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

Abstract

Structural characterizations of three different solid–gas reaction products, recently obtained from abraded solid-state silicate free radicals reacting with two isotopically enriched methane gases, 13CH4 and CD4 (a possible sink for methane on MARS) and with 13CO2, are derived from various dynamic solid-state NMR experiments. These include cross-polarization/depolarization zero-cross times (ZCTs), variable temperature (VT) NMR to study 3-site jump CH3/CD3 activation energies (Ea), and 13CO2/13CH3 molecular species as a spy to determine the approximate diameters for the channel structures for some of these structures. Literature Ea data indicate that l-alanine and 4-CH3-phenanthrene exhibit the highest known Ea values (= 20–22.6 kJ/mol) for CH3 3-site jump motions. The ZCTs for these two compounds are 120 and 162 μs, respectively, indicative of the high Ea values for CH3/CD3 groups. Determination of Ea for 4-CD3-phenanthrene by low-temperature 2H MAS NMR experiments supplemented the previously reported liquid-state Ea value (Ea = 21 kJ/mol) for 4-CH3-phenanthrene. Finally, such experiments also revealed the structural difference for the free-radical reaction products with 13CH4 and CD4, i.e, a change from helical to chain structure.