Reconstruction of the (011) surface on alpha-quartz: a semiclassical Ab initio molecular dynamics study.
Lopes-PEM; Demchuk-E; Mackerell-AD Jr.
Int J Quant Chem Jan 2009; 109(1)(Spec Iss SI):50-64
Ab initio molecular dynamics simulations have been performed on the (011) surface of -quartz at room-temperature using density functional theory. Pristine surface was obtained by homolytic fracture of alpha-quartz crystal, leaving exposed SiO· radicals, following which the surface was allowed to spontaneously reconstruct during the simulation. Reconstruction events occurred at different timings but in most cases produced similar geometries. The most common motif consisted of a fused seven-member ring. Formation of this structure proceeded through reaction of two adjacent O· radicals, followed by linking of one of the reactive O· with a nearby silicon. Other structures, like Si2O2, were also observed. In the newly formed structures some of the silicon atoms had pentacoordinated geometry, usually distorted between pure bipyramidal, trigonal, and square pyramidal. In a few cases oxygen atoms also became tricoordinated. Formation of new bonds was investigated by analyzing the electronic structure of the system along the reaction path, and specifically the localization of unpaired electrons was deduced from the spin-density. Because most reactions involved triplet singlet transitions, a time-dependent density functional theory was used to determine the crossing point of the two potential energy surfaces. Plotted density of states was also used to compare the electronic structures of the initial and reconstructed surfaces. Reconstruction originated new states from O-px and O-pz orbitals. Stability analysis of the reconstructed surface was performed at the PBE level and 7-member rings were found to be more stable than fused rings with pentacoordinated silicon. Hypervalent silicon and oxygen atoms were also found to exist. Nevertheless, specialization of (011) surface appears to be less reactive compared to (001), which suggests a lesser toxicity of rhombohedral form on crystalline silica.
Biological-factors; Biological-systems; Health-hazards; Protective-coatings; Protective-measures; Silicates; Silicon-compounds; Silicosis;
Author Keywords: silica toxicity; quartz; silicosis; molecular dynamics; density functional theory; surfaces
Pedro E. M. Lopes, National Institute for Occupational Safety and Health, CDC, Morgantown, WV 26505
Issue of Publication
International Journal of Quantum Chemistry