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Mechanical Methods (mechanical + methods)

Distribution by Scientific Domains
Chemistry64%
Medical Sciences11%
Earth and Environmental Science11%

Kinds of Mechanical Methods

  • molecular mechanical methods
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  • quantum mechanical methods
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    Selected Abstracts

    Enol Forms of 1,3-Indanedione, Their Stabilization by Strong Hydrogen Bonding, and Zwitterion-Assisted Interconversion

    EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 14 2010
    Mark Sigalov
    Abstract By analyzing NMR spectroscopic data, and supported by IR, UV/Vis, Raman, dielectrometry, and DFT techniques, a comprehensive study of the 1:2 adducts of picolinaldehyde and 1,3-indanediones is presented. The parent indanedione derivative 5 exists in an equilibrium between all-keto and enol forms, the latter being stabilized by an intramolecularO,H···N hydrogen bond. Only the all-keto form was observed in the 5,6-dimethoxy compound 6, whereas solely the enol tautomer was observed with its 5,6-dichloro analogue 7. Polar solvents and low temperatures shift the equilibrium towards the enol tautomer in 5. The structure of adduct 8, formed with isonicotinaldehyde, prevents the formation of intramolecular O,H···N hydrogen bonds and thus it exists in the all-keto form in low polar solvents. However, in DMSO solutions it adopts a zwitterionic form with a strong anionic O,···H···O hydrogen bond. Thus, the enol form in indanedione adducts was unequivocally characterized in solution and the factors that determine the keto,enol tautomerism, namely electronic effects, solvent, temperature, and intramolecular hydrogen bonds, have been methodically studied by spectroscopic and quantum mechanical methods. [source]

    Electrodynamic Disaggregation: Does it Affect Apatite Fission-Track and (U-Th)/He Analyses?

    GEOSTANDARDS & GEOANALYTICAL RESEARCH, Issue 1 2010
    Jörg Giese
    désagrégation électrodynamique; analyse des traces de fission sur apatite; recuit; analyse (U-Th)/He; diffusion de l'He Apatite fission-track and (U-Th)/He analyses require the liberation of intact idiomorphic apatite grains from rock samples. While routinely being carried out by mechanical methods, electrodynamic disaggregation (ED) offers an alternative approach. The high-voltage discharges produced during the ED process create localised temperature peaks (10000 K) along a narrow plasma channel. In apatite, such high temperatures could potentially reduce the length of fission tracks, which start to anneal at temperatures > 60 °C, and could also enhance He diffusion, which becomes significant at 30,40 °C over geological time scales. A comparison of fission-track analyses and (U-Th)/He ages of apatites prepared both by mechanical (jaw crusher, disk mill) and ED processing provides a way of determining whether heating during the latter method has any significant effect. Apatites from three samples of different geological settings (an orthogneiss from Madagascar, the Fish Canyon Tuff, and a muscovite-gneiss from Greece) yielded statistically identical track length distributions compared to samples prepared mechanically. Additionally, (U-Th)/He ages of apatites from a leucogranite from Morocco prepared by both methods were indistinguishable. These first results indicated that during electrodynamic disaggregation apatite crystals were not heated enough to partially anneal the fission tracks or induce significant diffusive loss of He. Les analyses des traces de fission et des rapports isotopiques (U-Th)/He sur apatite nécessitent la séparation de grains intacts et automorphes d'apatite à partir des échantillons de roche. La désagrégation électrodynamique (DE) offre une approche alternative aux méthodes mécaniques utilisées actuellement en routine. Les décharges de haute tension produite pendant le processus de DE entrainent la formation de pics de température (10000 K) localisés le long d'un étroit canal de plasma. Dans l'apatite, de telles températures peuvent potentiellement réduire la longueur des traces de fission, qui commencent à recuire à des températures > 60 °C, et peuvent aussi favoriser la diffusion de l'Hélium, qui devient significative à des températures de 30,40 °C sur des échelles de temps géologiques. Une comparaison des résultats obtenus à partir des analyses des traces de fission et des âges (U-Th)/He sur des apatites séparées par des moyens mécaniques (concasseur à mâchoires, broyeur à disque) et sur d'autres séparées par la méthode de désagrégation électrodynamique offre un moyen de déterminer si le chauffage lié,à la seconde méthode a un effet significatif. Les apatites séparées par la méthode DE à partir de trois échantillons provenant de différents contextes géologiques (un orthogneiss de Madagascar, le tuf de Fish Canyon et un gneiss à muscovite de Grèce) fournissent des distributions de longueurs de trace de fission statistiquement identiques par rapport à des échantillons préparés mécaniquement. En outre, des âges (U-Th)/He obtenus à partir d'apatites provenant d'un leucogranite du Maroc et préparées par les deux méthodes (DE et mécaniques) sont indiscernables. Ces premiers résultats indiquent que, pendant la désagrégation électrodynamique, les cristaux d'apatite n'ont pas été suffisamment chauffés pour soit recuire partiellement les traces de fission soit provoquer une perte significative par diffusion de l'He. [source]

