Vibrational and electronic spectra of matrix-isolated para-dichlorobenzene radical cations

Abstract

A new pulsed-glow discharge technique has been used to generate the radical cations of para-dichlorobenzene (PDCB) and trap them in an argon matrix at 12 K. Three visible absorption bands with origins at 520.5, 490.6, and 322.4 nm are shown to be consistent with gas-phase photoelectron results and intermediate neglect of differential overlap/spectroscopy calculations. Resonance Raman and fluorescence spectra compare well with previous gas-phase electron impact excited emission and freon matrix (77 K) studies. The infrared spectrum of the PDCB cation has been measured for the first time and shows bands at 843, 986, 1110, and 1429 cm−1, all of which correlate well, in intensity, with the 520 nm electronic band. Ab initio self-consistent-field level calculations on the PDCB neutral and radical cation species reproduce the spectra well, after scaling is performed. It is shown that, upon ionization, the C–Cl bond force constants and the ring central C–C bond force constants increase, while the force constants for the ring C–C bonds adjacent to the C–Cl bond decrease. This accounts for the observed blue shift for certain bands and the red shift for certain others when PDCB is ionized.