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.