Hans Jørgen Jakobsen

NMR and EPR Studies of Free-Radical Intermediates from Experiments Mimicking the Winds on Mars: A Sink for Methane and Other Gases

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A new kind of solid gas chemical reactions has been investigated using solid-state powder H-2, C-13, and Si-29 NMR and EPR spectroscopies. These studies involve reactions between a silicate-created Si free-radical intermediate and a few ordinary gases such as isotopically H-2-, C-13-, and O-17-enriched methane ((CH4)-C-13 and CD4), carbon dioxide ((CO2)-C-13), hydrogen (H-2(2)) and oxygen (O-17(2)). The solid-state Si free-radical intermediate and gas reaction and silicate products are formed in a specially designed rotating apparatus, which by mechanical tumbling mimics the winds and collision speed of the mineral particles on Mars. It is shown that the "hard" quartz (SiO2) or corundum (alpha-AL(2)O(3)) grain particles, used to simulate the collision particles in a rotating Pyrex (borosilicate) reaction flask, act as an abrasive on the "soft" Pyrex flask and thereby create a silicate Si free-radical intermediate. EPR studies show that this radical is identical to the silicate radical intensively investigated by EPR from gamma-irradiation of Pyrex and other glasses and shown to constitute a submicrostructure of either silicate chains or helices. The intensity of the silicate Si free-radical EPR signal for the reaction product is strongly reduced or even disappears by performing the tumbling of the abrasive grain particles in an atmosphere of methane or other gases. C-13{H-1} and Si-29{H-1} CP/MAS NMR experiments of the reaction product with methane gas show the presence of one -bond CH3-Si equivalent to and HO-Si equivalent to covalent bonds in its structure and that sieving the product leads to a sensitivity enhancement by a factor of similar to 25. A flexible helical structure for the silicate Si free-radical intermediate is indicated by the preliminary results for the product resulting from the reaction with (CO2)-C-13, (encapsulation of the gas) and the indication of a congested methyl group in the product from reaction with methane.

OriginalsprogEngelsk
TidsskriftThe Journal of Physical Chemistry Part C
Vol/bind120
Nummer45
Sider (fra-til)26138-26149
Antal sider12
ISSN1932-7447
DOI
StatusUdgivet - 2016

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