Stability Constants and a New 1820 cm –1 Absorption Band of (H 2 O) 3 H + and (H 2 O) 4 H + in Liquid Acetonitrile Reveal Asymmetric Proton Hydration

Abstract The protonated water trimer (H2O)3H+ and tetramer (H2O)4H+ are largely symmetric in the gas phase and strongly asymmetric when solvated in polar liquid solutions (as-(H2O)3H+ and as-(H2O)4H+). as-(H2O)3H+ features a shared-proton motif (a deformed Zundel arrangement) deformed by a strong interaction with a third water molecule. The fourth water molecule in both the star-like and linear (H2O)4H+ arrangements was found computationally to interact less strongly with the proton than the two water–water interactions within the as-(H2O)3H+ core-structure complex, thereby making the solution-phase structure of (H2O)4H+ highly asymmetric. We have tested these theoretical predictions by probing protonated water clusters (H2O)nH+ with predetermined sizes n = 2–6, in acetonitrile. We observe for as-(H2O)3H+ a previously unreported weak IR absorption band at 1820 cm–1, which also appears in the higher n = 4–6 complexes. The stability constants of protonated water clusters decrease as the cluster size increases from 2 to 3 and then to 4 water molecules. The weakest interaction in the 4-molecule water cluster resembles water–water interactions in bulk water predicted by theory. We argue, based on our new experimental evidence, that the as-(H2O)3H+ arrangement, which is a central part of the as-(H2O)4H+ arrangements, best represents the core structure of the hydrated proton in aqueous solutions with far-reaching consequences for all fields of chemistry.