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.

Nitro-nitrite isomerization and transition state switching in the dissociation of ionized nitromethane: a threshold photoelectron-photoion coincidence spectroscopy study

Threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy has been employed to investigate the competition between bond cleavage and rearrangement reactions in the dissociation of ionized nitromethane, 1. Modeling TPEPICO breakdown diagrams with a combination of RRKM theory and ab initio calculations at the G3 level of theory allowed the derivation of the activation energy for the isomerisation of 1 to ionized methyl nitrite, 2, 82 kJ mol(-1). In addition, evidence was found for a transition state switch in the bond cleavage reaction in 1 leading to CH(3)(*) + NO(2)(+). As internal energy increases, the effective transition state for this reaction becomes tighter (i.e. is characterized by a lower entropy of activation, Delta(double dagger)S). Fitted thresholds for NO(+) and CH(2)OHO(+) ions, originating from the isomeric methyl nitrite ion, are consistent with G3 level ab initio calculations.