Solution and solid state NMR studies of fluorine tagging reagents
A series of studies are presented in which fluorine tagging reagents are used to analyze complex mixtures for compounds containing active hydrogen functional groups (e.g., hydroxyl, amine, thiol, and carboxylic groups). The existence of these derivatized functional groups is determined by utilizing a number of solution and solid-state nuclear magnetic resonance (NMR) techniques.
In solution NMR studies p-fluorobenzoyl chloride was the fluorine tagging reagent of choice because of a large ¹⁹F chemical shift range for the different derivatized substrates (~10 ppm) and generally good reaction yields. Various classes of sterol and amino acid p-fluorobenzoyl derivatives were characterized on the basis of their ¹⁹F NMR isotropic chemical shifts. The presence (or absence) of hydroxyl, amine, and carboxylic acid functional groups in coal extract and pyrolysis products was also determined.
The versatility of the p-fluorobenzoyl chloride as the fluorine tagging reagent in ¹⁹F NMR was enhanced by: a) enriching the carbonyl carbon of the acid chloride with labeled ¹³C isotope, thus synthesizing a dual ¹⁹F and ¹³C NMR sensitive reagent and b) using the reagent in conjunction with LC-NMR. The extension to either technique added another dimension to the NMR spectral data obtained from the p-fluorobenzoyl tagging reagent in solution NMR.
Finally, preliminary data is presented illustrating how fluorine tagging reagents may be used to study functional groups (and atoms present in the immediate proximity of the group) existing on solid material utilizing solid-state NMR. Functional groups on the solid material are tagged with a fluorinated reagent. The sample is then analyzed using solid-state NMR with cross-polarization (CP), magic angle spinning (MAS), and high-power proton decoupling. The ¹⁹F dipolar coupling interactions, created by the presence of the fluorine tag, attenuate signals for these nuclei in the immediate proximity of the tagged site. A series of 1-adamantanol, steroid, and silica gel fluorinated derivatives show that the effects of the ¹⁹F dipolar interactions were modulated by complex anisotropic molecular motions (i.e., solid system with little motion, exhibiting greater signal attenuations due to ¹⁹F dipolar coupling).