Direct Observation of Polaritonic Chemistry by Nuclear Magnetic Resonance Spectroscopy

Abstract

Polaritonic chemistry is emerging as a powerful approach to modifying the properties and reactivity of molecules and materials. However, probing how the electronics and dynamics of molecular systems change under strong coupling has been challenging due to the narrow range of spectroscopic techniques that can be applied in situ. Here we develop microfluidic optical cavities for vibrational strong coupling (VSC) that are compatible with nuclear magnetic resonance (NMR) spectroscopy using standard liquid NMR tubes. VSC is shown to influence the equilibrium between two conformations of a molecular balance sensitive to London dispersion forces, revealing an apparent change in the equilibrium constant under VSC. In all compounds studied, VSC does not induce detectable changes in chemical shifts, J‐couplings, or spin‐lattice relaxation times. This unexpected finding indicates that VSC does not substantially affect molecular electron density distributions, and in turn has profound implications for the possible mechanisms at play in polaritonic chemistry under VSC and suggests that the emergence of collective behavior is critical.

Cavity Quantum Electrodynamics Enables para- and ortho-Selective Electrophilic Bromination of Nitrobenzene

“…While a general theory for how the electronic density changes inside the cavity remains unknown, it will be the subject of future work to figure out design principles for at least a given class of reactions. On the other hand, in the collective VSC regime (when ω c ≈ 0.1 eV), recent experiments 55 show that the nuclear magnetic resonance (NMR) spectrum of molecules is not modified inside the cavity (i.e., there are no apparent NMR shifts under VSC), implying that the electronic density is not perturbed by the cavity under VSC conditions. This is an important distinction between the ESC and VSC coupling regimes.…”

Unraveling a Cavity-Induced Molecular Polarization Mechanism from Collective Vibrational Strong Coupling

“…However, the average cavityinduced polarization remains rather small, which seems in agreement with recent NMR experiments. 62 Notice this collectively induced local mechanism occurs without external driving; i.e., the sole presence of a thermal bath is sufficient.…”

“…The continuous distribution automatically implies the existence of hotspots within the molecular ensemble, where the collective coupling can strongly polarize single molecules and thus significantly alter their chemical properties. However, the average cavity-induced polarization remains rather small, which seems in agreement with recent NMR experiments . Notice this collectively induced local mechanism occurs without external driving; i.e., the sole presence of a thermal bath is sufficient.…”

“…However, the average cavity-induced polarization remains rather small, which seems in agreement with recent NMR experiments. 62 Notice this collectively induced local mechanism occurs without external driving; i.e., the sole presence of a thermal bath is sufficient. The detailed study of the physical properties of the hotspots and the polarization glass (e.g., thermodynamics, implied time scales, distribution, and frustration effects) will be left for future work, since, in analogy with spin-glasses, 61 these are most likely highly nontrivial, as well as strongly interconnected theoretical aspects, which will require considerable efforts.…”