Infrared predissociation spectroscopy of cold trapped C₂H−(H₂) and C₂H−(D₂)

Abstract

The vibrational spectra of cryogenically cooled acetylide anions, C₂H⁻, tagged with H₂ and D₂, are obtained over a wide spectral range spanning 350–3500 cm⁻¹. Using the FELIX infrared free electron laser in combination with the 22-pole ion trap instrument FELion, infrared predissociation (IRPD) spectroscopy of C₂H−(H₂) and C₂⁢H⁻(D₂) complexes have been performed and compared to high-level ab initio calculations. The IRPD spectrum of the H₂-tagged complex exhibits two transitions at 511(2) cm⁻¹ and 1790(3) cm⁻¹, which are assigned to the fundamental C≡C–H bending and C≡C stretching modes of C₂⁢H⁻, respectively; a third spectral feature at 1014(2) cm⁻¹ is ascribed to the first overtone of the C≡C–H bending mode of C₂⁢H⁻. Theoretical vibrational configuration interaction calculations for C₂⁢H⁻ have been carried out on a four-mode potential energy surface at the CCSD(T)-F12/CVQZ-F12 level of theory. These results, as well as previously published computational results, validate the assignments. The influence of the H₂/D₂ messenger molecules on the band positions is found to be small. However, for C₂⁢H−(H₂), additional spectral features are observed in the 700–900 cm⁻¹ range, which are interpreted as overtone and/or combination bands of intermolecular vibrational modes between the H₂ messenger molecule and the C₂H⁻ anion. Finally, a transition at 2860(3) cm⁻¹ is assigned to the fundamental D–D stretching vibration inside the C₂H⁻(D₂) complex.