Unraveling the Ultrafast Photochemical Dynamics of Nitrobenzene in Aqueous Solution

Abstract

Nitroaromatic compounds are major constituents of the brown carbon aerosol particles in the troposphere that absorb near-ultraviolet (UV) and visible solar radiation and have a profound effect on the Earth's climate. The primary sources of brown carbon include biomass burning, forest fires, and residential burning of biofuels, and an important secondary source is photochemistry in aqueous cloud and fog droplets. Nitrobenzene is the smallest nitroaromatic molecule and a model for the photochemical behavior of larger nitroaromatic compounds. Despite the obvious importance of its droplet photochemistry to the atmospheric environment, there have not been any detailed studies of the ultrafast photochemical dynamics of nitrobenzene in aqueous solution. Here, we combine femtosecond transient absorption spectroscopy, time-resolved infrared spectroscopy, and quantum chemistry calculations to investigate the primary steps following the near-UV (λ ≥ 340 nm) photoexcitation of aqueous nitrobenzene. To understand the role of the surrounding water molecules in the photochemical dynamics of nitrobenzene, we compare the results of these investigations with analogous measurements in solutions of methanol, acetonitrile, and cyclohexane. We find that vibrational energy transfer to the aqueous environment quenches internal excitation, and therefore, unlike the gas phase, we do not observe any evidence for formation of photoproducts on timescales up to 500 ns. We also find that hydrogen bonding between nitrobenzene and surrounding water molecules slows the S₁/S₀ internal conversion process.

Figure 1

UV–visible spectra of nitrobenzene in water (15 mM, purple), cyclohexane (104 mM, red), acetonitrile (68 mM, green), and methanol (102 mM, blue). Inset: expanded view (×10) of the UV–visible spectra in the 300–400 nm range. The dashed vertical lines mark the wavelengths employed in this work. Data have been normalized to the maximum of the band centered around 250–280 nm.

Figure 2

Transient absorption spectra of 15 mM nitrobenzene in water at specified pump–probe delays following photoexcitation at 355 nm. Letters A–E label the peaks in the structured triplet ESA (see text).

Figure 3

Kinetic traces and associated kinetic fits of the transient absorption spectrum of 15 mM nitrobenzene in water after photoexcitation at 355 nm. Peaks A, B, and C are assigned to the triplet vibrational structure illustrated in Figure 2. S₁ represents the decay of the ESA from the pump-excited S₁ state. The broad feature centered around 650 nm is derived from the basis function fitting in the 598–650 nm window. Data have been normalized to the maximum ΔA.

Figure 4

TRIR spectra of 16 mM nitrobenzene in d₂-water at specified pump–probe delays following photoexcitation at 340 nm (left) and 260 nm (right) for the feature centered around 1360 cm^–1, which is a NO symmetric stretch.

Figure 5

Kinetic traces and associated kinetic fits of the TRIR spectra of 16 mM nitrobenzene in d₂-water after photoexcitation at 340 (red) and 260 nm (blue). Ground-state recovery signals are monitored around 1360 cm^–1. Data have been normalized to the maximum ΔA.

Figure 6

CAM-B3LYP/Def2-SVP GD3BJ microsolvated structures of nitrobenzene using (a) water, (b) cyclohexane, (c) acetonitrile, and (d) methanol. Black dashed lines indicate possible hydrogen bonds.

Figure 7

Schematic representation of the relaxation processes following photoexcitation at 355 nm in our TAS experiments (left) and at 340 and 260 nm in our TRIR experiments (right). Note that T₁ and T₂ are nearly degenerate.