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Experimental Ones (experimental + ones)
Selected AbstractsHydrogen Balmer Spectrum from a High-Pressure Arc Discharge: RevisitedCONTRIBUTIONS TO PLASMA PHYSICS, Issue 4-5 2007B. Omar Abstract The interpretation of hydrogen Balmer spectra emitted from a high-pressure arc discharge (Radtke and Günther, Contrib. Plasma Phys. 26, 143 (1986)) is re-examined. Assuming local thermodynamic equilibrium, synthetic Balmer spectra are calculated for given temperature and density conditions. Radiation transport is accounted for using a one-dimensional plasma layer model. The lineshape of bound-bound transitions is determined using a microscopic quantum-statistical approach. Free-free and free-bound contributions are added by taking expressions from literature. Comparing the synthetic spectra with experimental ones, plasma temperature and density conditions are inferred. The plasma parameters are confronted with theoretical results for the compositions of dense plasma. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Syntheses and spectroscopic investigation of some cyclophosphazanes: Analysis of pseudo-triplet splittingHETEROATOM CHEMISTRY, Issue 4 2006Khodayar Gholivand Some new phosphoramidates, 1,3, and the corresponding cyclophosphazanes, 4,6, with formula Cl2P(p -NHC6H4CH3) 1, Cl2P(O)(p -NHC6H4NO2) 2, (CH3)2NP(O)Cl(p -NHC6H4CH3)3, [ClP(p -NC6H4CH3)]24, [ClP(O)(p -NC6H4NO2)]25, and [(CH3)2NP(O)(p -NC6H4CH3)]26 were synthesized and characterized by 1H, 13C, 31P NMR, IR, mass spectroscopy, and elemental analysis. A pseudo-triplet signal was observed in the 1H NMR spectrum of molecule 6 for the N(CH3)2 protons. The A6A, 6X2 spin system was suggested for the pseudo-triplet pattern of 3JPNCH coupling in this molecule. Ab initio calculations were performed at the HF and B3LYP levels of theory with 6-311G** standard basis set on the geometry of compound 6. Also, the NMR chemical shift calculations were done to compare the computed results with the experimental ones. The calculated results are in good agreement with experimental data. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:337,343, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/hc.20229 [source] Hybrid modelling for simulation of scour and flow patterns in laboratory flumesINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN FLUIDS, Issue 3 2010Aytac Guven Abstract In this study, a hybrid numerical,mathematical model is developed, which simulates the temporal evolution of local scour and flow patterns in laboratory flumes. The mathematical module calculates the shape of the scour hole by solving an integro-parabolic equation. The numerical module simulates the two-dimensional laterally averaged flow patterns in the corresponding scour hole. The hybrid model is verified by experimental data taken from the literature. The hybrid model predicted the temporal evolution of local scour and simulated the flow patterns in and around the scoured zone in good agreement with the experimental ones. The proposed hybrid model proved to be efficient when used in complex local scour and turbulent flow problems. Copyright © 2009 John Wiley & Sons, Ltd. [source] Substituent effects on O,H and S,H bond dissociation enthalpies of disubstituted phenols and thiophenolsINTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 4 2008Daniel J. V. A. dos Santos Abstract The O,H and S,H homolytic bond dissociation enthalpies of a set of disubstituted phenols and thiophenols (NH2, OH, CH3, Cl, CF3, and NO2) have been computed by a density functional theory procedure with the 6-311++G(d,p) basis set. A very good agreement between our results and available experimental ones is observed. The effect of substituents on structure, charges and BDEs are investigated and their correlation with Hammett parameters is studied. