Seismic collapse capacity spectra for simple structures vulnerable to dynamic instabilities

In this study collapse capacity spectra based on various definitions of the seismic intensity are set in contrast and evaluated. The presented collapse capacities for highly inelastic non-deteriorating single-degree-of-freedom (SDOF) systems, which are vulnerable to the destabilizing effect of gravity loads, are derived for a near-fault set of ground motions with distinct pulse characteristics. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Interplay between degenerate convergence of semigroups and asymptotic analysis: a study of a singularly perturbed abstract telegraph system

Singularly perturbed evolution equations may be analyzed by various techniques. In this paper, we focus on ideas of asymptotic analysis and those akin to the Trotter–Kato approximation theorems. Using as an example an abstract telegraph-type system, we show how these two approaches intertwine and complement each other. The paper also may be seen as a continuation of Kato’s project (in Turbululence and Navier–Stokes Equations Theory. Proc. Conf. Univ. Paris-Sud, Orsay 1975, Lect. Notes Math., Springer, Berlin, vol 565, pp. 104–112, 1976) taking account of recent results. On leave from Lublin University of Technology.     

Density functional study of the O2 binding to [CuI(TPAR)]+ (TPA = tris(2-pyridylmethyl)amine) in THF and EtCN

Density functional theory using the B3LYP hybrid functional has been employed to study the formation of [Cu(II)(TPA(H))(O2-)]+ and [Cu(II)(TPA(MeO))(O2-)]+ (TPA = tris(2-pyridylmethyl)amine) in two different solvents, THF and EtCN. The thermodynamics of solvent coordination as well as that of the overall reactions with O2 has been computed. The formations of [Cu(II)(TPA(H))(O2-)]+ in THF and of [Cu(II)(TPA(MeO))(O2-)]+ in both THF and EtCN are found to be initiated from the [Cu(I)(TPA(R))]+ species, that is, the Cu complex possessing an empty coordination site. In contrast, the formation of [Cu(II)(TPA(H))(O2-)]+ in EtCN is found to be initiated from the [Cu(I)(TPA(H))(EtCN)]+ species, that is, one solvent molecule being coordinated to Cu(I). In general, good agreement is found between theoretical and experimental results. The high accuracy of the B3LYP functional in reproducing experimental thermodynamic data for the present type of transition metal complexes is demonstrated by the fact that the differences between measured and computed thermodynamic parameters (DeltaG degree, DeltaH degrees , and -TDeltaS degree, in most cases are less than 2.0 kcal mol(-1). An attempt was made to investigate the kinetics of the formation of [Cu(II)(TPA(H))(O2-)]+ in THF and EtCN. Computed free energies of activation, DeltaG, are in good agreement with experimental results. However, an analysis of the partitioning of the free energy barriers in enthalpic and entropic contributions indicates that the computationally studied reaction pathway might differ from the one observed experimentally.     

Electronic spectroscopy of the 3d Rydberg states of NO-Rg (Rg=Ne,Ar,Kr,Xe) van der Waals complexes

We have employed (2+1) resonance-enhanced multiphoton ionization spectroscopy to record electronic absorption spectra of NO-Rg (Rg=Ne,Ar,Kr) van der Waals complexes. The nitric oxide molecule is the chromophore, and the excitation corresponds to an electron being promoted from the 2ppi* orbital to 3dsigma, 3dpi, and 3ddelta Rydberg states. We review the ordering of the 3dlambda states of NO and use this as a basis for discussing the 3d components in the NO-Rg complexes, in terms of the interactions between the Rydberg electron, the core, and the Rg atom. Predissociation of the H' 2Pi state occurs through the F2Delta state for NO-Ar and NO-Kr, and this will be considered. We shall also outline problems encountered when trying to record similar spectra for NO-Xe, related to the presence of atomic Xe resonances.     

Directed synthesis of noncentrosymmetric molybdates using composition space analysis

A systematic investigation of the factors governing the reaction product composition, hydrogen bonding, and symmetry was conducted in the MoO3/3-aminoquinuclidine/H2O system. Composition space analysis was performed through 36 individual reactions under mild hydrothermal conditions using racemic 3-aminoquinuclidine. Single crystals of three new compounds, [C7H16N2][Mo3O10] x H2O, [C7H16N2]2[Mo8O26] x H2O, and [C7H16N2]2[Mo8O26] x 4 H2O, were grown. The relative phase stabilities for these products are dependent upon the reactant mole fractions in the initial reaction gel. This phase stability information was used to direct the synthesis of two new noncentrosymmetric compounds, using either (S)-(-)-3-aminoquinuclidine dihydrochloride or (R)-(+)-3-aminoquinuclidine dihydrochloride. [(R)-C7H16N2]2[Mo8O26] and [(S)-C7H16N2]2[Mo8O26] both crystallize in the noncentrosymmetric space group P2(1) (No. 4), which has the polar crystal class 2 (C2). The second-harmonic generation activities were measured on sieved powders. The structure-directing properties of the molybdate components in each compound were determined using bond valence sums. The structures of all five compounds were determined using single-crystal X-ray diffraction.     

