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Lenz, Annika
Publications (10 of 13) Show all publications
Lenz, A., Pohl, A., Ojamäe, L. & Persson, P. (2012). Computational study of the catalytic effect of platinum on the decomposition of DNT. International Journal of Quantum Chemistry, 112(7), 1852-1858
Open this publication in new window or tab >>Computational study of the catalytic effect of platinum on the decomposition of DNT
2012 (English)In: International Journal of Quantum Chemistry, ISSN 0020-7608, E-ISSN 1097-461X, Vol. 112, no 7, p. 1852-1858Article in journal (Refereed) Published
Abstract [en]

The catalytic decomposition of dinitrotoluene (DNT; 3-4-DNT), a by-product of the explosive trinitrotoluene (trotyl), on a platinum surface is investigated computationally. Reaction paths have been computed for a DNT molecule interacting with a Pt-cluster under varying temperatures using quantum-chemical density functional theory. Two possible initiation steps where DNT split either into nitro-tolyl and NO2, or in nitro-tolyl-oxidanyl and NO, are considered. The energy barrier for the catalytic process is found to decrease significantly for the Pt catalyzed reaction compared with the uncatalyzed reaction.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2012
Keywords
density functional theory, sensors, catalysis, reaction path, half life
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-76183 (URN)10.1002/qua.23164 (DOI)000300503100011 ()
Available from: 2012-03-31 Created: 2012-03-30 Last updated: 2017-12-07
Lenz, A. & Ojamäe, L. (2011). Structures of the I-, II- and H-Methane Clathrates and the Ice−Methane Clathrate Phase Transition from Quantum-Chemical Modeling with Force-Field Thermal Corrections. Journal of Physical Chemistry A, 115(23), 6169-6176
Open this publication in new window or tab >>Structures of the I-, II- and H-Methane Clathrates and the Ice−Methane Clathrate Phase Transition from Quantum-Chemical Modeling with Force-Field Thermal Corrections
2011 (English)In: Journal of Physical Chemistry A, ISSN 1089-5639, E-ISSN 1520-5215, Vol. 115, no 23, p. 6169-6176Article in journal (Refereed) Published
Abstract [en]

Methane hydrates with the three clathrate structures I, II and H are studied by quantumchemicalmethods. The periodic B3LYP computations are combined with force-field methodsfor the thermal energy corrections. The pressure dependencies for the crystal structures, latticeenergies and guest molecule interactions are derived. Quantum-chemical geometryoptimizations predict too small cell volumes compared to experimental data, but includingzero-point energy and thermal energy the cell volume increases and the correct densities areobtained. Phase diagram for the three structures are investigated, and phase transitions werefound at 5 GPa for the MH-I–MH-II transition and at 10 GPa for the MH-II–MH-H transition.

Place, publisher, year, edition, pages
ACS Publications, 2011
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-53177 (URN)10.1021/jp111328v (DOI)
Note
The original title of this article was: "Structure and phase transitions of I-, II- and H- methane clathrates and ice from quantum-chemical B3LYP computations with corrections for thermal effects".Available from: 2010-01-18 Created: 2010-01-18 Last updated: 2017-12-12Bibliographically approved
Lenz, A., Kariis, H., Pohl, A., Persson, P. & Ojamäe, L. (2011). The electronic structure and reflectivity of PEDOT:PSS from density functional theory. Chemical Physics, 384(03-jan), 44-51
Open this publication in new window or tab >>The electronic structure and reflectivity of PEDOT:PSS from density functional theory
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2011 (English)In: Chemical Physics, ISSN 0301-0104, E-ISSN 1873-4421, Vol. 384, no 03-jan, p. 44-51Article in journal (Refereed) Published
Abstract [en]

The geometric and electronic structure of condensed phase organic conducting polymer PEDOT:PSS blends has been investigated by periodic density functional theory (DFT) calculations with a generalized-gradient approximation (GGA) functional, and a plane wave basis set. The influence of the degree of doping of the PEDOT polymer on structural and optical parameters such as the reflectivity, absorbance, conductivity, dielectric function, refractive index and the energy-loss function is studied. A flip from the benzoid to the quinoid structure is observed in the calculations when the neutral PEDOT is doped by negatively charged PSS. Also the optical properties are affected by the doping. In particular, the reflectivity was found to be very sensitive to the degree of doping, where higher doping implies higher reflectivity. The reflectivity is highly anisotropic, with the dominant contribution stemming from the direction parallel to the PEDOT polymer chain.

