J. Phys. Chem. 1989, 93, 3397-3410 3397 Helium Bonding in Singly and Doubly Charged First-Row Diatomic Cations Hex"+ (X = Li-Ne; n = 1, 2) Gernot Frenking,*?+ Molecular Research Institute, 701 Welch Road, Palo Alto, California 94304 Wolfram Koch,' Institut fur Organische Chemie, Technische Universitat Berlin, D- 1000 Berlin 12, West Germany Dieter Cremer,* Jiirgen Gauss, Institut fur Organische Chemie, Universitat Koln, Greinstrasse 4, 0-5000 Koln 41, West Germany and Joel F. Liebman* Department of Chemistry, University of Maryland, Baltimore County Campus, Baltimore, Maryland 21 228 (Received: February 1. 1988; In Final Form: August 4, 1988) With the use of ab initio calculations at the MP4(SDTQ)/6-3 1 lG(2df,2pd)//MP2/6-3lG(d,p) level of theory, equilibrium geometries, dissociation energies, and vibrational frequencies are reported for singly and doubly charged Hexn+cations (X = Li-Ne; n = 1,2). The calculations were performed for the electronic ground states and selected excited states of HexR+. The trends in the interatomic distances and bond strengths of the helium bonds are discussed in terms of donor-acceptor interactions between neutral He as the donor and the cationic X"+ fragment as the acceptor. In addition, the mechanism of bonding is analyzed by utilizing the properties of the calculated electron density p(r) and its associated Laplace concentration -V2p(r). Hex+ ions in their ground states represent van der Waals complexes stabilized by charge-induced dipole interactions. In those excited states in which X+ becomes a stronger acceptor, covalent bonds are predicted. In contrast to the singly charged Hex+ ground-state species, all Hex2+dications investigated in this work can be considered as covalently bonded molecules. Calculated properties of Hex* such as interatomic distances re and dissociation energies De are nicely explained within the donor-acceptor model. The results show that the electronic structure of X"+ is more important for the stability of the corresponding Hexn+system than the positive charge of X"+. Introduction Several theoretical studies of diatomic cations Hex"+ containing In recent theoretical studies1v2of the structures, stabilities, and first-row elements X have been published, although most work bonding of singly and doubly charged cations as well as neutral has been carried out at the Hartree-Fock level only.s HeLi+ was molecules containing helium, it was predicted that He may be the subject of sqveral theoretical studies6 with four of them"~d~f~h bound very strongly in positively charged compounds such as carried out with explicit inclusion of correlation energy. One HeCCHeZ+,HeCCZ+, and HeCCH' and that even a neutral He pseudopotential study for HeLP has been reported.6i HeLi+ is compound, HeBeO, is stable toward dissociation. The analysis one of the very few Hex"' systems investigated here for which of the helium bonds in a variety of molecules showed that the experimental data are available: Results from scattering exper- strength of the He-X bond is primarily determined by its electronic iments' indicate a potential well of 1.1-1.6 kcal/mol at equilibrium structure rather than the charge or electronegativity of X. Only when X provides low-lying empty orbitals suitable for donor- (1) (a) Koch, W.; Frenking, G. Int. J. Mass Spectrom. Ion Processes 1986, acceptor interactions with the weak electron donor He is a helium 74, 133. (b) Koch, W.; Frenking, G. J. Chem. SOC.,Chem. Commun. 1986, 1095. (c) Koch, W.; Frenking, G.; Luke, B. T. Chem. Phys. Lett. 1987,139, bond formed. For example, the He,X bonds in HeCCHe2+, 149. HeNNHeZ+,and HeOOHe2+become much weaker in this order, (2) Koch, W.; Frenking, G.; Gauss, J.; Cremer, D.; Collins, J. R. J. Am. which is opposite to changes in the H,X bond strength encountered Chem. SOC.1987, 109, 5917. for the isoelectronic hydrogen compounds HCCH, HNNH, and (3) A systematic comparison of isoelectronic hydrogen and helium (1 +) compounds is presented by the following: Frenking, G.; Koch, W.; Liebman, HOOH.2 This may be explained by the increase in the number J. F. In Molecular Structure and Energetics: Isoelectronic and Plemeioe- of occupied u orbitals from CCz+ to NN2+ and OOz+. The lectronic Reasoning, Liebman, J. F., Greenberg, A., Eds.; VCH Publishers: dominant principle of helium bonding, that is, donor-acceptor New York, in press. interactions, is clearly different from that of hydrogen chemi~try.~ (4) (a) Bartlett, N. The Chemistry offheNoble Gases; Elsevier: Am- sterdam, 1971. (b) Holloway, J. H. Chem. Br. 1987, 658. (c) Holloway, J. Helium chemistry is also different from the chemistry of the H. Noble Gas Chemistry; Methuen: London, 1968. heavier noble-gas elements. For example, for xenon strong bonds (5) A compilation of experimental and theoretical studies on noble-gas are found only when Xe combines with strongly electronegative compounds including those of helium up to 1976 is presented by: Hawkins, elements such as fluorine and ~xygen.