Exploiting noninnocent (E,E)-dibenzylideneacetone (dba) effects in palladium(0)-mediated cross-coupling reactions: modulation of the electronic properties of dba affects catalyst activity and stability in ligand and ligand-free reaction systems

The reactivity of palladium(0) complexes, [Pd(0) (2)(dba-n,n'-Z)(3)] (n,n'-Z=4,4'-F; 4,4'-CF(3); 4,4'-H; 4,4'-MeO) and [Pd(0)(dba-n,n'-Z)(2)] (n,n'-Z=4,4'-CF(3); 4,4'-H; 3,3',5,5'-OMe), used as precursor catalysts with suitable donor ligands (e.g. phosphines, N-heterocyclic carbenes), has been correlated in several palladium(0)-mediated cross-coupling processes. Increasing the electron density on the aryl moiety of the dba-n,n'-Z ligand increases the overall catalytic activity in the majority of these processes. This effect primarily derives from destabilization of the L(n)Pd(0)-eta(2)-dba interaction (in dpi-pi* synergic bonding, n=1 or 2), which ultimately increases the global concentration of catalytically active L(n)Pd(0) available for reaction with aryl halide in the first committed step in the general catalytic cycle(s) (oxidative addition). Decreasing electron density on the aryl moiety of the dba-n,n'-Z ligand stabilizes the Pd(0)-eta(2)-dba interaction, reducing catalytic activity. The specific type of dba-n,n'-Z ligand appears to also play a stabilizing role in the catalytic cycle, preventing Pd agglomeration, and increasing catalyst longevity. A subtle balance therefore exists between the L(n)Pd(0) concentration (and the associated catalytic activity) and catalyst longevity. Changing the type of dba-n,n'-Z ligand controls the concentration of L(n)Pd(0) and the rate of the oxidative addition step, and not other intimate steps within the catalytic cycle(s), for example, transmetallation (or carbopalladation) and reductive elimination. The role of dba-n,n'-Z ligands in Heck arylation is more convoluted and dependent on the alkene substrate employed, although trends have emerged. Changes in the structure of dba-n,n'-Z had a minimal affect on Buchwald-Hartwig aryl amination processes. A secondary Michael reaction of dba-n,n'-Z with amine and/or base effectively lessens its interference in the catalytic cycle.     

Isocratic and gradient elution chromatography: a comparison in terms of speed, retention reproducibility and quantitation

Chromatographers are cautioned to avoid gradient elution when isocratic elution will do. In this work, we compared the analytical properties of gradient and isocratic separations of a sample which can be done quite readily under isocratic conditions. We found that gradient elution gave a shorter overall analysis with similar resolution of the critical pair compared to isocratic elution without sacrificing repeatability in retention time, peak area and peak height or linearity of the calibration curve. We also obtained acceptable repeatability in peak area/height and linearity of calibrations curves for a sample that required gradient elution using a practical baseline subtraction technique. Based on these results and related work which show that columns can be reequilibrated by flushing with less than two column volumes of the initial eluent, we conclude that many of the reasons given to avoid gradient elution deserve serious reconsideration, especially for those samples which are easily separated isocratically. However, we believe isocratic elution will remain preferable when: (1) the sample contains less than 10 weakly retained components (i.e. the last peak elutes with k' < 5) or (2) the gradient baseline impedes trace analysis.     

