Title

Short strong hydrogen bonds studied by inelastic neutron scattering and computational methods

Date of Award

2003

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry

Advisor(s)

Bruce S. Hudson

Keywords

Hydrogen bonds, Neutron scattering, Bond energies, Potassium hydrogen bistrifluoroacetate

Subject Categories

Chemistry | Physical Sciences and Mathematics

Abstract

Hydrogen bonding can be simply defined as any interaction between molecules that involve the participation of hydrogen, or stated in another fashion: "a hydrogen bond exists when a hydrogen atom H is bonded to more than one other atom". Within this simple definition is a diverse range of interactions that is difficult to explain utilizing conventional ionic and covalent bonding. Hydrogen bonds that exhibit large bond energies are termed short, strong hydrogen bonds and possess unique potential surfaces. These potential surfaces are termed low barrier because the energy barrier that separates the potential wells is lower or equal to the zero point level. Little or no energy is required to move the hydrogen from one well to the other. This potential surface requires special attention in calculating quantum mechanical solutions for these systems due to the quartic shape of these short-strong hydrogen bond potential surfaces.

In this thesis I have examined several short, strong hydrogen bonded systems using inelastic neutron scattering and have calculated the corresponding neutron vibrational spectra. I have also made a detailed investigation into the potential surface of the strong hydrogen bond in potassium hydrogen bistrifluoroacetate. Results have shown that it is possible to calculate accurate structural coordinates and vibrational spectra that agree with the experimental. The calculations give an incorrect energy minimization resulting in incorrect vibrational band placement in the inelastic neutron spectrum from the use of a harmonic fit of an anharmonic potential surface. The anharmonic potential surface resultant from the barrier between double wells positioned below the zero point level is calculated for the hydrogen bond. This can be correctly modeled by calculating the potential surface using a fixed OO distance, and solving the Schrödinger equation along this potential. This is the first comparison of neutron vibrational spectra and calculated spectra to provide an understanding of the limitations of computational methods to examine strong hydrogen bonds. This is a new and powerful tool to accurately examine the strength and structure of strong hydrogen bonds.

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