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008 [source] An efficient method for analyzing nonuniformly coupled microstrip linesINTERNATIONAL JOURNAL OF RF AND MICROWAVE COMPUTER-AIDED ENGINEERING, Issue 2 2005Dengpeng Chen Abstract This article presents an efficient method for analyzing nonuniformly coupled microstrip lines. By choosing a modal-transformation matrix, the coupled nonlinear differential equations describing the symmetric nonuniformly coupled microstrip lines are decoupled using even- and odd-mode parameters; the original problem is thus transformed into two single nonuniform transmission lines. A power-law function of arbitrary order and having two adjustable parameters is chosen to better approximate the equation coefficients. Closed-form ABCD matrix solutions are obtained and used to calculate the S -parameters of nonuniformly coupled microstrip lines. Numerical results for two examples are compared with those from a full-wave commercial package and experimental ones in the literature in order to demonstrate the accuracy and efficiency of this method. This highly efficient method is employed to optimize a cosine-shape 10-dB codirectional coupler, which has good return loss and high directivity performance over a wide frequency range. © 2005 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2005. [source] Crystal structure prediction of organic pigments: quinacridone as an exampleJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 1 2007N. Panina The structures of the ,, , and , polymorphs of quinacridone (Pigment Violet 19) were predicted using Polymorph Predictor software in combination with X-ray powder diffraction patterns of limited quality. After generation and energy minimization of the possible structures, their powder patterns were compared with the experimental ones. On this basis, candidate structures for the polymorphs were chosen from the list of all structures. Rietveld refinement was used to validate the choice of structures. The predicted structure of the , polymorph is in accordance with the experimental structure published previously. Three possible structures for the , polymorph are proposed on the basis of X-ray powder patterns comparison. It is shown that the , structure in the Cambridge Structural Database is likely to be in error, and a new , structure is proposed. The present work demonstrates a method to obtain crystal structures of industrially important pigments when only a low-quality X-ray powder diffraction pattern is available. [source] Theoretical versus experimental geometries in S-bridged manganese carbonyl complexesJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 4 2003Juan F. Van der Maelen Uría The experimental geometry obtained from single-crystal X-ray diffraction for a number of binuclear S-bridged manganese complexes is compared with the results of theoretical calculations made at the ab initio level by using Hartree,Fock and density functional theory methods with medium-size and large basis sets. The optimized geometries obtained were somewhat relaxed when compared with the experimental ones, with very similar bond and torsion angles but longer bond lengths. The mean square deviation for bond lengths (angles) was found to be between 0.046,Ĺ (1.1°) and 0.004,Ĺ (0.7°) depending on the theoretical model used. [source] Newly developed basis sets for density functional calculationsJOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2005S. Chiodo Abstract Optimized contracted Gaussian basis sets of double-zeta valence polarized (DZVP) quality for first-row transition metals are presented. The DZVP functions were optimized using the PWP86 generalized gradient approximation (GGA) functional and the B3LYP hybrid functional. For a careful analysis of the basis sets performance the transition metal atoms and cations excitation energies were calculated and compared with the experimental ones. The calculated values were also compared with those obtained using the previously available DZVP basis sets developed at the local-density functional level. Because the new basis sets work better than the previous ones, possible reasons of this behavior are analyzed. The newly developed basis sets also provide a good estimation of other atomic properties such as ionization energies. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 175,183, 2005 [source] CONCENTRATION BOUNDARY CONDITIONS IN THE THEORETICAL ANALYSIS OF CONVECTIVE DRYING PROCESSJOURNAL OF FOOD PROCESS ENGINEERING, Issue 5 2007AHMET KAYA ABSTRACT In the theoretical analysis of convective drying process, two boundary conditions are common for concentration: constant concentration and convection. In this study, these two boundary conditions were comparatively examined by comparing theoretical results obtained with regard to experimental ones. Pumpkin (Cucurbita pepo) was considered as the product to be dried while air was the drying medium. The drying characteristics of pumpkin were determined for