C-13 NMR lineshapes of acetone adsorbed on silica

C-13 NMR data, obtained as a function of temperature with magic-angle spinn ing (MAS) and tither cross polarization or direct polarization, are reporte d on acetone and a sample of acetone: tan approximately equal mixture with C-13 labels at C-l or C-2) adsorbed on dry silica gel. Various contribution s to the observed linewidths and T-2(C) values are considered in terms of a previously established model of the acetone/SiO2 system; in that model, ac etone species are in equilibrium between a physisorbed-acetone (non-hydroge n-bonded) state and a state consisting of acetone units that are hydrogen b onded to silanol moieties on the silica surface. Spin dynamics simulations are useful in interpreting the effects of variations of experimental parame ters. It is concluded that the main linewidth contributions, which increase at lower temperatures, are: (a) a dispersion of chemical shifts in the hyd rogen-bonded state, associated with the inhomogeneous character of the sili ca surface; (b) the interference between MAS averaging of the chemical shif t anisotropy (especially for the carbonyl carbon) and molecular motion and/ or chemical exchange; and (c) chemical exchange broadening. Prominence of t he last of these contributions is most consistent with data obtained as a f unction of magnetic field strength, MAS speed, and temperature. (C) 2000 Ac ademic Press.