polyhedral oligomeric silsesquioxane (poss) cages with endohedral metal hydrides motivation...

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  • Polyhedral Oligomeric Silsesquioxane (POSS) cages with endohedral metal hydrides Motivation Polyhedral Oligomeric Silsesquioxane (POSS) cages are of major interest as building blocks for nano-structured hybrid materials and nanocomposites [1-3]. POSS molecules may be employed to increase the interfacial area and to tune domain distances in solar cells based on conjugated polymers and fullerenes [1]. In this contribution, we investigate encapsulation of metal hydrides as a means to alter the energetic and electronic properties of three basic POSS cages ( T n with n = 8, 10, 12) in a controlled way. As an additional benefit, POSS cages with internal metal hydrides might prove to be efficient as novel media of hydrogen storage. AlH 3, a light metal-hydride (left: Al crystals, right: AlH 3 crystals) Fullerene Cages L.Gagliardi performed a computational study of metal-hydrides encapsulated in within fullerenes, such as ZrH 16 @ C 60 and 2TiH 16 @ C 114 [4]. Method The interaction between metal hydride cores and POSS cages was explored by using various ab initio and density functional theory (DFT) techniques [5], with ab initio procedures ranging from HF (Hartree-Fock) to MP2 (Moeller-Plesset perturbation theory at second order). A variety of metal-hydrides could be stored in T 12 cages. However, the more conventional T 8 cage is more easily fabricated. PtH 4 within T 12 Optimization performed using frequency analysis at the B3LYP/CEP-31G level FeH 2 within T 8 Results were found at the HF/CEP-121G level. Geometric optimizations were confirmed using MP2 and B3LYP. Results Equilibrium structures were obtained for systems of the form MH m @T n [(SiO 3/2 H) n ], with m = 2, 3, 4, n = 8, 10, 12, and M = transition metal elements in Group IVB, VIB, VIII, IIB. The potential surface inside the less studied T 12 cage turned out to be relatively flat, and equilibrium geometries were obtained for a wide range of large metal hydrides. Zero-point corrected stabilization energies E for T 10 based systems were found to be negative, corresponding to an exothermal encapsulation process, in 4 cases: For MH m molecules inside the T 8 and T 10 cages, no stability was found for m>2. For m=2, cage symmetry change associated with elongation along the MH 2 axis was found to occur in T 8 cage. Some common structures of MH2@T10 complexes Due to the size restriction associated with T 8, no systems of the form MH m @T 8 with m > 2 were found to converge. Some periodic arrangements of T 8 and MH 2 @T 8 were included and proven to be stable: Summary A wide variety of POSS cages with enclosed metal hydrides were shown to be stable by ab initio computation. For RuH 2, PdH 2, OsH 2, PtH 2 encapsulation into T 10 was found to be an exothermal process. An effort to analyze POSS polymers with endohedral metal hydrides was initialized. Xiqiao Wang, John Corn, Frank Hagelberg Department of Physics and Astronomy East Tennessee State University Johnson City, TN 37614 References: [1] F.Wang, X.Lu, C.He, J.Mat.Chem. 21, 2775 (2011) [2] D. Hossain, F.Hagelberg, C.Pittman, S.Saebo, J.Inorg. Org.Pol.Mat. 20, 1574 (2010) [3] D. Hossain, C. Pittman Jr., F. Hagelberg, S. Saebo, J.Phys.Chem.C, 112, 16070 (2008) [4] L.Gagliardi, J.Chem.Theo.Comp. 1, 1172 (2005) [5] M.J.Frisch et al. Gaussian, Rev. B.01, Gaussian Inc., Wallingford, CT (2004) Acknowledgment: Support by TN-SCORE (NSF EPS 1004083) is gratefully acknowledged. System Method E [Hartree] RuH2@T 10 MP2/CEP-121G -0.024 ~ -0.030 PdH2@T 10 MP2/CEP-121G -0.007 OsH2@T 10 MP2/CEP-121G -0.012 PtH2@T 10 MP2/CEP-121G -0.006


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