Chemistry Publications Chemistry 7-2009 Density Functional Studies on the Complexation and Spectroscopy of Uranyl Ligated with Acetonitrile and Acetone Derivatives George Schoendorff Iowa State University Theresa Lynn Windus Iowa State University, [email protected] Wibe A. de Jong Pacific oN rthwest National Laboratory Follow this and additional works at: http://lib.dr.iastate.edu/chem_pubs Part of the Chemistry Commons The ompc lete bibliographic information for this item can be found at http://lib.dr.iastate.edu/ chem_pubs/920. For information on how to cite this item, please visit http://lib.dr.iastate.edu/ howtocite.html. This Article is brought to you for free and open access by the Chemistry at Iowa State University Digital Repository. It has been accepted for inclusion in Chemistry Publications by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. Density Functional Studies on the Complexation and Spectroscopy of Uranyl Ligated with Acetonitrile and Acetone Derivatives Abstract The oorc dination of nitrile (acetonitrile, propionitrile, and benzonitrile) and carbonyl (formaldehyde, acetaldehyde, and acetone) ligands to the uranyl dication (UO22+) has been examined using density functional theory (DFT) utilizing relativistic effective core potentials (RECPs). Complexes containing up to six ligands have been modeled in the gas phase for all ligands except formaldehyde, for which no minimum could be found. A comparison of relative binding energies indicates that 5-coordinate complexes are predominant, while 6-coordinate complexes involving propionitrile and acetone ligands might be possible. Additionally, the relative binding energy and the weakening of the uranyl bond is related to the size of the ligand, and in general, nitriles bind more strongly to uranyl than carbonyls. Disciplines Chemistry Comments Reprinted (adapted) with permission from Journal of Physical Chemistry A 113 (2009): 12525, doi:10.1021/ jp9038623. This article is available at Iowa State University Digital Repository: http://lib.dr.iastate.edu/chem_pubs/920 J. Phys. Chem. A 2009, 113, 12525–12531 12525 Density Functional Studies on the Complexation and Spectroscopy of Uranyl Ligated with Acetonitrile and Acetone Derivatives† George Schoendorff,‡ Theresa L. Windus,*,‡ and Wibe A. de Jong§,| Department of Chemistry, Iowa State UniVersity and Ames Laboratory, Ames Iowa 50011, and EnVironmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 ReceiVed: April 27, 2009; ReVised Manuscript ReceiVed: June 9, 2009 The coordination of nitrile (acetonitrile, propionitrile, and benzonitrile) and carbonyl (formaldehyde, 2+ acetaldehyde, and acetone) ligands to the uranyl dication (UO2 ) has been examined using density functional theory (DFT) utilizing relativistic effective core potentials (RECPs). Complexes containing up to six ligands have been modeled in the gas phase for all ligands except formaldehyde, for which no minimum could be found. A comparison of relative binding energies indicates that 5-coordinate complexes are predominant, while 6-coordinate complexes involving propionitrile and acetone ligands might be possible. Additionally, the relative binding energy and the weakening of the uranyl bond is related to the size of the ligand, and in general, nitriles bind more strongly to uranyl than carbonyls. Introduction as a proving ground for the current computational methodologies and to improve upon them. In a series of joint experimental Understanding the chemical properties of uranium species 13,18-21 in the environment is a key issue for the U.S. Department of and computational papers it was shown that vibrational Energy: to understand speciation in waste tanks at nuclear stretching frequencies of the actinide species and the relative weapons production sites and to understand the transport of energetics calculated with density functional theory (DFT) are actinides in the subsurface environment. Uranium generally in good agreement with experimental data. 2+ This paper reports the results of ab initio calculations on exists as a uranyl dication (UO2 ) that can readily form complexes with various anions. The uranyl chemistry is de- acetonitrile and its derivatives propionitrile and benzonitrile and pendent on pH and available anions, and multiple species can acetone and its derivatives acetaldehyde and formaldehyde. In their gas-phase experiments of the nitrile series, Van Stipdonk often exist in equilibrium. Uranyl species can also interact with 12 2+ et al. were able to isolate the [UO2(L)n] complexes (with n mineral surfaces and form new species or undergo redox ) - ) - processes. This complex chemistry complicates the interpretation 1 5 for acetonitrile and n 2 5 for propionitrile and of experimental measurements. benzonitrile), and they studied the intrinsic reactions with water Molecular-scale modeling using computational chemistry molecules. From acetonitrile to benzonitrile, the ligands have methodologies, combined