(596a) Studies of Molecular Motions of Simple Organic Amine Groups Covalently Attached to MCM-41 by Solid-State 2H MAS NMR and Suspension NMR

Amine-functionalized ordered mesoporous silica composites have attracted interest for their potential application in heterogeneous catalysis. Here, we will discuss the molecular motions, the dispersion, and the interactions of these groups with silica surfaces by solid-state deuterium (2H) NMR [1]. Methylamine, benzylamine, and piperazine with labeled deuterons were attached to MCM-41 by post-synthetic grafting. Magic angle spinning measurements were employed to improve sensitivity given the low deuterium content of the samples. The results provide clear insights about the motion / dynamics of the different surface groups. For instance the deuterons of the methylamine and benzylamine groups attached to MCM-41 rapidly rotate anisotropically on the NMR time scale (~ 1 microsecond), while the deuterons of the piperazine group on MCM-41 appear rigid. Hexamethyldisilazane (HMDS) treatment to remove the surface silanol groups changes the rates of the motions based on the increased intensity of the observed signals and decreased line width. The results indicate that the HMDS capping decreases the possibility of hydrogen bonding between the propylamine and surface silanols, which restricted the mobility of the deuterons in functional fragments. Deuterated HMDS groups were also investigated, and these rapidly reorient on the microsecond time scale, even down to temperatures of 170 K.

Recently, high-resolution (HR) MAS suspension NMR spectroscopy has proven to be an efficient method to study the mobility of surface species [2]. In the current work suspension NMR will be used to investigate the materials described above in the presence of varying amounts of solvent (nitromethane). The suspension NMR spectroscopy of amine-OMS with nitromethane solvent will show the molecular dynamics of functionalized organic groups on OMS surface in the ?real reaction? conditions, and show how solvent modulates amine ? solvent interactions.

Reference:

1. Rohr-Schmidt, K.S., Spiess, H. W., Multidimensional Solid-State NMR and Polymers. 1994, London: Academic.

2. Posset, T., F. Rominger, and J. Blumel, Immobilization of bisphosphinoamine linkers on silica: Identification of previously unrecognized byproducts via (31)p CP/MAS and suspension HR-MAS studies. Chemistry of Materials, 2005. 17(3): p. 586-595.