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Light Noble Gas Chemistry

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Light Noble Gas Chemistry 4240 J. Am. Chem. SOC.1990, 112, 4240-4256 conversions. The strongly adsorbed alkyl species, however, did identified as acetylide and alkyl groups. Recombination of the appear to participate in the formation of products. The extension adsorbed alkyl species and hydrogenation of ethylene occurred of the various NMR techniques to allow data accumulation at rapidly at room temperature and formed weakly adsorbed ethane low temperatures will help uncover the operable mechanism in and cis- and trans-2-butene that subsequently hydrogenated to the adsorption and reaction of ethylene on silica-supported ru- butane. The formation of C-C bonds is postulated to take place thenium. through a metallocycle adsorbed species. The strongly adsorbed species identified as acetylide was not V. Conclusions appreciably consumed in the formation of products, although it NMR allows detection of chemisorbed species and also weakly may have served as a host for other reactions. Finally, spin adsorbed molecules at coverages higher than those used in counting revealed that there was one carbon in the strongly ad- spectroscopies employing ultrahigh vacuum conditions. The ap- sorbed layer for each surface ruthenium atom. plication of 13C high-resolution NMR techniques allows the formation of a coherent picture of the chemistry of a relatively Acknowledgment. This work was supported by the US. De- complex system by determining the structure and abundance of partment of Energy, Office of Basic Energy Sciences, Contract molecular species adsorbed on the catalyst surface. The 13C Number W-7405-ENG-82. One of the authors (J.C.K.) ac- CP/ MAS technique allows simultaneous observation of the knowledges the financial support of the Amoco foundation. transformations of chemisorbed and weakly adsorbed molecules. Additional support was obtained from the Iowa State University Direct I3C excitation allows quantitative measurements of the Engineering Research Institute. various species present. From these experiments we observed the decomposition of Registry No. H2C=CH2, 74-85-1 ; Ru, 7440-18-8; CH3CH3,74-84-0; ethylene at room temperature to form strongly adsorbed species CH,(CH2)2CH,, 106-97-8; trans-butene, 624-64-6; cis-butene, 590-1 8-1. Light Noble Gas Chemistry: Structures, Stabilities, and Bonding of Helium, Neon and Argon Compounds Gernot Frenking,*.t-laWolfram Koch,lb Felix Reichel,lEand Dieter Cremer*,ld Contribution from the Institut fur Organische Chemie, Universitat Marburg, Hans- Meerwein-Strasse, D-3550 Marburg, West Germany, the Institute for Superconducting and Applied Mathematics, IBM Scientific Center, Tiergartenstrasse 15, 0-6900 Heidelberg, West Germany, the Institut fur Organische Chemie, Universitat Koln, Greinstrasse 4, 0-5000 Koln 41, West Germany, and Theoretical Chemistry, University of Goteborg, Kemighrde 3, S-41296 Goteborg, Sweden. Received July 10, 1989 Abstract: Theoretically determined geometries are reported for the light noble gas ions Ng2C2+,Ng2NZ+, Ng202+, NgCCNg2+, NgCCH+, NgCN+, and NgNC+ (Ng = He, Ne, Ar) at the MP2/6-31G(d,p) level of theory. In a few cases, optimizations were carried out at CASSCF/6-31G(d,p). The thermodynamic stability of the Ng compounds is investigated at MP4- (SDTQ)/6-31 IG(2df,2pd) for Ng = He, Ne and at MP4(SDTQ)/6-31 lG(d,p) for Ng = Ar. The structures and stabilities of the molecules are discussed in terms of donor-acceptor interactions between Ng and the respective fragment cation, by using molecular orbital arguments and utilizing the analysis of the electron density distribution and its associated Laplace field. Generally, there is an increase in Ng,X binding interactions of a noble gas molecule NgX with increasing atomic size of Ng. In some cases the Ne,X stabilization energies are slightly smaller than the corresponding He,X values because. of repulsive p-T interactions in the neon compounds. The argon molecules are in all cases significantly stronger bound. 1. Introduction There are two other noble gas elements, neon and argon, which In a recent theoretical study of compounds containing the most have resisted so far all attempts to force them into binding with inert chemical element helium, we found2 that He can form strong other atoms to form a stable compound? Little is known about chemical bonds in ions and may even be bound in the ground state neon and argon chemi~try.