Motional heterogeneity in single-site silica-supported species revealed by deuteron NMR

Abstract

The molecular dynamics of [–SiDMe₂] grafted on two amorphous silica materials, mesoporous SBA and non-porous Aerosil, was investigated by deuteron (²H) solid-state NMR spectroscopy. Quadrupole echo (QE), quadrupole Carr–Purcell–Meiboom–Gill (QCPMG) and magic angle spinning (MAS) spectra were recorded as a function of temperature. These were analyzed to determine the rates and trajectories of molecular motion of the surface species. The dynamics were modelled as a composite two frame motion with independent rotations around the two Si–O bonds. In the first frame there are fast three-site jumps of the –SiDMe₂group described by a single rate (k₁) and unequal populations of the tetrahedral sites, such that the ratio D : Me : Me is around 1 : 4 : 4. In the second frame, the Si–O axis makes small step, nearest-neighbour jumps at a rate k₂ along an arc defined by the rim of a cone with a fixed half-angle. Both rates were found to be in the fast motional regime (k_1,2 > 10¹⁰ s⁻¹) throughout the experimentally accessible temperature range, 190–350 K. The experimental data are compatible only with models that include a distribution of arc lengths, λ, in the second frame. The best fit of the simulations to the experimental data yields the distributions of the arc length. The results unequivocally demonstrate that even though the sites all have the same average environment, as reported by the isotropic chemical shifts, the dynamics of the grafted species are microscopically spatially heterogeneous with different molecules on the surface having different ranges of motional trajectories and populations. Furthermore, a clear difference in dynamic behavior is observed between the two silica supports, the motion being more constrained on the mesoporous SBA. This differential mobility is possibly due to differences in surface roughness and to the pore structure of SBA compared with the smoother surface of Aerosil.