorus and distinct electronegativities of hydrogen,nitrogen and phosphorus.     

The Structures and Stability of HNOS Isomers

Potential energy surface of HNOS system is investigated by means of MP2 method with 6-311 G(d,p) basis set.The energy for each minimum and saddle point on the potential energy surface is corrected at the QCISD(T)/6-311 G(3df,2p) level of theory with zero-point vibrational energy included.As a result ,eighteen isomers are theoretically predicted and cis-HNSO is found to be global minimum on the potential energy surface,Wherein,fourteen isomers are considered as kinetically stable species,and should be experimentally observed.Comparisons are made for HNOS system with its analogues,HNO2 and NHS2.The nature of bonding and isomers‘ stability of HNOS system are similar to HNS2.The obvious similarities and discrepancies among HNOS,HNO2 and HNS2 are attributed to the hypervalent capacity of sulfur,oxygen and nitrogen atoms.     

Structures and Stability of HNS_2 Isomers

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Efficient asymmetric protonation of enolates with readily accessible chiral α-sulfinyl alcohols

The efficient asymmetric protonation of lithium enolates of 2-alkylcycloalkanones (87–96% ee) with readily accessible chiral α-sulfinyl alcohols is described. Optimal stereoselection is achieved for each lithium enolate at a different reaction temperature in the range −40 to −100°C.     

Stability of mechanical properties of molecular layer–deposited alucone

We report on the effect of aging on the mechanical properties of molecular layer–deposited (MLD) thin films. We studied the mechanical failure of the films during uniaxial tensile testing and observed a sixfold difference in the crack-onset strain (COS) and related flexibility within the first two days after the samples were exposed to ambient air. The MLD films made using trimethylaluminum and ethylene glycol are notorious for exhibiting structural changes after the fabrication; we show that these changes are detrimental for mechanical robustness of the films. This information aids to plan the handling or the protection of these films to achieve better performance with these materials. The interfacial shear strains and COSs of the shortly air-exposed 300-nm-thick films were observed to be roughly 0.3% and 1.8%, respectively. These values are the highest reported so far for hybrid organic–inorganic MLD thin films and would extrapolate to about 14% COS for 5-nm-thick film, indicating potential applications as interfacial adhesion layer for films on polymer substrates and as a protective coating in battery applications.     

Stability of pyrrolidone derivatives against γ-ray irradiation

To evaluate the stability of N-alkylated pyrrolidone derivatives(NRPs),which are supposed to be used as precipitants for U(VI) and Pu(IV,VI) species in HNO 3 media,under irradiation environment,some candidate NRPs were irradiated by γ-ray.Irradiation to HNO 3 solutions up to 6 mol dm 3(= M) containing 2 M N-n-butyl-2-pyrrolidone(NBP),one of NRPs with lower hydrophobicity,has revealed that the residual ratios of NBP in the samples of HNO 3 up to 3 M decreased identically and linearly.Approximately 20% of NBP was found to be degraded after the irradiation at 1 MGy.It was also found that the decrease in the precipitation ratio of UO 2 2+(P.R.,%) was gentle and that the P.R.values were relatively in accordance with the residual ratios of NBP.On the other hand,the degradation of the samples irradiated in 6 M HNO 3 was found more distinguished.It was proposed from the analyses of degraded compounds that the degradation of NBP in HNO 3 by γ-ray irradiation started from the cleavage of the pyrrolidone ring by the addition of oxygen atom originating from HNO 3,followed by the formation of chain compounds by the successive addition of oxygen,leading to the generation of oxalic acid and acetic acid.The stability of other NRPs in 3 M HNO 3 was evaluated to be nearly identical with that of NBP except lower P.R.values of the samples containing NRPs with higher hydrophobicity irradiated at more than 0.5 MGy.