    Energy decomposition scheme for combined ab initio quantum mechanical / molecular mechanical methods

    INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY, Issue 3 2005
    Imre Berente
    Abstract A new energy decomposition scheme is presented which paves the way toward the accurate and simple treatment of boundary atoms in combined ab initio quantum mechanical / molecular mechanical methods. We extend the wave function beyond the quantum region to a few atoms of the molecular mechanical region, which are linked directly to boundary atoms. Furthermore, we apply an approximate decomposition scheme, which allows calculating the total energy in terms of one-center atomic contributions. Comparisons with reference ab initio calculations are made, and good agreement is obtained for geometry parameters referring to CC, CC, and CX (XO, S, N) bonds at the boundary, as well as for the rotational energy curve of n -butane. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005 [source]

    Conceptual, Qualitative, and Quantitative Theories of 1,3-Dipolar and Diels,Alder Cycloadditions Used in Synthesis

    ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 16-17 2006
    Daniel
    Abstract The application and performance of conceptual and qualitative theories and quantitative quantum mechanical methods to the study of mechanism, reactivity, and selectivity of 1,3-dipolar and Diels,Alder cycloadditions are reviewed. This review emphasizes the application of conceptual density functional theory (DFT) for predicting reactivity and regioselectivity, and highly accurate quantum mechanical methods for predicting barrier heights and reaction energetics. Applications of computations to solvation effects, metal and organocatalysis, are also described. [source]

    Fermentation characteristics and microbial growth promoted by diets including two-phase olive cake in continuous fermenters

    JOURNAL OF ANIMAL PHYSIOLOGY AND NUTRITION, Issue 1 2008
    A. Moumen
    Summary Two-phase olive cake (2POC) is the by-product obtained from the so called ,two-phase' procedure to extract olive oil by mechanical methods. After the mechanical extraction the 2POC is dried and most of the remaining oil extracted by chemical means. The production of the crude by-product may reach more than 4 millions t/year in Spain (MAPA, 2003), most of it in areas (Southeast) with shortage of pastures and conventional feeds for ruminants. Six continuous fermenters, inoculated with ruminal liquor from wethers or goats, were fed lucerne hay (LH); LH plus a concentrate including dried two-phase olive cake (LHCO) and; diet LHCO added with polyethylene glycol (LHCOP). The highest pH values and ammonia N (NH3 -N) output were found in fermenters fed diet LH (6.19 and 6.35 for pH, and 53.7 and 68.9 mg NH3N/day, respectively, in fermentes inoculated with rumen liquor from sheep and goats) without differences due to the inoculum origin. The digestibility of carbohydrates (CHO) was affected (p < 0.001) by inoculum (67.0 and 58.8%, respectively, for goats and wethers) the lowest values being for diet LHCO (53.2 and 57.0% with inoculum from wethers and goats, respectively). The main volatile fatty acid (VFA) was acetic acid with higher (p < 0.01) values in fermenters with inoculum from goats than from wethers (80.2 and 63.0 mmol/day respectively). The efficiency of bacterial protein synthesis (EBS) was not different (p > 0.05) with inoculum from wethers and goats [26.4 and 28.1 and 35.2 and 33.5 g bacterial N/kg digested CHO, respectively, obtained by using diamino pimelic acid (DAPA) and purine bases (PB) as microbial markers]. The lowest (p < 0.05) values were found in fermenters fed diets LHCOP and LH, estimated, respectively, from DAPA and PB (21.9 and 29.0 g bacterial N/kg digested CHO). The substitution of a part of lucerne hay by a concentrate including dried 2POC does not seem to cause important differences in efficiency of VFA production. Results concerning bacterial protein synthesis are not so clear since values estimated from DAPA and PB did not show similar trends. Neither inoculum origin nor PEG had important effects on fermentation characteristics. The DAPA could be an adequate marker in continuous fermenters, with comparative purposes, as protozoa are not present and, estimated protein synthesis values are similar to those obtained in vivo with similar diets using urinary excretion of PD (Yáñez Ruiz et al., 2004b). Further research is needed to state the optimal proportion of 2POC in practical diets for ruminants at both maintenance and production stages. [source]

