Environment Rearrangement Slows Down the Tunneling Rates of Proton Transfer in Sandwich-Like Molecular Clusters.

We present the measurement of tunneling splitting for double proton transfer in a propiolic acid-formic acid dimer embedded within sandwich-like molecular clusters with fluorobenzene and helium, utilizing rotational spectroscopy. We systematically compare our new experimental results with previously measured experimental tunneling splittings for similar carboxylic acid molecular clusters, namely a propiolic acid-formic acid dimer and a formic acid dimer with and without fluorobenzene. The data reveal a consistent trend: each added molecular layer further suppresses the tunneling rate, with the ternary FA-PA@PhF cluster exhibiting a dramatically reduced splitting (50.89 MHz) relative to the bare dimer (291.43 MHz), and an additional decrease upon helium tagging (47.58 MHz). The comparison is also supported with theory, using ring-polymer instanton theory and by solving effective one-dimensional Schrödinger equation models. The observed trend of decreasing tunneling rates with increasing environment complexity suggests that the environment acts as a penalty on proton transfer, as explained by the Marcus-type environment reorganization model. These findings provide not only the most complex molecular system to date with spectroscopically resolved proton tunneling, but also a general conceptual framework for understanding and potentially engineering quantum proton dynamics in tailored molecular environments.