Thesis defense [before December 2013]
Licentiate thesis: IR and Raman Spectra of HOD in D2O
by
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Europe/Stockholm
Seminarierummet, Hus 12
Seminarierummet, Hus 12
Description
Water is one of the most important substances on earth, without it life as we know it would be impossible. Although it is molecularly a simple liquid its microscopic structure is still not properly understood.
Experiments like x-ray and neutron diffraction, Infrared and Raman spectroscopies and x-ray absorption (XAS) give valuable information about the structure of water and puts constraints on the structures that can reproduce the experimental data. In this thesis methods of computing Infrared and Raman spectra of dilute HOD in D2O are developed and discussed. Density Functional Theory is used to compute anharmonic vibrational frequencies for the OH stretch which can be assumed to be relatively uncoupled to the other vibrational modes. A faster and more approximative alternative is the E-field approximation, where the frequency is modeled by the projected electric field felt by the proton from the surrounding molecules, represented by point charges. Several structure models of water are investigated; structures taken from Molecular Dynamics (MD) simulations, structures that were fitted to reproduce diffraction data by the Empirical Potential Structure Refinement (EPSR) method, and structures that we ourselves fitted to both diffraction data and an E-field distribution coming from an MD simulation using the Reverse Monte Carlo (RMC) method. Some of these structures were made very asymmetric, having on the average two hydrogen bonds per molecule instead of close to four. This was done to test the constraints set by the data on the structures, and because recent x-ray absorption and x-ray emission experiments seem to support such structures.
Diffraction data and the E-field representation of the Raman spectrum were seen to not impose strong enough constraints to exclude neither symmetric nor asymmetric structures, since both of these could be fitted. However, none of the structures reproduced the experimental x-ray absorption spectrum. It is shown that the E-field approximation, which relies on empirical fitting between calculated frequencies and E-fields, is quite model dependent. The form of the mapping between the frequency and E-field is examined and it is concluded that a quadratic mapping is superior to a linear one.
Experiments like x-ray and neutron diffraction, Infrared and Raman spectroscopies and x-ray absorption (XAS) give valuable information about the structure of water and puts constraints on the structures that can reproduce the experimental data. In this thesis methods of computing Infrared and Raman spectra of dilute HOD in D2O are developed and discussed. Density Functional Theory is used to compute anharmonic vibrational frequencies for the OH stretch which can be assumed to be relatively uncoupled to the other vibrational modes. A faster and more approximative alternative is the E-field approximation, where the frequency is modeled by the projected electric field felt by the proton from the surrounding molecules, represented by point charges. Several structure models of water are investigated; structures taken from Molecular Dynamics (MD) simulations, structures that were fitted to reproduce diffraction data by the Empirical Potential Structure Refinement (EPSR) method, and structures that we ourselves fitted to both diffraction data and an E-field distribution coming from an MD simulation using the Reverse Monte Carlo (RMC) method. Some of these structures were made very asymmetric, having on the average two hydrogen bonds per molecule instead of close to four. This was done to test the constraints set by the data on the structures, and because recent x-ray absorption and x-ray emission experiments seem to support such structures.
Diffraction data and the E-field representation of the Raman spectrum were seen to not impose strong enough constraints to exclude neither symmetric nor asymmetric structures, since both of these could be fitted. However, none of the structures reproduced the experimental x-ray absorption spectrum. It is shown that the E-field approximation, which relies on empirical fitting between calculated frequencies and E-fields, is quite model dependent. The form of the mapping between the frequency and E-field is examined and it is concluded that a quadratic mapping is superior to a linear one.