Structures and Stabilities of (MgO)_n Nanoclusters

Abstract

Global minima for (MgO)n structures were optimized using a tree growth-hybrid genetic algorithm in conjunction with MNDO/MNDO/d semiempirical molecular orbital calculations followed by density functional theory geometry optimizations with the B3LYP functional. New lowest energy isomers were found for a number of (MgO)n clusters. The most stable isomers for (MgO)n (n > 3) are 3-dimensional. For n < 20, hexagonal tubular (MgO)n structures are more favored in energy than the cubic structures. The cubic structures and their variations dominate after n = 20. For the cubic isomers, increasing the size of the cluster in any dimension improves the stability. The effectiveness of increasing the size of the cluster in a specific dimension to improve stability diminishes as the size in that dimension increases. For cubic structures of the same size, the most compact cubic structure is expected to be the more stable cubic structure. The average Mg-O bond distance and coordination number both increase as n increases. The calculated average Mg-O bond distance is 2.055 Å at n = 40, slightly smaller than the bulk value of 2.104 Å. The average coordination number is predicted to be 4.6 for the lowest energy (MgO)40 as compared to the bulk value of 6. As n increases, the normalized clustering energy ΔE(n) for the (MgO)n increases and the slope of the ΔE(n) vs n curve decreases. The value of ΔE(40) is predicted to be 150 kcal/mol, as compared to the bulk value ΔE(∞) = 176 kcal/mol. The electronic properties of the clusters are presented and the reactive sites are predicted to be at the corners.

Gas-phase spectra of MgO molecules: a possible connection from gas-phase molecules to planet formation

“…The slightly larger harmonic, diagonal (MgO)3 force constants, displayed in Table 2 of the SI, also corroborate the growth in stabilization for larger clusters of (MgO)n. Since periclase or bulk magnesium oxide is a stable ionic lattice, such an increase in stabilization for larger clusters is fully expected. As a result, the larger clusters are behaving more like the mineral with each addition of a monomer unit (Chen, Felmy & Dixon 2014).…”

“…The relative energies and structural data for the gaseous clusters of (MgO)n (n = 1 − 40), have been previously calculated with density functional theory (Chen, Xu & Zhang 2008;Chen, Felmy & Dixon 2014;Feitoza et al 2017) showing patterns of aggregation. Most notably, the face-centered cubic patterns of (MgO)n begin to emerge at n = 4 as a cube built from two, stacked (MgO)2 structures.…”

“…Most notably, the face-centered cubic patterns of (MgO)n begin to emerge at n = 4 as a cube built from two, stacked (MgO)2 structures. However, depending upon the n value, cubic and hexagonal isomers vie for the minimum energy structure (Chen, Felmy & Dixon 2014). In any case, spectral data for the smallest of these clusters will assist in the characterization of magnesium oxide forms in the laboratory and potentially even in mapping the physical properties of protoplanetary discs.…”

Synthesis Target Structures for Alkaline Earth Oxide Clusters

“…For n = 16 and, most importantly n = 18 and n = 24, as at these sizes it is again possible to form a barrel motif, magnesium oxide clusters also preferentially adopt a cuboid, rocksalt cut structure. The barrel is the second-lowest energy structure for (MgO) 18 and while it is not ranked in the top 10 structures for (MgO) 24 , many of the lowest-energy structures are partially barrel-like. The case of n = 18 is also noteworthy as there are two possible cuboid configurations composed of either 4 × 3 × 3 or 6 × 3 × 2 atoms.…”

“…Comparison with previous computational results for magnesium oxide and calcium oxide clusters [10][11][12][13][14][15][16][17][18][19], as well as a comparison of theory with infrared data [20,21], will also be discussed below. A comparison with results for alkali halides is also offered.…”