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Infrared Study of Lignin: Reexamination of Aryl-Alkyl Ether C—O Stretching Peak Assignments

Infrared (IR) C-O ether stretching peak assignments of beta- O-4 interunit bonds and methoxyl groups have been assigned by isotope labeling. The C aryl -O peak for both types of ethers was typically found at 1262-1224 cm -1 , but syringyl and 3,5-dimethoxyl derivatives gave C aryl -O peaks of 1328-1295 cm -1 and 1254-1204 cm -1 . The C alkyl -O peak for all ethers was found at 1047-1004 cm -1 . These assignments are similar to values reported in general IR tables for aryl-alkyl ethers, except for the unusually high C aryl -O stretch for syringyl and 3,5- dimethoxyl compounds. IR shift tables also often list a peak at 1176 cm -1 associated with the C alkyl -O bond of aryl-alkyl ethers. It is doubtful the peaks observed in this region are due to the C alkyl -O bond.