Vibrational Frequencies for Model Silicates: Extensions Beyond Molecular Properties
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VIBRATIONAL FREQUENCIES FOR MODEL SILICATES: EXTENSIONS BEYOND MOLECULAR PROPERTIES Kim F. Ferris and Steven M. Risser Pacific Northwest Laboratory Richland, WA 99352 The investigation of surface properties for ceramic materials often focuses on molecular properties, both in terms of model systems and methods. Typically, we use molecular species (i.e. SiO 4 H 4 , H 3 SiOH) to represent silica surfaces, often resulting in poor prediction of absorption phenomena. Dielectric effects, even when approximated by electrostatic and dipolar interactions, can have significant effects on the charge distribution and surface absorption characteristics of model molecular complexes. In this paper, we report on the effect of the surrounding matrix on the harmonic vibrational frequencies by employing reaction field techniques in electronic structure calculations. Comparisons of molecular-based to reaction field affected properties will be made to illustrate the extensions of the molecular to the extended network domain. INTRODUCTION The investigation of complex systems such as gas-solid interfaces often span multiple time and spatial scales, making rigorous approaches difficult. These problems are often exacerbated when subtle electronic and energetic interactions such as hydrogen bonding are the controlling process. The absorption of water onto silica surfaces has been frequently studied both from theoretical and experimental standpoints [1]. The key interaction is hydrogen bonding of water onto the surface silanol groups, requiring ab initio methods for consistent theoretical prediction of structures and energetics. Unfortunately, the use of such methods invokes a high degree of computational intensity necessitating the use of small molecule analogs. Thermodynamic quantities are typically extracted through statistical means. However, the classical application of gas phase statistical mechanics is not appropriate for extended network systems as there are significant differences in terms of translational and rotational degrees of freedom. As a first order approximation, Sauer [2] proposed the rigid cluster model for surface reactivity in which only those components dealing with free molecular motion are considered to contribute to the molecular partition function. For example, we might use the reaction H 3 SiOH + H 2 0 -> H 3 SiOH...OH 2 as a prototype for water absorption onto silica. As both H 3 SiOH and H 3 SiOH-..OH 2 are considered models for extended surfaces, their primary contribution to the partition function are considered to be their vibrational components. For free (molecular) water, all degrees of freedom are available, and would contribute to the partition function. In terms of energetics, small molecule analogs for silica surfaces such as H 3 SiOH and Si(OH)4 both offer interaction energy predictions in general agreement with available experimental data. However, this model does not include near neighbor influences and results in large discrepancies for surface absorption of small molecules using simple Langmuir mode
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