The hydrogen bond network in I-beta cellulose as observed by infrared spectrometry

FT-IR spectra of I-beta cellulose were recorded on films made of hydrotherm ally treated Valonia microcrystals. Polarized spectra of these not complete ly disordered systems allowed to define the spectra along the cellulose c c hain axis as well as along a perpendicular axis which may slightly vary fro m one sample to another one. Weakening and ruptures of some hydrogen bonds upon heating the samples at 115 degrees C, as well as evaporation of D2O mo lecules and H/D exchanges after immersion in a D2O vapor were followed spec troscopically. A critical analysis of the spectra allowed to propose a deta iled assignment for most of the bands at wavenumbers higher than 800 cm(-1) . From this analysis it appeared that the majority (more than 2/3) of C2O2H alcohols were involved in weak hydrogen bonds or perhaps even not hydrogen -bonded at all. On the other hand, the minority of C2O2H established a hydr ogen bond with the O6 atom of an adjacent primary alcohol of the same chain . This particular hydrogen bond was the strongest found in these crystals. With the proposed assignment, hydroxymethyl moieties were found adopting th ree conformations (a dominant one and two minor) allowing the formation of different hydrogen bonds on adjacent chains. These conformations correspond ed to three slightly different C4-C5-C6-O6 (chi) dihedral angles. Most prob ably the primary alcohols that accept a hydrogen bond from the adjacent C2O 2H alcohols were not the ones which adopt the dominant conformation. (C) 20 00 Elsevier Science B.V. All rights reserved.