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J. Phys (j + phy)
Selected AbstractsOptimal separation times for electrical field flow fractionation with Couette flowsELECTROPHORESIS, Issue 20 2008Jennifer Pascal Abstract The prediction of optimal times of separation as a function of the applied electrical field and cation valence have been studied for the case of field flow fractionation [Martin M., Giddings J. C., J. Phys. Chem. 1981, 85, 727] with charged solutes. These predictions can be very useful to a priori design or identify optimal operating conditions for a Couette-based device for field flow fractionation when the orthogonal field is an electrical field. Mathematically friendly relationships are obtained by applying the method of spatial averaging to the solute species continuity equation; this is accomplished after the role of the capillary geometrical dimensions on the applied electrical field equations has been assessed [Oyanader M. A., Arce P., Electrophoresis 2005; 26, 2857]. Moreover, explicit analytical expressions are derived for the effective parameters, i.e. diffusivity and convective velocity as functions of the applied (orthogonal) electrical field. These effective transport parameters are used to study the effect of the cation valence of the solutes and of the magnitude of the applied orthogonal electrical field on the values of the optimal time of separation. These parameters play a significant role in controlling the optimal separation time, leading to a family of minimum values, for particular magnitudes of the applied orthogonal electrical field. [source] Deformations caused by the movements of shear and tensile faultsINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 12 2001Guang Y. Sheu Abstract Earlier solutions of deformations resulting from the movements of shear and tensile faults in a half space (Bull. Seismol. Soc. Amer. 1985; 75:1135, 1992; 82:1018) have been revised in view of cross-anisotropic stress,strain relationships. The dislocation theory (Canad. J. Phys. 1958; 36:192) is reviewed and the displacement field due to a concentrated force in an anisotropic half space is solved analytically for developing the current research. A fault is simulated as a point source of strain nuclei in applying the dislocation theory. Data (Terr. Atmos. Oceanic Sci. 2000; 11(3):591, 631) that were used to study the Chi-Chi earthquake (ML=7.3; 1999/9/21 AM 1:47) are introduced to compare the solution with the isotropic results. Results indicate that the anisotropy of stress,strain relationships does affect the results of predicted deformations. Copyright © 2001 John Wiley & Sons, Ltd. [source] Shock tube study of 1,3,5-triazine dissociation and relaxation and relaxation of pyrazineINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2010Hui Xu The three-body dissociation of 1,3,5-triazine (s-triazine, s-C3H3N3 , 3HCN) has been observed in incident shock waves with the laser-schlieren technique. The experiments use 5% triazine/Kr and cover 1630,2350 K for 100,600 Torr. These experiments show dissociation rates with strong falloff and a slight but fully expected pressure dependence. The dissociation is without secondary reaction save for a possible, but rather unlikely, contribution from the isomerization HCN , HNC. Electronic structure calculations of the transition-state properties (G3B3, HL1, Eo = 84.6 kcal/mol) are used to construct a Rice,Ramsperger,Kassel,Marcus (RRKM) model whose fit to the rate measurements suggests a ,,E,down of 1200 cm,1. However, a seemingly better fit is achieved using the barrier of 81 kcal/mol proposed by Dyakov et al. (J. Phys. Chem. A 2007, 111, 9591,9599). With this barrier k, (s,1) = 5.3 × 1016 exp(,86.6(kcal/mol)/RT), and the fit now accepts the more routine ,,E,down = 126(T/298)0.9. It seems the dissociation most likely occurs by a direct, one-step, "triple" dissociation to 3HCN, although the present experiments cannot rule out a multistep process. Vibrational relaxation of the triazine was also examined in 5% and 20% mixtures with Kr over 770,1500 K for pressures between 6 and 14 Torr. Relaxation is very fast, with a slight inverse temperature dependence, P, rising from 100 to 200 ns-atm over the full temperature range. Integrated gradients are in good accord with calculated total changes in density, indicating a single exponential relaxation. A separate investigation of relaxation in the related molecule pyrazine (500,1300 K, in 1% and 5% in Kr, between 13 and 66 Torr) is included. Again relaxation is rapid, but here the temperature dependence seems more normal, the relaxation times decreasing slightly with temperature. