Hydrogen Bonding between Acetone and Silica Gel, as Studied by NMR

13 C NMR spectra and relaxation parameters ( T 1 H, T 1 C, T 1 ρ H, T CH ) were measured via CP-MAS and DP-MAS techniques as a function of acetone loading level in acetone/SiO 2 samples at 25 °C. T 1 H and T 1 ρ H values were also measured via 29 Si CP-MAS experiments. Peak positions and relaxation parameters are averaged by rapid exchange between different acetone interaction sites on the surface. The data were analyzed in terms of a surface equilibrium of adsorbed acetone units between hydrogen-bonding and non-hydrogen-bonding sites. Data analysis by this model yielded the surface concentration of hydrogen-bonding (silanol) sites (1.5 nm -2 ) and the equilibrium constant ( K = 3.0 nm 2 ). Variable-temperature experiments (124−297 K) at three loading levels yielded values of Δ H ° (−5.8 ± 0.3 kJ mol -1 ) and Δ S ° (−12.5 ± 0.7 J K -1 mol -1 ). A self-consistent, qualitative interpretation of measured relaxation parameters is made in terms of the relative mobilities of hydrogen-bonded (less mobile) and non-hydrogen-bonded (more mobile) acetone on the SiO 2 surface.