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

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.