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CORRESPONDENCE Comments on “Performance of COSMO-RS with Sigma Profiles from Different Model Chemistries” Andreas Klamt COSMOlogic GmbH&CoKG, LeVerkusen, Germany, and Institute of Physical and Theoretical Chemistry, UniVersity of Regensburg, Germany Sir: Mu, Rarey, and Gmehling presented in their article1 a broad comparison between two different re-implementations of the COSMO-RS model, in combination with different quantum chemical (QC) levels and programs for the generation of the underlying σ-profiles. In the Introduction section, the authors write that the COSMO model2 would imbed the solute into an ideal conductor, while the dielectric continuum models would consider finite dielectric constants. This is incorrect. COSMO itself, as a standalone model, also treats finite dielectric media, but replaces the dielectric boundary condition by a slightly approximate but highly robust and efficient scaled conductor boundary condition. Only in the context of COSMO-RS do the underlying COSMO calculations need to be performed in an ideal conductor. Next, the authors are using the different QC levels in combination with COSMO-RS parametrizations that have been derived on just one (or none) of the considered QC levels. The COSMO-SAC re-implementation3 was parametrized on DMol3 BP/DNP COSMO files,4 as it was the first DFT-based COSMORS implementation by Klamt et al.,5 reported many years earlier. So it is not a surprise that COSMO-SAC performs worse with the Turbomole or Gaussian-based BP-TZVP COSMO files mostly considered in this article, nor should it be surprising that COSMO-RS(Ol),6 which was developed on just these QC levels, performs better with these QC levels. It is perhaps more surprising that the overall differences are quite small. No conclusion can be drawn from such comparison regarding the overall suitability of a certain quantum level for COSMO-RS. Such conclusions require that COSMO-RS become individually parametrized for each QC level. Especially, based on our experience with COSMO-RS, we cannot support the assumption made in the conclusions, that B3LYP should be especially wellsuited for COSMO-RS. The detailed consideration of the performance on different compound classes may be interesting, but no conclusions should be drawn from these overall small differences, whether the slight COSMO-RS modifications or QC levels have any advantages for one or the other class. The only really significant discrepancy appears to be the surprisingly good performance of COSMO-SAC, with respect to amine-water systems, while COSMO-RS(Ol) shows large errors for these systems, as they are reported for the original COSMO-RS implementations by Klamt et al.5 In general, the other differences probably more reflect that some compound classes or properties have been better or less well-represented in the respective parametrization of COSMO-RS and, therefore, perform better or worse with one method or the other. It should be emphasized that the entire
Figure 1. Plot showing σ-profiles of an ether and an ester, compared to alkane and water.
dataset used for this comparison is quite exactly the training data set of COSMO-RS(Ol) and that, therefore, a slightly better performance of COSMO-RS(Ol) is not at all surprising. Furthermore, it should be noted that the exclusion of systems with very high activity coefficients generally is a slight advantage for the UNIFAC methods and for COSMO-RS(Ol), because the former are known to have problems with very high activity coefficients, and the latter was parametrized without very high activity coefficients. I am quite surprised that the authors support their finding that infinite dilution activity coefficients tend to be underestimated by COSMO-RS with two citations of papers on which I am coauthor, since I cannot find such a statement or trend in the evaluation study of Putnam et al.7 Even more, in our COSMOSPACE paper,8 we point out that the self-consistent COSMOSPACE equations used in COSMO-RS are better able to describe the steep increase of infinite dilution activity coefficients at the very low concentration than the UNIQUAC approximation does, which is used in the UNIFAC method. In general, we find evenly distributed underestimations and overestimations with our COSMO-RS implementations. Hence, this does not appear to be a general feature of COSMO-RS, as the authors speculate. Their statement “Hydrogen bonding is included in COSMORS in a very simple way, which does not account for the fact
10.1021/ie0712535 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/09/2008
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that between two donor-acceptor sites only one hydrogen bond can be formed. A realistic modeling of hydrogen bonding requires association models,” is incorrect. Depending on the exact definition of the threshold and the effective contact area of a thermodynamic contact, water has ∼2 donors and ∼2 acceptors in COSMO-RS, alcohols typically have 1 donor and ∼1.7 acceptors (due to the slightly less accessible acceptors), and acetone has ∼2 acceptors. Since each piece of surface can be just bound in one surface contact at a time in a statistical sense, we would just get a maximum of 2 hydrogen bonds per molecule in pure alcohols, depending on temperature, and 4 hydrogen bonds in water, as association theory would tell us. By this, I do not want to say that COSMO-RS is great in describing hydrogen bonding. This part of the surface interactions definitely is less rigorously founded than the electrostatic part and deserves improvements. But the analysis of the authors regarding the origin of the weakness is incorrect, and not at all supported by the paper presented here. It is misleading to classify ethers and esters as nonpolar, as done in the sentence “For nonpolar-nonpolar binary systems (e.g., the binary systems that contain hydrocarbons, ethers, or esters), ...”. Ethers and esters clearly have significant dipole moments, and the COSMO-RS σ-profiles (see Figure 1) vividly show that ethers and esters have surface area in the strongly negatively polar range, i.e., in the positive σ range. It may be that such a classification has become usual in the context of group-interaction models such as UNIFAC, which are the main area of experience of the authors, and which do not tell anything
about the physical nature of interactions, but in the context of COSMO-RS it is not at all acceptable to classify ethers and esters as nonpolar, since their main interactions are of electrostatic (misfit) nature. Literature Cited (1) Mu, T.; Rarey, J.; Gmehling, J. Performance of COSMO-RS with Sigma Profiles from Different Model Chemistries. Ind. Eng. Chem. Res. 2007, 46 (20), 6612-6629. (2) Klamt, A.; Schu¨u¨rmann, G. COSMOsA New Approach to Dielectric Screening in Solvents with Explicit Expressions for the Screening Energy and Its Gradient. J. Chem. Soc., Perkin Trans. 1993, 2 (5), 799-805. (3) Lin, S. T.; Sandler, S. I. A Priori Phase Equilibrium Prediction from a Segment Contribution Solvation Model. Ind. Eng. Chem. Res. 2002, 41 (5), 899-913. (4) Andzelm, J.; Ko¨lmel, C.; Klamt, A. Incorporation of Solvation Effects into Density Functional Calculations of Molecular Energies and Geometries. J. Chem. Phys. 1995, 103, 9312-9320. (5) Klamt, A.; Jonas, V.; Burger, T.; Lohrenz, J. C. W. Refinement and Parameterization of COSMO-RS. J. Phys. Chem. A 1998, 102 (26), 50745085. (6) Grensemann, H.; Gmehling, J. Performance of a Conductor-like Screening Model for Real Solvents Model in Comparison to Classical Group Contribution Methods. Ind. Eng. Chem. Res. 2005, 44 (5), 1610-1624. (7) Putnam, R.; Taylor, R.; Klamt, Eckert, A. F.; Schiller, M. Prediction of Infinite Dilution Activity Coefficients Using COSMO-RS. Ind. Eng. Chem. Res. 2004, 42, 3635-3641. (8) Klamt, A.; Krooshof, G. J. P.; Taylor, R. COSMOSPACE: Alternative to Conventional Activity-Coefficient Models. AIChE J. 2002, 48 (10), 2332-2349.
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