Thursday, May 10, 2012

Synthesis and formation mechanism of hydrogenated boron clusters B12Hn with controlled hydrogen content

Maximiliano De La Higuera Macías

     We present the formation of hydrogen-content-controlled B12 Hn+  clusters through the decomposition and ion-molecule reactions of the decaborane _ B10 H14_  and diborane _ B2 H6_ molecules in an external quadrupole static attraction ion trap. The hydrogen- and boron-contents of the B10−y Hx+  cluster are controlled by charge transfer from ambient gas ions. In the process of ionization, a certain number of hydrogen and boron atoms are detached from decaborane ions by the energy caused by charge transfer. The energy caused by the ion-molecule reactions also induces H atom detachment. Ambient gas of Ar leads to the selective generation of B10 H6+ . The B10 H6+ clusters react with B2 H6  molecules, resulting in the selective formation of B12 H8+  clusters. Ambientgas of Ne _ He_  leads to the generation of B10−y Hx+  clusters with x =4–10 and y =0–1 _ with x =2–10 and y =0–2_ , resulting in the formation of B12 Hn+  clusters with n =4–8 _n =2,4–8_ . The introduction of ambient gas also increases the production of clusters. PBE0/ 6-311+G_ d_/ / B3LYP/ 6-31G_ d_ -level density functional theory calculations are conducted to investigate the structure and the mechanism of formation of B10−y Hx+  and B12 Hn+  clusters.
     The analysis of the mass spectrum of B10−y Hx+  ions, which are generated by charge transfer from noble gas ions _ Ar, Ne, and He_  to B10 H14  molecules in the EQSIT, revealed that B10 H6 +  is generated with Ar, B10 H4–12 +  and B9 Hx +  are generated with Ne, and B10 H2–12+ , B9 Hx+ , and B8 Hx+  are generated with He. The B10 Hx+  ions react with B2 H6  molecules, and the analysis of the mass spectrum shows that B12 Hn+  with n =8, n =4–8, and n =2–8 are produced withthe ambient gas of Ar, Ne, and He, respectively. PBE0/ 6-311+G_ d_/ / B3LYP/ 6-31G_ d_ -level DFT calculations were conducted to investigate the ionization process of B10 H14 . On the basis of the experimentally observed derivative ions of B10 H14 and their calculated energies, the charge transfer energy _ECT_  was estimated. ECT  is generated when charge is transferred from noble gas ions to B10 H14  molecules. When ECT  is in the range of 0.96–12.00 eV for He, 0.96–9.18 eV for Ne, and 3.94–5.36 for Ar, the computationally expected products are in agreement with the experimental result. The formation process of B12 Hn+  was also calculated.
     The reaction energies, _E_x_ , of B10 Hx+  and B2 H6 were calculated. _E _ 6, 4, and 2_  were estimated to be 3.85, 6.38, and 7.50 eV. The calculations of the pathway of hydrogen detachment from icosahedral B12 Hx+6+ _x =6,4,2_ indicated that B12 Hn +  with n =8, 6, and 4 are produced by the detachment of two hydrogen molecules with ambient gas of Ar, Ne, and He, respectively. The remaining ECT  is considered to be the reason of the formation of B12 Hn +  clusters with fewer H atoms in the experiment than in the prediction. The introduction of ambient gas was shown to be effective for producing B- and H-atom-controlled ions and clusters. The DFT calculation of the reaction process of B12 Hn+  indicates that further reducing of the hydrogen atoms in decaborane ion leads to the formation of planar B12 Hn+  with n =0–3. This means that control of the number of the H atoms in decaborane ion leads to the control of the structures of boron clusters. In addition, the production of clusters increases dramatically upon introducing the ambient gas. These results open up the possibility of fabricating nanostructured materials by the deposition of clusters.

 Toshihiko Kanayama, et al. "Synthesis And Formation Mechanism Of Hydrogenated Boron Clusters B12hn With Controlled Hydrogen Content." Journal Of Chemical Physics 133.7 (2010): 074305. Academic Search Complete. Web. 10 May 2012.

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