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C: Surfaces, Interfaces, Porous Materials, and Catalysis
Size-dependent Polymorphism in Aluminum Carbide Cluster Anions AlC : Formation of Acetylide-Containing Structures n
2–
Kazuyuki Tsuruoka, Kiichirou Koyasu, Shinichi Hirabayashi, Masahiko Ichihashi, and Tatsuya Tsukuda J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12767 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018
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The Journal of Physical Chemistry
Size-Dependent Polymorphism in Aluminum Carbide Cluster Anions AlnC2–: Formation of Acetylide-Containing Structures Kazuyuki Tsuruoka,1 Kiichirou Koyasu,1,2 Shinichi Hirabayashi,3 Masahiko Ichihashi,4 Tatsuya Tsukuda1,2* 1
Department of Chemistry, School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
2
Elements Strategy Initiative for Catalysts and Batteries (ESICB), Kyoto University, Katsura, Kyoto 615-8520, Japan East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan
3 4
Cluster Research Laboratory, Toyota Technological Institute: in East Tokyo Laboratory, Genesis Research Institute, Inc., 717-86 Futamata, Ichikawa, Chiba 272-0001, Japan ABSTRACT: Aluminum carbide clusters anions AlnC2– (n = 5–13) were observed as the most dominant products in gas-phase reactions of laser-ablated Aln– with organic molecules, such as methanol, ethanol, pentane, acetonitrile or acetone. Density functional theory calculations predicted two possible isomeric structures for AlnC2–: isomers in which two carbons are dissociated (type D) as in the case of the bulk aluminum carbide and novel isomers in which two carbons form an acetylide-like C2 unit. The latter isomers are further categorized into three types depending on the location of the C2 unit: the C2 unit is encapsulated within the Al cage (type I), contained in the surface of Al clusters (type S), or attached to the surface of Al clusters (type O). Size-dependent behavior of the adiabatic electron affinities of AlnC2 determined by photoelectron spectroscopy was explained in terms of polymorphism as a function of size (n): type I for n = 5–8, type D for n = 9–11, type D or O for n = 12, and type O for n = 13. The tendency in which the position of the C2 unit was shifted from inside to outside with the increase in n was ascribed to the balance between the stabilizations gained by forming the Al–C bonds and the Al–Al bonds. The smaller AlnC2– clusters (n = 5–8) prefer to surround the acetylide-like C2 unit with the Al atoms so as to maximize the number of the Al–C bonds, while larger ones (n = 12 and 13) prefer to attach the C2 unit onto the surface of the Al clusters so as to maximize the number of the Al–Al bonds.
Much effort has been made for the chemical synthesis of stable Al clusters. For example, Al12, Al13, Al50, and Al77 with closed electronic structures have been successfully synthesized by protection with pentamethylcyclopentadiene, bis(trimethylsilyl)amine, or dendrimer.23,24 Theoretical studies suggested the possibility that Al13– can be stabilized by vinylpyrrolidone25 and tetrahydrofuran.26 During the survey of the binding affinity of Aln– with various organic molecules (pentane, ethanol, acetone, …) to find suitable protecting ligands, we serendipitously found that carbon-containing aluminum clusters AlnC1– and AlnC2– were generated as the main reaction products. Although the formation of AlnC1– and AlnC2– in the gas phase has been reported previously,27–30 the structural information is limited to AlnC1–.30,31 The mass peaks for Al7C1– and Al24C1– showed high abundances in a series of AlnC1–.27–30 It was proposed that the C atom is surrounded by Al atoms for n = 5–13 by theoretical calculation.30 The present study investigated the geometric and electronic structures of AlnC2– using photoelectron spectroscopy and DFT calculation. The results demonstrated polymorphism of AlnC2– depending on the cluster size n. More interestingly, we herein proposed the formation of novel structural isomers containing an acetylide-like unit.
!"#$%&'()*+&,(%# Metal clusters composed of fewer than hundred metal atoms have size-specific geometric and electronic structures that are significantly different from those of the corresponding bulk.1,2 For example, they frequently take non-closest packed structures such as icosahedron and have quantized electronic structures with an energy gap larger than the thermal energy. Because of such unique structures, metal clusters exhibit non-scalable, distinct physicochemical properties,3 making them as attractive targets of materials science. Aluminum cluster is one of the prototypical systems that have been studied extensively to reveal correlation between structures and properties.4 The well-known magic cluster Al13– has an icosahedral structure. A variety of structures including a deformed octupole, a decahedron, an fcc nanocrystal, and a face-sharing biicosahedron have been theoretically proposed for another magic cluster Al23–.5–7 These magic clusters Al13– and Al23– have closed electronic shells with 40 and 70 valence electrons, respectively:8,9 individual constituent Al atoms provide three valence electrons due to s–p hybridization.10 The magic clusters Al13– and Al23– do not react with O2 whereas other Al cluster anions are easily combusted.3,4,11,12 Because of the high stability and closed electronic shell structure, Al13– has been viewed as a rare-gas like superatom, a stable building unit for novel nanomaterials.13–16 So far, Al13-based compounds, such as Al13I–,17 NaAl13,18 KAl13,19 HAl13,20 CuAl13,21 and Al13(OAl)1,2– (Ref. 22) have been produced in the gas phase.
-"#./&0()1# A. Experimental
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The Journal of Physical Chemistry Supporting Information. Full citations for Ref. 36, mass spectra and photoelectron spectra obtained under other conditions and additional calculation results (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
eV, is plausible. The AEA plot can be explained by 12c(D). However, we cannot exclude the possibility of other isomers, 12b(O) and 12d(I), in which the C2 unit is attached to a pentagonal bipyramidal Al12 core and encapsulated by an icosahedral Al12 cage, respectively, because their calculated AEA values are similar to the experimental values. The C2-encapsulated structure was theoretically predicted for neutral Al12C2.46 n = 13. The most stable structural motif for n = 13 is classified into type O. The icosahedral motif of Al13– is deformed into a pentagonal bipyramidal Al13 core in 13a(O). This suggests that the C2 unit acts as a template for regulating of the Al13 core. The formation of 13a(O) is also supported by the AEA value (Figure 4). In summary, AlnC2– shows polymorphism as a function of n: type I for n = 5–8, type D for n = 9–11, type D or O for n = 12, and type O for n = 13. The novel finding of this paper is the production of AlnC2– in which C2 is contained as an acetylide unit (types I, S and O) given that carbon is contained as an atomic form in the aluminum carbide in the bulk. The size-dependent evolution of structures and positions of the C2 unit in AlnC2– is explained by the balance between the stabilizations gained by formation of the Al–C and Al–Al bonds and destabilization caused by dissociation of the C–C bond. The preferential formation of type I structures for n = 5–8 indicates that the formation of the Al–C bonds is a determining factor of the structures. However, type D structure is energetically favored more than type I structure at n = 9. This suggests that Al9C2– gains more stabilization energy with respect to Al8C2– by increasing the number of the Al–C bonds with the sacrifice of dissociation of the C–C bond than by increasing the number of the Al–Al bonds by attaching an Al atom on the surface of the Al cage. The preferential formation of type D structures up to n = 12 indicates that the formation of the Al–C bonds around the dissociated C atoms is a determining factor of the structures. Type O structures are preferred for n = 12 and 13 so as to maximize the number of the Al–Al bonds while retaining the C2 unit.
