Acid−Base Chemistry on Crystalline Ice:  HCl + NH₃

The chemical interaction of (i) HCl, (ii) NH 3, and (iii) HCl + NH 3 with crystalline ice has been examined as a function of temperature ( T = 80−150 K) utilizing FTIR spectroscopy. The infrared spectra reveal the formation of ionic hydrates for HCl and molecular hydrates for NH 3 . The acid−base reaction leads to the formation of the ammonium ion, NH 4 + . While it is produced to a limited extent between 80 and 140 K, NH 4 + is a dominant species above 140 K. All species appear as lower amorphous hydrates for T = 80−140 K. Dramatic changes in the spectra occur above 140 K as higher crystalline hydrates are formed. This temperature coincides with the amorphous to crystalline phase transformation of ice. This change appears to be due to a greater number of water molecules on the surface that are available for hydration and crystallization.