Molecular dynamics of pyridine adsorbed on the silica surface

The molecular dynamics of pyridine adsorbed onto dry high-surface-area silica gel was studied using solid-state deuterium NMR spectroscopy. Selectively and totally deuterated pyridine samples were used to identify the spectral signatures associated with each ring location and to identify the dynamical modes present in the surface adsorbed species. Loading levels corresponding to 0.8 and 0.2 monolayer coverage were studied. At 174 K and 0.8 monolayer, the adsorbed pyridine molecules are found in three motional modes: a fraction is found to be static, a larger fraction is found executing fast discontinuous rotation (180° ring flips) about the pyridine Cy-N axis, and a very small fraction is found in fast continuous rotation (diffusion) about the pyridine Cy-N axis, all referred to a 5 x 10 -6 s time scale. No evidence of O-H-N rotation about the Si-O bond ("swinging arm" motion) is observed. The pyridine flips and rotations permit measurement of the angle between the Cy-N axis and each of the C α -D and C β -D axes are found to be 55.9 ± 0.3° and 60.4 ± 0.5°, respectively. As the temperature is lowered, the fraction of static pyridine rings increases, while the fractions of flipping and freely rotating pyridine rings decrease. At 78 K and 0.8 monolayer, pyridine ring flip motion is still observed, implying a very low potential barrier to discontinuous rotation for some of the surface population. At 221 K, the flipping rings dominate, rotating rings are a minority, and no static rings remain. Above 221 K, the chemical exchange rate among pyridine molecules becomes significant and further motional averaging is observed. At 221 K and below, the spectra show no contributions corresponding to intermediate-rate motions; the notable absence of intermediate-rate patterns is explained in terms of a previously proposed model based on a broad distribution of motional correlation times. The surface structures and dynamics with 0.2 monolayer are found to be substantially similar to those found at 0.8 monolayer. These dynamical data are interpreted in terms of the structure of the silica surface as probed by pyridine molecules.