The forced hydrolysis reaction of aqueous aluminum ion (Al 3+ ) is of critical importance in Al chemistry, but its microscopic mechanism has long been neglected. Herein, density functional calculations reveal an external OH − -induced barrierless proton dissociation mechanism for the forced hydrolysis of Al 3+ (aq). Dynamic reaction pathway modeling results show that the barrierless deprotonations induced by the second-or third-shell external OH − proceed via the concerted proton transfer through H-bond wires connected to the coordinated waters, and the inducing ability of the external OH − decreases with increasing hydration layers between Al(H 2 O) 6 3+ and the external OH − . The OH − -induced forced hydrolysis mechanism of Al 3+ (aq) is quite different from its self-hydrolysis mechanism without OH − . The inducing ability is a unique characteristic of OH − , rather than other anions such as F − or Cl − .aluminium ion, barrierless proton dissociation, density functional theory, forced hydrolysis,The hydrolysis chemistry of aluminum (Al) is central to environmental science, human health and industry activities. [1][2][3] Extensive efforts have been devoted to exploring the self and forced hydrolysis reactions of Al 3+ (aq) in aqueous solution that form various monomeric and polymeric hydrolytic Al species. [4] Unlike the self-hydrolysis of Al 3+ (aq) in acidic solutions whose detailed reaction steps have been carefully studied using both experimental [5,6] and theoretical [7][8][9][10][11] methods, the microscopic reaction kinetics for the forced hydrolysis of Al 3+ (aq) under elevated pH conditions such as base titration is rarely considered, and the role of hydroxyl in the formation mechanisms of the hydrolytic Al species remains poorly understood. This has hindered the precision studies of the hydrolysis-polymerization mechanisms of Al species.Conventionally, the self-hydrolysis of Al 3+ (aq) is viewed as the spontaneous proton removal on Al 3+ inner-shell coordinated waters with the assistance of adjacent solvent water molecules, [12] while the forced hydrolysis of Al 3+ (aq) involves the participation of external OH − . The firstorder forced hydrolysis of Al 3+ (aq), which acts as the first reaction step in forming polymeric hydroxyl Al species upon base titration, is usually expressed as: [13,14] Al 3 + + OH − ! AlðOHÞ 2 +AlðH 2 OÞ 6 3 + + OH − ! AlðOHÞðH 2 OÞ 5 2 + + H 2 O:In aqueous solution, the Al 3+ ion exists mainly in the hydrated form of Al(H 2 O) 6 3+ rather than the naked Al 3+ , thus, Equation (2) is more suitable to describe the reaction in aqueous solution. The reaction may proceed via the pathway that a proton on an Al(H 2 O) 6 3+ inner-shell coordinated water dissociates and moves into OH − , or in other words, the external OH − captures the proton. Herein, this possible reaction pathway is modeled using the density functional theory-quantum chemical cluster model (DFT-CM) method, [15,16] in order to elucidate the molecular mechanism of the forced hydrolysis reaction of Al 3+ (aq) in aqueou...