with experimental observations, has an increased capability to donate electron density to the uranyl. been demonstrated to provide a fundamental understanding of Similarly, by eliminating the methyl groups on acetone, the the complex chemistry of actinides in the condensed phase. Over electron-donating capability is reduced, which should be the years various computational studies on model systems, with reflected in the structure and vibrational spectroscopy of uranyl. or without the inclusion of an approximate description of the The vibrational spectra of the uranyl acetone complexes in the molecule’s environment, have been reported in the literature.1-7 gas-phase have been measured, whereas those of the acetalde- For example, in our recent computational modeling study of hyde and formaldehyde complexes have not. The reaction of gas-phase uranyl carbonate, nitrate, and acetate complexes8 we formaldehyde with uranium has been studied experimentally by Gibson et al.,22 while Senanayake et al. studied the reaction showed that the calculated structures and vibrational frequencies 23 are in generally good agreement with experimental data obtained on the surface of UO2 crystals. We will present the coordina- in the solution and solid-state environment. tion, vibrational frequencies, and the binding and dissociation 2+ ) - ) One of the key issues in computational chemistry is the energetics of the [UO2(L)n] (n 1 6, L formaldehyde validation of the basic methodologies and calculated results for (Form), acetaldehyde (Aca), acetone (Ace), acetonitrile (Acn), molecules. For uranyl complexes, we have relied on highly propionitrile (Pn), and benzonitrile (Bzn)) complexes. These accurate benchmark calculations on the free uranyl in lieu of results provide the groundwork for a subsequent study on the available experimental data.9-11 Over the last couple of years, reaction of water molecules with these species, which will enable Groenewold and co-workers published results of measurements the direct comparison of our calculations with the previous on various uranyl complexes in the gas phase.12-17 These mentioned gas-phase experiments. experiments provide the computational chemistry community Details of the Calculations with a wealth of experimental benchmark data that can be used All calculations were performed with the NWChem software † Part of the “Russell M. Pitzer Festschrift”. suite24,25 using DFT. The choice of functional and basis sets is * Corresponding author. E-mail: [email protected]. based on a previous systematic study where fully relativistic ‡ Iowa State University and Ames Laboratory. § E-mail: [email protected]. coupled cluster singles and doubles with perturbative triples | 2+ Pacific Northwest National Laboratory. (CCSD(T)) benchmark calculations on UO2 were compared 10.1021/jp9038623 CCC: $40.75 2009 American Chemical Society Published on Web 07/02/2009 12526 J. Phys. Chem. A, Vol. 113, No. 45, 2009 Schoendorff et al. TABLE 1: Calculated Uranyl UdO and CdN Bond Lengths (in Å) and Associated Stretching Frequencies (in cm-1)ofthe Acetonitrile, Propionitrile, and Benzonitrile Complexes molecular complex UdO bond length UO2 symmetric stretch UO2 asymmetric stretch CdN bond length CdN stretch 2+ [UO2] 1.702 1028 1131 acetonitrile 1.161 2320 1-Acn 1.722 986 1084 1.175 2216 2-Acn 1.735 960 1053 1.168 2253 (a), 2271 (s) 3-Acn 1.745 941 1031 1.164 2284 (a), 2284 (a), 2299 (s) 4-Acn 1.754 923 1011 1.162 2301 (a), 2302 (a), 2302 (a), 2314 (s) 5-Acn 1.759 914 1001 1.160 2319 (a), 2319 (a), 2321 (a), 2321 (a), 2330 (s) 6-Acn 1.761 892 977 1.159 2329 (a), 2330 (a), 2330 (a), 2331 (a), 2331 (a), 2337 (s) propionitrile 1.161 2309 1-Pn 1.724 974 1078 1.177 2171 2-Pn 1.737 955 1048 1.170 2225 (a), 2244 (s) 3-Pn 1.747 938 1027 1.166 2261 (a), 2261 (a), 2278 (s) 4-Pn 1.756 922 1007 1.163 2281 (a), 2282 (a), 2282 (a), 2295 (s) 5-Pn 1.760 913 997 1.161 2302 (a), 2302 (a), 2304 (a), 2304 (a), 2314 (s) 6-Pn 1.767 891 974 1.160 2315 (a), 2315 (a), 2315 (a), 2316 (a), 2316 (a), 2322 (s) benzonitrile 1.164 2289 1-Bzn 1.733 958 1058 1.183 2121 2-Bzn 1.747 931 1027 1.177 2182 (a), 2196 (s) 3-Bzn 1.755 916 1010 1.172 2221 (a), 2221 (a), 2241 (s) 4-Bzn 1.763 905 993 1.169 2241 (a), 2245 (a), 2245 (a), 2266 (s) 5-Bzn 1.766 898 984 1.165 2267 (a), 2267 (a), 2272 (a), 2272 (a), 2287 (s) 6-Bzn 1.770 883 968 1.163 2286 (a), 2287 (a), 2287 (a), 2289 (a), 2289 (a), 2297 (s) to various DFT functionals and basis set choices.10 The local NsUsN angles are always evenly spaced with the 3-coordinate density approximation (LDA)26,27 was used to determine the complex having a NsUsN angle of 120°, the 4-coordinate structures and frequencies, and energies were calculated using complex 90°, and the 5-coordinate complex 72°. The 2-coor- the B3LYP28,29 functional at the LDA optimized geometry. For dinate complex, however, is an exception with a NsUsN angle uranium the small core Stuttgart relativistic effective core of 104°.