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 211,220, 2010 [source] A practical valence bond method: A configuration interaction method approach with perturbation theoretic facilityJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 4 2004Lingchun Song Abstract The previously developed valence bond configuration interaction (VBCI) method (Wu, W.; Song, L.; Cao, Z.; Zhang, Q.; Shaik, S., J. Phys. Chem. A, 2002, 105, 2721) that borrows the general CI philosophy of the MO theory, is further extended in this article, and its methodological features are improved, resulting in three accurate and cost-effective procedures: (a) the effect of quadruplet excitation is incorporated using the Davidson correction, such that the new procedure reduces size consistency problems, with due improvement in the quality of the computational results. (b) A cost-effective procedure, named VBCI(D, S), is introduced. It includes doubly excited structures for active electrons and singly excited structures for inactive pairs. The computational results of VBCI(D, S) match those of VBCISD with much less computational effort than VBCISD. (c) Finally, a second-order perturbation theory is utilized as a means of configuration selection, and lead to considerable reduction of the computational cost, with little or no loss in accuracy. Applications of the new procedures to bond energies and barriers of chemical reactions are presented and discussed. © 2004 Wiley Periodicals, Inc. J Comput Chem 25: 472,478, 2004 [source] Vibrational analysis of Ni(II)- and Cu(II)-octamethylchlorin by polarized resonance Raman and Fourier transform infrared spectroscopyJOURNAL OF RAMAN SPECTROSCOPY, Issue 6-7 2001Robert J. Lipski We measured the polarized resonance Raman spectra of Cu(II)-2,2,7,8,12,13,17,18-octamethylchlorin in CS2 at various excitation wavenumbers in a spectral region covering the Qy, Qx and Bx optical absorption bands. Additionally, we measured the FTIR-Raman spectrum of the highly overcrowded spectral region between 1300 and 1450 cm,1. The spectral decomposition was carried out by a self-consistent global fit to all spectra obtained. The thus identified Raman and IR lines were assigned by comparison with the resonance Raman spectra of Cu(II)-octaethylporphyrin, by utilizing their depolarization ratio dispersions and by a normal mode analysis. The latter was based on a modified transferable molecular mechanics force field of Ni(II)-octaethylporphyrin [E. Unger, M. Beck, R.J. Lipski, W. Dreybrodt, C.J. Medforth, K.M. Smith and R. Schweitzer-Stenner, J. Phys. Chem. B103, 10229 (1999)]. A comparison of normal mode patterns obtained for Cu(II)-octamethylchlorin and Cu(II)-octaethylporphyrin revealed that some modes are significantly distorted by the reduction of the pyrrole ring, in accordance with results which Boldt et al. reported earlier for Ni(II)-octaethylchlorin [N.J. Boldt, F.J. Donohoe, R.R. Birge and D.F. Bocian, J. Am. Chem. Soc.109, 2284 (1987)]. In contrast to conclusions drawn from this study, however, the results of our vibrational analysis and several further lines of evidence suggest that the normal modes of corresponding chlorines and porphyrins are still comparable, because they display contributions from the same local coordinates. Thus, the classical normal mode classification developed for metalloporphyrins is also applicable to metallochlorins. Finally, we performed a preliminary analysis of the absorption spectrum and the resonance excitation profiles and depolarization ratio dispersions of some Raman lines. The results show that the electronic properties of Cu(II)-octamethylchlorin can still be described in terms of Gouterman's four orbital model [M. Gouterman, J. Chem. Phys.30, 1139 (1959)]. In regions of the Q bands, Raman scattering of A1 modes is determined by interferences between Franck, Condon coupling and interstate Herzberg, Teller coupling between Qx(Qy) and Bx(By) states. The B2 modes are resonance enhanced by Herzberg, Teller coupling between Qx and Qy and between Qx(Qy) and By(Bx). Franck, Condon coupling of A1 modes with large contributions from C,Cm stretching vibrations is comparatively strong for Qx. This is interpreted as reflecting the expansion of the chlorin macrocycle by an electronic transition into this excited state. Copyright © 2001 John Wiley & Sons, Ltd. [source] FTIR Spectroscopic and Theoretical Study of the Photochemistry of Matrix-isolated CoumarinPHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2007Nihal Ku The infrared spectrum of monomeric unsubstituted coumarin (C9H6O2; 2H -1-benzopyran-2-one), isolated in solid argon at 10 K is presented and assigned. The UV-induced (, > 200 nm) unimolecular photochemistry of the matrix-isolated compound was studied experimentally. Three main photoreactions were observed: (a) decarboxylation of the compound and formation of benzocyclobutadiene and CO2, with the Dewar form of coumarin as intermediate; (b) isomerization of the compound, leading to production of a conjugated ketene; and (c) decarbonylation, leading to formation of CO and benzofuran complex. Further decomposition of benzofuran to produce ethynol is suggested. Photochannels (a) and (b) correspond to those previously observed for matrix-isolated ,- pyrone and its sulfur analogs (Phys. Chem. Chem. Phys. 2004, 6, 929; J. Phys. Chem. A 2006, 110, 6415), while route (c) is similar to the UV-induced photochemistry of coumarin in the gaseous phase (J. Phys. Chem. A 2000, 104, 1095). Interpretation of the experimental data is supported by extensive calculations performed at the B3LYP/6-311++G(d,p), MP2/6-31G(d,p) and MP2/6-311++G(d,p) levels. [source] The Complex of Apomyoglobin with the Fluorescent Dye Coumarin 153,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 5 2004P. K. Chowdhury ABSTRACT Understanding a protein's dielectric response requires both a theoretical model and a well-defined experimental