J. Phys. Chem. A XXXX, xxx, 000 A He@Mo6Cl8F6: A Stable Complex of Helium Wenli Zou,† Yang Liu,† Wenjian Liu,‡ Ting Wang,‡ and James E. Boggs*,† Institute for Theoretical Chemistry, Chemistry and Biochemistry Department, the UniVersity of Texas at Austin, Austin, Texas 78712-0165, and Institute of Theoretical and Computational Chemistry, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: August 26, 2009; ReVised Manuscript ReceiVed: October 22, 2009 The electronic structure and chemical stability of the endo helium cluster, He@Mo6Cl8F6, were investigated carefully by using density function theory. The results show that the cluster is significantly different from typical van der Waals systems: the bond distance between helium and molybdenum is only about 1.89 Å. Moreover, the bonding analysis clearly reveals considerable charge and bond order on the helium atom and bond order for He-Mo. The dissociation of He@Mo6Cl8F6 to He + Mo6Cl8F6 is prohibited by a barrier of 0.86 eV (19.8 kcal/mol), indicating that the cluster is chemically stable. However, no covalent He-Mo bonding was found so it is an analogue of He@adam. Comparison was also made with the isoelectronic system of 2- [Mo6Cl8F6] . Introduction anion, Grochala recently calculated some derivatives by the - Since xenon hexafluoroplatinate was experimentally prepared density functional theory (DFT) and post Hartree Fock in 1962,1 many rare-gas compounds of Kr, Xe, and Rn have methods and found that CsFHeO and N(CH3)4FHeO may 19 been found,2 thereby leading to a new field of research. exist at a very low temperature. For the latter, the Especially in 2000, HArF was also prepared at low temperature,3 dissociation energy in the He-O bond is computed as large which is hitherto the only stable covalent compound of Ar. It as half an electronvolt, but a crossing of the singlet-triplet is well-known that, due to larger polarizability, the heavier rare- electronic states and the decomposition in the bending gas elements are more readily bound than helium and neon. direction decrease the kinetic stability considerably.19 In - Up to now, several types of helium compounds have been our test calculations of FHeO and CsFHeO, typical reported experimentally, including cations (e.g., HeH+, which covalent molecular orbitals can be found between He and was found 70 years ago4), quasi-molecules in excited states (e.g., O as well as He and F. However, these compounds are not HeH5), and weakly bound van der Waals molecules (e.g., helium so simple from a topological analysis point of view. The in carbon cages6 and the metal-helium system HgHe7). Yet, positive Laplacians (∇2F) of the electron density suggest stable or metastable neutral covalent compounds of helium are there are no covalent interactions between the atoms, still unknown experimentally. Nevertheless, some new kinds meaning that they are not real covalent compounds. On of neutral complexes of helium were theoretically predicted in the contrary, the negative values of the H(r) function the literature to be stable or metastable, including the following: suggested by Cremer and Kraka [ref 8c and references • Metastable complexes with uncommonly strong van der therein] indicate that the He-O bonds are covalent. The 8–16 Waals interactions. For such molecules, the calculated same should also be true for N(CH3)4FHeO. binding energies of helium only amount to a few kcal/mol, • Covalent compounds. The only example is HHeF20 pres- and the topological analysis of the electron density distribu- ently. It was considered to be metastable with a strong tion shows that there is no covalent bond formed on helium. covalent H-He bond,20 but a high-level ab initio calculation For example, HeBeO.8–10 by MRCI showed that the barrier between HHeF and H + • Pressure-induced complexes, which are a special kind of He + F is only 0.5 kcal/mol.21 Subsequent quantum endohedrals (i.e., atom-in-cage systems). An example is dynamics treatments indicated that its lifetime is extremely the complex of He in adamantane (He@adam).17 It was short, only 157 fs,21 though maybe somewhat longer in found that the centered helium atom is protected by a barrier pressurized solid helium.22 to dissociation of as large as 1.7 eV (39.2 kcal/mol). In addition, the HHeCCH molecule was also predicted to be Different from most of the helium-in-cage systems (for stable by the B3LYP/6-31G** method with a barrier of 15 kcal/ instance, see ref 6), there are very strong repulsive mol between HHeCCH and He + HCCH.23 Our topological interactions between helium and carbon atoms. The topo- analysis, by using different functionals and basis sets, also