QM/MM Study of Aqueous Solvation of the Uranyl Fluoride [UO 2 F 4 2 ] Complex IVAN INFANTE, LUCAS VISSCHER Department of Theoretical Chemistry, Faculty of Sciences, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands Received 1 July 2003; Accepted 21 August 2003 Abstract: The aqueous solvation of the uranylfluoride complex [UO 2 F 4 2- ] was studied using full quantum mechanical (QM) and hybrid QM/molecular mechanics (MM) methods. Inclusion of a complete first solvation shell was found necessary to reproduce the experimentally observed heptacoordination of uranium. An efficient and accurate compu- tational model is proposed that consists of structure optimization of the coordinated uranium complex as QM region, followed by single-point full QM calculations to compute relative energies. This method is proven feasible for studies of large solvated actinide complexes. © 2003 Wiley Periodicals, Inc. J Comput Chem 25: 386 –392, 2004 Key words: QM/MM; uranyl; ZORA; bonding interaction Introduction Trying to achieve quantitative understanding of the rich chemistry of the actinides is challenging for both experimental and theoret- ical chemists. Experimental work is difficult due to the precautions that need to be taken due to the radioactivity and high toxicity of the materials. The interpretation of experiments in terms of a simple model is further far from trivial because many orbitals may contribute to chemical bonding. 1 This situation calls for invocation of theoretical methods both as an aid in the interpretation of the experiment and as an independent tool to study details of reaction mechanisms. The most accurate of the theoretical methods is in principle the fully relativistic four-component wave function ap- proach combined with a coupled-cluster expansion to describe the correlations in the electronic motion. 2 One can, however, only apply this method for rather small molecules. The description of the larger actinide complexes requires the introduction of approx- imations in both the relativistic and the electron correlation treat- ment. Here, we will focus on one of the most popular methods, the zero-order regular approximation 3 (ZORA) of the Dirac equation, combined with gradient corrected density functional theory (DFT). This computational model has been proven to be applicable to rather large molecules with an accuracy of a few kJ/mol in the energy and a few pm in typical bond lengths. The migration and complexation of actinides in aqueous solu- tion is of central importance in many situations of practical inter- est. 4–7 To describe such solvated complexes that are often in strongly acidic or alkaline solution, 8 we want to augment the ZORA model with a reliable description of solvation effects. The purpose of the present work is to assess the reliability of a ZORA– DFT quantum mechanical (QM)/molecular mechanics (MM) ap- proach in studies of solvated actinides. We chose to concentrate on the modeling of the uranyl ion that forms the basis for several stable complexes. This molecule has been extensively studied in the gas phase using a variety of computational models. 9 –13 Coordination in a solvent was studied by Schreckenbach et al., 14 who considered four hydroxy ions. We will focus on the UO 2 F 4 2- complex, 15 in which the uranyl ion is equatorially coordinated by four fluorine atoms, because the recent EXAFS experiment by Vallet et al. 16 provides experimental infor- mation to which to compare our computed structures. Because Vallet et al. also did a theoretical investigation to determine the effect of a first shell of three water molecules on the structure of the UO 2 F 4 2- their work serves as a good starting point for our investigation on the inclusion of explicit solvent layers. We expect that this detailed description of hydrogen bonding and other mi- croscopic interactions will improve upon the description of sol- vated uranyl complexes by continuum models. As a secondary ob- jective we can also gauge the reliability of the QM/MM scheme itself by comparison with full QM calculations of the solvent shells. Experimentally, the [UO 2 F 4 (H 2 O) 2- ] complex is known to be heptacoordinated in solution, i.e., with one water molecule being coordinated to the uranium atom. Because theoretically the hypo- thetical gas-phase (isolated) complex is a logical reference point, we started by analyzing the conformations of the [UO 2 F 4 (H 2 O) 2- ] complex in both gas and liquid phases. This offers a conceptual Correspondence to: L. Visscher; e-mail: visscher@chem.vu.nl © 2003 Wiley Periodicals, Inc.