    Comparison of basis set effects and the performance of ab initio and DFT methods for probing equilibrium fluctuations

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 2 2007
    Ross C. Walker
    Abstract The electronic absorption and emission spectra of large molecules reflect the extent and timescale of electron-vibration coupling and therefore the extent and timescale of relaxation/reorganization in response to a perturbation. In this paper, we present a comparison of the calculated absorption and emission spectra of NADH in liver alcohol dehydrogenase (LADH), using quantum mechanical/molecular mechanical methods, in which we vary the QM component. Specifically, we have looked at the influence of basis set (STO-3G, 3-21G*, 6-31G*, CC-pVDZ, and 6-311G**), as well as the influence of applying the DFT TD-B3LYP and ab initio TD-HF and CIS methods to the calculation of absorption/emission spectra and the reorganization energy (Stokes shift). The ab initio TD-HF and CIS methods reproduce the experimentally determined Stokes shift and spectral profiles to a high level of agreement, while the TD-B3LYP method significantly underestimates the Stokes shift, by 45%. We comment on the origin of this problem and suggest that ab initio methods may be naturally more suited to predicting molecular behavior away from equilibrium geometries. © 2006 Wiley Periodicals, Inc. J Comput Chem 28: 478,490, 2007 [source]

    Charge distribution in the water molecule,A comparison of methods

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 1 2005
    F. Martin
    Abstract The charge distribution in the water molecule has been analyzed using a broad variety of basis sets, four different quantum mechanical methods (Hartree,Fock, Becke3LYP, MP2, and QCISD), and six population analysis methods (Mulliken, NPA, AIM, CHELPG, Merz,Kollman, and Resp). The influence of the molecular structure on the calculated atomic charges has been studied using small perturbations of the experimentally determined structure. © 2004 Wiley Periodicals, Inc. J Comput Chem 26: 97,105, 2005 [source]

    Comparison of linear-scaling semiempirical methods and combined quantum mechanical/molecular mechanical methods for enzymic reactions.

