4.6 Article

Structures and Energetics of (MgCO3)n Clusters (n ≤ 16)

Journal

JOURNAL OF PHYSICAL CHEMISTRY A
Volume 119, Issue 14, Pages 3419-3428

Publisher

AMER CHEMICAL SOC
DOI: 10.1021/jp511823k

Keywords

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Funding

  1. Geosciences Research Program in the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences Biosciences
  2. Office of Science of the U.S. Department of Energy [DE-AC05-00OR22725]
  3. Robert Ramsay Chair Fund of The University of Alabama

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There is significant interest in the role of carbonate minerals for the storage of CO2 and the role of prenucleation dusters in their formation. Global minima for (MgCO3)(n) (n <= 16) 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. The most stable isomers for (MgCO3)(n) (n < 5) are approximately 2-dimensional. Mg can be bonded to one or two 0 atoms of a CO32-, and the 1-O bonding scheme is more favored as the cluster becomes larger. The average C-Mg coordination number increases as the cluster size increases, and at n = 16, the average C-Mg coordination number was calculated to be 5.2. The normalized dissociation energy to form monomers increases as n increases. At n = 16, the normalized dissociation energy is calculated to be 116.2 kcal/mol, as compared to the bulk value of 153.9 kcal/mol. The adiabatic reaction energies for the recombination reactions of (MgO)(n) clusters and CO2 to form (MgCO3)(n) were calculated. The exothermicity of the normalized recombination energy < RE >(CO2) decreases as n increases and converged to the experimental bulk limit rapidly. The normalized recombination energy < RE >(CO2) was calculated to be -52.2 kcal/mol for the monomer and -30.7 kcal/mol for n = 16, as compared to the experimental value of -27.9 kcal/mol for the solid phase reaction. Infrared spectra for the lowest energy isomers were calculated, and absorption bands in the previous experimental infrared studies were assigned with our density functional theory predictions. The C-13, O-17, and Mg-25 NMR chemical shifts for the clusters were predicted. The results provide insights into the structural and energetic transitions from nanoclusters of (MgCO3)(n) to the bulk and the spectroscopic properties of clusters for their experimental identification.

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