various values of drying air parameters, including temperature, velocity and relative humidity. Sorption isotherms of the dried pumpkin were also determined for different temperatures and water activities. The values of the effective moisture diffusivity, Deff, and the convective mass transfer coefficient, hm, were predicted, and these values were found to agree fairly well with those available in the existing literature. PRACTICAL APPLICATIONS Convective drying as well as other drying techniques are used in order to preserve and store agricultural products for longer periods by removing some of their moisture content. Drying is a complicated process involving simultaneous heat and mass transfer under transient conditions. Understanding the heat and mass transfer in the product will help to improve drying process parameters and hence the quality. [source] Modeling and simulation of polymeric nanocapsule formation by emulsion diffusion methodAICHE JOURNAL, Issue 8 2009M. Hassou Abstract The objective of this work is to develop a predictive dynamical model of a nanoencapsulation process using an emulsion diffusion method. This model describes the formation of the polymeric membrane around the oil droplet and its size reduction due to the solvent diffusion. To this end, we assume that the phase separation is only due to purely diffusive mechanism. This work is illustrated on the formation of poly-,-caprolactone (PCL) around the oily core formed of labrafac from the initial homogeneous polymer-solvent-nonsolvent solution (PCL, ethyl acetate, and labrafac). The polymeric membrane formation, the size reduction of the nanocapsule after the solvent diffusion, and the morphology of nanocapsules are experimentally investigated. It is shown that the results obtained in simulation from the dynamical model are in agreement with the experimental ones. The model is then used to predict the effect of the initial composition on the nanocapsule morphology. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source] Evaluation of maximal bite force in temporomandibular disorders patientsJOURNAL OF ORAL REHABILITATION, Issue 8 2006E. M. KOGAWA summary, The aim of this study was to evaluate the maximum bite force in temporomandibular disorders (TMD) patients. Two hundred women were equally divided into four groups: myogenic TMD, articular TMD, mixed TMD and control. The maximum bite force was measured in the first molar area, on both sides, in two sessions, using an IDDK (Kratos) Model digital dynamometer, adapted to oral conditions. Three-way anova, Tukey and Pearson correlation tests were used for the statistical analysis. The level of statistical significance was given when P , 0·05. The maximal bite force values were significantly higher in the control group than in the experimental ones (P = 0·00), with no significant differences between sides. Higher values were obtained in the second session (P = 0·001). Indeed, moderate negative correlation was found between age and bite force, when articular, mixed groups and all groups together were evaluated. A moderate negative correlation was also detected between TMD severity and the maximal bite force values for myogenic, mixed and all groups together. Authors concluded that the presence of masticatory muscle pain and/or TMJ inflammation can play a role in maximum bite force. The mechanisms involved in this process, however, are not well understood and deserve further investigation. [source] Structural and vibrational study of 2-(2,- furyl)-4,5-1H -dihydroimidazoleJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 12 2009Juan Zinczuk Abstract In this study 2-(2,-furyl)-4,5-1H -dihydroimidazole (1) was prepared and then characterized by infrared, Raman, and multidimensional nuclear magnetic resonance (NMR) spectroscopies. The crystal and molecular structures of 1 were determined by X-ray diffraction methods. The density functional theory (DFT) and second-order Mřller,Plesset theory (MP2) with Pople's basis set show that there are two conformers for the title molecule that have been theoretically determined in the gas phase, and that only one of them, conformer I, is present in the solid phase. NMR spectra observed for 1 were successfully compared with the calculated chemical shifts at the B3LYP/6-311++G** level theorized for this conformer. The harmonic vibrational frequencies for the optimized geometry of the latter conformer