Proceedings - Interdisciplinary Center for Nanotoxicity

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146 Conference on Current Trends in Computational Chemistry 2009 Including Explicit Water Molecules in DFT‐based pKa Predictions: Can the Theoretical Slope be Reproduced? Shuming Zhang, Jon Baker, Peter Pulay Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, AR 72701 A popular method for predicting pKa values of organic molecules is to establish empirical linear relations between calculated deprotonation energies and experimental pKa values. In virtually all such calculations, the empirically observed slope of the pKa vs. ΔE relation is significantly less than the theoretical value, (2.303RT) ‐1 which is 0.73 mol/kcal at room temperature. For instance, in our continuum solvation calculations, the empirical slope for a set of 115 carboxylic acids was only 0.23 mol/kcal, although the calculated pKa values matched the experimental data very well. There has been much speculation about the cause of this phenomenon. Although the ΔE – pKa relation neglects entropic effects, they are expected to largely cancel out. The main potential cause for the strange behavior of the fitted slope is specific water‐solute interactions that cannot be described accurately by continuum solvation models. We used our previously developed pKa prediction protocol (OLYP/6‐311+G(d,p)//3‐ 21G(d) with the COSMO solvation model) to investigate the effect of including one or two explicit water molecules. Waters were added either to both the ionized and unionized forms, or only to the ionized. Our calculations included carboxylic acids, phenols, alcohols, amines, anilines and pyridines. We found that the slopes for organic acids (especially for carboxylic acids) got closer to the theoretical value when explicit water molecules were added to both the neutral molecule and the anion. However, explicit water molecules had almost no effect on the slopes for organic bases. Adding explicit waters only to the ions produces intermediate results. Unfortunately calculations with explicit water molecules have much larger errors than using continuum solvation models alone, and are also much more expensive. Consequently they are not suitable for large‐scale pKa calculations. Possible reasons for the decrease in accuracy with explicit water molecules will be discussed.
Conference on Current Trends in Computational Chemistry 2009 Thioglycolic acid on the gold (111) surface and Raman vibrational spectrum Jian‐Ge Zhou and Quinton L. Williams Department of Physics, Atmospheric Sciences, and Geoscience, Jackson State University, Jackson, MS 39217, USA The interaction of thioglycolic acid (HSCH 2 COOH) with the Au(111) surface is investigaged, and it is found that at the low coverage the molecule lies down on the substrate. If the mercaptan‐hydrogen atom is eliminated, the resulting SC H 2 COOH molecule is randomly oriented on the surface. If the carboxylic acid group in the HSCH 2 COOH molecule is deprotonated instead, the HSCH 2 COO − molecule lies down on the surface. However, when the mercaptan‐hydrogen atom in the HSCH 2 COO − molecule is removed, the resulting SCH 2 COO − molecule rises up to a certain level on the substrate. The calculated Raman vibrational spectrum deciphers which compounds and atomic displacements contribute to the corresponding frequencies [1]. We thus propose a consistent mechanism for the deposition of thioglycolic acid on the Au(111) surface. References [1] A. Krolikowska et. al., Sur. Sci. 532, 227 (2003). 147
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Organizing Committee Shonda Allen-H
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Schedule of Events Conference on Cu
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Proceedings - Interdisciplinary Center for Nanotoxicity

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