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 14 2002

    Abstract QM/MM methods have been developed as a computationally feasible solution to QM simulation of chemical processes, such as enzyme-catalyzed reactions, within a more approximate MM representation of the condensed-phase environment. However, there has been no independent method for checking the quality of this representation, especially for highly nonisotropic protein environments such as those surrounding enzyme active sites. Hence, the validity of QM/MM methods is largely untested. Here we use the possibility of performing all-QM calculations at the semiempirical PM3 level with a linear-scaling method (MOZYME) to assess the performance of a QM/MM method (PM3/AMBER94 force field). Using two model pathways for the hydride-ion transfer reaction of the enzyme dihydrofolate reductase studied previously (Titmuss et al., Chem Phys Lett 2000, 320, 169,176), we have analyzed the reaction energy contributions (QM, QM/MM, and MM) from the QM/MM results and compared them with analogous-region components calculated via an energy partitioning scheme implemented into MOZYME. This analysis further divided the MOZYME components into Coulomb, resonance and exchange energy terms. For the model in which the MM coordinates are kept fixed during the reaction, we find that the MOZYME and QM/MM total energy profiles agree very well, but that there are significant differences in the energy components. Most significantly there is a large change (,16 kcal/mol) in the MOZYME MM component due to polarization of the MM region surrounding the active site, and which arises mostly from MM atoms close to (<10 Å) the active-site QM region, which is not modelled explicitly by our QM/MM method. However, for the model where the MM coordinates are allowed to vary during the reaction, we find large differences in the MOZYME and QM/MM total energy profiles, with a discrepancy of 52 kcal/mol between the relative reaction (product,reactant) energies. This is largely due to a difference in the MM energies of 58 kcal/mol, of which we can attribute ,40 kcal/mol to geometry effects in the MM region and the remainder, as before, to MM region polarization. Contrary to the fixed-geometry model, there is no correlation of the MM energy changes with distance from the QM region, nor are they contributed by only a few residues. Overall, the results suggest that merely extending the size of the QM region in the QM/MM calculation is not a universal solution to the MOZYME- and QM/MM-method differences. They also suggest that attaching physical significance to MOZYME Coulomb, resonance and exchange components is problematic. Although we conclude that it would be possible to reparameterize the QM/MM force field to reproduce MOZYME energies, a better way to account for both the effects of the protein environment and known deficiencies in semiempirical methods would be to parameterize the force field based on data from DFT or ab initio QM linear-scaling calculations. Such a force field could be used efficiently in MD simulations to calculate free energies. © 2002 Wiley Periodicals, Inc. J Comput Chem 23: 1314,1322, 2002 [source]

    Direct hydride transfer in the reaction mechanism of quinoprotein alcohol dehydrogenases: a quantum mechanical investigation

    JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2001
    A. Jongejan
    Abstract Oxidation of alcohols by direct hydride transfer to the pyrroloquinoline quinone (PQQ) cofactor of quinoprotein alcohol dehydrogenases has been studied using ab initio quantum mechanical methods. Energies and geometries were calculated at the 6-31G(d,p) level of theory. Comparison of the results obtained for PQQ and several derivatives with available structural and spectroscopic data served to judge the feasibility of the calculations. The role of calcium in the enzymatic reaction mechanism has been investigated. Transition state searches have been conducted at the semiempirical and STO-3G(d) level of theory. It is concluded that hydride transfer from the C,-position of the substrate alcohol (or aldehyde) directly to the C(5) carbon of PQQ is energetically feasible. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1732,1749, 2001 [source]

    VISCOSITY AND QUALITY OF TOMATO JUICE AS AFFECTED BY PROCESSING METHODS

    JOURNAL OF FOOD QUALITY, Issue 6 2007
    CHARANJIT KAUR
    ABSTRACT Quality and viscosity of tomato juice is strongly governed by mechanical and thermal abuse during processing. The effect of processing and storage duration on the viscosity and quality parameters of tomato juice was evaluated in the present work. Tomato juice was obtained by two different methods, normal method (NM) and variable method (VM), and was heated by both conventional and microwave hot break treatments. Tomato juice was evaluated for various quality characteristics including precipitate weight ratio, degree of serum separation (DOSS), Brookfield viscosity, lycopene levels, vitamin C, phenols and antioxidant activity. Changes observed in several quality parameters during storage were statistically insignificant. The methods and processing had the greatest contribution toward viscosity, whereas storage duration had no significant effects. Viscosity parameters viz precipitate weight, DOSS and Bostwick consistency revealed that juice processed by VM was more viscous than that of NM, irrespective of the hot break processing used. Ascorbic acid and lycopene content varied significantly with mechanical methods and processing conditions. However, phenolic content and antioxidant activity remained stable. The variable process represents an improvement over the conventional by enabling tomato processors to improve the consistency of the tomato product. PRACTICAL APPLICATIONS Quartering or slicing tomatoes followed by hot break processing techniques can significantly improve the quality of tomato juice in terms of reduced degree of serum separation and precipitate weight ratio. The modified technology could be considered as a viable and economical method for small- and medium-scale tomato processors with limited resources and facilities at their disposal. [source]

