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 center dot center dot center dot 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 angstrom), 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 center dot center dot center dot 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 center dot center dot center dot O hydrogen bondings. These strongly support the existence of CH2 center dot center dot center dot O=C hydrogen bondings in PHV. PHV shows three crystalline C=O stretching bands at 1740, 1725, and 1721 cm(-1), suggesting that the C=O groups are in three different environments in the PHV crystal. The frequencies of the 1725 and 1721 cm(-1) bands are very close to that of the crystalline C=O stretching band of PHB (1723 cm-1). We have assigned these two C=O stretching bands of PHV to the C=O group that has hydrogen bonds with both side chain and main chain CH2 groups and that has a hydrogen bond with one of the CH2 groups. Temperature-dependent increases in the a and b lattice parameters of PHV. indicate that the directions of C-H center dot center dot center dot O=C hydrogen bondings are almost in parallel to the (110) diffraction, which is the chain folding direction of PHV. PHB has a particular C-H center dot center dot center dot O=C hydrogen bonding between the C=O group and the CH3 group along the a axis ((110) to (1 (1) over bar0) direction), which is the same direction as its chain folding direction. The difference in the chain folding direction between PHV and PHB may come from the difference in the inter- or intramolecular interactions in their crystal structures.