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Title: Parameterizing Potential-Derived Charge-Dependent Cavity Radii for Continuum Solvation Models: Aqueous Solutes with Oxo, Hydroxo, Amino, and Methyl Functionalities

Abstract

In recent years, computer processing capabilities have improved, allowing computationally demanding solvation models to become practically implemented. Since the determination of experimental solvation free energies for short-lived intermediates such as radicals may be difficult, expensive, and time consuming, developing computational solvation models has become an effective alternative approach. In this work, the COSMO (COnductor like Screening MOdel) continuum solvation model was used to predict solvation free energies of a set of solutes with oxo, hydroxyl, amino, and methyl functional groups in the solvent water (ε=78.39). The continuum solvation model requires a cavity to be defined around a solute to predict solvation free energy (ΔGs*). To reproduce experimental values, the size and shape of the cavity must reflect the strength of solute/solvent interactions. The cavity is defined as interlocking spheres around atoms or groups of atoms in the solute. The sphere radii are defined by linear functions parameterized in terms of potential derived charges known as CHELPG charges. A training set of neutral and ionic solutes was created, and coefficients in the radii definitions were fitted to reproduce experimental ΔGs* values by minimizing residuals between experimental and calculated ΔGs* values for compounds in the training set. Data used in fitting themore » radii definitions’ coefficients was generated by performing electronic structure and solvation computations, using varying cavity sizes. The calculations were done using Density Functional Theory with the B3LYP functional and 6-311+G** basis set in the Gaussian98 package of programs. Radii definitions reproduce ΔGs* of neutrals and singly-charged ions in training set to within experimental uncertainty and accurately predict ΔGs* of some compounds outside the training set. The mean unsigned error of this training set is 0.18 kcal/mol. These findings suggest that the protocol described here for developing potential derived charge dependent cavity definitions may successfully be extended to more functional groups to increase the applicability of the scheme and obtain greater accuracy in using continuum solvation models in predicting equilibrium properties of aqueous solutes.« less

Authors:
;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
993378
Report Number(s):
PNNL-SA-46271
KL0101000; TRN: US201023%%221
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Undergraduate Research, VI:147; Journal Volume: 6
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; SOLVATION; MATHEMATICAL MODELS; FREE ENERGY; C CODES; HYDROXY COMPOUNDS; AMINES; OXYGEN COMPOUNDS; METHYL RADICALS; DENSITY FUNCTIONAL METHOD; WATER; Continuum; solvation; models