    Carbon sequestration in soils of central Asia,

    LAND DEGRADATION AND DEVELOPMENT, Issue 6 2004
    R. Lal
    Abstract Problems of frequent drought stress, low soil organic carbon (SOC) concentration, low aggregation, susceptibility to compaction, salinization and accelerated soil erosion in dry regions are accentuated by removal of crop residues, mechanical methods of seedbed preparation, summer clean fallowing and overgrazing, and excessive irrigation. The attendant soil degradation and desertification lead to depletion of SOC, decline in biomass production, eutrophication/pollution of waters and emission of greenhouse gases. Adoption of conservation agriculture, based on the use of crop residue mulch and no till farming, can conserve water, reduce soil erosion, improve soil structure, enhance SOC concentration, and reduce the rate of enrichment of atmospheric CO2. The rate of SOC sequestration with conversion to conservation agriculture, elimination of summer fallowing and growing forages/cover crops may be 100 to 200,kg,ha,1,y,1 in coarse-textured soils of semiarid regions and 150 to 300,kg,ha,1,y,1 in heavy-textured soils of the subhumid regions. The potential of soil C sequestration in central Asia is 10 to 22,Tg,C,y,1 (16±8,Tg,C,y,1) for about 50 years, and it represents 20,per,cent of the CO2 emissions by fossil fuel combustion. Copyright © 2004 John Wiley & Sons, Ltd. [source]

    Crazing and Fracture in Polymers: Micro-Mechanisms and Effect of Molecular Variables

    MACROMOLECULAR SYMPOSIA, Issue 1 2004
    H.H. Kausch
    Abstract The influence of the primary molecular parameters chain configuration, architecture and molecular weight (MW) on the mode of mechanical breakdown is discussed for two series of (amorphous) thermoplastic polymers, methyl methacrylate glutarimide copolymers and amorphous semi-aromatic polyamides. Structural and dynamic analyses and fracture mechanical methods applied to such adequately chemically modified (glassy) polymers permit us to show and to explain the effect of intrinsic variables on local molecular motions and on the competition between chain scission, disentanglement and segmental slip, which in turn determine the dominant mode of instability and plastic behaviour. Above a critical molecular weight, toughness depends most strongly on the entanglement density; a positive effect of the intensity of sub-Tg relaxations and in-chain cooperative motions on the toughness of these materials is clearly evident. [source]

    Renaissance in chiroptical spectroscopic methods for molecular structure determination

    THE CHEMICAL RECORD, Issue 2 2007
    Prasad L. Polavarapu
    Abstract Two of the chiroptical spectroscopic methods, namely optical rotatory dispersion (ORD) and electronic circular dichroism (ECD), have been around for several decades. But their use in determining the absolute configuration and predominant conformation is gaining renewed interest with the availability of quantum mechanical methods for predicting ORD and ECD. Two other methods, namely vibrational circular dichroism (VCD) and vibrational Raman optical activity (VROA), are relatively new and offer convenient approaches for deducing the structural information in chiral molecules. With the availability of quantum mechanical programs for predicting VCD and VROA, these methods have attracted numerous new researchers to this area. This review summarizes the latest developments in these four areas and provides examples where more than one method has been used to confirm the information obtained from individual methods. © 2007 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 7: 125,136; 2007: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20117 [source]

    Early mobilization after total knee replacement reduces the incidence of deep venous thrombosis

