Ab initio metadynamics simulations of oxygen/ligand interactions in organoaluminum clusters

Abstract

. Prior to reaction with the aluminum core, simulations suggest that the oxygen undergoes a hindered crossing of the steric barrier presented by the outer ligand monolayer. A combination of two collective variables based on aluminum/oxygen distance and lateral oxygen displacement was found to produce distinct reactant, product, and transition states for this process. In the methylated cluster with Cp * ligands, a broad transition state of 45 kJ/mol was observed due to direct steric interactions with the ligand groups and considerable oxygen reorientation. In the non-methylated cluster the ligands distort away from the oxidizer, resulting in a barrier of roughly 34 kJ/mol with minimal O 2 reorientation. A study of the oxygen/cluster system fixed in a triplet multiplicity suggests that the spin state does not affect the initial steric interaction with the ligands.The metadynamics approach appears to be a promising means of analyzing the initial steps of such oxidation reactions for ligand-protected clusters. [http://dx

Growth of metalloid aluminum clusters on graphene vacancies

“…[26][27][28]36,37 In addition to organometallic bonds, these clusters contain many metal-metal bonds in which the Al is in a low formal oxidation state. Synthesis relies on AlX (X = Cl, Br, or I) monovalent aluminum halide precursors formed in a specialized cocondensation apparatus.…”

Modeling the stability and growth of metalloid clusters for energetic materials

“…Following this step, the oxygen molecule splits as it interacts with the aluminum core, forming a temporarily stable oxidized system with Al-O bonds reminiscent of aluminum(III) oxides. [11,12] Here we expand these simulations to understand the subsequent decomposition of the cluster, for comparison with preliminary thermally programmed reactivity (TPR) experiments by Bowen and coworkers on gas-phase Al 4 Cp * 4 . [13] In this work, the pre- viously observed dissociation of O 2 inside the Al 4 tetramer is followed by a complete detachment of a single AlCp * unit, a proton transfer, and removal of Cp * ligand.…”

“…All these systems are synthesized from AlX (X=Cl, Br, or I) monovalent aluminum halide precursors formed in a specialized cocondensation apparatus. [14,15,16,17,18] 12 . These clusters crystallize into low-symmetry molecular crystals that are very airsensitive; in this report we focus on isolated clusters for comparison with recent gas-phase cluster studies in ultra-high vacuum.…”

“…[5] There has been considerable recent interest in developing metal/organic structures that address these issues and might allow for systems with fast reaction kinetics that still retain some of the high energy density of pure metals. [6,7,8,9,10,11,12] At the micron and nanoparticle scales, organic layers are being pursued as an alternative passivation to the native oxide layer, [6,9] or as a means of assembling metal particles into structured energetic materials. In recent work we have considered low-valence metalloid clusters as materials that may allow metal oxidation on even faster timescales than coated nanoparticles.…”

“…In recent work we have considered low-valence metalloid clusters as materials that may allow metal oxidation on even faster timescales than coated nanoparticles. [7,11,12] Several experimental efforts have also begun on lab-scale synthesis of ligated metalloid clusters that could be tailored for energetics applications. [13] In these proceedings we discuss simulations of the oxidation and unimolecular thermal decomposition of aluminum clusters ligated with cyclopentadienyl (Cp) and pentamethylcyclopentadienyl (Cp * ) ligands.…”