Place, publisher, year, edition, pages
Elsevier Science B.V., Amsterdam., 2011
Keywords
PEDOT:PSS; Conducting polymer; Quantum-chemical calculations
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-69856 (URN)10.1016/j.chemphys.2011.05.003 (DOI)000291472400006 ()
Note
Original Publication: Annika Lenz, Hans Kariis, Anna Pohl, Petter Persson and Lars Ojamäe, The electronic structure and reflectivity of PEDOT:PSS from density functional theory, 2011, Chemical Physics, (384), 03-jan, 44-51. http://dx.doi.org/10.1016/j.chemphys.2011.05.003 Copyright: Elsevier Science B.V., Amsterdam. http://www.elsevier.com/ Available from: 2011-08-10 Created: 2011-08-08 Last updated: 2017-12-08
Lenz, A. & Ojamäe, L. (2010). Computational studies of the stability of the (H2O)100 nanodrop. Journal of Molecular Structure: THEOCHEM, 944(1-3), 163-167
Open this publication in new window or tab >>Computational studies of the stability of the (H2O)100 nanodrop
2010 (English)In: Journal of Molecular Structure: THEOCHEM, ISSN 0166-1280, Vol. 944, no 1-3, p. 163-167Article in journal (Refereed) Published
Abstract [en]

The stability of the (H2O)100 nanodrop, experimentally known from a polyoxomolybdatecrystal structure (Müller et al. Inorg. Chem. Commun., 2003, 6, 52) and other structuresinferred from clathrate structures, are studied by quantum-chemical B3LYP computations.The free energies are compared to the trends for smaller clusters with 15-30 molecules. Forthe small clusters both cage-based structures and denser structures with a larger number of Hbondsobtained by an evolutionary algorithm (Bandow and Hartke, J. Phys. Chem. A, 2006,110, 5809) are used. The dense structures are most often found to be lower in electronicenergy. The cage-based structures, to which the structure of the experimentally found(H2O)100 cluster can be categorized, become more stable when Gibbs free energy is calculatedat 298 K. Additional cage-based clusters in the 35-81 molecular range were constructed forcomparison. The experimental cluster with 100 molecules (C2h/Ci-symmetry for oxygens/allatoms) and the constructed cluster with 42 molecules are found to be lower in energy than aplausible overall trend. The (H2O)42 cluster has an extraordinary high symmetry (S6), evenwhen the hydrogens are considered. The (H2O)100 cluster is the only of the studied clusters forwhich ΔG is negative at 298 K.

Keywords
Water clusters, Quantum-chemical computations, Hydrogen bonding, B3LYP calculations, Gibbs free energy
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-53176 (URN)10.1016/j.theochem.2009.12.033 (DOI)000275688200022 ()
Note
Original Publication: Annika Lenz and Lars Ojamäe, Computational studies of the stability of the (H2O)100 nanodrop, 2010, Journal of Molecular Structure: THEOCHEM, (944), 1-3, 163-167. http://dx.doi.org/10.1016/j.theochem.2009.12.033 Copyright: Elsevier Science B.V., Amsterdam http://www.elsevier.com/ Available from: 2010-01-18 Created: 2010-01-18 Last updated: 2017-12-12
Huang, C., Wikfeldt, K. T., Tokushima, T., Nordlund, D., Harada, Y., Bergmann, U., . . . Nilsson, A. (2010). Letter: Reply to Soper et al.: Fluctuations in water around a bimodal distribution of local hydrogen-bonded structural motifs [Letter to the editor]. Proceedings of the National Academy of Sciences of the United States of America, 107(12), E45-E45
Open this publication in new window or tab >>Letter: Reply to Soper et al.: Fluctuations in water around a bimodal distribution of local hydrogen-bonded structural motifs
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2010 (English)In: Proceedings of the National Academy of Sciences of the United States of America, ISSN 0027-8424, E-ISSN 1091-6490, Vol. 107, no 12, p. E45-E45Article in journal, Letter (Other academic) Published
Abstract [en]

