Further Study on the Distribution of Substituent Group in Cellulose Acetate by 13C{1H}NMR Analysis: Assignment of Carbonyl Carbon Peaks

Abstract

An attempt was made to determine an unsolved problem in peak assignment of carbonyl carbon region in 13C NMR spectra for cellulose acetate. For this purpose, Kamide and Okajima’s method for determination of the distribution of acetyl group among three hydroxyl groups in a glucopyranose unit was reexamined carefully using a 200 MHz FT-NMR spectrometer, adequate and the most reasonable assignment at present was made on the carbonyl carbon peaks in NMR spectra, applying a low-power selective decoupling method to acetyl methyl proton located at specific carbon position. All three NMR peaks in the carbonyl carbon region were assigned to be attributed to the acetyl groups located at C6, C3, and C2 positions, from the low magnetic field side. The previous Kamide and Okajima’s assignment on C3 and C2 peaks was found to be erroneoulsy made by neglecting an overlapping of the three peaks due to low resolving power in the 1H and 13C NMR spectrometer used by them. The new and complete assignments were in good agreement with those by Miyamoto et al. The method of Miyamoto et al. was examined in detail in the light of the validity of the assignment of carbonyl carbon peaks for not fully substituted cellulose acetate.