Role of Water in Stabilizing Ferricyanide Trianion and Ion-Induced Effects to the Hydrogen-Bonding Water Network at Long Distance

Abstract

Structures and reactivities of gaseous Fe(CN)(6)(3-)(H(2)O)n were investigated using infrared photodissociation (IRPD) kinetics, spectroscopy, and computational chemistry in order to gain insights into how water stabilizes highly charged anions. Fe(CN)(6)(3-)(H(2)O)(8) is the smallest hydrated cluster produced by electrospray ionization, and blackbody infrared dissociation of this ion results in loss of an electron and formation of smaller dianion clusters. Fe(CN)(6)(3-)(H(2)O)(7) is produced by the higher activation conditions of IRPD, and this ion dissociates both by loss of an electron and by loss of a water molecule. Comparisons of IRPD spectra to those of computed low-energy structures for Fe(CN)(6)(3-)(H(2)O)(8) indicate that water molecules either form two hydrogen bonds to the trianion or form one hydrogen bond to the ion and one to another water molecule. Magic numbers are observed for Fe(CN)(6)(3-)(H(2)O)n for n between 58 and 60, and the IRPD spectrum of the n = 60 cluster shows stronger water molecule hydrogen-bonding than that of the n = 61 cluster, consistent with the significantly higher stability of the former. Remarkably, neither cluster has a band corresponding to a free O-H stretch, and this band is not observed for clusters until n ≥ 70, indicating that this trianion significantly affects the hydrogen-bonding network of water molecules well beyond the second and even third solvation shells. These results provide new insights into the role of water in stabilizing high-valency anions and how these ions can pattern the structure of water even at long distances.

Nanometer patterning of water by tetraanionic ferrocyanide stabilized in aqueous nanodrops

“…Three isolated species without hydrate distributions are observed, Fe(CN) 5 2– ( m / z = 92.9), Fe(CN) 4 2– ( m / z = 79.9) and Fe(CN) 3 – ( m / z = 133.9), which are formed via sequential ligand loss from the ferricyanide dianion. 21 A summary of all hydrated species formed by nESI is given in Table 1 and the isolated distributions of each hydrated ion are shown in Fig. 2.…”

“…27 Recent results show hydrates of the ferricyanide trianion, Fe(CN) 6 3– (H 2 O) n , are unstable to electron ejection with a critical cluster size of n c = 8, which IRPD spectroscopy and theory show is still within the first solvation shell of the ion. 21 Although isolated P 3 O 9 3– and Co(NO 2 ) 6 3– have been reported previously, 28 subsequent experiments show n c = 6 is required to stabilize P 3 O 9 3– with respect to charge separation and no evidence for isolated Co(NO 2 ) 6 3– was found under identical experimental conditions for forming P 3 O 9 3– (H 2 O) n and Fe(CN) 6 3– (H 2 O) n . 21 Therefore, Fe(CN) 6 3– and P 3 O 9 3– are the only small, high-charge density trianions that have been observed in gas phase aqueous clusters and at minimum require first-shell hydration for thermodynamic stability.…”

“…37,38 Using IRPD spectroscopy of mass-selected, ion-containing water clusters, orientation effects on water by a single solvated ion free of any counterion contributions can be measured. 21,39,40 The sensitivity of this technique is not limited by the ratio of water molecules to ions. As a result, ion–water interactions can be measured to a hypothetical infinite dilution.…”

“…IRPD spectra for SO 4 2– and Fe(CN) 6 3– show that the non-bonded O–H stretch is absent with droplet radii up to ∼0.7 and 0.8 nm, respectively, as a result of long-distance orientation effects by the MCAs. 21,39 …”

Making Mass Spectrometry See the Light: The Promises and Challenges of Cryogenic Infrared Ion Spectroscopy as a Bioanalytical Technique

“…FTICR traps have already been demonstrated in cryogenic experiments, at least down to 133K [54, 55]. A disadvantage of the induced current detection scheme for FTICR is the lower sensitivity vis-á-vis electron multiplier detection (by about two orders of magnitude) [56].…”