~In two recent papers we have in- of a neutral molecule, Le., HeBeO. The most important criterion vestigated the electronic structure and bonding in diatomic cations for a potential binding partner of helium is its electronic structure, Hex+: NeX+, and ArX" with X being a first-row element rather than its electronegativity or positive charge. The structures and stabilities of He compounds could be rationalized by using (I) (a) Universitat Marburg. (b) IBM Scientific Center. (c) Universitat the model of a donor-acceptor complex.2 Helium bonds can be KBln. (d) University of GBteborg. very strong with a dissociation energy of up to 90 kcal/mol, if (2) Koch, W.; Frenking, G.; Gauss, J.; Cremer, D.; Collins, J. R. J. Am. the binding partner provides low-lying empty u orbitals. The Chem. SOC.1987, 109, 5917. electronic structure of the molecules has been investigated with (3) Frenking, G.;Koch, W.; Gauss, J.; Cremer, D. J. Am. Chem. SOC. the aid of an electron density analysis. This revealed covalent 1988, 110, 8007. (4) Weakly bound neutral van der Waals complexes and clathrates con- He,C bonds in several cations and dications.2 The neutral com- taining noble gas elements are known for many years. See ref 5 and Holloway, pound HeBeO was found, however, to be an unusually stable (Do J. H. Noble-Gas Chemistry; Methuen, London, 1968. = 3 kcal/mol) van der Waals complex where the attractive in- (5) Frenking, G.; Cremer, D. In Srrucrure and Bonding, Springer: Hei- teractions are dominated by charge-induced dipole intera~tions.~J delberg, 1990; Vol. 73. (6) Frenking, G.; Koch, W.; Gauss, J.; Cremer, D.; Liebmann, J. F. J. Phys. Chem. 1989, 93, 3397. 'Part of this work was performed at the Molecular Research Institute, Palo (7) Frenking, G.;Koch, W.; Gauss, J.; Cremer, D.; Liebmann, J. F. J. Alto, CA. Phys. Chem. 1989, 93, 3410. 0002-7863/90/ 15 12-4240$02.50/0 0 1990 American Chemical Society Light Noble Cas Chemistry J. Am. Chem. SOC.,Vol. 112, No. 11, 1990 4241 He 0 Is Is Ar Figure 1. Perspective drawings of the HF/6-31G(d,p) Laplace concentration, -V2p(r), of (a) He(%), (b) Ne(%), and (c) Ar(lS). Inner shell and valence shell concentrations are indicated. Note that the function value is cut off above and below predetermined values to improve the representation. Li-Ne. The electronic ground states of the diatomic ions are in spectively, and the program COLOGNE.^^ As a standard, molecular most cases weakly bound van der Waals complexes held together geometries have been optimized both at the Hartree-Fock (HF) and the by charge-induced dipole interactions, although in some cases second-order Mdler-Plesset (MP2) perturbation" level employing the covalent bonding has been predicted on the basis of an electron 6-31G(d,p) basis set. These levels of theory are denoted by HF/6-31G- density analy~is.~ particular, (X'Z+) ArF+ has calculated (d,p) and MP2/6-3 lG(d,p). Unless otherwise noted, all structures re- In been ported here have been verified as true minima on the potential energy with a dissociation energy 0,of 49 f 3 kcai/m~l~.~which might surface with only positive eigenvalues of the force-constant matrix. be sufficient to form a stable salt compound.* Most of the excited Geometry optimizations were carried out by using analytical gradients. states of NgX+ have been predicted to be covalently bo~nd.~.~ Harmonic vibrational frequencies and zero-point energies (ZPE) are In this paper we extend our previous investigation of He com- determined at MP2/6-31G(d,p) and, in some cases, at HF/6-31G(d,p). pounds2 to the heavier elements Ne and Ar. We discuss calculated The ZPE values have been scaled uniformly by a factor of 0.87 and 0.93 results for NgX molecules which in the case of Ng = He were for the two levels of theory used.I4 Utilizing the geometries obtained found2 to be strongly bound. We present a systematic comparison at MP2/6-3 lG(d,p), single-point energies are calculated with a larger of analogous helium, neon, and argon compounds based on ab basis set and fourth-order Mdler-Plesset pertuibation the0ry.l) For the initio calculations and the analysis of the electron density dis- argon compounds, the theoretical level is MP4(SDTQ)/6-3 1 lG(d,p) and for the helium and neon species it is MP4(SDTQ)/6-31 IG(2df,2pd). tribution of the computed molecular structures. The principle Unless otherwise specified, energy values in the text are given at these questions which we address are the following. theoretical levels. For a few structures, CASSCFI~ calculations have been (a) Are there thermodynamically stable He, Ne, and Ar com- carried out with a full-valence active space and a 6-31G(d,p) basis set pounds? (b) What are the differences for a given structure NgX by using the program GAMESS.'~ The geometries have been obtained at when Ng is either He, Ne, or Ar? (c) What are the stabilities of Ne and Ar compounds compared to those of He molecules? (d) Is the model of donor-acceptor interactions as valid for ex- (1 I) (a) Binkley, J. S.; Frisch, M. J.; DeFrees, D. J.; Raghavachari, K.; plaining neon and argon structures as it is for helium compounds? Whiteside, R. A.; Schlegel, H. B.; Fluder, E.
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