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  • Tunega, Daniel  (8)
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  • 1
    Language: English
    In: Geoderma, Jan, 2014, Vol.213, p.115(9)
    Description: To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.geoderma.2013.08.001 Byline: Adelia J.A. Aquino, Daniel Tunega, Gabriele E. Schaumann, Georg Haberhauer, Martin H. Gerzabek, Hans Lischka Abstract: An overview of the variety of processes induced by the aluminum cation interacting with carboxyl and carboxylate groups is given by means of quantum chemical density functional theory (DFT) calculations. Different hydration states of Al.sup.3+ ranging from the hexaaquo complex down to the unhydrated cation and direct/indirect bonding with the polar groups are considered. The calculations reflect the amphoteric character of the hydrated aluminum complex showing in most cases its acidic character via proton transfer from the water molecules of the hydration shell to the carboxylate group, but in some cases also deprotonation of the carboxyl group. Several additional processes are observed such as interconversion of bidentate and monodentate bonding by the carboxyl/carboxylate groups and strong hydrogen bonding between proton transfer partners. Comparison with analogous previous investigations on cation bridges induced by calcium and sodium shows the pronounced activity of the triply charged aluminum cation. The importance of the strong polarizing and bridging power of the aluminum cation for soil organic matter with low exchange capacities and a low concentration of charged groups is discussed. Article History: Received 19 February 2013; Revised 24 July 2013; Accepted 3 August 2013
    Keywords: Hydrogen -- Analysis ; Protons -- Analysis ; Humic Acids -- Analysis
    ISSN: 0016-7061
    Source: Cengage Learning, Inc.
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  • 2
    Language: English
    In: International Journal of Quantum Chemistry, 06/2011, Vol.111(7-8), pp.1531-1542
    ISSN: International Journal of Quantum Chemistry
    E-ISSN: 00207608
    E-ISSN: 1097461X
    Source: Wiley (via CrossRef)
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  • 3
    Language: English
    In: Geoderma, January 2014, Vol.213, pp.115-123
    Description: An overview of the variety of processes induced by the aluminum cation interacting with carboxyl and carboxylate groups is given by means of quantum chemical density functional theory (DFT) calculations. Different hydration states of Al ranging from the hexaaquo complex down to the unhydrated cation and direct/indirect bonding with the polar groups are considered. The calculations reflect the amphoteric character of the hydrated aluminum complex showing in most cases its acidic character via proton transfer from the water molecules of the hydration shell to the carboxylate group, but in some cases also deprotonation of the carboxyl group. Several additional processes are observed such as interconversion of bidentate and monodentate bonding by the carboxyl/carboxylate groups and strong hydrogen bonding between proton transfer partners. Comparison with analogous previous investigations on cation bridges induced by calcium and sodium shows the pronounced activity of the triply charged aluminum cation. The importance of the strong polarizing and bridging power of the aluminum cation for soil organic matter with low exchange capacities and a low concentration of charged groups is discussed.
    Keywords: Humic Substances ; Cation Bridges ; Water Bridges ; Molecular Simulation ; Density Functional Theory ; Proton Transfer ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 4
    Language: English
    In: Environmental science & technology, 01 October 2011, Vol.45(19), pp.8411-9
    Description: The stabilizing effect of water molecule bridges on polar regions in humic substances (HSs) has been investigated by means of molecular dynamics (MD) simulations. The purpose of these investigations was to show the effect of water molecular bridges (WAMB) for cross-linking distant locations of hydrophilic groups. For this purpose, a tetramer of undecanoid fatty acids connected to a network of water molecules has been constructed, which serve as a model for spatially fixed aliphatic chains in HSs terminated by a polar (carboxyl) group. The effect of environmental polarity has been investigated by using solvents of low and medium polarity in force-field MD. A nonpolar environment simulated by n-hexane was chosen to mimic the stability of WAMB in a hydrophilic hotspot surrounded by a nonpolar environment, while the more polar acetonitrile environment was chosen to simulate a more even distribution of polarity around the carboxylic groups and the water molecules. The dynamics simulations show that the rigidity of the oligomer chains is significantly enhanced as soon as the water cluster is large enough to comprise all four carboxyl groups. Increasing the temperature leads to evaporization processes which destabilize the rigidity of the tetramer-water cluster. Embedding it into the nonpolar environment introduces a pronounced cage effect which significantly impedes removal of water molecules from the cluster region. On the other hand, a polar environment facilitates their diffusion from the polar region. One important consequence of these simulations is that although the local water network is the stabilizing factor for the organic matter matrix, the degree of stabilization is additionally affected by the presence of nonpolar surroundings.
