Nuclear Quantum Effects on the Organic Bifurcation Reaction in Microsolvated Water Clusters: Ring-Polymer Molecular...

Solvent environments often reshape reaction mechanisms compared to those obtained in the gas phase or in nonpolar solvents. Recently, explicit solvation models-where individual solvent molecules are treated-have been increasingly employed in simulations of organic reactions to capture the dynamic influence of solvent motions. In aqueous systems, the incorporation of nuclear quantum effects (NQEs) is particularly crucial for accurately describing both structural and dynamical features. Here, we investigate the bifurcation reaction between 2-aminoacrolein and 1,3-butadiene in microsolvated (H₂O)_n clusters (n = 5, 15, 45) using ring-polymer molecular dynamics (RPMD), and compare the results with our previous classical molecular dynamics (classical MD) simulations. The branching fractions obtained from RPMD trajectories exhibit an increased tendency toward the minor (4 + 2) product pathway-equivalently, a lower fraction of the dominant (4 + 3) channel-compared with classical MD, owing to zero-point energy contributions distributed across all vibrational modes of the system. Moreover, RPMD reveals significantly accelerated proton-transfer events, indicating that nuclear quantum effects, including zero-point energy and proton delocalization, contribute substantially even at 300 K. These findings demonstrate that reliable prediction of aqueous branching behavior and proton-transfer kinetics requires both explicit solvation and rigorous inclusion of NQEs.