The surface orientations and conformations of solvated molecules at air-water interfaces can significantly affect the chemistry of many systems and are of particular importance for atmospheric aerosol. Vibrational sum-frequency (VSF) spectroscopy, a surface sensitive technique, can provide information about the populations and behaviors of molecules at such interfaces. However, the numerous contributions are coupled together in the observed VSF signal, making experimental spectra challenging to interpret. Computational methods can be used to decouple contributions and better understand the orientations and conformations at the interface. Spectra are calculated by using response properties from a library of density functional theory (DFT) structures that span the range of structures found in molecular dynamics calculations. While the method works well for calculating VSF spectra of reasonably large solvent molecules, only recently have effects of solvation been addressed through microsolvation of the DFT structures. Progress towards continued improvements to the methodology is presented, consisting of both simulation of spectra of additional organic molecules as well as water where both first and second solvation shell environments are included in the DFT structures. Including these structures provides a more complete depiction of coordinated water and allows for representation of how organic modes convolve with modes from these various water environments. Experimental spectra are compared as a metric for evaluating the improved methodology while using various MD force fields and water models. This work aims to further our ability to accurately simulate VSF spectra for increasingly large and complex systems of organics at aqueous interfaces, such as those in secondary organic aerosol.

Methodology improvements for calculation of vibrational sum-frequency spectra of aqueous organic interfaces

251st National Meeting (2016)

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