Deuterium nuclear magnetic resonance study of solid deoxycholic acid adducts: Structure, dynamics, and photochemical reactivity

We report on an ²H NMR study of polycrystalline powders of several deoxycholic acid (DOCA) inclusion compounds between 150 and 350 K. The information obtained relates primarily to the conformation, flexibility, and mobility of the guest molecules, their intrachannel orientation and site multiplicity, and the effect of a photochemical guest-host reaction on these characteristics. With acetopheonone, fluoroacetophenone, acetic acid, p- and o-xylene, and acetone rapid internal methyl rotation persists at all temperatures investigated. The C-CD₃ bonds were found to fluctuate about a mean orientation within the channel. The amplitude of these fluctuations was found to be site specific, temperature dependent, and suppressible upon cooling and following occurrence of the guest-host reaction. Ring inversion was observed with tetrahydrofuran (THF), cyclohexane, and cyclohexanone. Discrete jumps about symmetry axes of the guest molecule or the host lattice were detected with acetone and o-xylene, the flip motion taking place between unequally populated sites. In some cases, such as the cyclohexane and acetic acid adducts, there are sites in the host lattice where, on the average, the guest molecules reorient isotropically.