n/a

National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-54769 (URN)10.1073/pnas.0913504107 (DOI)000275898300083 ()
Available from: 2010-04-09 Created: 2010-04-09 Last updated: 2017-12-12Bibliographically approved
Lenz, A. & Ojamäe, L. (2009). A theoretical study of water equilibria: The cluster distribution versus temperature and pressure for (H2O)n, n=1–60, and ice. Journal of Chemical Physics, 131(13), 134302-134302-13
Open this publication in new window or tab >>A theoretical study of water equilibria: The cluster distribution versus temperature and pressure for (H2O)n, n=1–60, and ice
2009 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 131, no 13, p. 134302-134302-13Article in journal (Refereed) Published
Abstract [en]

The size distribution of water clusters at equilibrium is studied using quantum-chemical calculations in combination with statistical thermodynamics. The necessary energetic data is obtained by quantum-chemical B3LYP computations and through extrapolations from the B3LYP results for the larger clusters. Clusters with up to 60 molecules are included in the equilibrium computations. Populations of different cluster sizes are calculated using both an ideal gas model with noninteracting clusters and a model where a correction for the interaction energy is included analogous to the van der Waals law. In standard vapor the majority of the water molecules are monomers. For the ideal gas model at 1 atm large clusters [56-mer (0–120 K) and 28-mer (100–260 K)] dominate at low temperatures and separate to smaller clusters [21–22-mer (170–280 K) and 4–6-mer (270–320 K) and to monomers (300–350 K)] when the temperature is increased. At lower pressure the transition from clusters to monomers lies at lower temperatures and fewer cluster sizes are formed. The computed size distribution exhibits enhanced peaks for the clusters consisting of 21 and 28 water molecules; these sizes are for protonated water clusters often referred to as magic numbers. If cluster-cluster interactions are included in the model the transition from clusters to monomers is sharper (i.e., occurs over a smaller temperature interval) than when the ideal-gas model is used. Clusters with 20–22 molecules dominate in the liquid region. When a large icelike cluster is included it will dominate for temperatures up to 325 K for the noninteracting clusters model. Thermodynamic properties (Cp, H) were calculated with in general good agreement with experimental values for the solid and gas phase. A formula for the number of H-bond topologies in a given cluster structure is derived. For the 20-mer it is shown that the number of topologies contributes to making the population of dodecahedron-shaped cluster larger than that of a lower-energy fused prism cluster at high temperatures.

Keywords
water, vapour, ice, quantum chemistry, statistical thermodynamics, hydrogen bonding
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:liu:diva-50778 (URN)10.1063/1.3239474 (DOI)
Available from: 2009-10-14 Created: 2009-10-14 Last updated: 2017-12-12
Huang, C., Wikfeldt, K. T., Tokushima, T., Nordlund, D., Harada, Y., Bergmann, U., . . . Nilsson, A. (2009). The inhomogeneous structure of water at ambient conditions. PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 106(36), 15214-15218
Open this publication in new window or tab >>The inhomogeneous structure of water at ambient conditions
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2009 (English)In: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, ISSN 0027-8424, Vol. 106, no 36, p. 15214-15218Article in journal (Refereed) Published
Abstract [en]

Small-angle X-ray scattering (SAXS) is used to demonstrate the presence of density fluctuations in ambient water on a physical length-scale of approximate to 1 nm; this is retained with decreasing temperature while the magnitude is enhanced. In contrast, the magnitude of fluctuations in a normal liquid, such as CCl4, exhibits no enhancement with decreasing temperature, as is also the case for water from molecular dynamics simulations under ambient conditions. Based on X-ray emission spectroscopy and X-ray Raman scattering data we propose that the density difference contrast in SAXS is due to fluctuations between tetrahedral-like and hydrogen-bond distorted structures related to, respectively, low and high density water. We combine our experimental observations to propose a model of water as a temperature-dependent, fluctuating equilibrium between the two types of local structures driven by incommensurate requirements for minimizing enthalpy (strong near-tetrahedral hydrogen-bonds) and maximizing entropy (non-directional H-bonds and disorder). The present results provide experimental evidence that the extreme differences anticipated in the hydrogen-bonding environment in the deeply supercooled regime surprisingly remain in bulk water even at conditions ranging from ambient up to close to the boiling point.

