The optical properties of dissolved organic matter (DOM) are of great interest to scientists and engineers, as they are used as surrogates for both concentration and properties of this material, including being used to predict photochemical reactivity. However, there is ongoing debate in the literature regarding two competing hypotheses that could explain the observed optical properties of DOM: a) charge transfer interactions or b) superposition of individual components. One aspect of this debate is centered around the potential for the optically-active portion of the DOM to be solvent accessible or not, which may explain certain properties of the optical response of DOM. <br/>In this work, computational modeling was used to ascertain the mechanisms involved in the formation of the three-dimensional configuration of DOM from model chromophores and fluorophores. Six models of DOM that fit to chemical characteristics of Suwannee River fulvic acid were generated. The difference in models was choice of molecular fragments and the molecular size of individual components. Molecular dynamic simulations were performed with three different solvents: water, methyl tert-butyl ether and tetrahydrofuran. Radius of gyration (RofG) as a parameter was used to estimate relative free energy of the models. Snapshots with RofG that corresponded to the lowest free energy were used to calculate solvent accessible surface area as well hydrophobic and hydrophilic surface area. Hydrogen bonding, and formation of π-staking clusters was also evaluated. The data indicates that radius of gyration of organic molecules assemblies that formed in different solvents varied between 11-18 Å. 3-7 hydrogen bonds could formed between different molecules. And the π-staking clusters were observed only in few snapshots. Overall, the results indicate that both chromophores and fluorophores within DOM are solvent accessible, suggesting that charge transfer interactions may be limited in their contributions to DOM optical properties.

Computational assessment of the role of three-dimensional configuration on the optical properties of dissolved organic matter

ACS Spring Meeting 2021

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