Computational Vibrational Sum Frequency Spectra of Formaldehyde and Hydroxymethanesulfonate at Aqueous Interfaces

Abstract

The identity and arrangement of aqueous species at the interface of atmospheric aerosol impacts aerosol properties including albedo and propensity to uptake additional gas phase species. Formaldehyde and sulfur dioxide are two common atmospheric species that alter (individually and in concert) aqueous atmospheric aerosol interfaces. Vibrational sum frequency (VSF) spectroscopy studies of planar aqueous formaldehyde solution surfaces have shown alteration during exposure to sulfur dioxide gas. Additional changes were observed once exposure was ceased. The results suggested the formation of a new organic species, hydroxymethanesulfonate (HMS), which acts as a thermodynamic sink for aqueous sulfur dioxide and lead to acidification of the aerosol particles. The coherent nature of the VSF response and its strong dependence on surface species present and their orientations, however, made definite vibrational assignments for exact species notoriously difficult. The focus of this paper is on elucidating the species and orientations that give rise to specific experimentally derived spectral features through VSF spectra calculated using a combination of classical molecular dynamics (MD) and density functional theory (DFT). The results demonstrate that the most prevalent surface species for a formaldehyde containing aqueous solution is hydrated formaldehyde in the form of methylene glycol. Calculated VSF spectral frequencies for the proposed product HMS are in agreement with experiment. Furthermore, changes in the experimental spectra both during and after the flow of sulfur dioxide are consistent with HMS in different interfacial orientations.

On the Rise: Experimental and Computational Vibrational Sum Frequency Spectroscopy Studies of Pyruvic Acid and Its Surface-Active Oligomer Species at the Air–Water Interface

“…Calculated VSF spectra generated using the computational methodology utilized here have been shown to reproduce the experimental VSF data for spectral regions arising from modes that are insensitive to solvation (such as CH stretching modes). ,− This technique references only the gas-phase DFT structures of the organics and does not include contributions arising from water nor variations in those organic modes sensitive to their solvation environment. For this reason, the calculated response of the OH stretching region cannot be relied upon to the reproduce the experimental spectra.…”

“…Vibrational sum frequency (VSF) spectroscopy is an inherently surface-selective nonlinear optical technique that provides a vibrational spectrum of surface-active molecules that are anisotropically ordered and noncentrosymmetric. − At asymmetric interfaces, VSF spectroscopy is a well-established technique that has been used extensively by the Richmond lab ,− and others − to study liquid interfaces. By overlapping a fixed visible and a tunable-frequency IR beam in time and space, a third sum frequency beam is generated, which is the sum of the incident frequencies.…”

“…These techniques also provide vital structural and behavioral insights about the simulated system, such as depth-dependent molecular partitioning. This approach has been shown to be robust, efficient, and highly adaptable for numerous and varied chemical systems when compared with experimental spectra. ,− …”

“…Molecular dynamics (MD) simulations are performed using the Amber 12 suite of programs using parameters and force fields derived in the same manner as in previous studies, ,− and starting configurations were generated by PACKMOL . In each simulation, a specific number of PA and water molecules are arranged to form a 30 Å cube within a 30 Å × 120 Å × 30 Å simulation box, with periodic boundary conditions, forming a water slab with two interfaces.…”

Vibrational Sum-Frequency Generation Spectroscopy in the Energy Representation from Dual-Level Molecular Dynamics Simulations

“…Most calculations have been carried out using classical MD simulations and parametric expressions for the molecular dipole moment and polarizability. , In a number of cases, ,− the classical MD trajectory provides the input to combined QM/MM computations of the molecular dipole moment and polarizability in which part of the system (typically the solute molecule) is treated quantum mechanically (QM), while the surrounding system (the solvent) is treated with a molecular mechanics (MM) force-field. More rigorous SFG calculations would require the use of first-principles MD simulations, but in practice, due to high computational cost and slow convergence of the time correlation function, only few simulations of this type have been reported so far, using either full ab initio − or QM/MM schemes …”