Size-Dependent Onset of Nitric Acid Dissociation in Cs⁺·(HNO₃)(H₂O)_n=0–11 Clusters at 20 K

Abstract

We report the water-mediated charge separation of nitric acid upon incorporation into size-selected Cs + •(HNO 3 )(H 2 O) n=0−11 clusters at 20 K. Dramatic spectral changes are observed in the n = 7−9 range that are traced to the formation of many isomeric structures associated with intermediate transfer of the acidic proton to the water network. This transfer is complete by n = 10, which exhibits much simpler vibrational band patterns consistent with those expected for a tricoordinated hydronium ion (the Eigen motif) along with the NO stretching bands predicted for a hydrated NO 3 − anion that is directly complexed to the Cs + cation. Theoretical analysis of the n = 10 spectrum indicates that the dissociated ions adopt a solvent-separated ion-pair configuration such that the Cs + and H 3 O + cations flank the NO 3 − anion in a microhydrated salt bridge. This charge separation motif is evidently assisted by the electrostatic stabilization of the product NO 3 − /H 3 O + ion pair by the proximal metal ion.

Disentangling Sum-Frequency Generation Spectra of the Water Bending Mode at Charged Aqueous Interfaces

“…This mode has been probed using vibrational spectroscopies, because it reports on the local structure of the hydrogen-bond network in water; when water is strongly (weakly) hydrogen-bonded, the frequency of the bending mode is blue-shifted (red-shifted). , Probing the bending mode of water has several advantages over probing the O–H stretch mode. Whereas the O–H stretch mode of water cannot be spectrally distinguished from other molecules containing OH-groups, the H–O–H water bending mode is specific to water. − Also, the vibrational coupling between bending modes has a limited impact on its spectral response, , in sharp contrast to the O–H stretch mode. , Furthermore, understanding the bending mode is essential to unveil the vibrational energy transfer from the O–H stretch mode of water and the amide mode of proteins to the local heat, − because the bending mode is believed to be an essential intermediate step to receive excess vibrational energy and release it to the local heat. ,− …”

Vibrational Signatures of HNO3 Acidity When Complexed with Microhydrated Alkali Metal Ions, M+·(HNO3)(H2O)n=5 (M = Li, K, Na, Rb, Cs), at 20 K

“…This metric was explored in detail, for example, in a study that illustrated how the frequency of the shared proton in a series of proton-bound dimers B 1 :H + :B 2 depends on the proton affinities of the Bronsted bases B 1 and B 2 . In the case of the Cs + ·(HNO 3 )­(H 2 O) n system, clusters formed by electrospray ionization can occur with one water molecule attached to the acid, while the remaining waters reside in the first hydration shell of the ion. , For the n = 1 system, the key bands and structure of this ternary complex are indicated in Figure b, where a red shift of 867 cm –1 is observed for ν OH a–w relative to the value in the bare ion Cs + ·(HNO 3 ) (blue, ν OH a in Figure a and navy blue, ν OH a–w in Figure b) . To put this in context, the addition of a water molecule to HNO 3 in carbon tetrachloride causes the OH stretch to red-shift relative to the solvated acid OH stretch ( CC l 4 ν OH a ) by only ∼ 385 cm –1 .…”

“…The unusual behavior of HNO 3 at the air–water interface is currently under investigation with a variety of theoretical and experimental methods. − Specifically, it has been proposed that this strong acid (under typical aqueous conditions) is only partially ionized near the water surface where the counterions are less efficiently solvated. ,, Understanding the nature of aqueous HNO 3 in pure water, as well as in the presence of electrolytes, has motivated several theoretical and experimental efforts designed to establish how its p K a depends on the local environment. − These involve the elucidation of HNO 3 behavior at the air–water interface, as well as in well-defined cluster systems (e.g., Cs + ·(HNO 3 )­(H 2 O) n =0–11 ). ,− The advantage of the cluster regime is that electronic structure calculations can be applied to harvest the structural information encoded in experimental vibrational band patterns obtained with size-selective, cryogenic photofragmentation mass spectrometry . Using this approach, for example, Mitra et al recently observed the cluster-size-dependent onset of the solvent-separated, NO 3 – /H 3 O + ion pair in the ∼20 K Cs + ·(HNO 3 )­(H 2 O) n system at n ∼ 10. Calculations revealed the critical role played by the proximal Cs + ion to stabilize the incipient NO 3 – conjugate base in a water network-mediated, Cs + /NO 3 – /H 3 O + salt bridge arrangement.…”

“…Thus, the ν OH a–w band in the isolated ternary Cs + ·(HNO 3 )­(H 2 O) complex lies 459 cm ‑1 below than that of the binary neutral complex HNO 3 -H 2 O, , reflecting the enhancement of HNO 3 acidity by the proximal ion. This local HNO 3 –H 2 O motif was observed to persist up to n = 6 in the Cs + ·(HNO 3 )­(H 2 O) n system, and the ν OH a–w band was observed to gradually blue-shift back toward the bare value as water molecules were added to the first hydration shell of the ion . Since the fundamental cause for this shift is traced to the electric field generated by the ion, one question raised by that observation is how the ν OH a–w red shift depends on the ionic radius of the metal ion.…”

Water Network Shape-Dependence of Local Interactions with the Microhydrated −NO2– and −CO2– Anionic Head Groups by Cold Ion Vibrational Spectroscopy

“…The vibrational band patterns displayed by size-selected M ± ·(H 2 O) n cluster ions arising from the molecular ions, M ± , as well as those arising from the water molecules, encode how the local interactions are modulated by the shapes of the water networks. , At a qualitative level, molecular ions can induce an electric dipole moment in the first hydration shell arising from partial orientation of the water molecules, which in turn imposes a solvent “reaction field” , on the ion. This solvent electric field can then distort the embedded ion by both electronic and structural contributions to its polarizability .…”