THE SIGNIFICANCE OF INTRAMOLECULAR HYDROGEN-BONDING IN THE BETA-O-4 LINKAGE OF LIGNIN

During our molecular mechanics investigation of the three-dimensional structure of lignin, the need for a more accurate model chemistry beca me apparent. Selected conformers of 2-phenoxy-1,3-propanediol, a simpl ified p-coumaryl beta-O-4 model structure, were analyzed with both MM3 (92) and ab initio (HF/6-31G) calculations. The poor agreement between these computational methods led to the proposal that hydrogen bonding to the aromatic ether oxygen changes the C-AR-O bond order which, in turn, is predicted to change the bond's rotational characteristics. Ab initio computations on methoxybenzene indicate a significant lowering of the rotation barrier due to an inter-molecular hydrogen-bonded wat er molecule. We suggest that, in the p-coumaryl beta-O-4 linkage, intr a-molecular hydrogen bonds from the alpha- and gamma-hydroxyl groups t o the ether oxygen effect the rotational characteristics, and thus the preferred orientation, of the aromatic ring.