A Structural and Vibrational Study of Uranium(III) Molecules by Density Functional Methods Laurent Joubert and Pascale Maldivi* Laboratoire de Reconnaissance Ionique, SerVice de Chimie Inorganique et Biologique (UMR 5046 CEA-CNRS-UJF), De ´ partement de Recherche Fondamentale sur la Matie ` re Condense ´ e, CEA-Grenoble, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France ReceiVed: June 18, 2001; In Final Form: July 14, 2001 In this paper, we present a theoretical investigation of structural and vibrational properties of selected gas- phase UX 3 (X ) F, Cl, Br, and I) and U(CH 3 ) 3 molecules by density functional methodologies or with a post Hartree-Fock MP2 perturbative approach. Relativistic scalar corrections have been explicitly included either by a frozen core approximation with a quasi-relativistic treatment (QR) of the valence electron shells or by energy-adjusted large core quasi-relativistic effective core potential (QRECP) scheme. The influence of the size of the core (large core, LC, or small core, SC) as well as of the addition of polarization functions has also been examined on one derivative, i.e., UCl 3 . MP2/LC-QRECP optimized geometries and vibrational frequencies are found in good agreement with the available estimated or experimental data. Among the different DFT approaches, the best agreement is obtained for DFT/QR computations which reproduce the experimental (or estimated) C 3V molecular structures of all the selected species. In contrast, the DFT/LC-QRECP approaches provide irregular results, strongly dependent on the choice of the functional. Nevertheless, the use of a small core pseudopotential greatly reduces the margin between MP2 and DFT/QRECP calculations. At this level, both “classical” nonlocal gradient corrected and hybrid density functionals provide reasonable results. The only exception concerns the B3LYP functional that is clearly inadequate for an effective treatment of electron correlation in open-shell molecular systems. 1. Introduction In the past decades, the chemistry of very heavy metals has been the subject of various and numerous experimental and theoretical investigations. 1-3 An important and particular aspect of this topic concerns the study of lanthanide- or actinide- ligand interactions for molecules involved in the nuclear waste disposal. In this context, there is a need for theoretical modeling and recently, various molecular dynamics, 4-6 ab initio, 6-16 and density functional theory (DFT) studies 16-27 have focused on the structural studies of large or small model molecular systems involved in extraction processes relevant to the nuclear industry. But a theoretical treatment of such species still represents a challenging task for modern quantum mechanics methods. Besides the well-known intrinsic difficulty of open-shell system calculations, an efficient treatment of relativistic and correlation effects has to be chosen. Since the development of accurate density functionals includ- ing gradient corrections and more recently a part of exact exchange, the DFT approach has proven to be a powerful alternative to the classic Hartree-Fock (HF) or post-HF methods for transition metal studies, in addition to a lesser computational effort. 28 Moreover, different approximations allow to take into account the main relativistic effects (mass-velocity and Darwin terms) within a DFT formalism, such as the use of relativistic effective core potentials 8 (RECP) or by a fully relativistic frozen core description combined with a perturbational scalar quasi- relativistic (QR) treatment of valence shells. 29-31 In particular, these methodologies have recently been successfully applied to the study of lanthanide trihalide systems for which experi- mental data are available. 17-19,23 The reproduction of experi- mental data was shown to be at least of the same quality or better, than post-HF/RECP methodologies. Our purpose is to investigate the reliability and the accuracy of these methodologies for 5f elements. More precisely, we will focus on the structural and vibrational properties of small reference uranium(III) complexes, i.e., UX 3 (X ) F, Cl, Br, I) and U(CH 3 ) 3 molecules. Moreover, post-HF calculations can be also reasonably considered for these small molecular systems. Thus we decided to perform also MP2 calculations, to compare with the DFT computations, as DFT/post-HF comparisons had already been made for LnX 3 species. 17,18 More accurate post- HF methods (MP4 and CCSD) have been attempted but they appeared to be too demanding in computer resources to be applied to such high spin unrestricted species (see below). The various quasi-relativistic/DFT methodologies that are described here involve: - gradient corrected (GGA) exchange and correlation func- tionals in combination with a frozen core and a Pauli or ZORA treatment (see details in the computational section); - a GGA exchange and correlation functional or self- consistent hybrid (SCH) functionals, including some exact exchange, combined with a large core (LC) RECP for the treatment of relativistic effects. More accurate calculations have also been carried out on UCl 3 to assess both the quality of the LC approximation (comparison with small core RECP calculations) and the influence of extended polarization functions on halogen atoms. 2. Computational Details Two different implementations of the Kohn-Sham (KS) methodology have been used to take into account the relativistic * Corresponding author. E-mail: Pmaldivi@cea.fr. 9068 J. Phys. Chem. A 2001, 105, 9068-9076 10.1021/jp012301g CCC: $20.00 © 2001 American Chemical Society Published on Web 09/07/2001