High-resolution Nuclear Magnetic Resonance Spectrum of a Methanol Lignin

Abstract

A SAMPLE of methanol lignin from Eucalyptus regnans¹, purified by counter-current distribution and possessing the empirical formula (on the basis of a C₉ unit) of C₉H₈.₁O₂.₇₀(OCH₃)_1.86, was examined by means of the high-resolution nuclear magnetic resonance technique. This lignin was shown to contain approximately equal numbers of guaiacyl and syringyl units and to have been methylated to the extent of 0.41 methoxyl groups per C₉ unit¹. The spectrum (Fig. 1) of an approximately 10 per cent solution in deuterochloroform was recorded on a Varian model A60 spectrometer operating at 60 Mc/s and was calibrated in parts per million from tetra-methyl silane as internal reference². The feature centred at 6.62 p.p.m. can be attributed to protons linked directly to aromatic nuclei and that centred at 3.85 p.p.m. to protons in methoxyl groups. When recorded under the same conditions the spectrum of vanillin showed the methoxyl feature at 3.88 p.p.m. and that of syringaldehyde at 3.93 p.p.m. In the foregoing empirical formula, hydrogen present as methoxyl accounts for about 41 per cent of the total, whereas the feature at 4.98–3.45 p.p.m. equals 46 per cent of the total. The remaining features are most likely to be mainly associated with protons attached directly to oxygen (hydroxyl groups) or to carbon atoms bearing negative substituents. Protons resonating at corresponding frequencies may be found in the spectra of lignans^3,4. There is a striking lack of features upfield from 2 p.p.m. where most aliphatic and alicyclic methyl, methylene and methine protons would be expected to occur⁵. In general, the proton magnetic resonance spectrum appears to be in agreement with the type of structure predicted by Freudenberg⁸.