Spectroscopic Probes of Low-Barrier Proton-Transfer Dynamics

The diverse discipline of molecular spectroscopy, which has profited tremendously from the advent of tunable laser sources, has transformed the scientific community’s knowledge of the microscopic world, allowing the study of perplexing quantum-chemical phenomena. Classically-hindered proton transfer, a multidimensional process mediated by nuclear-quantum effects (e.g., potential-barrier tunneling), is a chemical transformation that forms the crux of all acid/base chemistry. Although extensive research efforts have aimed to establish the paradigms that govern this transformation, a full understanding has proven elusive with questions continuing to emerge.Exploring the proton-transfer reaction, and the related concept of hydrogen bonding, has benefitted greatly from investigations of model systems where the hydron migration is facilitated by a symmetric double-minimum potential well. In such molecular species, the spectroscopic signature of tunneling-induced bifurcations gives a direct measure of reaction rates, thus enabling the extraction of dynamical information. This thesis focuses on a relatively unexplored member of this group, 6-hydroxy-2- formylfulvene or HFF, which exhibits a quasi-linear reaction site on a conjugated framework – a structural arrangement that engenders a low-barrier hydrogen bonding (LBHBing) motif. Additionally, HFF has been suggested to experience a drastic quenching in dynamics accompanying π*←π electronic excitation that has been attributed to a substantial change in reaction mechanism whereby the strictly planar reaction coordinate in the X1A1 state transforms into an out-of-plane pathway involving substantial heavy- atom motion in the A1B2 (π∗π) state. The unique structural and dynamical characteristics of HFF create a potent platform for studying the effects of isotopic substitution and vibrational excitation on tunneling phenomena as presented in this thesis. The origin band of HFF and its monodeuterated isotopolog, HFF-d, were probed using polarization-resolved degenerate four-wave mixing (DFWM), an absorption-based technique that provides near-rotational resolution. This enabled the measurement of tunneling-induced bifurcations for the vibrationless A1B2 states of HFF and HFF-d, yielding Δ = 0.1009(43) cm-1 and Δ = 0.074(10) cm-1, respectively. These values imply a small deuterium kinetic isotope effect (DKIE) of Λ = 1.36 (relative to the analogous ground-state value of Λ = 3.44) that can be rationalized by considering the substantial heavy-atom motion (and the corresponding large effective mass) that is involved in the excited-state proton-transfer process, which dominates the reaction and makes the change in the mass of the shuttling hydron less consequential. Similar DFWM studies also were performed for two higher-energy vibronic bands of A1B2 (π∗π) HFF and HFF-d, ν4(a1), a chelate-ring breathing mode, and ν7(b2), a chelate-ring deformation mode. Although vibrational excitation can have a substantial effect on proton-transfer dynamics, the two studied modes did not couple effectively to the reaction coordinate and, therefore, resulted in minimal changes to measured tunneling splittings, thereby highlighting the distinct nature of the multidimensional out-of-plane tunneling mechanism that governs the π * ← π excited state

VIBRATIONAL SPECIFICITY AND ISOTOPIC DEPENDENCE OF PROTON-TRANSFER DYNAMICS IN ELECTRONICALLY EXCITED 6-HYDROXY-2-FORMYLFULVENE

The vibrational specificity and isotopic dependence of hindered proton-transfer dynamics have been explored in the lowest-lying singlet excited state, \textit{\~{A}}$^{1}$B$_{2}$ ($\pi$$^{*}$$\pi$), of 6-hydroxy-2-formylfulvene (HFF) and its monodeuterated isotopolog (HFF-\textit{d}). Both systems have been probed under bulk-gas conditions by employing polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy, where judicious selection of incident and detected polarization geometries served to alleviate spectral complexity and to allow for the quantitative extraction of rotation-tunneling information. The observed $>$1000-fold decrease in tunneling rate that accompanies the $\pi$$^{*}$$\leftarrow$ $\pi$ electron promotion (transitioning from ultrafast ground-state dynamics\footnote{Z. N. Vealey, L. Foguel and P. H. Vaccaro, \textit{J. Phys. Chem. Lett.} \textbf{9}, 4949 (2018)} to near-complete quenching of analogous excited-state behavior) makes HFF a compelling model system for investigating the nuanced nature of low-barrier hydrogen bonding and its ability to regulate attendant hydron-migration events. A thorough analysis of low-energy vibronic bands in the \textit{\~{A}}$^{1}$B$_{2}$ manifold will be presented, with the dependence of unimolecular reactivity on heavy atom motion and isotopic modification being discussed in the context of structural predictions emerging from high-level quantum-chemical calculations

