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

Abstract

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–CO 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 CO stretching at 1550 cm⁻¹, C–C stretching at 1045 cm⁻¹ and C–CO, C–CN 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–CO bending/CO 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 CN 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.