HeI photoelectron and ab initio study of acetyl cyanide and its cation

A HeI photoelectron spectrum of acetyl cyanide (pyruvonitrile: CH 3 COCN) in the gas phase has been observed for the first time in the region up to 19 eV, and interpreted in terms of a total of eight ionization bands on the basis of ab initio MP2/6-31G(d) and G3 calculations. The first ionization energy is much lower (1.8 eV) than the second one in spite of nearly the same orbital energies. The first and second adiabatic ionization energies have well been reproduced especially by the G3 calculations, attributable to {15a′(n O, π′ CN )} −1 and {3a″(π CN , π CO )} −1 , respectively. The molecular geometries and vibrational frequencies at the neutral ground state ( 1 S 0 ) as well as at the cationic states ( 2 D 0 , 2 D 1 ) were also calculated, in good agreement with available experimental data.