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USING ULTRAFAST VIBRATIONAL SPECTROSCOPY FOR A COMPREHENSIVE UNDERSTANDING OF STRUCTURAL AND ROTATIONAL MOTIONS FOR WATER TO PROTIC IONIC LIQUIDS

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 (Unpublished)

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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:
CreatorsEmailPitt UsernameORCID
Johnson, Clintoncaj52@pitt.educaj520000-0002-4141-9495
ETD Committee:
TitleMemberEmail AddressPitt UsernameORCID
Committee ChairGarrett-Roe, Seansgr@pitt.edusgr0000-0001-6199-8773
Committee MemberWaldeck, David H.dave@pitt.edu0000-0003-2982-0929
Committee MemberLaaser, Jennifer L.j.laaser@pitt.edu0000-0002-0551-9659
Committee MemberLambrecht, Daniel S.dlambrecht@fgcu.edu0000-0001-5326-0234
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 2020 14:09
URI: http://d-scholarship.pitt.edu/id/eprint/39494

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