~In this paper, we present D. T.; Falconer, W. E.; Bartlett, N. Noble-Gas Chemistry. A Bibliography: 1962-1976; IFI/Plenum: New York, 1978. the results of our ab initio study on first-row mono- and dications (6) (a) Catlow, C. W.; McDowell, M. R. C.;Kaufman, J. J.; Sachs, L. M.; Hexn+(n = 1, 2) comprising a "first-row sweep" of X from Li Chang, E. S. J. Phys. B 1970, 3, 833. (b) Krauss, M.; Maldonado, P.; Wahl, to Ne. We calculated the ground states and, for most systems, A. C. J. Chem. Phys. 1971, 54, 4944. (c) Hariharan, P. C.; Staemmler, V. the first excited states of Hex"'. The results, in particular the Chem. Phys. 1976,15,409. (d) Senff, U. E.; Burton, P. G. Mol. Phys. 1986, 58, 637. (e) Pyykko, P.; Sundholm, D.; Laaksonen, 1. Mol. Phys. 1987,60, interatomic distances re and dissociation energies De, are discussed 597. (f) Tatewaki, H.; Tanaka, K.; Ohno, Y.; Nkamura, T. Mol. Phys. 1984, in terms of donor-acceptor interactions. 53, 233. (g) Viehland, L. A. Chem. Phys. 1983, 78,279. (h) Cooper, D. L.; Gerratt, J.; Raimondi, M. Mol. Phys. 1985, 56, 611. (i) Fuentealba, P. J. Phys. B 1986, L235. 'Present address: Fachbereich Chemie, Universitat Marburg, Hans- (7) (a) Dalgano, A,; McDowell, M. R. C.; Williams, A. Phil. Trans. R. Meerwein-Strasse, D-3550 Marburg, West Germany. SOC.London, A 1958, 250, 41 1. (b) Mason, E. A,; Schamp, H. W., Jr. Ann. *Present address: IBM Wissenschaftliches Zentrum, Tiergartenstrasse 15, Phys. 1958, 4, 233. (c) Polark-Dingels, P.; Rajan, M. S.; Gislason, E. A. J. D-6900 Heidelberg, West Germany. Chem. Phys. 1982, 77, 3983. 0022-365418912093-3397$01.50/0 0 1989 American Chemical Society 3398 The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 Frenking et al. geometries between 1.95 and 2.22 A. No theoretical or experi- HeO+, HeF+ and by Cooper and WilsonIobabout HeCd, He", mental data have been published for doubly charged HeLi2+. Heon+compare the trends in interatomic distances and disso- In case of HeBe", some theoretical studies have been carried ciation energies. In the latter paper, the authors conclude that out for doubly charged HeBe2+,8one of themsd at a correlated "The various trends in stability are easily understood in terms of level. Only one LCAO study has been reported for HeB+,9but the effective nuclear charges on the two centres."Iob As shown none have been reported for the dication HeB2+. later in the present paper, this conclusion is misleading because A number of theoretical studies are available for HeCn+.lJ0 it compares the ground states of HeC+ and HeN+ with the excited Hartree-Fock calculations on HeC+ and HeC2+have been re- state of HeO'. It will become clear from our calculated results ported by Harrison et al.Ioa and by Cooper and Wilson,Iob who for ground and excited states of Hex"+ that the trends in inter- also presented results for singly and doubly charged HeN"+ and atomic distances and dissociation energies are primarily a function Heon+. Correlated studies have recently been presented for of the electronic state of Xn+ rather than the electronic charge. HeC' Iav2,loc and HeC2+,Ibv2and a CASSCF investigation of the This is true for very weakly bounded systems with binding energies low-lying electronic states of HeC2+was published by two of the of 1 kcal/mol or less as well as strongly bound, covalent molecules. present authors.Ic By application of the model of donor-acceptor interactions in Liebman and Allenlla,bdiscussed HeN+ in several electronic helium molecules, useful predictions on bonding and the nature states based on LCAO calculations, and they compared their of He,X"+ interactions can be made. results for HeN+ with HeO+ and HeF+. Other theoretical data We want to comment on the accuracy of the calculated data on HeN+ are available, at both the SCFIoband correlated level.1b,2 presented here. Our theoretical results for the equilibrium ge- The only results for doubly charged HeN2+have been reported ometries and dissociation energies of Hexd cations are certainly by Cooper and Wilson.Iob more reliable and accurate than earlier studies carried out mostly The potential energy curves for all valence states of HeO+ have at the Hartree-Fock LCAO le~eI.~~~-'~~'~In a recent theoretical been calculated with a minimum basis set and full configuration study" of the atomization energies of hydrides XH, (X = Li-F) interaction by Augustin et Several LCAO'obJ1,'2b,cstudies at a comparable level of theory, dissociation energies Dohave been as well as correlation corrected1b,2calculations on HeO+ have been predicted that differ from experiment by less than 2 kcal/mol. published. In addition, two theoretical studies of doubly charged However, the calculated stabilization energies of some Hex+ He02+are known to us.lob,lZc cations in their ground states are in the range of 2 kcal/mol.