A highly active palladium catalyst for intermolecular hydroamination. Factors that control reactivity and additions of functionalized anilines to dienes and vinylarenes

We report a catalyst for intermolecular hydroamination of vinylarenes that is substantially more active for this process than catalysts published previously. With this more reactive catalyst, we demonstrate that additions of amines to vinylarenes and dienes occur in the presence of potentially reactive functional groups, such as ketones with enolizable hydrogens, free alcohols, free carboxylic acids, free amides, nitriles, and esters. The catalyst for these reactions is generated from [Pd(eta(3)-allyl)Cl](2) (with or without added AgOTf) or [Pd(CH(3)CN)(4)](BF(4))(2) and Xantphos (9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene), which generates complexes with large P-Pd-P bite angles. Studies on the rate of the C-N bond-forming step that occurs by attack of amine on an eta(3)-phenethyl and an eta(3)-allyl complex were conducted to determine the effect of the bite angle on the rate of this nucleophilic attack. Studies on model eta(3)-benzyl complexes containing various bisphosphines showed that the nucleophilic attack was faster for complexes containing larger P-Pd-P bite angles. Studies of substituted unsymmetrical and unsubstituted symmetrical model eta(3)-allyl complexes showed that nucleophilic attack on complexes ligated by Xantphos was faster than on complexes bearing ligands with smaller bite angles and that nucleophilic attack on unsymmetrical allyl complexes with larger bite angle ligands was faster than on unsymmetrical allyl complexes with smaller bite angle ligands. However, monitoring of catalytic reactions of dienes by (31)P NMR spectroscopy showed that the concentration of active catalyst was the major factor that controlled rates for reactions of symmetrical dienes catalyzed by complexes of phosphines with smaller bite angles. The identity of the counterion also affected the rate of attack: reactions of allylpalladium complexes with chloride counterion occurred faster than reactions of allylpalladium complexes with triflate or tetrafluoroborate counterion. As is often observed, the dynamics of the allyl and benzyl complexes also depended on the identity of the counterion.     

A comparative mechanistic analysis of the stereoselectivity trends observed in the oxidation of chiral oxazolidinone-functionalized enecarbamates by singlet oxygen, ozone, and triazolinedione

The stereoselectivity in the reactions of the E/Z enecarbamates 1, equipped with the oxazolidinone chiral auxiliary, has been examined for singlet oxygen (¹O₂), ozone (O₃), and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) in a variety of solvents as a function of temperature. The oxidative cleavage of the alkenyl functionality by ¹O₂ and O₃ releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (% ee) as well as the sense (R vs S) of the stereoselectivity in the MDB formation depends on the electronic nature of the oxidant. A high stereoselectivity, substantially dependent on solvent and temperature, is displayed for the reactions with ¹O₂, whereas the ground-state reactants O₃ and PTAD are rather unaffected by solvent and temperature variations. The present comparative analysis clearly substantiates our hypothesis of stereoselective vibrational quenching of the attacking ¹O₂, whereas O₃ and PTAD are only subject to steric impositions. The electronically excited ¹O₂ is sensitive to all three stereochemically relevant structural characteristics embodied in the chiral enecarbamates, namely the R/S configuration at the C₄ position of the oxazolidinone chiral auxiliary, the Z/E geometry of the ‘alkene’ functionality, and R/S configuration at the C_3′ position of the enecarbamate side chain.     

1H, 13C, and 73Ge NMR spectral analysis of substituted aryltrimethylgermanes

NMR chemical shifts of 1H, 13C, and 73Ge are reported for a series of monosubstituted aromatic trimethylgermanes of the type XC6H4Ge(CH3)3; X = p-N(CH3)2, p-OCH3, p-OC2H5, p-C(CH3)3, p-Si(CH3)3, p-Ge(CH3)3, p-Sn(CH3)3, p-CH3, m-CH3, -H, m-OCH3, p-Cl, p-Br, m-F, m-CF3, p-CF3, o-OCH3, and o-CH3. The relatively narrow 73Ge resonances show a strong correlation with Hammett sigma constants, with a correlation coefficient of 0.976 and 0.876 for 73Ge chemical shifts in meta- and para-substituted derivatives, respectively. The 13C chemical shifts of the methyl carbons bonded to germanium also display a relationship, with correlation coefficients of 0.904, 0.993, and 0.911 for para-, meta- and all derivatives, respectively. Comparisons of the Hammett plots for the homologous series XC6H4M(CH3)3; M = C, Si, Ge, Sn, show that, in general, correlation coefficients decrease while slopes increase significantly down the group, presumably reflecting the corresponding increase in chemical shift range of the group 14 atom. The Hammett constant derived for the p-Ge(CH3)3 group of +0.13 compares with the NMR-derived constants of -0.12 for p-C(CH3)3, +0.14 for p-Si(CH3)3, and -0.14 for p-Sn(CH3)3. The indication of electron release by carbon and tin can be rationalized through traditional hyperconjugative arguments for carbon and by the low electronegativity and consequent inductive effect of tin. The small electron attraction suggested by the positive constants for silicon and germanium can be simply, and perhaps naively, attributed to pi-acceptor interactions with the benzene ring.