DFT study on structure and stability of clusters

Dehydrogenation of diborane on small Nbn+ clusters

The reactivity of Nbn+ (1 ≤ n ≤ 21) clusters with B2H6 is studied by using a self-developed multiple-ion laminar flow tube reactor combined with a triple quadrupole mass spectrometer (MIFT-TQMS). The Nbn+ clusters were generated by a magnetron sputtering source and reacted with the B2H6 gas under fully thermalized conditions in the downstream flow tube where the reaction time was accurately controlled and adjustable. The complete and partial dehydrogenation products NbnB1-4+ and NbnB1-4H1,2,4+ were detected, indicative of the removal of H2 and likely BHx moieties. Interestingly, these NbnB1-4+ and NbnB1-4H1,2,4+ products are limited to 3 ≤ n ≤ 6, suggesting that the small Nbn+ clusters are relatively more reactive than the larger Nbn>6+ clusters under the same conditions. By varying the B2H6 gas concentrations and the reactant doses introduced into the flow tube, and by changing the reaction time, we performed a detailed analysis of the reaction dynamics in combination with the DFT-calculated thermodynamics. It is demonstrated that the lack of cooperative active sites on the Nb1+ cations accounts for the weakened dehydrogenation efficiency. Nb2+ forms partial dehydrogenation products at a faster rate. In contrast, the Nbn>6+ clusters are subject to more flexible vibrational relaxation which disperse the energy gain of B2H6-adsorption and thus are unable to overcome the energy barriers for subsequent hydrogen atom transfer and H2 release.

Structural transformations in boron clusters induced by metal doping

In the last decades, experimental techniques in conjunction with theoretical analyses have revealed the surprising structural diversity of boron clusters. Although the 2D to 3D transition thresholds are well-established, there is no certainty about the factors that determine the geometry adopted by these systems. The structural transformation induced by doping usually yields a minimum energy structure with a boron skeleton entirely different from that of the bare cluster. This review summarizes those clusters no larger than 40 boron atoms where one or two dopants show a radical transformation of the structure. Although the structures of these systems are not easy to predict, they often adopt familiar shapes such as umbrella-like, wheel, tubular, and cages in various cases.

The Studies on Structure and Stability of CaBn Clusters

Calcium-boron systems have excellent properties of hardness, strength, and chemical stability, and we studied a series of CaB_n clusters to investigate their structures and relative stability. The results showed the most stable structures of CaB_n clusters are not planar. The B atoms tend to get together and form the planar ring to stabilize the structure, and the Ca atoms are coordinated to the periphery of the formations. The average binding energy (E_b), fragmentation energy (E_F), second-order energy difference (Δ₂E), adiabatic detachment energy (ADE), and adiabatic electron affinity (AEA) of the CaB_n clusters were calculated to investigate the relative stability and the ability of removing or obtaining an electron. As shown by the results, E_F and Δ₂E values had obvious odd-even alteration as n increased, which indicated that the formations CaB₄, CaB₆, and CaB₈ were more stable. The ADE values for CaB_n clusters with even values of n were higher than those with odd values of n, which indicated CaB_n clusters with even values of n had difficultly removing an electron. The AEA values of CaB₃ and CaB₇ were larger than the others, which meant CaB₃ and CaB₇ easily obtained an electron. These results provide a useful reference for understanding the formation mechanism and stability of the alkaline earth metal boride as well as guidance for synthesizing the CaB_n clusters.

Density functional theory study on structure and stability of BeBn+ clusters

RATIONALE

Boride compounds hold promise for broad applications in the field of optoelectronics due to their high-temperature resistant, corrosion resistant and antioxidant properties. In order to reveal the formation mechanism of alkali and alkaline earth metal doped boron clusters, theoretical studies of these systems are required.

METHODS

All the possible geometrical structures of BeB_n⁺ clusters (n = 1-8) were optimized at the B3LYP/6-311+G(d) level; the harmonic vibration frequencies were obtained to examine the true stability and give the zero-point vibration energy at that theoretical level. The single point energies of all the structures were computed at the CCSD(T)/aug-cc-pVDZ level. For the most stable structures, the average binding energy (E_b ), the fragmentation energy (E_F ) and second-order difference of total energy (Δ₂ E) were used to evaluate the relative stability of clusters.

RESULTS

Most of the BeB_n⁺ clusters are planar in structure; the B atoms tend to aggregate to form a boron ring, and the coordinating Be atoms are on the periphery of the clusters. The fragmentation energy and second-order difference of total energy show that there is an obvious odd-even alteration as n increases, and local-maxima when n is odd.

CONCLUSIONS

A systematic theoretical investigation on the geometries, stabilities and electronic properties of BeB_n⁺ clusters has been carried out where n = 1-8. The results provide a useful reference for understanding the formation mechanism and stability of these clusters, as well as guidance for finding larger size clusters.