Gas Phase Hydrolysis and Oxo‐Exchange of Actinide Dioxide Cations: Elucidating Intrinsic Chemistry from Protactinium to Einsteinium

Abstract

Gas‐phase bimolecular reactions of metal cations with water provide insights into intrinsic characteristics of hydrolysis. For the actinide dioxide cations, actinyl(V) AnO2+, melding of experiment and computation provides insights into trends for hydrolysis, as well as for oxo‐exchange between actinyls and water that proceeds by a hydrolysis pathway. Here this line of inquiry is further extended into the actinide series with CCSD(T) computations of potential energy surfaces, for the reaction pathway for oxo‐exchange through hydrolysis of nine actinyl(V) ions, from PaO2+ to EsO2+. The computed surfaces are in accord with previous experimental results for oxo‐exchange, and furthermore predict spontaneous exchange for CmO2+, BkO2+, CfO2+ and EsO2+, but not for AmO2+. Natural Bond Order analysis of the species involved in both hydrolysis and oxo‐exchange reveals an inverse correlation between the barrier to hydrolysis and the charge on the actinide centre, q(An). Based on this correlation, it can be concluded that hydrolysis, and related phenomena such as oxo‐exchange, become less favourable as the charge on the metal centre decreases. The new results provide a straightforward rationalization of trends across a wide swathe of the actinide series.

Coupled Cluster Studies of Platinum–Actinide Interactions. Thermochemistry of PtAnOn+ (n = 0–2 and An = U, Np, Pu)

“…The final BDEs are generally in good agreement with the experimental values of Marçalo and Gibson given the relatively large uncertainties of the latter. The present results are also consistent with previously reported CCSD­(T) values …”

“…The present results are also consistent with previously reported CCSD(T) values. 69 III.C. Thermochemistry of PtAnO n+ .…”

Intrinsic reactivity of [OUCH]+: Apparent synthesis of [OUS]+ by reaction with CS2

“…Addition of a single 18 O atom is easy to rationalize using the pathways shown in Scheme 2a. Incorporation of a second 18 O label suggests that the oxo‐ligand in [OUS] + is labile, which would be consistent with several previous gas‐phase studies of oxo‐ligand exchange of actinyl species 17–19 . The observation that [UO 2 ] + is created spontaneously by reaction of [OUS] + with O 2 indicates that the appearance of the species in the spectra shown in Figure 1 represents reaction of both [O=U≡CH] + and [OUS] + (once formed by reaction with CS 2 ) with background O 2 in the ion trap.…”

“…Incorporation of a second 18 O label suggests that the oxo-ligand in [OUS] + is labile, which would be consistent with several previous gas-phase studies of oxo-ligand exchange of actinyl species. [17][18][19] The observation that…”

Chemical Understanding of Actinide Separations

“…144 Similarly, study of the interactions of actinide oxide cations with simple ligand molecules have provided detailed information on energetics, geometry and vibrational frequencies of those clusters. 145,146 Comparison to experimental data shows that, while quantitative accuracy---a remarkable feat itself---is typically still a challenge, trends across the actinide series in accessible properties, such as vibrational frequencies and bond dissociation enthalpies, 147 can often be obtained through calculation. 118 For more complex systems, quantitative accuracy remains a challenge.…”