Infrared Spectra of HCl−H₂O Clusters in Helium Nanodroplets

Small (HCl) m (H 2 O) n clusters have been assembled in He droplets, and their spectra in the HCl stretch range (2500−3000 cm −1 ) have been obtained. In a recent He droplet study, a band at 2670 cm −1 was assigned to the dissociated H 3 O + (H 2 O) 3 Cl − ion pair. In this work, we have revised the assignment of this band to a cyclic hydrogen-bonded form of the (HCl) 2 (H 2 O) 2 cluster based on careful measurements of the pickup pressure dependence as well as the transition moment angles associated with the HCl stretch vibrations. A number of vibrational bands due to small mixed clusters have also been observed. As the number of the captured water molecules increases, a broad feature appears that spans the 2550−2800 cm −1 range. The possible origin of this spectral broadening in large (HCl) m (H 2 O) n clusters is discussed.