Exploring the Influence of Non-Covalent Interactions on Tunneling Phenomena via Cryogenic Ion Vibrational Spectroscopy
Final Report Abstract
Quantum mechanical tunneling offers an alternative pathway to the classical thermal one, by penetrating the energetic barrier separating reactant(s) from product(s). It impacts all reactions (albeit to widely differing degrees), thus making it paramount to understand its guiding principles in order to more accurately predict the selectivity and kinetics of reactions under different reaction conditions. To this effect, tunneling phenomena involving the motions of both light hydrogen atoms as well as heavier atoms like carbon have been extensively investigated in neutral molecules; however, tunneling reactions involving ionic species have rarely been studied. This project addresses this gap in fundamental knowledge about the impact of quantum mechanical tunneling on the reactivity of ionic species via cryogenic ion vibrational spectroscopy. The prototypical phenyl anion (phenide) was chosen as model system for investigating the potential contribution of intramolecular hydrogen tunneling to the reactivity of an ionic species. This choice reflects both the importance of the phenides as a synthon often used in organic synthesis (in the form of e.g. phenyllithium) as well as the reported propensity of substituted phenides to undergo hydrogen shifts, inferred from prior mass spectrometric studies. In addition, phenides are readily accessible upon gas-phase decarboxylation of benzoates via collision-induced dissociation. In order to detect the (otherwise degenerate) circumambulatory charge migration in phenide(s) via infrared spectroscopy, a positional marker in the form of a benzoyl moiety was utilized. Thus, the different isomers resulting from the charge migration (i.e. hydrogen shifts) could be unambiguously identified via their unique vibrational spectra. It was found that the investigated phenides 5 and 6 do not rearrange at cryogenic temperatures, retaining the charge at the carbon atom that carried the carboxylate group prior to decarboxylation. However, decarboxylation at room temperature results in a mixture of both this nascent isomer as well as one where the charge has migrated to a position in which it is most stabilized, i.e. ortho to the benzoyl substituent. This o-benzyol phenide 4, however, appears to only be metastable as it undergoes a facile intramolecular nucleophilic attack to the pendant phenyl group, forming 7. Quantumchemical calculations suggest that the circumabulatory charge migration proceeds via consecutive 1,2-H shifts through significant activation barriers of ~ 65 kcal mol-1, while the subsequent ring closure affords only ~ 11 kcal mol-1. The low mass of the migrating hydrogen atom and the small distances it is displaced during these rearrangements suggest a potential contribution from quantummechanical tunneling. Control experiments with perdeuterated 4BBA (d9-3) gave preliminary evidence for a large kinetic isotope effect, hallmark of tunneling reactions, but may be skewed by the massselectivity of the experiments masking H/D-exchange of this substrate. The latter may indicate proton shuttling across the phenide scaffold via e.g. H2O and necessitates further experiments in the future.
Publications
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Structures and Chemical Rearrangements of Benzoate Derivatives Following Gas Phase Decarboxylation. Journal of the American Society for Mass Spectrometry, 33(10), 1914-1920.
Perez, Evan H.; Schleif, Tim; Messinger, Joseph P.; Rullán, Buxó Anna G.; Moss, Olivia C.; Greis, Kim & Johnson, Mark A.
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”Hydrogen tunneling in the 1,2-H shifts of phenyl anions in gas phase” at Quantum Atomic and Molecular Tunneling Systems (QAMTS), 15.-20.5.2022, Canmore, Canada
Tim Schleif
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Characterization of Oxidation Products from HOCl Uptake by Microhydrated Methionine Anions Using Cryogenic Ion Vibrational Spectroscopy. The Journal of Physical Chemistry A, 127(19), 4269-4276.
Stropoli, Santino J.; Greis, Kim; Schleif, Tim & Johnson, Mark A.
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Isomer-Specific Heavy-Atom Tunneling in the Ring Expansion of Fluorenylazirines. The Journal of Organic Chemistry, 88(13), 7893-7900.
Rowen, Julien F.; Beyer, Frederike; Schleif, Tim & Sander, Wolfram
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“Characterization of ionic products from ESI droplets using cryogenic ion chemistry and spectroscopy”, AFOSR MURI workshop on Mechanistic Studies of Microdroplet Chemistry, 26.-28.1.2023, San Diego, USA
Tim Schleif
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“Heavy-Atom Tunneling - Impact on Reactivity of Organic Molecules and Solvation Control of Tunneling Reactions” at Gordon Research Seminar and Conference, 24.06.2023 – 30.06.2023, Holderness, USA
Tim Schleif
