Noble Gas Matrices May Change the Electronic Structure of Trapped Molecules: The UO2(Ng)4 [Ng=Ne, Ar] Case

Abstract

In the last two decades matrix isolation techniques have achieved a notable success for several types of spectroscopy, ranging from infrared, optical absorption, laser-induced fluorescence and electron-spin resonance spectroscopy. Their major accomplishment lies on the possibility of embedding a guest molecule inside a host that acts as spectator and is inert towards any type of chemical interaction at very low temperatures (4–12 K). The most successful isolations occur when the gas used to trap the guest molecule is a light noble gas, known for its reluctance to form any type of chemical bond. The recent discoveries of stable noble gas complexes, such as HArF and NgAuF (Ng=Ar, Kr, Xe), have spurred new questions regarding the actual inertness of Ng hosts in matrix isolation, especially when actinide molecules, capable of forming bonds possibly involving the close-lying 5f, 6d, and 7s orbitals, are trapped. In particular, the molecules CUO and UO2 have challenged this paradigm, having shown in recent years a very probable noble-gas-induced ground state reversal going from Ne to Ar as the isolating gases. On investigating the UO2 molecule in more detail, Andrews et al. in 2000 noticed a large frequency shift for the asymmetric stretching mode at n=776 cm!1 in solid Ar, as compared to solid Ne at n=915 cm!1.[8] Such a significant shift can only be explained with the help of highly accurate calculations by exploring several electronic states and matching the measured asymmetric U!O stretch vibration with the computed ones. Following this procedure, the two possible candidates to the ground-state reversal have been recognized in the F2u (ground state in gas phase) and the H4g states, with their values being computed at n=919 and 824 cm!1, respectively, by using density functional theory (DFT). Subsequently, Heaven et al. have recorded fluorescence spectra attributed to UO2 in solid Ar and concluded that the pattern of low-lying electronic excited states is consistent with their previous gas-phase measurements, where they established the F2u as the ground state and stated that reversal of the ground state in solid Ar is unlikely. As a consequence of this contradicting situation, theoretical chemists carried out state-of-the-art calculations pinpointing the F2u state as the ground state and describing with great precision the excitation spectrum of the UO2 molecule. However, the calculations have been done for the bare molecule, certainly a good approximation for the gas phase, but not for the matrix. Now with improvement of computer architecture, we can use computationally intensive methods to study the electronic excited states of the UO2 molecule with a shell of noble gas atoms. In this work, we decided to run CASSCF/ CASPT2 calculations on the UO2(Ng)4 [Ng=Ne, Ar] complex, where the four noble gas atoms are displaced along the equatorial plane at a fixed equal U!Ng bond length, thus enforcing a D4h symmetry (D2h in the calculations). We decided to keep the inversion center to distinguish between the ungerade and gerade states and to allow maximum computational advantage. See the Supporting Information for more details. In Figure 1, the SO-CASPT2 potential energy surface (PES) computed with MOLCAS is depicted for the most [a] Prof. Dr. L. Gagliardi Department of Chemistry University of Minnesota and Supercomputing Institute 207 Pleasant St. SE, Minneapolis, MN 55455 (USA) Fax: (+1)6126 267 541 E-mail : gagliard@umn.edu [b] Dr. I. Infante Kimika Fakultatea Euskal Herriko Unibertsitatea and Donostia International Physics Center (DIPC) P.K. 1072, 20080 Donostia, Euskadi (Spain) [c] Prof. Dr. L. Andrews Department of Chemistry, University of Virginia Charlottesville, Virginia 22904-4319 (USA) [d] Dr. X. Wang Department of Chemistry, Tongji University 200092 Shanghai (China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002549.