Dissociation of H2 on carbon doped aluminum cluster Al6C

Reconsidering the Structures of C2 Al4 - and C2 Al5. Chemistry 2023;29:e202301338. [PMID: 37498677 DOI: 10.1002/chem.202301338]

The study of C2 Al4 -/0 and C2 Al5 -/0 was conducted using anion photoelectron spectroscopy and quantum chemical computations. The present findings reveal that C2 Al4 - has a boat-like structure, with a single C2 unit surrounded by four aluminum atoms. In contrast, the neutral C2 Al4 species adopts a D2h planar structure with two planar tetracoordinate carbon (ptC) units, consistent with previous reports. Furthermore, the global minimum isomer of C2 Al5 - adopts a D3h symmetry, where the C2 unit interacts with five aluminum atoms. It was also found that a lower symmetry structure of C2 Al5 - , where all five aluminum atoms are located on the same side of the C2 unit, albeit slightly higher in energy compared to the D3h structure. These computations show that the D3h structure of C2 Al5 - is highly stable, exhibiting a large HOMO-LUMO gap.

Investigating the role of carbon doping on the structural and energetic properties of small aluminum clusters using quantum Monte Carlo

In this study, we investigate the energetics of small aluminum clusters doped with a carbon atom using several computational methods, including diffusion quantum Monte Carlo, Hartree-Fock (HF), and density functional theory. We calculate the lowest energy structure, total ground-state energy, electron population distribution, binding energy, and dissociation energy as a function of the cluster size of the carbon-doped aluminum clusters compared with the undoped ones. The obtained results show that carbon doping enhances the stability of the clusters mainly due to the electrostatic and exchange interactions from the HF contribution gain. The calculations also indicate that the dissociation energy required to remove the doped carbon atom is much larger than that required to remove an aluminum atom from the doped clusters. In general, our results are consistent with available theoretical and experimental data.

Structural Evolution of Carbon-Doped Aluminum Clusters AlnC- (n = 6-15): Anion Photoelectron Spectroscopy and Theoretical Calculations

Carbon-doped aluminum cluster anions, Al_nC^- (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of Al_nC^- (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al₆C^- is a carbon endohedral triangular prism. The Al₇C^- anion is a magic cluster with high stability. The structures of Al_7-9C^- can be viewed as the additional aluminum atoms attached around the triangular prism Al₆C^-. Two isomers of Al₁₀C^- have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al₉C^- with the carbon atom located in a pentagonal bipyramid. The Al₁₁C^- anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al_12-14C^- can be viewed as evolving from Al₁₁C^- by adding aluminum atoms to interact with the carbon atom. In Al₁₅C^-, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al₁₃ unit can be identified as a part of the geometric structure of Al₁₅C^-.

Shape Symmetrization and IR-Spectral Enhancement of Aluminum Clusters via Doping with a Carbon Core

Composite nanosystems are a class of objects with interesting and potentially useful properties. Here we study mixed-composition species representing interfaces at the molecular level between such technologically relevant materials as carbon and aluminum. Specifically, core-shell C₈@Al_n (n = 16, 18) species and their isomers with the core and relaxed-shell attached outside are investigated at a DFT level in terms of structures and stabilities, charge distributions and polarities, and IR spectra and electron affinities. Among the interesting findings is the possibility of bringing the aluminum cluster into a more symmetric shape (thus making a convenient building block) via insertion of a suitable molecular-carbon skeleton. Another notable feature is the system-selective dependence of polarity on spin multiplicity, suggesting possible molecular-electronic applications. The IR spectra of the composite species are much brighter compared to those of the separated components and are highly focused for the core-shell isomers. A related aspect of interest is the apparent reflections of the system structural details in the IR spectra features (line intensities and separations) via related vibrations, facilitating an experimental analysis of the structure and detection of the species formation and transformation as well as potentially enabling the means of achieving desirable optical characteristics via a geometric design.

Photoelectron spectroscopy and theoretical study of AlnC5-/0 (n = 1-5) clusters: structural evolution, relative stability of star-like clusters, and planar tetracoordinate carbon structures

The AlnC5- (n = 1-5) clusters were detected in the gas-phase and were investigated via mass-selected anion photoelectron spectroscopy. The structures of AlnC5-/0 (n = 1-5) were explored by theoretical calculations. It is found that the structures of AlC5-/0 and Al2C5-/0 are linear while those of Al3C5-/0, Al4C5-/0, and Al5C5-/0 are two-dimensional. The most stable structures of AlC5-/0 and Al2C5-/0 are linear with the Al atoms attached to the ends of C5 chain. The most stable structures of Al3C5-/0 can be viewed as three Al atoms interacting with a nonlinear C5 chain. The most stable structure of Al4C5- anion is a planar structure composed of a C2 unit, a C3 unit, and two Al2 units, while that of the neutral Al4C5 cluster has four Al atoms connected to different positions of a distorted C5 chain. The global minimum structures of Al5C5-/0 are planar structures composed of an Al4C quadrilateral, two C2 groups, and an Al atom connected to two C2 groups. Planar tetracoordinate carbon (ptC) has been identified in the structures of both anionic and neutral Al5C5. It is worth mentioning that the star-like structure of Al5C5- is slightly higher in energy than the ground state structure. The comparison of theoretical calculations with the experimental spectra indicates the star-like structure of Al5C5- may also appear in our experiments.

The DFT study of Si-doped Pd6Si clusters for selective acetylene hydrogenation reaction

Recently, it has been demonstrated that Si-modified Pd catalyst shows excellent selectivity and produces less amount of green oil than the unmodified Pd catalyst in acetylene hydrogenation. Motivated by experiment works, we systematically investigate the mechanism of the selective acetylene hydrogenation reactions over pristine Pd₇ and Si-doped Pd₆Si clusters by using the B3LYP method of density functional theory. Our result confirms that both the Pd₇ and Pd₆Si clusters catalytic hydrogenation of acetylene are mainly through two different pathways and a series of intermediates can be transformed into each other by proton transfer, which link the two independent reaction paths into a network path. Among these reaction paths, activation energies for all steps of the reaction have been calculated and it is illustrated that the lowest activation energy in the process of ethylene generation are 22.59 kcal/mol for Pd₇ cluster and 11.25 kcal/mol for Pd₆Si cluster, which indicate that the Pd₆Si cluster perform better than Pd₇ cluster in the aspect of catalytic activity. Besides, it has also been demonstrated that the selectivity for the reaction is enhanced after doping a Si atom.

Covalent versus Ionic Bonding in Al-C Clusters

The low-energy structures of Al_nC_m (n = 4, 6; m = 1-4) are determined by using the genetic algorithm combined with density functional theory and the QCISD models. The electronic structures and bonding features are analyzed through the density of states (DOS), valence molecular orbitals (MOs), and electron localization function (ELF). The results show that the carbon atoms tend to aggregate and sit at the center of the clusters. The C-C bond lengths in most cases agree with the double C═C bond. Because of the large difference between the electronegativities of carbon and aluminum atoms, almost all of the 3p electrons of Al transfer to C atoms. The 3s orbitals of Al and the 2s2p orbitals of C form bonding and antibonding orbitals; the bonding orbitals correspond to the covalent C-Al bonds, and the antibonding orbitals form lone pair electrons on the outer side of Al atoms. The lone pair electrons form large local dipole moments and enhance the electrostatic interactions between C and Al atoms. Planar geometry and multiconnection are prominent structural patterns in small Al_nC_m clusters. However, the multiconnection does not correspond to multicenter chemical bonding. There are multicenter bonds, but they are much weaker than the σ C-Al bonds.