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

Abstract

A new kind of solid–gas chemical reactions has been investigated using solid-state powder 2H, 13C, and 29Si NMR and EPR spectroscopies. These studies involve reactions between a silicate-created Si free-radical intermediate and a few ordinary gases such as isotopically 2H-, 13C-, and 17O-enriched methane (13CH4 and CD4), carbon dioxide (13CO2), hydrogen (2H2), and oxygen (17O2). 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 (α-Al2O3) 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 γ-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. 13C­{1H} and 29Si­{1H} CP/MAS NMR experiments of the reaction product with methane gas show the presence of one-bond CH3–Si≡ and HO–Si≡ covalent bonds in its structure and that sieving the product leads to a sensitivity enhancement by a factor of ∼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 13CO2 (encapsulation of the gas) and the indication of a congested methyl group in the product from reaction with methane.

Dynamic Solid-State NMR Experiments Reveal Structural Changes for a Methyl Silicate Nanostructure on Deuterium Substitution

“…Analysis of the 2 H MAS NMR spectra was performed employing the STARS software package 29 and the upgraded version also used and described in a recent article. 1 The quadrupole coupling and CSA parameters are defined by…”

“…From this kind of silicate free radical intermediate and its reaction products with methane and other gases, created from our terrestrial experimental simulations of the Mars wind erosion on its mineral, a possible sink for methane on Mars could recently be proposed. 1 To obtain additional information and insight into the structure of the free radical silicate nanomaterial and its reaction products, we have undertaken new terrestrial solidstate NMR experiments. Here we report solid-state roomtemperature (RT) MAS and very low variable-temperature (VT) MAS NMR dynamic experimental results on the methane reaction products for the two isotopic 13 CH 4 and CD 4 gases, recently isolated in small quantities (∼100 mg) following sieving to enhance their solid concentrations 1 and (2), respectively, in the present study.…”

“…1 To obtain additional information and insight into the structure of the free radical silicate nanomaterial and its reaction products, we have undertaken new terrestrial solidstate NMR experiments. Here we report solid-state roomtemperature (RT) MAS and very low variable-temperature (VT) MAS NMR dynamic experimental results on the methane reaction products for the two isotopic 13 CH 4 and CD 4 gases, recently isolated in small quantities (∼100 mg) following sieving to enhance their solid concentrations 1 and (2), respectively, in the present study. The solid-state RT data are obtained from an array of 13 C{ 1 H} cross-polarization− depolarization (CP/DP) MAS experiments for the 13 CH 4 reaction product (1), which has been shown to contain both one-bond 13 CH 3 −Si≡ and HO−Si≡ covalent bonds.…”

“…18 The E a values of 22.6 and 20.0 kJ mol −1 for CD 3 -L-alanine (8) were determined from low-temperature solid-state VT 2 H static NMR spin−lattice relaxation study 17 and line-shape analysis, 19 respectively, while the value E a = 21.1 kJ/mol for 4-CH 3 -phenanthrene (4) 18 was obtained from VT 1 H spin− lattice relaxation time (T 1 ) measurements using a degassed liquid-state sample. To complement and compare this latter E a value determined from the liquid-state 1 H NMR measurements with the static solid-state VT 2 H NMR results for CD 3 -Lalanine (8), 17,19 we have in addition to the CP/DP investigations performed solid-state VT 2 H dynamic experiments, which employ MAS as opposed to static NMR. These investigations are performed on (iii) the CD 3 group for a synthesized deuterated sample of 4-CD 3 -phenanthrene ( 5) and (iv) for the one-bond CD 3 −Si≡ species of the CD 4 reaction product (2).…”

“…Finally, we have taken further advantage of our recent RT 13 C MAS NMR study of the reaction product (named (3)) resulting from reaction between the recently produced silicate free-radical intermediate 1 and 13 C-enriched 13 CO 2 gas, which shows that the CO 2 molecules get encapsulated within the silicate structure. 1 Thus, here we use the 13 CO 2 molecule as a spy or molecular probe to obtain additional information on this structure following two recent reports 26,27 on CO 2 molecules confined in the micropores of solids.…”