Switching Site Selectivity in Alkoxyamine Hydration: From Lone-Pair Direction to Solvent Network Dominance.

Abstract

Predicting site selectivity during the earliest stages of solvation in multifunctional organic molecules remains a complex challenge in chemical physics. In this work, we use high-resolution rotational spectroscopy and quantum-chemical calculations to decode the hydration landscape of N,N-diethylacetyloxyamine (DEAcA), a molecule featuring competing sp3 nitrogen and carbonyl oxygen acceptors. While the isolated monomer preferentially adopts a syn configuration driven by the electric dipole moment minimization and the synergistic alleviation of oxygen-oxygen electrostatic repulsion, microsolvation ((H2O)n, n = 1-3) reveals an exclusive preference for the highly basic nitrogen site. We demonstrate that the 14N nuclear quadrupole coupling (NQC) constants provide direct experimental evidence of the cooperative polarization that strengthens the nascent water chain. Crucially, at the tetrahydrated level (n = 4), we identify a sharp transition in the solvation regime: the global minimum shifts to a cyclic water tetramer anchored at the carbonyl oxygen. This structural pivot marks the boundary where localized stereoelectronic control yields to the collective stabilization of the solvent network. These findings provide a rigorous benchmark for modeling the transition from molecular recognition to bulk-like solvation in complex organic systems.