Sequential solvation of HCl in argon: High resolution infrared spectroscopy of ArnHCl (n=1,2,3)

Abstract

High-resolution near-infrared spectra of the vHCl=1←0 fundamental stretch in Ar2HCl and Ar3HCl have been characterized using a slit-jet infrared spectrometer. Analysis of the jet-cooled, rotationally resolved spectra (i) permits unambiguous identification of the cluster size, (ii) provides vibrationally averaged geometries in the vHCl=1 excited state, and (iii) allows the vibrational shift of the HCl chromophore to be measured as a function of the number of Ar atoms in the complex. The equilibrium structures of ArnHCl (n=1–3) clusters calculated using accurate Ar–Ar and Ar–HCl pair potentials are consistent with the vibrationally averaged structures inferred spectroscopically. The vibrational red-shifts for ArnHCl (n=1–3) reflect a near-linear dependence on the number of Ar atoms, which is qualitatively reproduced by simple classical calculations on vHCl=0 and 1 pairwise additive potential surfaces. Theoretical predictions of the ArnHCl red-shifts in a fcc lattice indicate good agreement with experimental matrix results. However, to achieve this asymptotic limit requires up to n≈54 Ar atoms; this underscores a clear sensitivity to non-nearest neighbor Ar–HCl interactions significantly outside the first solvation shell. Finally, for smaller ArnHCl clusters with only one solvation shell (n=12), the potentials predict an energetic preference for HCl in surface vs interior sites. Analysis indicates that this effect is predominantly due to Ar/HCl size mismatch, which destabilizes the nearest neighbor Ar shell for HCl solvated in the center of the cluster.