Photochemical synthesis of simple organic free radicals on simulated planetary surfaces—An ESR study

Details Photochemical synthesis of simple organic free radicals on simulated planetary surfaces—An ESR study Photochemical synthesis of simple organic free radicals on simulated planetary surfaces—An ESR study

Jan 1975

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1975orli....6...61t&link_type=abstract

Origins of Life, Volume 6, Issue 1-2, pp. 61-73

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Electron spin resonance spectroscopy provided evidence for formation of hydroxyl radicals during ultraviolet photolysis (254 nm) at -170°C of H2O adsorbed on silica gel or of silica gel alone. The carboxyl radical was observed when CO or CO2 or a mixture of CO and CO2 adsorbed on silica gel at -170°C was irradiated. The ESR signals of these radicals slowly disappeared when the irradiated samples were warmed to room temperature. However, re-irradiation of CO or CO2, or the mixture CO and CO2 on silica gel at room temperature then produced a new species, the carbon dioxide anion radical, which slowly decayed and which was identical with that produced by direct photolysis of formic acid adsorbed on silica gel. The primary photochemical process may involve formation of hydrogen and hydroxyl radicals by means of (1) photodissociation of H2O physically adsorbed on the silica gel, or (2) absorption of the excitation energy by the silica gel surface with subsequent cleavage of the silanol bonds, or (3) dissociation of H2O molecules through photosensitization by the surfaces or a combination of (1) to (3). Subsequent reactions of these radicals with adsorbed CO or CO2 or both yield carboxyl radicals, CO2H, the precursors of formic acid. Our results confirm the formation of formic acid under simulated. Martian conditions and provide a mechanistic basis for gauging the potential importance of gas-solid photochemistry for chemical evolution on other extraterrestrial bodies, on the primitive earth and on dust grains in the interstellar medium.

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