ORCID

0009-0008-8176-7655

Date of Award

6-25-2024

Author's School

Graduate School of Arts and Sciences

Author's Department

Chemistry

Degree Name

Doctor of Philosophy (PhD)

Degree Type

Dissertation

Abstract

Hydrogen bonding (H-bond) interactions are essential in dictating structures and properties of chemical and biological systems and are pivotal mediators in charge transport reactions and chemical recognition. Accurate description of H-bonds requires thorough characterization of the shared proton’s potential energy surface (PES) and molecular dynamics. Ultrafast nonlinear spectroscopic techniques, in particular, transient absorption (TA) and two dimensional infrared (2D IR) spectroscopy, have proven themselves valuable tools for probing molecular structures and dynamics mediated by H-bonding interactions. Herein, I describe the implementation of TA and 2D IR experiments to investigate the vibrational dynamics of the intramolecular H-bond in a series of β-diketones and the intermolecular H-bonding in meso Octamethylcalix[4]pyrrole (OMCP) complexed with chloride and bromide ions in dichloromethane (DCM) and chloroform (trichloromethane, TCM). Our investigation of β-diketones revealed surprisingly anharmonic shared proton PES’s resulting in strong intermode coupling. Polarization-sensitive TA measurements in the OH and OD stretch regions revealed biexponential behavior with notable isotopic differences in orientational dynamics. Modeling showed that the fast (<200 fs) orientational relaxation timescale can be accounted for by small angle rattling motions of the H/D atom along the H-bond along with fast energy relaxation out of the probed OH/OD stretch modes. The long-time component of the orientational relaxation were measured to occur on ∼600 fs and ∼2 ps for the light and deuterated isotopologues, respectively, with small discrepancies observed depending on the intramolecular H bond distance and asymmetry. The long-time components were interpreted in terms of H/D atom transfer events driven by collective intramolecular structural rearrangements, providing lower limit timescales for the H/D atom transfer in β-diketones at room temperature. Ultrafast IR spectroscopic analysis of OMCP∙Cl‒ and OMCP∙Br‒ in DCM and TCM revealed spectral and dynamical differences arising from both the identity of the halide and the solvent. OMCP∙Br‒ complexes in both solvents displayed slower intermolecular vibrational relaxation dynamics, consistent with weaker H-bonding interactions with OMCP compared to chloride. To our surprise, more perturbative solvent effects were observed in TCM, despite DCM’s larger dielectric constant. Appearance of cross peak features between OMCP·X‒ complexes and TCM and long-lived transient signatures suggested strong solvent coupling interactions in TCM solvent. OMCP∙Cl‒ complexes displayed more rapid and larger structural fluctuations compared to bromide complexes due to higher sensitivity to the evolving solvation shell. The results raise questions regarding the accuracy of electrostatic models often used to describe anion binding interactions with macrocyclic receptors and the possible role of countercation effects in solvents with low dielectric constants.

Language

English (en)

Chair and Committee

Joseph Fournier

Committee Members

Richard Mabbs

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