Vibrational Signatures of HNO 3 Acidity When Complexed with Microhydrated Alkali Metal Ions, M + ·(HNO 3 )(H 2 O) n =5 (M = Li, K, Na, Rb, Cs), at 20 K.

The speciation of strong acids like HNO ₃ under conditions of restricted hydration is an important factor in the rates of chemical reactions at the air-water interface. Here, we explore the trade-offs at play when HNO ₃ is attached to alkali ions (Li ⁺ -Cs ⁺ ) with four water molecules in their primary hydration shells. This is achieved by analyzing the vibrational spectra of the M ⁺ ·(HNO ₃ )(H ₂ O) ₅ clusters cooled to about 20 K in a cryogenic photofragmentation mass spectrometer. The local acidity of the acidic OH group is estimated by the extent of the red shift in its stretching frequency when attached to a single water molecule. The persistence of this local structural motif (HNO ₃ -H ₂ O) in all of these alkali metal clusters enables us to determine the competition between the effect of the direct complexation of the acid with the cation, which acts to enhance acidity, and the role of the water network in the first hydration shell around the ions, which acts to counter (screen) the intrinsic effect of the ion. Analysis of the vibrational features associated with the acid molecule, as well as those of the water network, reveals how cooperative interactions in the microhydration regime conspire to effectively offset the intrinsic enhancement of HNO ₃ acidity afforded by attachment to the smaller cations.