system. The former has already been proposed by Song (J. Chem. Phys. 116, 9359 [2002]). We suggest that the latter is provided by the complex of coumarin 153 (C153) with apomyoglobin (ApoMb). C153 has been exhaustively studied and has proven to be an excellent probe of the solvation dynamics of polar solvents. Myoglobin is one of the most thoroughly studied proteins. Myoglobins from a wide range of species have been subject to X-ray structural analysis and site-directed mutagenesis. Here, we demonstrate the existence of a robust C153-apomyglobin system by means of molecular dynamics simulations, equilibrium binding studies using a Job's plot and capillary electrophoresis, circular dichroism and time-resolved fluorescence. The reorganization energy of C153 bound to ApoMb is compared with that of C153 in bulk solvent using the method of Jordanides et al. (J. Phys. Chem. B 103, 7995 [1999]). [source] Is Cyclopropane Really the ,-Aromatic Paradigm?CHEMISTRY - A EUROPEAN JOURNAL, Issue 38 2009Wei Wu Prof. Abstract Dewar proposed the ,-aromaticity concept to explain the seemingly anomalous energetic and magnetic behavior of cyclopropane in 1979. While a detailed, but indirect energetic evaluation in 1986 raised doubts,"There is no need to involve ,,-aromaticity',",other analyses, also indirect, resulted in wide-ranging estimates of the ,-aromatic stabilization energy. Moreover, the aromatic character of "in-plane", "double", and cyclically delocalized ,-electron systems now seems well established in many types of molecules. Nevertheless, the most recent analysis of the magnetic properties of cyclopropane (S. Pelloni, P. Lazzeretti, R. Zanasi, J. Phys. Chem. A2007, 111, 8163,8169) challenged the existence of an induced ,-ring current, and provided alternative explanations for the abnormal magnetic behavior. Likewise, the present study, which evaluates the ,-aromatic stabilization of cyclopropane directly for the first time, fails to find evidence for a significant energetic effect. According to ab initio valence bond (VB) computations at the VBSCF/cc-PVTZ level, the ,-aromatic stabilization energy of cyclopropane is, at most, 3.5,kcal,mol,1 relative to propane, and is close to zero when n -butane is used as reference. Trisilacyclopropane also has very little ,-aromatic stabilization, compared to Si3H8 (6.3,kcal,mol,1) and Si4H10 (4.2,kcal,mol,1). Alternative interpretations of the energetic behavior of cyclopropane (and of cyclobutane, as well as their silicon counterparts) are supported. [source] Voltammetric Manifestation of the Ultraslow Dynamics at the Interface between Water and an Ionic Liquid,CHEMPHYSCHEM, Issue 13 2010Prof. Takashi Kakiuchi Abstract The ultraslow relaxation (on the order of minutes) of the electrical double-layer structure, related to a change in the phase-boundary potential across the interface between water (W) and the ionic liquid (IL) trioctylmethylammonium bis(nonafluorobutanesufonyl)amide ([TOMA+][C4C4N,]) (Y. Yasui et al., J. Phys. Chem. B. 2009, 113, 3273), appears to be invisible in the transfer of tetrapropylammonium ions across the [TOMA+][C4C4N,]|W interface, provided that the charging current, which shows an unusual dependence on the voltage scan rate, is subtracted to obtain the faradaic current. This counterintuitive observation can be explained by the differences in the timescales of the fast and slow components of the relaxation dynamics of the electrical double layer on the IL side (ms and min). In contrast, the effect of the slow dynamics becomes surfaced in ion-transfer voltammetry when the ion is surface-active. The transfer of pentadecafluorooctanoate across the [TOMA+][C4C4N,]|W interface is irreversible, which is attributable to the self-inhibition of pentadecafluorooctanoate ions transferred to the IL phase. This process is likely to be affected by the ultraslow structural change of the IL side of the interface. [source] Photoelectrochemical Behavior of Nanostructured WO3 Thin-Film Electrodes: The Oxidation of Formic AcidCHEMPHYSCHEM, Issue 12 2006Damián Monllor-Satoca Abstract Nanostructured tungsten trioxide thin-film electrodes are prepared on conducting glass substrates by either potentiostatic electrodeposition from aqueous solutions of peroxotungstic acid or direct deposition of WO3 slurries. Once treated thermally in air at 450,°C, the electrodes are found to be composed of monoclinic WO3 grains with a particle size around 30,40 nm. The photoelectrochemical behavior of these electrodes in 1,M HClO4 apparently reveals a low degree of electron,hole recombination. Upon addition of formic acid, the electrode showed the current multiplication phenomenon together with a shift of the photocurrent onset potential toward less positive values. Photoelectrochemical experiments devised on the basis of a kinetic model reported recently [I. Mora-Seró, T. Lana-Villarreal, J. Bisquert, A. Pitarch, R. Gómez, P. Salvador, J. Phys. Chem. B2005, 109, 3371] showed that an interfacial mechanism of inelastic, direct hole transfer takes place in the photooxidation of formic acid. This behavior is attributed to the tendency of formic acid molecules to be specifically adsorbed on the WO3 nanoparticles, as evidenced by attenuated total reflection infrared spectroscopy. [source] | |||||||||||||