MICROSOLVATION AND THE EFFECTS OF NON-COVALENT INTERACTIONS ON INTRAMOLECULAR DYNAMICS

Physicochemical processes brought about by non-covalent interactions between neighboring molecules are undeniably of crucial importance in the world around us, being responsible for effects ranging from the subtle (yet precise) control of biomolecular recognition events to the very existence of condensed phases. Of particular interest is the differential ability of distinct non-covalent forces, such as those mediated by dispersion-dominated aryl ($pi$-$pi$) coupling and electrostatically-driven hydrogen bonding, to affect unimolecular transformations by altering potential surface topographies and the nature of reaction coordinates. A concerted experimental and computational investigation of “microsolvation” (solvation at the molecular level) has been undertaken to elucidate the site-specific coupling between solute and solvent degrees of freedom, as well as attendant consequences for the efficiency and pathway of intrinsic proton-transfer dynamics. Targeted species have been synthesized in situ under “cold” supersonic free-jet expansion conditions (textit{T}$_{rot}$ $approx$ 1-2K) by complexing an active (proton-transfer) substrate with various ligands (e.g., water isotopologs and benzene derivatives) for which competing interaction mechanisms can lead to unique binding motifs. A series of fluorescence-based spectroscopic measurements have been performed on binary adducts formed with the prototypical 6-hydroxy-2-formylfulvene (HFF) system, where a quasi-linear intramolecular O–H···O bond and a zero-point energy that straddles the proton-transfer barrier crest synergistically yield the largest tunneling-induced splitting ever reported for the ground electronic state of an isolated neutral molecule. Such characteristics afford a localized metric for unraveling incipient changes in unimolecular reactivity, with comparison of experimentally observed and quantum-chemical predicted rovibronic landscapes serving to discriminate complexes built upon electrostatic (hydrogen-bonding) and dispersive (aryl-coupling) forces

VIBRATIONAL SPECIFICITY OF PROTON-TRANSFER DYNAMICS IN ELECTRONICALLY EXCITED 6-HYDROXY-2-FORMYLFULVENE

The transduction of protons between distinct donor and acceptor sites, as often directed by attendant hydrogen bonds, is a ubiquitous chemical transformation essential for all of acid/base chemistry, yet the role of selective nuclear displacements in this deceptively simple process remains a topic of active investigation. Polarization-resolved degenerate four-wave mixing (DFWM) spectroscopy was employed to investigate the vibrational specificity of proton-transfer dynamics in the lowest-lying singlet excited state, \textit{\~{A}}$^{1}$B$_{2}$ ($\pi$$^{*}$$\pi$), of 6-hydroxy-2-formylfulvene (HFF), with judicious selection of incident and detected polarization geometries allowing for the quantitative extraction of refined rotation-tunneling parameters. While the zero-point level of the ground electronic state [\textit{\~{X}}$^{1}$A$_{1}$] straddles the barrier crest for hydron migration and thus affords a prototypical example of low-barrier hydrogen bonding,\footnote{Z. N. Vealey, L. Foguel and P. H. Vaccaro, \textit{J. Phys. Chem. Lett.} \textbf{9}, 4949 (2018).} this model system also has been found to undergo a dramatic change in dynamics upon $\pi$$^{*}$$\leftarrow$ $\pi$ electron promotion – as reflected by a $>$1000-fold decrease in tunneling rates.\footnote{Z. N. Vealey, L. Foguel and P. H. Vaccaro, \textit{J. Phys. Chem. A.} \textbf{123}, 6506 (2019).} This talk will present complementary experimental and computational analyses of vibronic levels in the pertinent \textit{\~{A}}$^{1}$B$_{2}$ ($\pi$$^{*}$$\pi$) manifold designed to unravel the dependence of unimolecular reactivity on heavy atom motion and to elucidate the disparate behavior observed for analogous \textit{\~{X}}$^{1}$A$_{1}$ features