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The Photochemistry of Crystalline Nitromethane under Static Pressure C

Fanetti, Samuele, Citroni, Margherita, Falsini, Naomi, Bini, Roberto
Journal of physical chemistry 2018 v.122 no.4 pp. 2023-2031
Fourier transform infrared spectroscopy, absorption, ammonium carbonate, carbon dioxide, irradiation, nitrous oxide, oxidation, photochemistry, prebiotics, prototypes, reaction mechanisms, spectral analysis, wavelengths
The high-pressure chemical reactivity of nitromethane, under irradiation with visible and near-UV laser light, was investigated by in situ FTIR spectroscopy in a diamond anvil cell. The reactivity was probed at different pressures (0.2, 1.2, 5.0, 15.3, and 28.0 GPa) with different excitation wavelengths (514, 458, and 350 nm), with all absorbed through a two-photon process. Insight into the reaction mechanism was gained by measuring the near-UV absorption spectrum of nitromethane as a function of pressure to 32 GPa, the threshold pressure above which it reacts spontaneously in the absence of electronic excitation. We were thus able to determine the pressure evolution of the two lowest-energy transitions (σ → π* and a singlet–triplet transition). The information obtained from the absorption spectra together with the reactivity data allowed us to locate the red absorption edge of the higher-energy π → π* transition and its pressure shift. The excitation of the σ → π* transition was not able to induce any photochemical reaction in the crystal, whereas the excitation of the π → π* transition was very effective at all the pressures probed. Comparing the products of the reactions induced at the different pressures, we observe a general trend: the higher the reaction pressure, the smaller the relative amount of small oxidized compounds such as CO₂, N₂O, and ammonium carbonate, and the larger the amount of N-methylformamide. The latter, a prototype prebiotic molecule containing the simplest peptidic bond, was reported as the main product in the purely pressure-induced reaction in the absence of light (P ≥ 32 GPa).