Five conformational isomers corresponding to minima in potential energy surfaces calculated with the B3LYP/6-311G** and MP2/6-311G** models lie within 6 kcal/mol of each other. The vertical ionization energies of the most stable isomer calculated with ab initio electron propagator theory in the P3/6-311G** approximation are in excellent agreement with experimental data from photoelectron spectroscopy. The chief components of the first four Dyson orbitals are a N lone pair, an O lone pair, pi(CO), and C-H lobes, respectively. Conformationally induced shifts of the vertical ionization energies are explained in terms of electrostatic and phase relationships in the Dyson orbitals.