An Experimental and Theoretical Study of the Valence Shell Electronic Structure of Nitromethane

Vibrationally resolved CH3NO2 and CD3NO2 photoelectron spectra and angular distribution parameters, β, have been measured in the photon energy range 20-80 eV. This allows a comprehensive investigation of conflicting literature interpretations of the two outermost photoelectron bands, complemented by high-level calculations characterizing the three O lone-pair-based orbital ionizations expected in this region. Franck-Condon simulations allow the regular vibrational progressions observed at the beginning of the first and second bands to be assigned as predominantly modes v5 and v6 of the D0 and D2 cation states, respectively. Irregular structuring observed midband in both bands is attributed to avoided crossings between the adiabatic D0/D1 and D1/D2 states as identified in potential surface cuts taken along the v5 and v6 normal mode coordinates. The associated D0/D1 and D1/D2 conical intersections were located and characterized, allowing possible vibronic coupling and its impact on the photoelectron spectra of the D0, D1, and D2 states to be discussed. This helps rationalize the apparently missing D1 ionization in this region, although some contribution by this state may be identifiable in the β-parameter spectra. Theoretical analysis of the complete valence region spectrum identifies the early breakdown of the independent electron model beyond these first two bands.