    Keywords: Molecular Dynamics Simulation ; Humic Substances -- Analysis ; Water -- Chemistry
    ISSN: 0013936X
    E-ISSN: 1520-5851
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  • 5
    Language: English
    In: Geoderma, 2011, Vol.169, pp.20-26
    Description: Molecular dynamics simulations have been performed by means of density functional theory with tight binding (DFTB) in order to describe the structure and the energetic stability of water bridges in humic substances (HS) model. This model is constructed from two parallel aliphatic chains geometrically restrained on one end and terminated with a carboxyl group on the other to supply the structural pattern for supramolecular contact of two HS chains through hydrogen bonds. Molecular dynamics simulations were used to analyze the interactions of the carboxylic groups with a variable number of water molecules up to 14 representing domains of micro hydration states of polar centers in humic acids. For the present geometrical arrangements of the model five water molecules form a stable bridge between the two carboxylic groups located at each aliphatic chain. The effect of environment through three solvents of different polarities (n-hexane, acetonitrile and water) was investigated. Distribution profiles of oxygen atoms of carboxyl and chain water molecules show that the environmental effect of the solvent with moderate polarity (acetonitrile) is most pronounced in exerting an ordering effect on the water bridge. Energy profiles for incremental addition of water molecules and hydrogen bond analysis demonstrate the remarkable stability of the five water complex as compared to all other models investigated in both gas phase and in acetonitrile. These findings correlate nicely with experimentally observed antiplasticizing effects of water bridges in organic matrices. ► Molecular modeling of humic substances at density functional theory level. ► Water bridges and hydrogen bonding as strong stabilizing factors in humic substances. ► Influence of solvent polarity on the structure of the water clusters. ► Dynamics simulations to describe energetic stability and environmental effects.
    Keywords: Geochemistry ; Soils ; Organic Materials ; Water ; Agriculture
    ISSN: 0016-7061
    E-ISSN: 1872-6259
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  • 6
    Language: English
    In: The Journal of Physical Chemistry C, 09/17/2009, Vol.113(37), pp.16468-16475
    ISSN: 1932-7447
    E-ISSN: 1932-7455
    Source: American Chemical Society (via CrossRef)
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  • 7
    Language: English
    In: Environmental Chemistry, 2019, Vol.16(7), p.541-552
    Description:  Immobilisation of organic chemicals in soil organic matter can strongly influence their availability in the environment. We show that the presence of water clusters, called water molecule bridges, hampers the release of organic molecules from soil organic matter. Moreover, water molecule bridges...
    Keywords: Thermal Stability ; Organic Matter ; Desorption ; Phenols ; Bridges ; Chemistry ; Sediments ; Molecules ; Samples ; Organic Soils ; Organic Soils ; Calorimetry ; Pharmaceutical Sciences ; Hypotheses ; Soil Structure ; Stability ; Bridges ; Molecular Structure ; Kinetics ; Soils ; Calorimetry ; Organic Matter ; Solvation ; Organic Chemicals ; Phenols ; Humidity ; Hydrogen Bonds ; Environmental Science ; Modelling ; Water Chemistry ; Kinetics ; Kinetics ; Immobilization ; Constants ; Organic Matter ; Molecular Modelling ; Hydrology ; Soil Organic Matter ; Soil Stabilization ; Differential Scanning Calorimetry ; Hydration ; Organic Matter ; Soils ; Modelling ; Environmental Conditions ; Immobilization ; Desorption ; Connecting ; Desorption ; Kinetics ; Desorption ; Phenols ; Phenols ; Soil Stabilization ; Thermal Stability ; Solvation ; Vapor Phases;
    ISSN: 1448-2517
    E-ISSN: 1449-8979
    Source: CSIRO Journals
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  • 8
    Language: English
    In: PLoS ONE, 01 January 2013, Vol.8(6), p.e65359
    Description: It is assumed to be common knowledge that multivalent cations cross-link soil organic matter (SOM) molecules via cation bridges (CaB). The concept has not been explicitly demonstrated in solid SOM by targeted experiments, yet. Therefore, the requirements for and characteristics of CaB remain unidentified. In this study, a combined experimental and molecular modeling approach was adopted to investigate the interaction of cations on a peat OM from physicochemical perspective. Before treatment with salt solutions of Al(3+), Ca(2+) or Na(+), respectively, the original exchangeable cations were removed using cation exchange resin. Cation treatment was conducted at two different values of pH prior to adjusting pH to 4.1. Cation sorption is slower (〉〉2 h) than deprotonation of functional groups (〈2 h) and was described by a Langmuir model. The maximum uptake increased with pH of cation addition and decreased with increasing cation valency. Sorption coefficients were similar for all cations and at both pH. This contradicts the general expectations for electrostatic interactions, suggesting that not only the interaction chemistry but also spatial distribution of functional groups in OM determines binding of cations in this peat. The reaction of contact angle, matrix rigidity due to water molecule bridges (WaMB) and molecular mobility of water (NMR analysis) suggested that cross-linking via CaB has low relevance in this peat. This unexpected finding is probably due to the low cation exchange capacity, resulting in low abundance of charged functionalities. Molecular modeling demonstrates that large average distances between functionalities (∼3 nm in this peat) cannot be bridged by CaB-WaMB associations. However, aging strongly increased matrix rigidity, suggesting successive increase of WaMB size to connect functionalities and thus increasing degree of cross-linking by CaB-WaMB associations. Results thus demonstrated that the physicochemical structure of OM is decisive for CaB and aging-induced structural reorganisation can enhance cross-link formation.
    Keywords: Sciences (General)
    E-ISSN: 1932-6203
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