Amorphous water ice is the dominant form of solid water in the interstellar medium. Low density amorphous (LDA) ice is characterized by the absence of long-range periodic structures, and its structure is more similar to liquid water than to crystalline ice. However, while in liquid water hydrogen bonds are exchanged on a picosecond time scale, LDA ice shows ultra-slow reorientation dynamics even at temperatures close to the glass-liquid transition, and at 3 K all this motion is expected to be completely frozen. In the laboratory, LDA ice is produced by slow vapor deposition of water on a cold window. These matrices are used in our laboratory to generate and trap reactive organic molecules such as radicals, carbenes, and carbenium ions, and to characterize these species by IR, UV-vis, and EPR spectroscopy.<br/>We studied several aryl carbenes and radicals to explore how LDA ice interacts with these highly reactive molecules, and how the chemical and spectroscopic properties of these species are modified by interactions with surrounding water molecules. We found that some carbenes are stabilized by hydrogen bonding, whereas others are protonated to produce the corresponding carbenium ions. Interestingly, highly reactive carbenium ions such as the benzhydryl cation are stable in neutral water at cryogenic temperatures. Otherwise, superacidic media are required for their stabilization and spectroscopic characterization.<br/>An alternative route to generate the cations is the photoionization of the corresponding arylradicals. The ionization potential of radicals in LDA water is considerably lowered compared to gas phase and inert gas matrices, which allowed us to generate the cations with near UV or visible light irradiation. Subsequent annealing of the LDA ice leads to diffusion and reaction of the trapped species. These processes might be of importance to the formation of complex molecules in icy objects in the solar system as well as in the interstellar medium.

Protonation and ionization of organic reactive molecules in low-density amorphous water ice

253rd National Meeting (2017)

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