13C NMR Lineshapes of Acetone Adsorbed on Silica

Abstract

13C NMR data, obtained as a function of temperature with magic-angle spinning (MAS) and either cross polarization or direct polarization, are reported on acetone and a sample of acetone (an approximately equal mixture with 13C labels at C-1 or C-2) adsorbed on dry silica gel. Various contributions to the observed linewidths and TC2 values are considered in terms of a previously established model of the acetone/SiO2 system; in that model, acetone species are in equilibrium between a physisorbed-acetone (non-hydrogen-bonded) state and a state consisting of acetone units that are hydrogen bonded to silanol moieties on the silica surface. Spin dynamics simulations are useful in interpreting the effects of variations of experimental parameters. It is concluded that the main linewidth contributions, which increase at lower temperatures, are: (a) a dispersion of chemical shifts in the hydrogen-bonded state, associated with the inhomogeneous character of the silica surface; (b) the interference between MAS averaging of the chemical shift anisotropy (especially for the carbonyl carbon) and molecular motion and/or chemical exchange; and (c) chemical exchange broadening. Prominence of the last of these contributions is most consistent with data obtained as a function of magnetic field strength, MAS speed, and temperature.