Exploring Molecular Dynamics of Adsorbed CO Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation.

Previous studies on CO adsorbents have mainly addressed the identification and quantification of adsorbed CO species in amine-modified porous materials. Investigation of molecular motion of CO species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin-lattice relaxation times ( ) by solid-state NMR. Rotational correlation times (τ) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen-Purcell-Pound theories. As expected, the τ values for the two physisorbed CO species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO species, the τ values of the CO species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit and τ values compatible with a CO layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and H-C heteronuclear dipolar couplings have also been estimated from measurements, for each adsorbed CO species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO dynamics in silica surfaces.