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 et al. , J. Electron Spectrosc. Relat. Phenom., 2000, 49 , 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' (-1) , 3a'' (-1) ) cationic states. It is shown that the vibrational profile of the first band (A') extending over 3500 cm -1 with a vibrational spacing of 500 cm -1 is not due solely to the overtones ( v = 0 → v ' = 1, 2, 3,…) of the C-CO bending mode as previously suggested but results from a collection of ( v = 0 → v ' = 1) transitions with frequencies multiple of 500 cm -1 associated with the CO stretching at 1550 cm -1 , C-C stretching at 1045 cm -1 and C-CO, C-CN bending modes at 370/500 cm -1 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 ( ω ≈ 150 cm -1 ) induced by the rotation (60 degrees) of the methyl group blurring the main vibrational progression ( ω ≈ 1115 cm -1 ) 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' (-1) and 3a'' (-1) 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 v = 0 → v = 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.