Johnson, Clinton
(2020)
USING ULTRAFAST VIBRATIONAL SPECTROSCOPY FOR A COMPREHENSIVE UNDERSTANDING OF STRUCTURAL AND ROTATIONAL MOTIONS FOR WATER TO PROTIC IONIC LIQUIDS.
Doctoral Dissertation, University of Pittsburgh.
(Unpublished)
Abstract
In this work, two-dimensional infrared (2D-IR) spectroscopy investigates the timescale of solvent fluctuations for proton and hydride transfers. To elucidate hydride transfer dynamics, the BH stretch of \ce{BH4-} is probed in various solvents from \ce{H2O} to ionic liquids (ILs). For proton transfer dynamics, a vibrational probe (\ce{SCN-}) explores the three-dimensional hydrogen bonding environment of a protic ionic liquid (PIL).
\ce{BH4-} is first investigated in increasing NaOH concentrations to develop a molecular understanding of suppressing the hydrogen evolution reaction. As the concentration increases, the timescale of frequency fluctuations decrease. Born Oppenheimer molecular dynamics (BOMD) simulations suggest that a crowding effect of ions around \ce{BH4-} inhibits the rearrangement of dihydrogen bonds between \ce{BH4-} and \ce{H2O}. To completely suppress the hydrogen evolution reaction, ILs with \ce{BH4-} as the anion are investigated. The linear and 2D-IR spectra of the antisymmetric BH stretch of \ce{BH4-} are complicated due to Fermi resonances. The narrow linear and 2D-IR linewidths of \ce{BH4-} in an IL allow a comprehensive assignment of all diagonal peaks and crosspeaks. Confirmed with a model Hamiltonian, two anharmonicities for the antisymmetric BH stretch of \ce{BH4-} are characterized.
Polarization- and temperature-dependent 2D-IR is employed to investigate the hydrogen bonding network of the PIL ethyl-ammonium nitrate (EAN). \ce{SCN-} experiences two hydrogen bonding subensembles in EAN as two separate vibrational relaxation times are resolved. Furthermore, the polarization-weighted frequency fluctuation correlation function can be separated into two components: structural spectral diffusion (SSD) and reorientation-induced spectral diffusion (RISD). For \ce{SCN-} in EAN, the timescales of frequency fluctuations are in the rotational limit as the SSD is unresolved. Temperature-dependent 2D-IR extracts the enthalpy and entropy of activation for frequency fluctuations. For \ce{SCN-} in EAN, the enthalpy of activation for rotational motions are similar as to \ce{SCN-} in \ce{H2O}, and this suggests that the breaking and forming of hydrogen bonds around \ce{SCN-} undergoes a similar mechanism in EAN as in \ce{H2O}.
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Details
| Item Type: |
University of Pittsburgh ETD
|
| Status: |
Unpublished |
| Creators/Authors: |
|
| ETD Committee: |
|
| Date: |
16 September 2020 |
| Date Type: |
Publication |
| Defense Date: |
7 July 2020 |
| Approval Date: |
16 September 2020 |
| Submission Date: |
31 July 2020 |
| Access Restriction: |
1 year -- Restrict access to University of Pittsburgh for a period of 1 year. |
| Number of Pages: |
214 |
| Institution: |
University of Pittsburgh |
| Schools and Programs: |
Dietrich School of Arts and Sciences > Chemistry |
| Degree: |
PhD - Doctor of Philosophy |
| Thesis Type: |
Doctoral Dissertation |
| Refereed: |
Yes |
| Uncontrolled Keywords: |
hydrogen bonding, ultrafast vibrational spectroscopy, protic ionic liquids, ionic liquids, two-dimensional infrared spectroscopy |
| Date Deposited: |
16 Sep 2020 14:09 |
| Last Modified: |
16 Sep 2021 05:15 |
| URI: |
http://d-scholarship.pitt.edu/id/eprint/39494 |
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