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

Abstract

Vibrationally resolved CH3NO2and CD3NO2photoelectron 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 v5and v6of the D0and D2cation states, respectively. Irregular structuring observed midband in both bands is attributed to avoided crossings between the adiabatic D0/D1and D1/D2states as identified in potential surface cuts taken along the v5and v6normal mode coordinates. The associated D0/D1and D1/D2conical intersections were located and characterized, allowing possible vibronic coupling and its impact on the photoelectron spectra of the D0, D1, and D2states to be discussed. This helps rationalize the apparently missing D1ionization 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.