logical analysis of electron density by the atoms in reveals a covalent H-He bond in HHeCCH. Unfortunately, our molecules (AIM) method does not support any chemical high-level MRCI and CCSD(T) calculations could not confirm bonds on helium,17a although this argument has been the existence of this configuration, so the artifactual stable disputed.17b HHeCCH molecule is completely a result of the defects of DFT. • Quasi-covalent compounds. In 2005 Hu et.al.18 predicted - On the other hand, during the past three decades, there have a helium-containing anion, FHeO . Inspired from this been a large number of reports of inorganic metallic cage clusters made up of main group elements24 or transition-metal * Corresponding author. Electronic mail: [email protected]. 25 † University of Texas at Austin. elements. Among these cage-like compounds, the endohedrals ‡ Peking University. have been attracting great attention because of their unique 10.1021/jp908254r CCC: $40.75 XXXX American Chemical Society B J. Phys. Chem. A, Vol. xxx, No. xx, XXXX Zou et al. TABLE 1: Optimized Geometries (in Å), HOMO (T2u or Eg), LUMO (Eu or Eu), and HOMO-LUMO Gap (ε,ineV) 2- of [Mo6Cl8F6] and He@Mo6Cl8F6 (X ) He or the Cage Center) cluster RX-Mo RMo-F RX-Cl EHOMO ELUMO ε 2- [Mo6Cl8F6] O3LYP-c/VTZ 1.821 1.978 3.044 0.01 3.90 3.89 2- a Figure 1. Optimized structures of [Mo6Cl8F6] (left) and He@Mo6Cl8F6 O3LYP-c/ANO-VQZ 0.17 4.04 3.87 (right) by O3LYP-c/VTZ. There are six molybdenum atoms lying at the 2c-SVWN5b 1.82 1.98 3.11 3.4c vertices of a regular octahedron, each connected with a fluorine atom, and exptd 1.834 1.994 3.046 ∼3.7 eight chlorine atoms at the vertices of a regular cube. For He@Mo6Cl8F6, He@Mo6Cl8F6 the helium atom occupies the center. O3LYP-c/VTZ 1.888 1.879 3.035 -7.36 -3.68 3.68 O3LYP-c/ANO-VQZa -7.29 -3.63 3.66 structures and novel properties. Up to now, the endo atoms a Computed at the O3LYP-c/VTZ structure. b Reference 28. reported experimentally range from light atoms to 5d transition- c Computed by the two-component relativistic SAOP potential at the 28 d metals (ref 26 and references therein). This raises an interesting optimized structure of 2c-SVWN5. The HOMO is Eg. Reference 27. See Supporting Information for the HOMO-LUMO gap. question: is there a stable and non van der Waals helium-in- metallic-cage compound? It is clear that, to capture the helium atom and form a decent helium-metal chemical bond, the empty as constants and are nearly independent of the functionals.34 2- metallic cage should be very compact with a relatively small Since O3LYP-c obtains the best structure for [Mo6Cl8F6] , the inner diameter. The hexanuclear M6 cluster, well-known in solid results should also be reliable for He@Mo6Cl8F6. 2- state chemistry, is a potential candidate. For [Mo6Cl8F6] , both The calculations with O3LYP-c were performed by using PC- experimental27 and theoretical28 researches show that the GAMESS35 and Gaussian 0331 (for analytical frequencies center-Mo distance is only about 1.8 Å. There are six only36), whereas GAMESS,37 modified versions of NWChem,38 molybdenum atoms lying at the vertices of a regular octahedron, and PC-GAMESS35 were used for the other functionals. At the each connected with a fluorine atom, and eight chlorine atoms optimized structures, the harmonic vibrational frequencies were 36 at the vertices of a regular cube (Oh symmetry, see Figure 1). obtained analytically and the vertical excitation energies were Since the center-Mo distance is much smaller than the van calculated with time-dependent DFT (TD-DFT). The natural der Waals lower limit between He and Mo (see below), we bond orbital (NBO) analysis was carried out by the NBO 39 envisage that the He@Mo6Cl8F6 cluster, obtained by replacing program via the PC-GAMESS interface, and the molecular 2- 40 the two extra electrons of [Mo6Cl8F6] by an endo helium atom, orbitals were plotted by using the MacMolPlt program. may also have a short He-Mo chemical bond. 2- Results and Discussion Here we investigate both [Mo6Cl8F6] and He@Mo6Cl8F6 2- by using DFT to find whether or not there is a covalent He- The structures of [Mo6Cl8F6] and He@Mo6Cl8F6 optimized in-cage structure. by using the hybrid O3LYP-c functional are plotted in Figure 1, where the helium atom occupies the center denoted as X. Computational Details The results are further summarized in Table 1. To have a first For the sake of computational efficiency, relativistic effective glance at the chemical stability, the energy gap between the core potentials (RECP) for F, Cl, and Mo were used in the highest occupied (HOMO) and the lowest unoccupied (LUMO) present calculations.