P-31 NMR-SPECTROSCOPY IN WOOD CHEMISTRY .4. LIGNIN MODELS - SPIN-LATTICE RELAXATION-TIMES AND SOLVENT EFFECTS IN P-31 NMR

A variety of lignin-like model compounds were derivatized with 1, 3, 2-dioxaphospholanyl chloride (I). Their phosphorus spin lattice relaxation times (T1) were of the order of 5 s for phosphorus nuclei attached to carboxylic acids while the relaxation times of phenol and alcohol derivatives were around 9 and 8 s respectively. The P-31 chemical shifts of the derivatives of monofunctional alcohols moved downfield when the samples were diluted with CDCl3 and upfield when diluted with pyridine. The concentration dependency of the P-31 chemical shifts of the phenol derivatives was considerably less than that of the alcohol derivatives. The carboxylic acid derivatives showed almost no concentration dependence.The P-31 chemical shifts most sensitive to solvent concentration were the primary hydroxyls in beta-0-4 model compounds of lignin. Alpha and guaiacyl hydroxyls were only slightly sensitive to solvent concentration. The resolution of the P-31 NMR spectra of lignins can be significantly improved by exploiting these solvent effects. Signals due to primary and secondary alcohols present in lignin may be readily resolved by appropriately adjusting the solvent concentration during spectral acquisition. The presence of the ionic salt, pyridine hydrochloride, appears to be responsible for the observed concentration dependence of the chemical shifts.