Quantum Chemical Modeling of Humic Acid/Air Equilibrium Partitioning of Organic Vapors

Abstract

Classical approaches for predicting soil organic matter partition coefficients of organic compounds require a calibration with experimental partition data and, for good accuracy, experimental compound descriptors. In this study we evaluate the quantum chemical model COSMO-RS in its COSMOtherm implementation for the prediction of about 200 experimental Leonardite humic acid/air partition coefficients without calibration or experimental compound descriptors, but simply based on molecular structures. For this purpose a Leonardite Humic Acid model monomer limited to 31 carbon atoms was derived from 13C NMR analysis, elemental analysis, and acidic function analysis provided in the literature. Altogether the COSMOtherm calculations showed a good performance and we conclude that it may become a very promising tool for the prediction of sorption in soil organic matter for compounds for which the molecular structure is the only reliable information available. COSMOtherm can be expected to be very robust with respectto new and complex compound structures because its calculations are based on a fundamental assessment of the underlying intermolecular forces. In contrast, other empirical models that are also based on the molecular structure of the sorbate have an application domain that is limited by their calibration data set that is often unknown to the user.

Nonadditive effects in the partitioning behavior of various aliphatic and aromatic molecules

“…This is also supported by another study that we have published recently [27]. This can be done in one of two ways: using a quantum chemical approach like COSMOtherm or a QSPR approach that accounts for nonadditive effects by empirically calibrated correction factors.…”

COSMO-RS: An Alternative to Simulation for Calculating Thermodynamic Properties of Liquid Mixtures

“…Reasonable estimates of vapor pressures, partition coefficients, and pK a values of very diverse compounds in often extremely complex environmental phases or at interfaces are of crucial importance for the estimation of the fate of chemical compounds entering the environment in various ways. Although simple classification and group contribution methods are still most widely accepted in this politically sensitive area, COSMO-RS is getting increasing attention in this context as a result of its broader, much more fundamental, and more general applicability to a wide range of environmentally relevant compounds and properties (97)(98)(99)(100)(101). An interesting example is the pK a of the industrially and environmentally important perfluoro-octanoic acid (PFOA).…”

Protonated o-semiquinone radical as a mimetic of the humic acids native radicals: A DFT approach to the molecular structure and EPR properties

“…Secondly, to investigate the effect of HA macromolecular structure, the semiquinones derived from 3,4-dihydroxy-antracene-1-carboxylic acid (34dha) were taken under study in addition to those derived from 34dhb. It should be mentioned that studies on the HA model compounds have proved to be invaluable source of information about these natural systems (Tossell, 2006(Tossell, , 2009Trout and Kubicki, 2006;Niederer and Goss, 2007;Cornard et al, 2008;Witwicki et al, 2008;Burk et al, 2009;Witwicki et al, 2009a,b;Xu et al, 2010). To the best of our knowledge, this is the first report on theoretical study of HA native radicals (and protonated semiquinones in general) dedicated to the influence of the chemical environment on their g tensors.…”