Infrared absorption spectroscopy of diacetylene ions trapped in solid argon

Abstract

The C₄H₂⁺ diacetylene radical cation has been generated in a pulsed jet electrical discharge through both a diacetylene/argon mixture and an acetylene/argon mixture. The product mixture was trapped on a 12 K cryostat window and studied via Fourier transform infrared absorption spectroscopy. The diacetylene cation was also produced via low energy electron bombardment of an effusive C₄H₂/Ar beam. Two new infrared bands at 3201.6 and 1827.9 cm⁻¹ have been identified as vibrations of the diacetylene cation, viz the ν₄ (σ_u) (C–H stretching) and the ν₅(σ_u) (CC stretching) modes, respectively. Geometry optimization and harmonic frequency calculations, carried out at various spin unrestricted levels (B3LYP, CCSD(T)) for spin doublet structures, indicate that, in its electronic ground state ²Π_g, the C₄H₂⁺ cation is linear. Three additional new bands at 2957.5, 1693.8 and 594.5 cm⁻¹ have been tentatively assigned to the C–H stretching, CC stretching and CC–H (in-plane) bending modes, respectively, of the nonlinear diacetylene anion (C₄H₂^–, ²B_u).