were calculated at the B3LYP/6-311++G** level in the approximation of the isolated molecule. For a complete assignment of the IR and Raman spectra in the solid phase of 1, DFT calculations were combined with Pulay´s scaled quantum mechanics force field (SQMFF) methodology to fit the theoretical frequency values to the experimental ones. Copyright © 2009 John Wiley & Sons, Ltd. [source] Enthalpies of formation and isomerization of cis - and trans -decalin, and their oxa-analogs by G3(MP2)//B3LYP calculationsJOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 8 2009Esko Taskinen Abstract G3(MP2)//B3LYP calculations have been carried out on trans - and cis -decalin, and their mono-, di-, tri-, and tetraoxa-analogs. The main purpose of the work was to obtain enthalpies of formation for these compounds, and to study the relative stabilities of the cis,trans and positional isomers of the various (poly)oxadecalins. Comparison of the computational enthalpies of formation with the respective experimental ones, known only for the decalins and 1,3,5,7-tetraoxadecalins, shows that in both cases the computational values are more negative than the experimental ones, the deviations being ,5 to ,7,kJ,mol,1 for the decalins and ,12 to ,17,kJ,mol,1 for the 1,3,5,7-tetraoxadecalins. The respective computational enthalpies of cis,trans isomerization, however, are in excellent to satisfactory agreement with the experimental data. The cis,trans enthalpy differences vary from +11.0,kJ,mol,1 for decalin to ,15.4,kJ,mol,1 for 1,4,5,8-tetraoxadecalin. Low relative enthalpy values were also calculated for the cis isomers of 1,8-dioxadecalin (,3.7,kJ,mol,1), 1,3,6-trioxadecalin (,4.6,kJ,mol,1), 1,3,8-trioxadecalin (,9.7,kJ,mol,1), 1,4,5- trioxadecalin (,5.6,kJ,mol,1), 1,3,5,8-tetraoxadecalin (,7.3,kJ,mol,1), and 1,3,6,8-tetraoxadecalin (,14.5,kJ,mol,1). Copyright © 2009 John Wiley & Sons, Ltd. [source] Improvement of rotational CARS thermometry in fuel-rich hydrocarbon flames by inclusion of N2 -H2 Raman line widthsJOURNAL OF RAMAN SPECTROSCOPY, Issue 7 2009Alexis Bohlin Abstract In rotational coherent anti-Stokes Raman spectroscopy (CARS) thermometry applied to air-fed flames, the temperature sensitivity mainly depends on the intensity distribution of the nitrogen spectral lines. Temperatures are estimated by numerical fitting of theoretical spectra to experimental ones, and one uncertainty in the calculation of theoretical CARS spectra for specific flame conditions is the accuracy in utilized line-broadening coefficients. In a previous article, self-broadened N2 -N2 line widths were considered in the spectral calculations as well as those of N2 -CO, N2 -CO2, N2 -H2O, and N2 -O2. In the present article, we also include N2 -H2 line widths calculated from a newly developed model, and it is shown that the evaluated temperature from flame spectra increases with increasing mole fractions of hydrogen. For example, in a very rich flame at , = 2.5, the use of available line-width data for all major species gives a temperature raise of 72 K at a temperature of ,1700 K, in comparison with using self-broadened N2 -N2 line widths only. Half of this temperature raise is related to the inclusion of N2 -H2 line widths. This article emphasizes the importance of using adequate line-broadening models for rotational CARS thermometry in flames. Copyright © 2009 John Wiley & Sons, Ltd. [source] Spectroscopic and computational studies on self-assembly complexes of bis(pyrrol-2- ylmethyleneamine) ligands linked by alkyl spacers with Cu(II)JOURNAL OF RAMAN SPECTROSCOPY, Issue 5 2007Wei Li Abstract Bis(pyrrol-2-ylmethyleneamine) ligands and their mononuclear monomeric and dinuclear dimeric self-assembly complexes with Cu(II) were investigated by means of IR and Raman spectroscopies and density functional theory. The ground-state geometries were calculated by using the BeckeLeeYangParr composite exchange-correlation functional (B3LYP) and a combined basis set (LanL2DZ for Cu; 6,31G(d) for C, H, N), and they were compared with the single-crystal X-ray diffraction (XRD) structures. The DFT-calculated CuN bond lengths are generally higher by 0.001,0.040 Ĺ than those determined through XRD. The vibrational spectra were also calculated at the same level of theory for the optimized geometries. The calculated wavenumbers were scaled by a uniform scaling factor and compared with the experimental fundamentals. The