Crystal Structures, Thermal Behaviors, and C-H···O=C Hydrogen Bondings of Poly(3-hydroxyvalerate) and Poly(3-hydroxybutyrate) Studied by Infrared Spectroscopy and X-ray Diffraction

We have investigated crystal structure and C−H···O═C hydrogen bonding by using infrared (IR) spectroscopy and wide-angle X-ray diffraction (WAXD) for poly(3-hydroxyvalerate) (PHV) in comparison with those of poly(3-hydroxybutyrate) (PHB). We have explored the distances between the C═O group in one helix and the CH3 or CH2 groups in the other helix of PHV by using X-ray crystallographic data. It has turned out that the distance between the C═O group and the CH3 group of PHV is longer than that of the van der Waals separation (2.72 A), while the distance between the C═O group and the two CH2 groups in the main and side chains of PHV is significantly shorter than that of the van der Waals separation. Therefore, it is very likely that PHV has C−H···O═C hydrogen bondings between the C═O group and the one or two CH2 groups in the main or side chains. Of particular note is that CH2 antisymmetric and symmetric stretching bands of PHV show large blue shifts, which are characteristic of C−H···O hydrogen bondings. T...