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

A HeI photoelectron spectrum of acetyl cyanide (pyruvonitrile: CH3COCN) in the gas phase has been observed for the first time in the region up to 19 e V, and interpreted in terms of a total of eight ionization bands on the bas is of ab initio MP2/6-31G(d) and G3 calculations. The first ionization ener gy is much lower (1.8 eV) than the second one in spite of nearly the same o rbital energies. The first and second adiabatic ionization energies have we ll been reproduced especially by the G3 calculations, attributable to {15a' (n(O,) pi'(CN))}(-1) and {3a "(pi (CN), pi (CO))}(-1), respectively. The mo lecular geometries and vibrational frequencies at the neutral ground state (S-1(0)) as well as at the cationic states (D-2(0), D-2(1)) were also calcu lated, in good agreement with available experimental data. (C) 2000 Elsevie r Science B.V. All rights reserved.