Citation Formats

Schwerdtfeger, Christine A, and Camaioni, Donald M. Parameterizing Potential-Derived Charge-Dependent Cavity Radii for Continuum Solvation Models: Aqueous Solutes with Oxo, Hydroxo, Amino, and Methyl Functionalities. United States: N. p., 2006. Web.
Schwerdtfeger, Christine A, & Camaioni, Donald M. Parameterizing Potential-Derived Charge-Dependent Cavity Radii for Continuum Solvation Models: Aqueous Solutes with Oxo, Hydroxo, Amino, and Methyl Functionalities. United States.
Schwerdtfeger, Christine A, and Camaioni, Donald M. Tue . "Parameterizing Potential-Derived Charge-Dependent Cavity Radii for Continuum Solvation Models: Aqueous Solutes with Oxo, Hydroxo, Amino, and Methyl Functionalities". United States. doi:.
@article{osti_993378,
title = {Parameterizing Potential-Derived Charge-Dependent Cavity Radii for Continuum Solvation Models: Aqueous Solutes with Oxo, Hydroxo, Amino, and Methyl Functionalities},
author = {Schwerdtfeger, Christine A and Camaioni, Donald M},
abstractNote = {In recent years, computer processing capabilities have improved, allowing computationally demanding solvation models to become practically implemented. Since the determination of experimental solvation free energies for short-lived intermediates such as radicals may be difficult, expensive, and time consuming, developing computational solvation models has become an effective alternative approach. In this work, the COSMO (COnductor like Screening MOdel) continuum solvation model was used to predict solvation free energies of a set of solutes with oxo, hydroxyl, amino, and methyl functional groups in the solvent water (ε=78.39). The continuum solvation model requires a cavity to be defined around a solute to predict solvation free energy (ΔGs*). To reproduce experimental values, the size and shape of the cavity must reflect the strength of solute/solvent interactions. The cavity is defined as interlocking spheres around atoms or groups of atoms in the solute. The sphere radii are defined by linear functions parameterized in terms of potential derived charges known as CHELPG charges. A training set of neutral and ionic solutes was created, and coefficients in the radii definitions were fitted to reproduce experimental ΔGs* values by minimizing residuals between experimental and calculated ΔGs* values for compounds in the training set. Data used in fitting the radii definitions’ coefficients was generated by performing electronic structure and solvation computations, using varying cavity sizes. The calculations were done using Density Functional Theory with the B3LYP functional and 6-311+G** basis set in the Gaussian98 package of programs. Radii definitions reproduce ΔGs* of neutrals and singly-charged ions in training set to within experimental uncertainty and accurately predict ΔGs* of some compounds outside the training set. The mean unsigned error of this training set is 0.18 kcal/mol. These findings suggest that the protocol described here for developing potential derived charge dependent cavity definitions may successfully be extended to more functional groups to increase the applicability of the scheme and obtain greater accuracy in using continuum solvation models in predicting equilibrium properties of aqueous solutes.},
doi = {},
journal = {Journal of Undergraduate Research, VI:147},
number = ,
volume = 6,
place = {United States},
year = {Tue Jan 31 00:00:00 EST 2006},
month = {Tue Jan 31 00:00:00 EST 2006}
}
  • Dielectric continuum solvation models are widely used because they are a computationally efficacious way to simulate equilibrium properties of solutes. With advances that allow for molecular-shaped cavities, they have reached a high level of accuracy, in particular for neutral solutes. However, benchmark tests show that existing schemes for defining cavities are unable to consistently predict accurately the effects of solvation on ions, especially anions. This work involves the further development of a protocol put forth earlier for defining the cavities of aqueous solutes, with resulting advances that are most striking for anions. Molecular cavities are defined as interlocked spheres aroundmore » atoms or groups of atoms in the solute, but the sphere radii are determined by simple empirically-based expressions involving the effective atomic charges of the solute atoms (derived from molecular electrostatic potential) and base radii. Both of these terms are optimized for the different types of atoms or functional groups in a training set of neutral and charged solutes. Parameters in these expressions for radii were fitted by minimizing residuals between calculated and measured standard free energies of solvation (ΔG s*), weighted by the uncertainty in the measured value. The calculations were performed using density functional theory with the B3LYP functional and the 6-311+G** basis set and the COnductor-like Screening MOdel (COSMO). The optimized radii definitions reproduce ΔG s* of neutral solutes and singly-charged ions in the training set to within experimental uncertainty and, more importantly, accurately predict ΔG s* of compounds outside the training set, in particular anions. Inherent to this approach, the cavity definitions reflect the strength of specific solute-water interactions. We surmise that this feature underlies the success of the model, referred to as the CD-COSMO model for Charge-Dependent (also Camaioni-Dupuis) COSMO model. These findings offer encouragement that we can keep extending this scheme to other functional groups and obtain better accuracy in using continuum solvation models to predict equilibrium properties of aqueous ionic solutes. The approach is illustrated for a number of test cases, including the determination of acidities of an amine base and a study of the tautomerization equilibrium of a zwitterionic molecule (glycine). The approach is also extended to calculating solvation energies of transition states toward a full characterization of reaction pathways in aqueous phase, here in S N2 exchange reactions. The calculated reactions barriers in aqueous solution are in excellent agreement with experimental values. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.« less
  • We applied our recently developed protocol of the conductor-like continuum model of solvation to describe the title reaction in aqueous solution. The model has the unique feature of the molecular cavity being dependent on the atomic charges in the solute, and can be extended naturally to transition states and reaction pathways. It was used to calculate the reaction energetics and reaction rate in solution for the title reaction. The rate of reaction calculated using canonical variational transition state theory CVT in the context of the equilibrium solvation path (ESP) approximation, and including correction for tunneling through the small curvature approximationmore » (SCT) was found to be 3.6 106 M-1 s-1, in very good agreement with experiment, These results suggest that the present protocol of the conductor-like continuum model of solvation with the charge-dependent cavity definition captures accurately the solvation effects at transition states and allows for quantitative estimates of reaction rates in solutions. This work was supported by the U.S. Department of Energy's (DOE) Office of Basic Energy Sciences, Chemical Sciences program. The Pacific Northwest National Laboratory is operated by Battelle for DOE.« less
  • Continuum solvation models enable efficient first principles calculations of chemical reactions in solution, but require extensive parametrization and fitting for each solvent and class of solute systems. Here, we examine the assumptions of continuum solvation models in detail and replace empirical terms with physical models in order to construct a minimally-empirical solvation model. Specifically, we derive solvent radii from the nonlocal dielectric response of the solvent from ab initio calculations, construct a closed-form and parameter-free weighted-density approximation for the free energy of the cavity formation, and employ a pair-potential approximation for the dispersion energy. We show that the resulting modelmore » with a single solvent-independent parameter: the electron density threshold (n c), and a single solvent-dependent parameter: the dispersion scale factor (s 6), reproduces solvation energies of organic molecules in water, chloroform, and carbon tetrachloride with RMS errors of 1.1, 0.6 and 0.5 kcal/mol, respectively. We additionally show that fitting the solvent-dependent s 6 parameter to the solvation energy of a single non-polar molecule does not substantially increase these errors. Parametrization of this model for other solvents, therefore, requires minimal effort and is possible without extensive databases of experimental solvation free energies.« less
  • Continuum solvation models enable efficient first principles calculations of chemical reactions in solution, but require extensive parametrization and fitting for each solvent and class of solute systems. Here, we examine the assumptions of continuum solvation models in detail and replace empirical terms with physical models in order to construct a minimally-empirical solvation model. Specifically, we derive solvent radii from the nonlocal dielectric response of the solvent from ab initio calculations, construct a closed-form and parameter-free weighted-density approximation for the free energy of the cavity formation, and employ a pair-potential approximation for the dispersion energy. We show that the resulting modelmore » with a single solvent-independent parameter: the electron density threshold (n{sub c}), and a single solvent-dependent parameter: the dispersion scale factor (s{sub 6}), reproduces solvation energies of organic molecules in water, chloroform, and carbon tetrachloride with RMS errors of 1.1, 0.6 and 0.5 kcal/mol, respectively. We additionally show that fitting the solvent-dependent s{sub 6} parameter to the solvation energy of a single non-polar molecule does not substantially increase these errors. Parametrization of this model for other solvents, therefore, requires minimal effort and is possible without extensive databases of experimental solvation free energies.« less
  • Abstract not provided.