predicted spectra are in good agreement with the experimental ones with the deviations generally less than 30 cm,1. In comparison with the spectra of the ligands, the coordination effect shifts the ,(CN) wavenumber by about 50 cm,1 toward a lower value. Because of the weak intermolecular CH···Cu hydrogen bond, the CuN stretching mode is shifted toward a lower wavenumber. Copyright © 2006 John Wiley & Sons, Ltd. [source] The critical condition for intrusion of a steel ball into sliding contact spaceLUBRICATION SCIENCE, Issue 5 2010Fuxun Li Abstract The intrusion tests of a ball into sliding contact space were carried out. A stainless steel ball with 3.175,mm radius was used as a sphere particle to study the critical intrusion condition of the ball into the contact space between the organic glass and the polyurethane rubber. The effect of intrusion angle of the contact space and the friction coefficients among the ball, the organic glass and the polyurethane rubber on the critical condition were studied experimentally. Then, the intrusion conditions for the ball into the contact space were analysed theoretically. The results showed that the calculated critical intrusion angles agreed well with the experimental ones. Finally, the intrusion diagram as a function of critical intrusion angle, friction coefficient µ1 and µ2 (here, µ1 is the friction coefficient between the ball and the polyurethane rubber, µ2 is the friction coefficient between the ball and the organic glass) was introduced to evaluate the intrusion performance of a ball into the sliding contact space. Copyright © 2010 John Wiley & Sons, Ltd. [source] QSAR Models for the Dermal Penetration of Polycyclic Aromatic Hydrocarbons Based on Gene Expression ProgrammingMOLECULAR INFORMATICS, Issue 7 2008Tao Wang Abstract Gene Expression Programming (GEP) is a novel machine learning technique. The GEP is used to build nonlinear quantitative structure activity relationship model for the prediction of the Percent of Applied Dose Dermally Absorbed (PADA) over 24,h for polycyclic aromatic hydrocarbons. This model is based on descriptors which are calculated from the molecular structure. Three descriptors are selected from the descriptors pool by Heuristic Method (HM) to build a multivariable linear model. The GEP method produced a nonlinear quantitative model with a correlation coefficient and a mean error of 0.92 and 4.70 for the training set, 0.91 and 7.65 for the test set, respectively. It is shown that the GEP predicted results are in good agreement with experimental ones. [source] Excited Singlet (S1)-state Interactions of Nile Red with Aromatic Amines,PHOTOCHEMISTRY & PHOTOBIOLOGY, Issue 2 2003J. Mohanty ABSTRACT Both steady-state (SS) and time-resolved (TR) studies show that the fluorescence of the dye Nile red (NR) is quenched by various aromatic amines (ArA). Bimolecular quenching constants (kq) from both SS and TR measurements are observed to match well, indicating that the interaction is dynamic in nature. The quenching interaction in the present systems has been attributed to electron transfer (ET) from ArA to excited NR, based on the variations in the kq values with the oxidation potentials of the amines. The kq values calculated within the framework of Marcus' outer-sphere ET theory at different free-energy changes (,G0) of the ET reactions match well with the experimental ones, supporting the ET mechanism in the systems studied. The reorganization energy (,) estimated from the correlation of the experimental and the calculated kq values is quite similar to the solvent reorganization energy (,s), calculated on the basis of the solvent dielectric continuum model along with the assumption that the reactants are the effective spheres. Although a modest error is involved in this ,s calculation, the similarity in , and ,s values suggests that the solvent reorganization plays a dominant role in governing the ET dynamics in the present systems. [source] Mixing efficiency in a pin mixing section for single-screw extrudersPOLYMER ENGINEERING & SCIENCE, Issue 6 2001W. G. Yao Non-Newtonian, non-isothermal, 3D finite-element simulation of mixing performance in a pin mixing section with different axial gaps in the pins has been carried out according to their realistic configurations. The quantitative evaluation of mixing ability was based on the theory of kinematics of fluid mixing. To learn and to compare the