Gas-Phase Chemistry of Actinides Ions: New Insights into the Reaction of UO + and UO 2+ with Water Maria del Carmen Michelini, Nino Russo,* and Emilia Sicilia Contribution from the Dipartimento di Chimica and Centro di Calcolo ad Alte Prestazioni per Elaborazioni Parallele e Distribuite-Centro d’Eccellenza MURST, UniVersita ` della Calabria, I-87030 ArcaVacata di Rende, Italy Received August 5, 2006; E-mail: nrusso@unical.it Abstract: The ability of uranium monoxide cations, UO + and UO 2+ , to activate the O-H bond of H2O was studied by using two different approaches of the density functional theory. First, relativistic small-core pseudopotentials were used together with B3LYP hybrid functional. In addition, frozen-core PW91-PW91 calculations were performed within the ZORA approximation. A close description of the reaction mechanisms leading to two different reaction products is presented, including all the involved minima and transition states. Different possible spin states were considered as well as the effect of spin-orbit interactions on the transition state barrier heights. The nature of the chemical bonding of the key minima and transition states was studied by using topological methodologies (ELF, AIM). The obtained results are compared with experimental data, as well as with previous studies on the reaction of the bare uranium cations with water, to analyze the influence of the oxo-ligand in reactivity. Introduction Uranium is a central element in actinide chemistry because of its importance in the treatment of nuclear waste. The radioactive waste contains a significant amount of actinides with long half-lives that are difficult to treat in a safe and cost- effective manner using presently available technology. To identify improved remediation strategies, our knowledge of the chemical and physical properties of actinide compounds must be advanced. As a consequence, theoretical studies capable of making reliable predictions of the properties of actinide compounds are of main importance. The current interest in gas- phase actinides chemistry is proved by some recent reviews on the subject. 1 The reaction products of gas-phase uranium and uranium monoxide cations with H 2 O have been studied by using different experimental techniques. 2-5 In particular, Jackson and col- laborators 2 studied the reaction of UO + and UO 2+ with H 2 O, using a quadrupole ion trap (QIT-MS) mass spectrometer. The reaction rate constants were determined by measuring the reaction rates at different partial pressures of the reagent gas. Schwarz et al. have analyzed the oxo ligand effect on the reactivity of U + in its reaction with different oxidizing reagents basing on fourier transform ion cyclotron mass spectrometry (FTICR-MS) experiments. 4 More recently, the oxidation reac- tion of dipositive actinide ions with different oxidants were performed by Gibson and collaborators, by means of FTICR- MS experiments. 5 In the case of the reaction between UO + and H 2 O, the following reaction products were detected: 2 Experimental results indicate that both reactions are exother- mic. The reaction between UO + and water was found to proceed at a rate of 1.5 ( 1 × 10 -11 cm 3 s -1 and showed a branching ratio of approximately 1:1, 2 in agreement with earlier experi- mental studies. 3 In contrast, in FTICR-MS experiments, the formation of UO 2 H + was not observed. 4 Different experimental works 1b,5,6 have demonstrated that QIT-MS/FTICR-MS con- trasting results are consequence of the different pressures used in the two techniques (10 -6 Torr for FTICR-MS, 10 -3 Torr in the case of QIT-MS experiments). FTICR-MS conditions results, therefore, in bimolecular processes in contrast to QIT- MS, which results in three-body processes. For the reaction of the doubled charged cation and water, similar reaction products were not observed at the conditions of the experiments. Considering that the oxidation of UO 2+ by O 2 is an exothermic process, 2,4,5 and taking into account the O-O and H 2 -O BDE (1) (a) Gibson, J. K. Int. J. Mass Spectrom. 2002, 214, 1. (b) Gibson, J. K.; Marc ¸ alo, J. Coord. Chem. ReV. 2006, 250, 776. (2) Jackson, G. P.; King, F. L.; Goeringer, D. E.; Duckworth, D. C. J. Phys. Chem. A 2002, 106, 7788. (Correction: Jackson, G. P.; King, F. L.; Goeringer, D. E.; Duckworth, D. C. J. Phys. Chem. A 2004, 108, 2139.) (3) Armentrout, P. B.; Beauchamp, J. L. Chem. Phys. 1980, 50, 27. (4) Cornehl, H. H.; Wesendrup, R.; Diefenbach, M.; Schwarz, H. Chem.- Eur. J. 1997, 3, 1083. (5) Gibson, J. K.; Haire, R. G.; Santos, M.; Marc ¸ alo, J.; Pires de Matos, A. J. Phys. Chem. A 2005, 109, 2768. (6) Jackson, G. P.; Gibson, J. K.; Duckworth, D. C. Int. J. Mass Spectrom. 2002, 220, 419. UO + + H 2 O f UO 2 + + H 2 (1) UO + + H 2 O f UO 2 H + + H (2) UO 2+ + H 2 O f UO 2 2+ + H 2 (3) UO 2+ + H 2 O f UO 2 H 2+ + H (4) Published on Web 03/20/2007 10.1021/ja065683i CCC: $37.00 © 2007 American Chemical Society J. AM. CHEM. SOC. 2007, 129, 4229-4239 9 4229