We have recorded the proton-decoupled natural-abundance 13C spectra (at 15.18 MHz) of phytol, phytyl acetate, and chlorophyll a. The 20 carbons of phytol yield 19 peaks. We have assigned 16 out of the 18 single-carbon resonances of phytol to specific carbons with the use of the chemical shift parameters of Grant and Paul, comparisons with the spectrum of pristane and of a phytol-pristane mixture, and 13C spin-lattice relaxation times (Tí) of individual carbons of neat phytol. Even some resonances separated by less than 0.1 ppm arising from structurally very similar carbons near the center of the phytol molecule were specifically assigned. The 13C resonances of phytyl acetate were assigned with the use of single-frequency off-resonance proton decoupling and a comparison with the spectrum of phytol. Phytyl acetate was then used as a model for identifying and assigning all the phytyl carbon resonances in the 13C spectrum of chlorophyll a dissolved in a chloroform-methanol mixture. The Tí values of neat phytol (at 52°) yielded information about the segmental motions in the branched phytyl chain. The observed behavior is compared with that of the unbranched 1-decanol molecule. While the effective rotational correlation time of the carbons of neat 1-decanol increases monotonically toward the hydroxyl end of the molecule, in the case of phytol there are localized deviations from monotonic behavior, as a result of branching and the presence of an olefinic bond.
ASJC Scopus subject areas
- Colloid and Surface Chemistry