EXAMPLES AND COMPARISON TO EXPERIMENT

Polycyclic aroatic hydrocarbons. Polycyclic aromatic hydrocarbons (PAHs) are useful test compounds because there are numerous compounds and isomers within this class of organic molecules, and there is a significant database of measured gas-phase vibrational spectra (Semmler et al. 1991). Unfortunately, this is not the case for many of the molecules of interest in geochemistry, such as aluminosilicates. 

Acetic acid. Another comparison of gas-phase experimental (Bertie and Michaelian 1982) and theoretical vibrational frequencies has been made for acetic acid (Kubicki et al. 1996a). In that study, energy minimizations and force constant analyses were performed with HF/3-21G and MP2/6-311+G (2 -order Mdler-Plessett electron correlation with triply-split valence orbitals, difihise functions on all heavy atoms, and polarization functions on all atoms) calculations in Gaussian 92 (Frisch et al. 1993). The resulting frequencies were scaled by 0.893 and 0.941, respectively (Foresman and Frisch 1996). A plot of the calculated frequencies resulted in a near perfect correlation to the experimental frequencies with slopes of approximately 1.0 and intercepts of approximately zero. Thus, at least for these neutral gas-phase molecules, the smaller (and more time-efficient) HF/3-21G calculations provided results that were just as useful as the more robust (and computationally intensive) MP2/6-311+G calculations after scaling was applied. 

Orthosilicic acid. Ideally, a comparison of observed and calculated silicate vibrational frequencies for gas-phase molecules would be the next step. However, gas-phase spectra of silicate molecules that may exist in aqueous solution or as components of silicate minerals and glasses are not available. Hence, to test the methodology used for 

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