posted on 2021-10-13, 07:04authored byViraj
C. Kirinda, Gopi Nath Vemuri, Nicholas G. Kress, Kaitlyn M. Flynn, Nuwanthika Dilrukshi Kumarage, Briana R. Schrage, David L. Tierney, Christopher J. Ziegler, C. Scott Hartley
1H NMR spectroscopy is
a powerful tool for the conformational
analysis of ortho-phenylene foldamers in solution.
However, as o-phenylenes are integrated into ever
more complex systems, we are reaching the limits of what can be analyzed
by 1H- and 13C-based NMR techniques. Here, we
explore fluorine labeling of o-phenylene oligomers
for analysis by 19F NMR spectroscopy. Two series of fluorinated
oligomers have been synthesized. Optimization of monomers for Suzuki
coupling enables an efficient stepwise oligomer synthesis. The oligomers
all adopt well-folded geometries in solution, as determined by 1H NMR spectroscopy and X-ray crystallography. 19F NMR experiments complement these methods well. The resolved singlets
of one-dimensional 19F{1H} spectra are very
useful for determining relative conformer populations. The additional
information from two-dimensional 19F NMR spectra is also
clearly valuable when making 1H assignments. The comparison
of 19F isotropic shielding predictions to experimental
chemical shifts is not, however, currently sufficient by itself to establish o-phenylene geometries.