Cellulose polymorphism study with sum-frequency-generation (SFG) vibration spectroscopy: identification of exocyclic CH2OH conformation and chain orientation

Sum-frequency-generation (SFG) vibration spectroscopy is a technique only sensitive to functional groups arranged without centrosymmetry. For crystalline cellulose, SFG can detect the C6H(2) and intra-chain hydrogen-bonded OH groups in the crystal. The geometries of these groups are sensitive to the hydrogen bonding network that stabilizes each cellulose polymorph. Therefore, SFG can distinguish cellulose polymorphs (I-beta, II, IIII and IIIII) which have different conformations of the exocyclic hydroxymethylene group or directionalities of glucan chains. The C6H(2) asymmetric stretching peaks at 2,944 cm(-1) for cellulose I-beta and 2,960 cm(-1) for cellulose II, IIII and IIIII corresponds to the trans-gauche (tg) and gauche-trans (gt) conformation, respectively. The SFG intensity of the stretch peak of intra-chain hydrogen-bonded O-H group implies that the chain arrangement in cellulose crystal is parallel in I-beta and IIII, and antiparallel in II and IIIII.