Isomer-specific cryogenic ion vibrational spectroscopy of the D₂ tagged Cs⁺(HNO₃)(H₂O)_{: N =0-2} complexes: Ion-driven enhancement of the acidic H-bond to water

We report how the binary HNO₃(H₂O) interaction is modified upon complexation with a nearby Cs⁺ ion. Isomer-selective IR photodissociation spectra of the D₂-tagged, ternary Cs⁺(HNO₃)H₂O cation confirms that two structural isomers are generated in the cryogenic ion source. In one of these, both HNO₃ and H₂O are directly coordinated to the ion, while in the other, the water molecule is attached to the OH group of the acid, which in turn binds to Cs⁺ with its -NO₂ group. The acidic OH stretching fundamental in the latter isomer displays a ∼300 cm^-1 red-shift relative to that in the neutral H-bonded van der Waals complex, HNO₃(H₂O). This behavior is analyzed with the aid of electronic structure calculations and discussed in the context of the increased effective acidity of HNO₃ in the presence of the cation.