Supporting Online Material for

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1 Supporting Online Material for Complementary Active Sites Cause Size-Selective Reactivity of Aluminum Cluster Anions with Water Patrick J. Roach, W. Hunter Woodward, A. W. Castleman Jr.,* Arthur C. Reber, Shiv N. Khanna *To whom correspondence should be addressed. Published 23 January 2009, Science 323, 492 (2009) DOI: /science This PDF file includes: Materials and Methods Figs. S1 to S5 References

2 Materials and Methods Experimental A Fast-flow reactor apparatus was used to monitor reactivity. The apparatus is depicted and described in Fig. S2. Deuterium oxide was used as a reactant as obtained from Cambridge Isotope Laboratories, Inc. (99.9%) to facilitate the identification of products. Similar results were obtained with H 2 O; therefore, no effort is made to distinguish isotopic differences and all mass spectra presented are of deuterated species. Theoretical First principles electronic structure investigations were carried out using a molecular orbital approach within a gradient corrected density functional framework. The molecular orbitals are expressed as a linear combination of atomic orbitals formed via a combination of Gaussian functions centered at the atomic sites. The exchange correlation contributions are included within a gradient corrected density functional formalism (GGA-PBE) (S1). The calculations were carried out, at an all electron level, using the Naval Research Laboratory Molecular Orbital Library (NRLMOL) developed by Pederson and co-workers (S2-S4). Here the Hamiltonian matrix elements are evaluated by numerical integration over a mesh of points. The basis sets, built from a variable number of primitive Gaussians, are based on a total-energy minimization for free atoms and are optimized for all electron density functional calculations (S4). The basis set for Al had 6s, 5p, and 3d Gaussians; that for O had 6s, 5p, and 3d Gaussians; and the basis set for H had 4s, 3p, and 1d Gaussians. The basis sets were supplemented with a diffuse d-gaussian to allow further variational freedom. For details of the codes and the basis sets, the reader is referred to earlier papers (S2-S4). The results displayed in Fig. 2 and 3 are global minimum for the pure aluminum clusters, and local minimum and transition state structures for the reaction pathways. Transition states have been calculated using constrained conjugate gradient optimizations. The local minimum calculations were performed using conjugate gradient optimizations.

3 Figures and Legends Figure S1 The affinity of aluminum cluster anions towards water, plotted as the sum of the ion products XAl n - normalized per the parent ion intensity

4 E Figure S2: The Fast Flow Reactor. Clusters are formed in a plasma that results from the pulsed laser vaporization of an aluminum rod in the presence of a continuous (~8000 standard cubic centimeter per minute) flow of helium (A). The clusters are entrained in a helium process gas and carried out of the source into a laminar flow-tube that is maintained at ~0.9 Torr. The clusters collisionally cool to the temperature of the flow-tube walls (room temperature) and interact with water, which is introduced through an annular stainless steel reaction gas inlet (B). Products are focused through an extraction orifice with a conical octopole [(C) (S5)] and guided through further differential pumping chambers (D) via radio-frequency octopoles and electrostatic lenses. Ions are steered orthogonally to remove neutrals using a quadrupole deflector energy filter and the clusters are mass selected via a quadrupole mass filter (E). All electronics are controlled and monitored using the Merlin system (Extrel, CMS) computer (F).

5 Figure S3: A) The calculated absolute HOMO and LUMO levels of the Al n - clusters as a function of size. B) The calculated binding energy when the O-H bond is intact and the proposed transition state energy, ΔE T, for the breaking of the O-H bond in the Al n (H 2 O) - complex.

6 Figure S4: Reactive Aluminum Clusters. Water Adsorption onto specific aluminum clusters, the mass of D 2 O is 20 amu, therefore +20 corresponds to a single D 2 O bound to the Aluminum cluster, and +40 corresponds to two D 2 O molecules bound to the aluminum cluster, etc. The non-labeled peaks are the larger aluminum clusters Al - n+1 and Al - n+2. Of specific interest, Al - 37 species with a filled superatomic shell of electrons adsorbs multiple waters.

7 Al Al Al 12 (H 2 O) - Al 17 (H 2 O) Figure S5: The calculated Hirschfeld charges (S6) on the Al - 12 and Al - 17 clusters before and after - water binds with the Lewis base sites (numbered in red). The Lewis acid site on Al 12 is the top site with e -, and is on Al After the water adds, the Lewis base active sites are further enhanced by the binding of water in both clusters. These calculations were done using an all electron calculation with the ADF DFT code (S7) using the PBE functional (S1) and a TZVP basis. References S1 J. P. Perdew, K. Burke, M. Ernzerhof. Phys. Rev. Lett (1996).

8 S2 M. R. Pederson, K. A. Jackson. Phys. Rev. B (1990) S3 K. Jackson, M. R. Pederson. Phys. Rev. B (1990). S4 D. Porezag, and M. R. Pederson. Phys. Rev. A S5. M.A. Rӧttgen, K. Judai, J-M Antonietti, U. Heiz S. Rauschenbach and K. Kern, Rev. Sci. Inst (2006) S6 F.L. Hirschfeld Theor Chim Acta (1977). S7 F.M.B. G. te Velde, E. J. Baerends, C. Fonseca Guerra, S. J. A. van Gisbergen, J. G. Snijders, T. Ziegler, J. Comp. Chem. 23, 22, 931 (2001).