Keywords
density fluctuations, liquid-liquid hypothesis, small angle X-ray scattering, water structure, X-ray spectroscopy
National Category
Natural Sciences
Identifiers
urn:nbn:se:liu:diva-20741 (URN)10.1073/pnas.0904743106 (DOI)
Note
Original Publication: C Huang, K T Wikfeldt, T Tokushima, D Nordlund, Y Harada, U Bergmann, M Niebuhr, T M Weiss, Y Horikawa, M Leetmaa, M P Ljungberg, O Takahashi, Annika Lenz, Lars Ojamäe, A P Lyubartsev, S Shin, L G M Pettersson and A Nilsson, The inhomogeneous structure of water at ambient conditions, 2009, PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, (106), 36, 15214-15218. http://dx.doi.org/10.1073/pnas.0904743106 Copyright: National Academy of Sciences http://www.nas.edu/ Available from: 2009-09-18 Created: 2009-09-18 Last updated: 2015-03-09
Lenz, A. (2009). Theoretical Investigations of Water Clusters, Ice Clathrates and Functionalized Nanoparticles. (Doctoral dissertation). Linköping: Linköping University Electronic Press
Open this publication in new window or tab >>Theoretical Investigations of Water Clusters, Ice Clathrates and Functionalized Nanoparticles
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Nanosized structures are of intermediate size between individual molecules and bulkmaterials which gives them several unique properties. At the same time their relative limitedsizes make them suitable for studies by the methods of computational chemistry. In this thesiswater clusters, ice clathrates and functionalized metal-oxide nanoparticles have been studiedby quantum-chemical calculations and statistical thermodynamics.

The stabilities of water clusters composed of up to 100 molecules have been investigated. Themultitude of possible H-bonded topologies and their importance for determining theproperties of the clusters have been highlighted. Several structural characteristics of thehydrogen bonded network have been examined and the structural factors that determine thestability of an H-bonded network have been identified. The stability of two kinds of oxygenframeworks for water clusters have been analyzed, taking into account thermal energy andentropy corrections. Clusters with many 4-coordinated molecules have been found to be lowerin energy at low temperatures whereas the clusters with less-coordinated molecules dominateat higher temperatures. The equilibrium size distribution of water clusters as a function oftemperature and pressure has been computed using statistical thermodynamics. Themicroscopic local structure of liquid water has been probed by utilizing information from thestudied water clusters. The average number of H-bonds in liquid water has been predicted byfitting calculated average IR spectra for different coordination types in water clusters toexperimental IR spectra.

Water can form an ice-like structure that encloses various molecules such as methane. Thesemethane hydrates are found naturally at the ocean floor and in permafrost regions and canconstitute a large unemployed energy resource as well as a source of an effective green-housegas. The pressure dependencies of the crystal structures, lattice energies and phase transitionsfor the three methane hydrates with the clathrate structures I, II and H have been mapped out.

Zinc oxide is a semiconducting material with interesting luminescence properties that can beutilized in optical devices, such as photodetectors, light emitting devices and biomarkers. Theeffect of water molecules adsorbed on the ZnO surface when adsorbing organic acids havebeen investigated. Changes in optical properties by the adsorption of carboxylic acids havebeen studied and compared with experimental results. Aromatic alcohols at TiO2 metal-oxidenanoparticles have been studied as model systems for dye-sensitizied solar cells. Adsorptiongeometries are predicted and the influence from the adsorbed molecules on the electronicproperties has been studied.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2009. p. 52
Series
Linköping Studies in Science and Technology. Dissertations, ISSN 0345-7524 ; 1254
National Category
Other Basic Medicine
Identifiers
urn:nbn:se:liu:diva-52746 (URN)978-91-7393-636-1 (ISBN)
Public defence
2009-06-09, Planck, Fysikhuset, Campus Valla, Linköpings universitet, Linköping, 10:15 (Swedish)
Opponent
Supervisors
Available from: 2010-01-18 Created: 2010-01-12 Last updated: 2020-02-19Bibliographically approved
Lenz, A., Selegård, L., Söderlind, F., Larsson, A., Holtz, P.-O., Uvdal, K., . . . Käll, P.-O. (2009). ZnO Nanoparticles Functionalized with Organic Acids: An Experimental and Quantum-Chemical Study. The Journal of Physical Chemistry C, 113(40), 17332-17341
Open this publication in new window or tab >>ZnO Nanoparticles Functionalized with Organic Acids: An Experimental and Quantum-Chemical Study
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2009 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 113, no 40, p. 17332-17341Article in journal (Refereed) Published
Abstract [en]

