On the Thermodynamic Stability of ArO 4 Roland Lindh,* ,† Wolfgang P. Kraemer, ‡ and Manfred Ka 1 mper § Dept of Chemical Physics, UniVersity of Lund, P.O. Box 124 S-221 00 Lund, Sweden, Max-Planck Institute of Astrophysics, Postfach 1523, D-85740 Garching, Germany, and Institut fu ¨ r Anorganische Chemie, der UniVersita ¨ t Mu ¨ nchen, D-80333 Mu ¨ nchen, Germany ReceiVed: April 22, 1999 The argon tetroxide molecule, ArO 4 , and the isoelectronically associated perchlorate, ClO 4 - , and sulfate, SO 4 2- , ions are investigated on different levels of ab initio theory. The equilibrium structures, harmonic vibrational frequencies, and heats of formation are computed applying density functional theory, second order Møller-Plesset perturbation, singles and doubles coupled-cluster with triples corrections, and Bruekner’s doubles coupled-cluster with triples corrections methods in conjunction with various one-particle basis sets. The calculations demonstrate that the description of the bond characteristics in argon tetroxide is sensitive to the applied level of theory. A careful analysis of the global potential energy surface shows that a stationary point exists for the ArO 4 complex corresponding to a local mininium. The calculated equilibrium Ar-O bond distance of 1.48 Å for this structure is slightly longer than the corresponding bond length of the perchlorate ion. Harmonic frequencies for ArO 4 obtained using Bruekner’s doubles coupled-cluster with triples corrections are found to have a similar pattern like those obtained for the isoelectronic series of ions SiO 4 4- , PO 4 3- , SO 4 2- , ClO 4 - . Using the concept of an isodesmic reaction, the enthalpy of formation of ArO 4 is determined to be endothermic by as much as 1246 kJ/mol. The present theoretically predicted strong endothermicity and the large Ar-O bond distance are in conflict with the monotonic trends obtained for the isoelectronic ions, but can be supported by other chemical extrapolation schemes. 1. Introduction Ab initio methods have served in the past as valuable tools assisting in the interpretation of experimental results. Especially the interpretation of molecular spectra with the help of ab initio calculations has been of fundamental importance. But in spite of its valuable contributions, theory does seem to take mostly a back-seat position in the sense that the experiment is performed first and that theory and its computational results are used only afterwards to rationalize the previous experimental findings. More recently, however, an increasing number of successful attempts have been made to reverse this process and to use ab initio approaches for predictive purposes. To do this, though, the theoretical chemist must be equipped with some chemical intuition to assure a reasonable success rate between reliable theoretical predictions and the consumed computer time. Almost all possible pair combinations of atoms from the periodic table have been investigated in the past by ab initio methods. These calculations were generally performed in a brute-force approach and resulted occasionally in predicting some previously unknown stable diatomic compounds. To a much lesser extent this has also been done for triatomic systems. However, as the number of atoms in a molecular ensemble grows, the brute force approach starts to become impossible, and simple empirical schemes are often useful to give directions for detailed theoretical studies. A valuable concept for this purpose is the isoelectronic or isosteric principle. It has been introduced by Langmuir in 1919 1 in order to rationalize similarities of molecular species consisting of an equal number of electrons and nuclei, and it was frequently used in the past to derive regularities for various groups of chemical compounds and to predict trends for unknown molecular properties. Among others, Pyykko ¨ and co-authors 2,3,4 have successfully applied this scheme, performing calculations at the MP2/6-31G* level of theory, and a number of so far unknown new species were actually predicted in these investigations. Making use of the isoelectronic principle in studying the ArO 4 complex, the attempt can be made to predict its structural parameters and thermodynamic stability by extrapolation from the isoelectronic series of ions SiO 4 4- , PO 4 3- , SO 4 2- , ClO 4 - . Essentially symmetrical tetrahedral geometrical structures are found for these anions with the four oxygens at the edges of the regular tetraeder and the third-period atoms (Si, P, S, Cl) at its center. The bond lengths between the central atoms and the oxygens are monotonically decreasing in the series because of the decreasing size of the central atoms with increasing nuclear charges and as a result of the continuous change of the bond character from SiO 4 4- with a predominant single-bond struc- ture to the other ions with increasing double-bond contributions. Fitting the experimentally determined bond lengths of the ions (silicate, 1.63 Å; phosphate, 1.55 Å; sulfate, 1.50 Å; perchlorate, 1.46 Å) vs. the charge by simple linear or exponential functions, the bond length in ArO 4 can be extrapolated to be approximately 1.41 Å. Similar extrapolations can in principle be done to predict the thermodynamic stability of the rare, gas compound using experimental enthalpies of formation for the isoelectronic anions. However, since these enthalpies of formation cannot be measured directly, the numbers available from the literature * Corresponding author. Tel.: 46 46 222 81 21. Fax: 46 46 222 4523 or 46 46 222 41 19. E-mail: Roland@signe.teokem.lu.se. † University of Lund. ‡ Max-Planck Institute of Astrophysics. § Institut fu ¨r Anorganische Chemie. 8295 J. Phys. Chem. A 1999, 103, 8295-8302 10.1021/jp991317s CCC: $18.00 © 1999 American Chemical Society Published on Web 09/29/1999