Knowledge Bank

The addition of HCl to the surface of crystalline ice clusters at cryogenic temperatures produces spectroscopic changes which can be related to the formation of the hydronium ion and the HCl acting as a molecular adsorbate. The hydronium ion is identified from its IR active modes below $2200cm-1$. Evidence for the HCl acting as a molecular adsorbate is its effect on the dangling-hydrogen (d-H) mode and the appearance of a $2500cm-1$ band. Analogous spectra for with the $HBr-H2O$ and the $DCl-D2O$ $systems1$ show two bands relative to the one band in the $2500cm-1$ region due to the $HCl-H2O$ interaction. The single band in the HCl system is attributed to the near overlap of the bands due to the molecular and ionic interaction of the HCl and $H2O$. Reference to matrix $isolation2,3$ and computational $work4$ will show the plausibility of the presence of the molecular complex. Shifting of the d-H band is a common effect for molecularly adsorbed species on the surface of $ice.5$ The shifting of the d-H to a position yet unobserved for common adsorbates gives strong evidence of the HCl acting as a molecular adsorbate.