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Are Recent Water Models Obtained by Fitting Diffraction Data Consistent with IR/Raman and X-ray Absorption Spectra?
Stockholm University, Faculty of Science, Department of Physics. (kemi)
Stockholm University, Faculty of Science, Department of Physics. (kemi)
Stockholm University, Faculty of Science, Department of Physics. (kemi)
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2006 (English)In: Journal of Chemical Physics, ISSN 0021-9606, E-ISSN 1089-7690, Vol. 125, 244510- p.Article in journal (Refereed) Published
Abstract [en]

X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data. ©2006 American Institute of Physics

Place, publisher, year, edition, pages
2006. Vol. 125, 244510- p.
Identifiers
URN: urn:nbn:se:su:diva-12384DOI: 10.1063/1.2408419OAI: oai:DiVA.org:su-12384DiVA: diva2:178904
Available from: 2008-01-15 Created: 2008-01-15 Last updated: 2010-05-07Bibliographically approved
In thesis
1. Structure Modeling with X-ray Absorption and Reverse Monte Carlo: Applications to Water
Open this publication in new window or tab >>Structure Modeling with X-ray Absorption and Reverse Monte Carlo: Applications to Water
2009 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Water is an important substance. It is part of us, of our environment, and is a fundamental prerequisite for the existence of life as we know it. The structure of water is still, after over 100 years of research on the subject, however under debate. In this thesis x-ray absorption spectroscopy (XAS) and reverse Monte Carlo (RMC) modeling are used to search for structural solutions of water consistent with many different experimental data sets, with emphasis on the combination of different experimental techniques for a reliable structure determination. Neutron and x-ray diffraction are analyzed in combination with the more recent synchrotron radiation based XAS. Geometrical criteria for H-bonding are implemented in RMC to drive the fits and allow to evaluate differently H-bonded structure models against the data. It is shown that the available diffraction data put little constraints on the type of H-bond topology or O-O-O tetrahedrality for the structure of liquid water. It is also demonstrated that classical MD simulations, using some of the most common interaction potentials for water, give rise to O-O and O-H pair-correlation functions with too sharp first peaks at too short distances to be in agreement with diffraction, and furthermore that requiring a large fraction of broken H-bonds is not in itself enough for a structure model to reproduce the experimental XAS. A contribution to the theoretical description of XAS is made by an in-depth investigation of important technical aspects of the TP-DFT spectrum calculations. A novel approach to RMC, applicable also to data that require a significant amount of computer time to evaluate, is developed which makes use of pre-computed properties from a large set of local geometries allowing RMC simulations directly on data from core-level spectroscopies such as XAS.

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2009. 83 p.
Keyword
water structure, x-ray absorption, spectroscopy, diffraction, reverse monte carlo, XAS, EXAFS, XANES, RMC, TP-DFT spectrum calculations
National Category
Physical Sciences
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-31475 (URN)978-91-7155-972-2 (ISBN)
Public defence
2009-12-11, FB53 AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4, 5 and 6: SubmittedAvailable from: 2009-11-19 Created: 2009-11-16 Last updated: 2011-04-26Bibliographically approved
2. Theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and surface adsorbates
Open this publication in new window or tab >>Theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and surface adsorbates
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis presents results of theoretical modeling of x-ray and vibrational spectroscopies applied to liquid water and to CO adsorbed on a Ni(100) surface. The Reverse Monte Carlo method is used to search for  water structures that reproduce diffraction, IR/Raman and x-ray absorption by fitting them to experimental data and imposed constraints. Some of the structures are created to have a large fraction of broken hydrogen bonds because recent x-ray absorption and emission studies have been seen to support the existence of such structures. In the fitting procedure a fast way of computing the IR/Raman spectrum for an isolated OH stretch is used, where the frequency is represented by the electric field projected in the direction of the stretch coordinate. This method is critically evaluated by comparing it to quantum chemical cluster calculations. Furthermore, the x-ray emission spectrum of water is investigated, the modeling of which is complicated by the necessity of including vibrational effects in the spectrum calculations due to a dissociative intermediate state. Based on the Kramers-Heisenberg formula a new semi-classical method is developed to include vibrational effects in x-ray emission calculations. The method is seen to work very well for a one-dimensional test system. Moreover, x-ray absorption and emission are implemented in a periodic Density Functional Theory code which is applied to ice and to the surface adsorbate system CO on Ni(100).

Abstract [sv]

Den här avhandlingen presenterar resultat av teoretisk modellering av röntgen- och vibrationella spektroskopier applicerade på flytande vatten och på CO adsorberat på en Ni(100) -yta. Reverse Monte Carlo-metoden används till att söka efter vattenstrukturer som reproducerar diffraktion, IR/Raman, röntgenabsorption och emission genom att anpassa strukturerna till experimentella data samt till pålagda restriktionsvillkor. Vissa av strukturerna är skapade så att de har en stor andel brutna vätebindningar eftersom nya röntgenabsorptions- och emissionsexperiment har setts stödja förekomsten av sådana strukturer. I anpassningsprocessen används en metod för att snabbt beräkna IR/Raman-spektrum för en isolerad OH-stretch, där frekvensen representeras av det elektriska fältet projicerat i stretch-koordinatens riktning.Vi utvärderar kritiskt denna metod genom att jämföra den med kvantkemiska klusterberäkningar. Vidare undersöks vattens röntgenemissionsspektrum, vars modellering kompliceras av nödvändigheten att inkludera vibrationella effekter i spektrumberäkningarna på grund av ett dissociativt intermediärt tillstånd. Baserat på Kramers-Heisenbergformeln utvecklas en ny semiklassisk metod som inkluderar vibrationella effekter. Metoden visar sig fungera mycket väl för ett endimensionellt testsystem. Dessutom implementerar vi röntgenabsorption och emission i en periodisk Täthetsfunktionalteorikod som vi sedan applicerar  på is och på ett ytadsorbatsystem: CO på Ni(100).

Place, publisher, year, edition, pages
Stockholm: Department of Physics, Stockholm University, 2010. 61 p.
Keyword
water, XAS, XES, IR, Raman
National Category
Atom and Molecular Physics and Optics
Research subject
Chemical Physics
Identifiers
urn:nbn:se:su:diva-38868 (URN)978-91-7447-096-3 (ISBN)
Public defence
2010-06-04, sal FR4, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript. Paper 6: Manuscript.Available from: 2010-05-11 Created: 2010-05-03 Last updated: 2011-11-14Bibliographically approved

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