Theoretical Investigation of Uranyl Dihydroxide: Oxo Ligand Exchange, Water Catalysis, and Vibrational Spectra Hrant P. Hratchian,* ,†,| Jason L. Sonnenberg, ‡,§,⊥ P. Jeffrey Hay, § Richard L. Martin, § Bruce E. Bursten,* ,‡ and H. Bernhard Schlegel † Department of Chemistry and Institute of Scientific Computing, Wayne State UniVersity, Detroit, Michigan 48202, Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210, and Theoretical DiVision, Los Alamos National Laboratory, Mail Stop B268, Los Alamos, New Mexico 87545 ReceiVed: May 18, 2005; In Final Form: July 25, 2005 Density functional theory is employed to investigate uranyl dihydroxide, UO 2 (OH) 2 , isomerization reaction energy barriers, including those occurring via proton shuttles. The ground-state structure of a uranyl dihydroxide complex containing a uranyl moiety with a near 90° OdUdO bond angle is reported for the first time. Furthermore, we predict the vibrational spectra of these compounds. Scalar-relativistic effects for uranium are treated by employing a relativistic effective core potential. 1. Introduction In the last quarter century of theoretical actinide chemistry, no class of compounds has received more attention than complexes of the uranyl dication, [UO 2 ] 2+ . 1-3 The formal f 0 nature and abundance of experimental data for this chemistry are primarily responsible for its popularity. One particularly interesting class of uranyl compounds is the set formed by complexation with hydroxide ligands. Uranyl hydroxide chem- istry has gained attention in experimental and theoretical communities due to its expected presence in uranium waste solutions. Much of the presented work in this area has focused on uranyl tetrahydroxide, which is the predominant mononuclear species in solutions with a pH greater than 11. Additionally, these compounds are pedagogically interesting because of the strong σ- and π-donor ability of the hydroxide ligand. 4 Uranyl dihydroxide is a known uranium oxide volatilization product formed in the presence of oxygen and water vapor 5 that might isomerize to form a structure containing an OdUdO bond angle near 90° in the gas phase. Throughout this paper, we refer to structures with a near 90° OdUdO angle as “bent” uranyls; configurations with OdUdO angles near 180° are referred to as “linear” uranyls. Using density functional theory (DFT) calculations, Tsushima and Reich examined two uranyl dihy- droxide complexes where both hydroxide hydrogens point toward the same oxo group and the remaining three U coordination sites are occupied by aqua ligands. 6 The m-UO 2 - (OH) 2 (H 2 O) 3 structure, where one aqua ligand is between the two OH - ligands, was found to be 0.5 kcal mol -1 higher in energy than the o-UO 2 (OH) 2 (H 2 O) 3 structure where the OH - ligands occupy neighboring coordination sites. Oda and Aoshima confirmed and extended this work by comparing calculated uranyl symmetric stretching frequencies to experimental Raman data. 7 They showed that a dihydroxide configuration with hydrogens pointing toward different oxo groups is slightly favored over the conformation with both hydrogens pointing toward the same oxo ligand. 8 Privalov et al. 9 also reported DFT results reproducing the gaseous UO 2 (OH) 2 entropy and heat capacity previously determined experimentally by Ebbinghaus using a third law treatment. 10 We also note theoretical work by Clavague ´ra-Sarrio et al. 11 that explored a comprehensive series of UO 2 X 2 complexes and found OH - ligands to be the most tightly bound. In this contribution, we use DFT calculations to investigate the electronic structure of UO 2 (OH) 2 and to study the energetic accessibility of bent UO 2 (OH) 2 isomers via oxo ligand exchange reactions, which has been suggested for UO 2 (OH) 4 in connection with solution chemistries. 12,13 Furthermore, water catalysis for these isomerization processes via proton shuttle reactions is considered, as are the computed vibrational spectra for the key UO 2 (OH) 2 isomers located on the potential energy surface. 2. Methods The Gaussian suite of electronic structure programs 14 was used for all calculations. Becke’s three-parameter hybrid functional (B3LYP), 15-18 which has been validated in a previous work by Hay and co-workers for uranyl complexes, 19 was employed throughout. To incorporate scalar-relativistic effects, the 60-electron Stuttgart U relativistic effective core potential was employed, 20 while spin-orbit effects have been ignored due to the formal f 0 nature of uranyl complexes. The most diffuse s, p, d, and f Gaussian functions of the associated uranium basis set were removed to generate the [7s 6p 5d 3f] basis, which was used previously. 21-23 The 6-31+G(d,p) basis 24-28 was utilized for the O and H centers. Ground-state and transition structures were optimized using standard methods 29-32 and verified by analytic frequency calculations ensuring that all structures correspond to potential energy surface minima and first-order saddle points, respectively. Using the damped velocity Verlet 33 and Hessian-based predictor-correc- tor 34,35 integrators of Hratchian and Schlegel, we also confirmed all transition structures reside on a pathway connecting ap- propriate reactant and product potential energy minima. * To whom correspondence should be addressed. E-mail: hhratchi@ indiana.edu (HPH); bursten@chemistry.ohio-state.edu (BEB). † Wayne State University. ‡ The Ohio State University. § Los Alamos National Laboratory. | Present address: Department of Chemistry, Indiana University, Bloom- ington, IN 47405. ⊥ Present address: Department of Chemistry, Wayne State University, Detroit, MI 48202. 8579 J. Phys. Chem. A 2005, 109, 8579-8586 10.1021/jp052616m CCC: $30.25 © 2005 American Chemical Society Published on Web 08/31/2005