OH Produced from o-nitrophenol photolysis: A combined experimental and theoretical investigation

As an important class of nitroaromatic compounds, nitrophenols are of particular interest since their presence in the environment has been acknowledged. Much work on the occurrence of nitrophenols in the environment, both in the gas and in the condensed phase, has been done, and nitrophenols have been discovered in different atmospheric compartments, such as ambient air, clouds, fog, and snow. Moreover, previous spectroscopic studies revealed that the strong absorption band of nitrophenols is in the atmospherically relevant UV region (300-400 nm), implying the photochemistry of nitrophenols might be important for the atmosphere. In the present work, photodissociation dynamics of o-nitrophenol (HOC6H4NO2) in the gas phase at different photolysis wavelengths (361-390 nm) is investigated, and the nascent OH radical is observed by the single-photon laser-induced fluorescence (LIF) technique. At all photolysis wavelengths, the OH radicals are formed in vibrationally cold state (υ″ = 0), and have similar rotational state distributions. The average rotational temperature for all photolysis wavelengths is approximately 970 ± 120 K, corresponding to a rotational energy of 1.9 ± 0.2 kcal mol-1. The spin-orbit and Λ-doublet states of the OH fragments formed in the dissociation are measured to be nonstatistical distributions. To get an insight into the dissociative mechanism leading to OH formation in the photolysis of o-nitrophenol, the potential energy surfaces of the OH-forming channels are mapped by ab initio theoretical calculations. According to both experimental and theoretical results, a possible mechanism for OH formation is proposed. It is suggested that electronically excited o-nitrophenol mostly relaxes to the lowest excited triplet state (T1) via intersystem crossing followed by intramolecular H transfer to form an aci-nitro isomer, from which OH elimination takes place by N-OH bond cleavage most likely. The transition state for the N-OH bond rupture channel is located on the T1 state and characterization of the dissociative state to be T1 state strongly supports much of our experimental findings.