local mixing performance in a standard screw and a pin mixing section, the local mixing efficiency distribution proposed by Ottino was calculated. Also, the RTDs of these mixers were calculated in an attemt to measure mixing. The integration of the two, namely, the integrating local mixing efficiency along a number of particle pathlines from entrance to exit, together with statistical treatment, which was referred as integral mixing efficiency, then gives a quantitative judgment of the total mixing ability of a continuous mixer. The calculated results showed a nonlinear dependence of the mixing ability of a pin mixing section on the axial gap of the pins. Finally, the calculation results were compared with the experimental ones obtained in our previous study. [source] Reactive Flow Model Parameter Estimation Using Genetic AlgorithmsPROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 3 2010Jose Baranda, Ribeiro Abstract An original real-coded genetic algorithm methodology that has been developed for the estimation of the parameters of the Tarver reactive flow model of shock initiation and detonation of heterogeneous solid explosives is described in detail. This methodology allows, in a single optimisation procedure and without the need for a starting solution, to search for the 15 parameters of the reaction rate law of the reactive flow model that fit the numerical results to the experimental ones. The developed methodology was applied and tested with an experimental situation, described in detail in the literature, involving the acceleration of a tantalum metal plate by an LX-17 explosive charge. The obtained parameters allow a very good description of the experimental results and are close to the ones originally used by Tarver and co-authors in their simulation of the phenomenon. [source] Calculation of the Energy of Explosives with a Partial Reaction Model.PROPELLANTS, EXPLOSIVES, PYROTECHNICS, Issue 1 2006Comparison with Cylinder Test Data Abstract The energy delivered by explosives is described by means of the useful expansion work along the isentrope of the detonation products. A thermodynamic code (W-DETCOM) is used, in which a partial reaction model has been implemented. In this model, the reacted fraction of the explosive in the detonation state is used as a fitting factor so that the calculated detonation velocity meets the experimental value. Calculations based on such a model have been carried out for a number of commercial explosives of ANFO and emulsion types. The BKW (Becker-Kistiakowsky-Wilson) equation of state is used for the detonation gases with the Sandia parameter set (BKWS). The energy delivered in the expansion (useful work) is calculated, and the values obtained are compared with the Gurney energies from cylinder test data at various expansion ratios. The expansion work values obtained are much more realistic than those from an ideal detonation calculation and, in most cases, the values predicted by the calculation are in good agreement with the experimental ones. [source] New model for the hydroxyapatite,octacalcium phosphate interfaceACTA CRYSTALLOGRAPHICA SECTION B, Issue 2 2003M. E. Fernández Some experimental results have indicated that hydroxyapatite (HA) and octacalcium phosphate (OCP) can form an epitaxic interface. Subsequently the OCP,HA interface has become of great biological interest in the context of mineralized tissue formation. In this work a new OCP,HA interface model based on Brown's proposed configuration [Brown (1962), Nature, 197, 1048,1050] and using the minimum interface free-energy optimization is presented. This new model is formed by half a unit cell of HA and one unit cell of OCP, as in Brown's model, but in our case [110] of HA is `glued' with [010] of OCP. Therefore, the relationship found was: [000]HA parallel to [001]OCP and [110]HA parallel to [010]OCP. Self-consistent field methods were used for the analysis of Brown's model and ours. It is shown that the atoms in our model have similar environments as in the HA and OCP unit cells and that, as a result of the differences between HA and OCP unit-cell parameters, this interface presents misfit-dislocation-like features. High-resolution transmission electron microscopy (HREM) simulated images for the new interface model have been included and, when they are compared with the experimental ones, the similarity is quite good. [source] Toward the Complete