Electronic State Spectroscopy of Nitromethane and Nitroethane

High-resolution photoabsorption cross-sections in the 3.7-10.8 eV energy range are reinvestigated for nitromethane (CH₃NO₂), while for nitroethane (C₂H₅NO₂), they are reported for the first time. New absorption features are observed for both molecules which have been assigned to vibronic excitations of valence, Rydberg, and mixed valence-Rydberg characters. In comparison with nitromethane, nitroethane shows mainly broad absorption bands with diffuse structures, which can be interpreted as a result of the side-chain effect contributing to an increased number of internal degrees of freedom. New theoretical quantum chemical calculations performed at the time-dependent density functional theory (TD-DFT) level were used to qualitatively help interpret the recorded photoabsorption spectra. From the photoabsorption cross-sections, photolysis lifetimes in the terrestrial atmosphere have been obtained for both compounds. Relevant internal conversion from Rydberg to valence character is noted for both molecules, while the nuclear dynamics of CH₃NO₂ and C₂H₅NO₂ along the C-N reaction coordinate have been evaluated through potential energy curves at the TD-DFT level of theory, showing that the pre-dissociative character is more prevalent in nitromethane than in nitroethane.