The infrared spectrum of acetylene–HF in helium nanodroplets

Rotationally resolved infrared spectra are reported for the T-shaped acetylene–HF complex in helium nanodroplets, corresponding to excitation of both the H–F and asymmetric C–H stretches. In comparison to the isolated complex, rotation about the A-axis is relatively unhindered by the helium, while overall rotation is slower by a factor of approximately two. An ab initio potential surface is also reported for this system. Although the bent complex, with acetylene acting as a hydrogen donor, is determined to be a local minimum, only the T-shaped isomer is experimentally observed, indicating that the barrier is too small to prevent the metastable bent structure from rearranging to the global minimum.