Electrochemical synthesis and physical characterization of ZnO nanoparticles functionalized with four different organic acids, three aromatic (benzoic, nicotinic, and trans-cinnamic acid) and one nonaromatic (formic acid), are reported. The functionalized nanoparticles have been characterized by X-ray powder diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, UV−vis, and photoluminescence spectroscopy. The adsorption of the organic acids at ZnO nanoparticles was further analyzed and interpreted using quantum-chemical density-functional theory computations. Successful functionalization of the nanoparticles was confirmed experimentally by the measured splitting of the carboxylic group stretching vibrations as well as by the N(1s) and C(1s) peaks from XPS. From a comparison between computed and experimental IR spectra, a bridging mode adsorption geometry was inferred. PL spectra exhibited a remarkably stronger near band edge emission for nanoparticles functionalized with formic acid as compared to the larger aromatic acids. From the quantum-chemical computations, this was interpreted to be due to the absence of aromatic adsorbate or surface states in the band gap of ZnO, caused by the formation of a complete monolayer of HCOOH. In the UV−vis spectra, strong charge-transfer transitions were observed.

Keywords
nanoparticles, ZnO, organic acids, adsorption, synthesis, XPS, UV-vis, quantum chemical calculations
National Category
Physical Chemistry
Identifiers
urn:nbn:se:liu:diva-50783 (URN)10.1021/jp905481v (DOI)
Available from: 2009-10-14 Created: 2009-10-14 Last updated: 2017-12-12
Lenz, A., Karlsson, M. & Ojamäe, L. (2008). Quantum-chemical investigations of phenol and larger aromatic molecules at the TiO2 anatase (101) surface. Journal of Physics, Conference Series, 117, 012020-(8 pp)
Open this publication in new window or tab >>Quantum-chemical investigations of phenol and larger aromatic molecules at the TiO2 anatase (101) surface
2008 (English)In: Journal of Physics, Conference Series, ISSN 1742-6588, E-ISSN 1742-6596, Vol. 117, p. 012020-(8 pp)Article in journal (Refereed) Published
Abstract [en]

Adsorption of aromatic molecules at the (101) surface of titanium dioxide anatase is studied by quantum-chemical B3LYP computations, where both cluster and periodic calculations were performed and compared. For phenol different adsorption modes at a TiO2 cluster were mapped out and the energetically most favourable conformation was used for investigation of the electronic structure, for periodic calculations, and as a mould for the adsorption modes of phenylmethanol, phenylethanol, naphthalen-2-ol, phenanthren-2-ol, pyren-2-ol and perylen-2-ol. The alcohols form a H-bond to a surface O and a O(molecule)-Ti bond. For the larger aromatic molecules their increasingly higher HOMO levels decrease the effective bad gap of the system. Inclusion of spacer groups as in phenylmethanol and phenylethanol results in higher adsorption energies and larger band gaps. The LUMOs for the adsorbates help visualize the electronic coupling to the surface. Comparison of the cluster with the periodic model indicates that the former describes the electronic coupling in a similar manner as the latter, although the former lacks in the description of the anatase substrate.

Keywords
TiO2, anatase, phenol, adsorption, nanoparticles
National Category
Theoretical Chemistry
Identifiers
urn:nbn:se:liu:diva-50664 (URN)10.1088/1742-6596/117/1/012020 (DOI)
Available from: 2009-10-13 Created: 2009-10-13 Last updated: 2017-12-12
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