Valence-shell ionization of acetyl cyanide: simulation of the photoelectron and infra-red spectra.

The vibrational envelopes of the first and second lines of the acetyl cyanide valence photoelectron spectrum [Katsumata , J. Electron Spectrosc. Relat. Phenom., 2000, , 113] in the gas phase have been simulated considering the Taylor expansion of the dipole moment from zero up to the second order as well as the changes of geometries/frequencies/normal modes between the initial neutral electronic ground state and the final (15a', 3a'') cationic states. It is shown that the vibrational profile of the first band (A') extending over 3500 cm with a vibrational spacing of 500 cm is not due solely to the overtones ( = 0 → ' = 1, 2, 3,…) of the C-CO bending mode as previously suggested but results from a collection of ( = 0 → ' = 1) transitions with frequencies multiple of 500 cm associated with the CO stretching at 1550 cm, C-C stretching at 1045 cm and C-CO, C-CN bending modes at 370/500 cm completed by combination bands. Our calculations also reveal that the structureless and asymmetric shape of the second band (A'') is due to the activation of the torsion mode at low-frequency ( ≈ 150 cm) induced by the rotation (60 degrees) of the methyl group blurring the main vibrational progression ( ≈ 1115 cm) corresponding to the cooperative motions of the methyl CH bending and C-CO bending/CO stretching. Infra-red spectra of the fundamental and both the 15a' and 3a'' cationic states were finally simulated. In contrast to the photoemission spectra, the infrared intensity of the CO stretching motion is very weak. The spectra are mainly dominated by the = 0 → = 1 transition of the CN stretching and the CH symmetric bending/stretching modes, providing complementary information between photoemission and infra-red spectroscopies to capture the nature of the cationic states in acetyl-cyanide.