Time-Dependent Density Functional Theory Investigation of the UV–Vis Spectra of Organonitrogen Chromophores in Brown Carbon

Abstract

The ability of brown carbon (BrC) in aerosols to absorb solar radiation is an important but highly uncertain factor in climate forcing. The uncertainties are partially due to incomplete characterization of BrC chromophores and lack of authentic standards to confirm light absorption. Organonitrogen species are crucial components in atmospheric aerosols, but their light-absorbing properties remain to be fully characterized. To facilitate the molecular characterization of BrC chromophores, time-dependent density functional theory (TD-DFT) based computational chemistry approaches were used in this study to predict the light absorption spectra of 16 organonitrogen species, including nitroaromatics, nitro-heterocyclic compounds, organonitrates, and Maillard-type reaction products in BrC. Effects of basis sets, functionals, solvation, and pH on light absorption properties of these compounds were evaluated. Predicted absorption spectra were compared with experimental measurements. Overall, the PBE0 and B3LYP functionals tend to outperform PBE and CAM-B3LYP on the predicted absorption spectra of studied compounds. Absorbance calculated in water and methanol (bulk solvents) varies up to 2 nm (0.03 eV). Absorbance calculated in gas phase (vacuum state) blue-shifts in comparison to solvation. Absorbance of weak acids (e.g., nitrophenols) is enhanced under basic conditions, and the absorption spectra can be predicted by the fractions of conjugate acid−base species. Results from this study demonstrate that a combined use of TD-DFT predictions and experimental measurements of light absorption can allow for a rapid and reliable determination of potential chromophores in BrC when authentic standards are not available.

Revealing the Role of Metals and Anions in Nitrophenol UV–Visible Spectroscopies and Their Atmospheric Implication

“…12 On the other hand, the λ max values depend on the solvents as well, and the λ max values of p-nitrophenol were measured to be 401 nm in water and 309 nm in methanol using a UV−vis spectrophotometer. 24 Moreover, the λ max values of p-nitrophenol were obtained at 295 nm (11 300 L mol −1 cm −1 ), 313−314 nm (9300 L mol −1 cm −1 ), and 403 nm (19 200 L mol −1 cm −1 ) at cyclohexane, water, and NaOH solution, respectively. 42 Meanwhile, the computational spectra of TD-DFT could vary depending on the levels of theory used.…”

“…using hybrid functionals (B3LYP, PBE0) showed better agreement with the experimentally observed values than the ones obtained by the pure functional (PBE). 24 The aim of this study is to investigate the UV−vis spectra in the molecular interactions between ions (metals and anions) and p-nitrophenol. The B3LYP-D3 method has been shown to give important contributions to weak interactions.…”

“…A previous study shows that absorption spectra of nitrophenols in the basic solution (pH = 11) were found to have longer and stronger wavelengths than the values in the acidic solution (pH = 2). 24 Meanwhile, the pH of atmospheric aerosols or droplets normally depends on emission sources and atmospheric processes. The previous studies have also shown that the pH of aerosol may influence the BrC light absorption.…”

“…It can calculate the electronically excited states of molecules when interacting with light. 24 The ultraviolet− visible (UV−vis) spectra (absorption wavelengths, oscillator strengths) were calculated by single-point calculations and a TD-DFT method on top of the B3LYP-D3/def2-TZVPoptimized structures. The UV−vis spectra were plotted with a Gaussian band shape, with a peak half-width at a half-height value of 0.33 eV (default value).…”

“…To explain the light absorption properties, the time-dependent (TD)-DFT approach is normally used. It can calculate the electronically excited states of molecules when interacting with light . The ultraviolet–visible (UV–vis) spectra (absorption wavelengths, oscillator strengths) were calculated by single-point calculations and a TD-DFT method on top of the B3LYP-D3/def2-TZVP-optimized structures.…”