Conformation and dynamics of organic tethers bound to MCM41-type surfaces from solid state NMR measurements

The conformation and motions of organic chains bound to the surface of MCM41-type mesoporous silica materials are investigated in molecular detail. 2D heteronuclear correlation measurements in conjunction with relaxation measurements are used to infer both the orientation and the motions of the organic chains on the silica surface. In octyl-MCM41, proximity of octyl tether carbons to silanols is observed with the exception of the terminal carbons, pointing to slight chain bending which positions the center carbons in the chain closer to neighboring silanols. In aminopropyl-MCM41, the carbon directly bound to the amine group is the closest to silanol protons and physisorbed water protons, indicating a tilted orientation of the tail allowing for the amine group to interact with the silanol. Motions in the two chains are characterized by using ¹³C longitudinal and rotating frame relaxation measurements. Substantial differences in the T ₁ and T _1Ï' times of the two materials are shown to be the result of differences in activation energy for respective rotation and libration motions between the two materials. The shorter tether in aminopropyl-MCM41, characterized by larger kinetic barriers for libration motions, demonstrates higher chain rigidity. The longer chain in octyl-MCM41 is characterized by faster librations and slower bond rotations in the center carbons. Evidence from structural and dynamical data jointly provides a detailed view of the chain behavior and underscores differences in the physical properties of the two tethers bound to the MCM41-type surface.