Solid-State CP/MAS 13C-NMR Spectra of Oligolides derived from 3-hydroxybutanoic acid

The solid-state CP/MAS ¹³C-NMR spectra (cross-polarization/magic-angle spinning ¹³C-NMR) of eight lower cyclic and one linear oligomers and several polymers of (R)-3-hydroxybutanoic acid (3-HB) are reported. The polymeric samples of different origin and molecular weight give remarkably similar and well resolved spectra, indicating considerable similarity in the conformations of the molecules and homegeneity in the solid-state environment. The crystalline cyclic oligomers 1 – 8 containing 3–9 units of 3-HB give very well resolved spectra. The number of nonequivalent positions in the solid state can be identified and is in accord with structures from X-ray diffraction where these were determined. The spectra of the oligolides become increasingly similar to those of the polymer as the ring size increases. This spectral evidence supports the view of a homogeneous and well defined conformation for the polymeric material (as proposed previously, based on other experiments).