MULTIPLE-QUANTUM NMR-STUDIES OF MONOMERIC BONDED SILICA PHASES

Covalently modified, bonded silica phases have been characterized by proton multiple-quantum (MQ) nuclear magnetic resonance (NMR) spectrometry. In the case of trimethylsilyl bonded phases, the loading has been shown to be homogeneous in that MQ coherence development plateaus at a value of approximately 10 correlated protons. Thus, at room temperature the MQ-NMR spin "counting" result is consistent with the fact that trimethylsilyl groups have 9 protons. The groups are dispersed such that intermolecular dipolar interactions are minimal, and the protons of individual trimethyl groups are behaving collectively as isolated clusters. Similar results are presented for a C8 monomeric phase (at -101-degrees-C) where isolated clusters of approximately 25 protons have been detected, and 23 are seen on the individual groups of the C8 monomeric phase. The mobility of the alkyl chain has been shown to vary depending upon the structure of the immobilized ligand. At room temperature the mobility of the C8 alkyl chain groups reduces the dipolar interaction among its protons. Therefore, MQ-NMR was run at the lower temperature to limit molecular motion while enhancing proton dipolar coupling.