    ANZ JOURNAL OF SURGERY, Issue 7-8 2009
    Sivashankar Chandrasekaran
    Abstract Both chemical and mechanical methods of prophylaxis have reduced the incidence of thromboembolic complications following total knee replacement (TKR). Only a few studies have shown that mobilization on the first post-operative day further reduces the incidence of thromboembolic phenomena. We conducted a prospective study to verify not only if early mobilization but also whether the distance mobilized on the first post-operative day after TKR reduced the incidence of thromboembolic complications. The incidence of deep venous thrombosis and pulmonary embolism were compared in 50 consecutive patients who underwent TKR from July 2006 following a change in the mobilization protocol with 50 consecutive patients who underwent TKR before the protocol was instigated. The mobilization protocol changed from strict bed rest the first post-operative day to mobilization on the first post-operative day. Mobilization was defined as sitting out of bed or walking for at least 15,30 min twice a day. The distance mobilized was accurately recorded by the physiotherapists. All patients underwent duplex scans of both lower limbs on the fourth post-operative day. There was a significant reduction in the incidence of thromboembolic complications in the mobilization group (seven in total) compared with the control group (16 in total) (P= 0.03). Furthermore, in the mobilization group the odds of developing a thromboemobloic complication was significantly reduced the greater the distance the patient mobilized (Chi-squared linear trend = 8.009, P= 0.0047). Early mobilization in the first 24 h after TKR is a cheap and effective way to reduce the incidence of post-operative deep venous thrombosis. [source]

    Trends in induction of labour, 1998,2007: A population-based study

    AUSTRALIAN AND NEW ZEALAND JOURNAL OF OBSTETRICS AND GYNAECOLOGY, Issue 6 2009
    N. M. MEALING
    Background:, Increasing rates of induction have been reported in the UK, the USA, Canada and Australia since the early 1990s; however, there is a lack of population-based studies on trends and pharmacological management of induction of labour. Aims:, To determine population trends in induction of labour and predictors of failed induction (in caesarean section, specifically for failure to progress with cervix dilation ,3 cm). Methods:, Trends in induction were determined for women in NSW who laboured at , 32 weeks from 1998 to 2007 (N = 739 904). To determine the predictors of failed induction, 92 359 deliveries of live singletons for whom linked birth and hospital data were available (2001,2005) were examined using logistic regression analysis. Results:, The rate of induction increased over the decade from 25.3 to 29.1%; however, among those induced with prostaglandin alone, it decreased from 33.5 to 23.8%. Oxytocin alone was the most commonly used labour induction agent overall (51%) and in most population subgroups. The predictors of failed induction in both nullipara and multipara included increasing maternal age, pre-term and post-term birth and the use of prostaglandin or mechanical methods of induction (neither oxytocin nor prostaglandin). Conclusions:, The pharmacological agents used for induction of labour have changed over the past decade. An important area for future research is to investigate how the dosage of oxytocin and prostaglandin affects pregnancy outcomes. [source]

    Intramolecular Charge Transfer in Pyrromethene Laser Dyes: Photophysical Behaviour of PM650

    CHEMPHYSCHEM, Issue 11 2004
    F. López Arbeloa Prof. Dr.
    Abstract Absorption and fluorescence (steady-state and time-correlated) techniques are used to study the photophysical characteristics of the pyrromethene 650 (PM650) dye. The presence of the cyano group at the 8 position considerably shifts the absorption and fluorescence bands to lower energies with respect to other related pyrromethene dyes; this is attributed to the strong electron-acceptor character of the cyano group, as is theoretically confirmed by quantum mechanical methods. The fluorescence properties of PM650 are intensively solvent-dependent. The fluorescence band is shifted to lower energies in polar/protic solutions, and the evolution of the corresponding wavelength with the solvent is analysed by a multicomponent linear regression. The fluorescence quantum yield and the lifetime strongly decrease in polar/protic solvents, which can be ascribed to an extra nonradiative deactivation, via an intramolecular charge-transfer state (ICT state), favoured in polar media. [source]