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Theoretical Treatments of the Effects of Low Frequency Vibrations on OH Stretches in Molecules and Ion-Water Complexes that Undergo Large Amplitude Motions

Dzugan, Laura C
http://rave.ohiolink.edu/etdc/view?acc_num=osu1492688748608717

Abstract Details

2017, Doctor of Philosophy, Ohio State University, Chemistry.
A vibrational spectrum of a molecule provides the frequencies and intensities associated with the vibrational motions in the molecule. As it is not always easy to assign specific vibrational motions to the peaks in measured spectra, theoretical calculations are used to assist in identifying the motions responsible for the observed transitions. The harmonic oscillator approximation is often used when calculating the frequencies and intensities for the vibrations in a molecule. However, anharmonicities can arise from strong couplings between vibrations. As such, adjustments to the calculations are needed to accurately evaluate the vibrational energy levels. This thesis uses several strategies to address the anharmonic effects in vibrational spectra, specifically those involving couplings between low frequency motions and high frequency OH or CH stretches. In the vibrational spectra of M2+ OH-(H2 O)n=4-6 (M = Mg, Ca) and the protonated water trimer and tetramer (H+(H2O)n=3,4), there are two types of bands in the OH stretch region; narrow peaks due to isolated OH stretches and a broadened feature that is attributed to OH stretches involved in H-bonding. The effects of low frequency motions on the H-bonded OH stretches were analyzed through calculations based on an adiabatic separation of the inter- and intramolecular vibrations. For these calculations, displacement geometries were sampled from the equilibrium structure based on the harmonic ground state probability amplitude using Monte Carlo sampling. Then only the frequencies and intensities for the OH stretches and HOH bends were calculated by constraining the remaining internal coordinates at their displaced geometries. As the calculated spectra had generally good agreement with the measured spectra, it was concluded that the H-bonded OH stretch frequencies were very sensitive to the placement of the water monomers within the molecule. However, the studies on the protonated water clusters proved to be more of a challenge due to other anharmonicities that this adiabatic approach did not take into account. To improve the analysis on the protonated water trimer and tetramer, the frequencies and intensities of the vibrations were calculated using second-order perturbation theory where the normal modes were defined as displacements of internal coordinates. From these calculations, the combination bands and overtones in the spectra were assigned and the effects of near degeneracies between vibrational states were analyzed. In the protonated water trimer, it was concluded that the H-bonded OH stretches strongly mix with combination bands and overtones that involve the motions of the hydronium core. The effects of tagging the protonated water tetramer with molecules of varying basicity was also investigated and the spectroscopic features during the proton transfer event were determined. Second-order perturbation theory was also used to calculate the vibrational spectrum of methyl hydroperoxide, CH3OOH for the range of 2νOH - 3νOH. In this study, the experimental spectra were obtained by detection of the OH radical in its vibrationless or first excited state following the vibrational excitation of the overtones and combination bands and the subsequent photodissociation along the OO bond. When the OH radical is in its vibrationless state, all the transitions in the 2νOH - 3νOH range are present, including the combination band of the 2νOH +νOOH. However, when the OH radical is in its first excited state, only the 2νOH and 3νOH states are present. This is of interest as the 2νOH+νOOH band also has quanta in the OH stretch overtone. The first goal of this study was to verify the peak assignments with second-order perturbation theory calculations. A near degeneracy between an HCH bend overtone and CH stretch needed to be treated with a degenerate perturbation theory approach. Coupling terms from the second-order correction to the Hamiltonian were also included for the CH stretch overtones in the degenerate perturbation theory analysis. The resulting calculated spectrum had generally good agreement with the measured spectrum for when the OH radical was in its vibrationless state. The absence of the 2νOH +νOOH peak when the OH radical is in its first excited state was investigated through analysis of the first-order correction to the wavefunctions for the 2νOH, 2νOH +νOOH, and 3νOH states and through analysis of the expectation values of the internal coordinates for these states. From these analyses it was determined that the coupling of HOOC torsion to the 2νOH +νOOH state indicated that the HOOC torsion was a mechanism for intramolecular vibrational relaxation for the 2νOH +νOOH state, which results in the absence of this peak from the spectrum when the OH radical is in its first excited state.
Anne McCoy (Advisor)
Claudio Turro (Advisor)
Sherwin Singer (Committee Member)
Elliot Paquette (Committee Member)
162 p.
Chemistry
proton transfer; protonated water clusters; ion-water complexes; low frequency motions; OH stretches; H-bonding

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Citations

  • Dzugan, L. C. (2017). Theoretical Treatments of the Effects of Low Frequency Vibrations on OH Stretches in Molecules and Ion-Water Complexes that Undergo Large Amplitude Motions [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492688748608717

    APA Style (7th edition)

  • Dzugan, Laura. Theoretical Treatments of the Effects of Low Frequency Vibrations on OH Stretches in Molecules and Ion-Water Complexes that Undergo Large Amplitude Motions. 2017. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1492688748608717.

    MLA Style (8th edition)

  • Dzugan, Laura. "Theoretical Treatments of the Effects of Low Frequency Vibrations on OH Stretches in Molecules and Ion-Water Complexes that Undergo Large Amplitude Motions." Doctoral dissertation, Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1492688748608717

    Chicago Manual of Style (17th edition)

osu1492688748608717
366
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