Prediction of the 1H and 13C NMR Spectra of Complex Organic Molecules by DFT Methods: Application to Natural SubstancesCHEMISTRY - A EUROPEAN JOURNAL, Issue 21 2006Alessandro Bagno Prof. Abstract The NMR parameters (1H and 13C chemical shifts and coupling constants) for a series of naturally occurring molecules have been calculated mostly with DFT methods, and their spectra compared with available experimental ones. The comparison includes strychnine as a test case, as well as some examples of recently isolated natural products (corianlactone, daphnipaxinin, boletunone B) featuring unusual and/or crowded structures and, in the case of boletunone B, being the subject of a recent revision. Whenever experimental spectra were obtained in polar solvents, the calculation of NMR parameters was also carried out with the Integral Equation-Formalism Polarizable Continuum Model (IEF-PCM) continuum method. The computed results generally show a good agreement with experiment, as judged not only by statistical parameters but also by visual comparison of line spectra. The origin of the remaining discrepancies is attributed to the incomplete modeling of conformational and specific solvent effects. [source] How Does the Trans,Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?CHEMPHYSCHEM, Issue 5 2010Giustiniano Tiberio Dr. Abstract The trans,cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion,inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans,cis photoisomerization quantum yield and decay times in various solvents (n -hexane, anisole, toluene, ethanol, and ethylene glycol), and obtain results comparable with experimental ones where available. A profound difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional,inversion pathway being the most important in solvents. [source] Gas-Phase Watson,Crick and Hoogsteen Isomers of the Nucleobase Mimic 9-Methyladenine,2-PyridoneCHEMPHYSCHEM, Issue 7 2006Jann A. Frey Abstract 2-Pyridone (pyridin-2-one) is a mimic of the uracil and thymine nucleobases, with only one NH and CO group. It provides a single H-bonding site, compared to three for the canonical pyrimidine nucleobases. Employing the supersonically cooled 9-methyladenine,2-pyridone (9MAd,2PY) complex, which is the simplest base pair to mimic adenine-uracil or adenine-thymine, we show that its gas-phase UV spectrum consists of contributions from two isomers. Based on the H-bonding sites of 9-methyladenine, these are the Watson,Crick and Hoogsteen forms. Combining two-color two-photon ionisation (2C-R2PI), UV-UV depletion and laser-induced fluorescence spectroscopies allows separation of the two band systems, revealing characteristic intermolecular in-plane vibrations of the two isomers. The calculated S0 and S1 intermolecular frequencies are in good agreement with the experimental ones. Ab initio calculations predict the Watson,Crick isomer to be slightly more stable (D0=,16.0 kcal,mol,1) than the Hoogsteen isomer (D0=,15.0 kcal,mol,1). The calculated free energies ,fG0 of the Watson,Crick and Hoogsteen isomers agree qualitatively with the experimental isomer concentration ratio of 3:1. [source] H/D exchange reactions;CHEMPHYSCHEM, Issue 6 2003sigma-bond metathesis; The mechanism of the H/D exchange reaction in alkane/hydrogen mixtures on silica-supported zirconium hydride was investigated by a modelling study using density functional theory (DFT) calculations. The electronic activation enthalpy (,H) for the CH bond activation step (TS3) was calculated to be around 92 kJ,mol,1, whereas it would be 258 kJ,mol,1for a direct exchange process (TS1, also called the kite TS). These data clearly speak in favour of the former as a mechanism for CH bond scrambling. Moreover, the calculated enthalpy of activation (,H) for H/D exchange in H2/D2mixtures (TS2) is 33.5 kJ,mol,1, which shows that this reaction is much faster than the H/D scrambling in alkane/H2mixtures, as shown experimentally. Additionally, the calculated activation entropies (For TS1,4,,S ranges between ,129 and ,174 J,mol,1,K,1) are very negative. Although the calculated activation entropies are also in full agreement with experimental data (,S=,113 J,mol,1,K,1), overall, the calculated activation enthalpies are much higher than the experimental ones. This suggests that the actual catalyst is probably more electrophilic than the model chosen for the calculations. [source] | ||||||||||||||||||||||||||||