lntracluster rearrangement of protonated nitric acid : Infrared spectroscopic studies of H + ( HN 0 3 ) ( H 2 0 ) n

The large differences between gas phase and solution phase basicities highlight the dramatic effects of solvation on ion chemistry. Strong aqueous acids such as HN03 can possess a proton affinity in the gas phase that is higher than that of H20. By clustering water molecules sequentially to a protonated nitric acid, the structure and properties of the ion will evolve towards the solvated form. Investigations of these hydrated cluster ions can provide insights into microscopic aspects of solvation. Protonation of nitric acid occurs in the liquid phase under nearly anhydrous conditions, e.g., in neat nitric acid and in concentrated sulfuric acid, by the reaction H+ +HN03 -.NOt +H20. 1 Upon addition of water, the nitronium ion (NOt> rapidly undergoes the reverse reaction and is therefore not present in aqueous solution. Experimental•3 and theoretical• studies have concluded that gas phase protonated nitric acid H2NOt possesses several isomers. The most stable form is a weakly bound complex of NOt and H20 and, as in nonaqueous liquids, the most basic site is on the OH group. Cacace eta/. 3 found evidence for a second isomer, (HOhNO+, which lies 10-20 kcal/mol higher and is formed by the protonation of a terminal oxygen atom of HN03 • Ab initio calculations by Lee and Rice confirmed the structures and relative stability of the two isomers, but the computed proton affinity for nitric acid is 182 ± 4 kcal!mol, in disagreement with Cacace's ICR bracketing measurement of 168 ± 3 kcal/mol and closer to an earlier measurement of 17 6 ± 7 kcal/mol. These values lead to large differences in the binding energy of the NO{(H20) complex (5-19 kcal/mol). The existence of hydrated H2NOt was inferred by Fehsenfeld et a!., who studied the ion chemistry of HN03 in a flowing afterglow apparatus. They observed rapid proton transfer from H30+ to HN03 , and attributed the decay of H2NOt to its association with H20. However, they were unable to detect hydrated clusters of protonated nitric acid, and postulated a fast reaction H+ (HN03 ) (Hp) +HP-H30+ (H20) +HN03 that would destroy the association products and convert NOt to HN03 • In an ab initio calculation, Grandinetti et a/. 6 predicted that the lowest energy structure of H+(HN03)(H20) is NOt(H20h. Kay eta/. formed the clusters H+(HN03)(HP)n (n=l-12) by ionization of neutral clusters in a beam 7 and attributed a minimum in the size distribution at n = 4 to the formation of solvated ion pairs in the neutral complexes, but made no conclusions regarding the ionic clusters. We have investigated the clusters H+(HN03)(H20)n for n = 0 to 4 by infrared vibrational predissociation spectroscopy. In this Communication, we present evidence for solvent-induced changes in the structure of protonated nitric acid.