It was proposed early on that the chromophores initially formed during irradiation were of the quinone, SUMMARY: Light-induced yellowing of lignocellulosic materials has been studied using 13 C-enriched DHP (dehydrogenation polymer), selectively 13 C-enriched at positions 1, 3, 4, and 5 in the aromatic ring, and quantitative solution state 13 C NMR spectroscopy. The NMR study confirmed the results of previous studies using side-chain labeled DHP, mainly that coniferyl alcohol end groups are

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... aded with the subsequent formation of a-carbonylic structures, e.g. vanillin end groups. The developed technique renders the potential to follow chemical changes of aromatic carbons 1, 3, 4, and 5, independently from each other. Although a drastic photoyellowing of the sheets was observed, definitive chemical shifts corresponding to quinoid carbons could not be discerned in the spectra. The reasons for this may be that the amount of quinoid carbons formed was too low to be detected, with the experimental setup used, or that the quinoid photoproducts, if formed, were not extractable with the solvent used or had reacted further forming non-quinoid structures. quinone methide, and cyclohexadienone type (Leary 1968a; Leary 1968b) . The observation that a sodium borohydride treatment of photo-aged paper made from mechanical pulp almost completely removed the formed chromophores also indicates that these chromophores contain carbonyl structures (like quinones, ketones, and aldehydes) (Hemmingson, Morgan 1990; Ragauskas 1993; Capretti et al. 1994; Schmidt, Heitner 1995; Pan, Ragauskas 1997) . Lin and Kringstad (1971) have irradiated lignin model compounds and have suggested that both o -quinones and p -quinones can be formed, although the latter to a lesser extent. Photoexposure of TMP and CTMP produced chromophores with absorption maxima at 350 and 420 nm (Heitner, Min 1987) . These chromophores were considered to be a methoxy-p -benzoquinone and an o -quinone with an alifatic substituent in the 2position. Others have stated that hydroquinones ( p -quinones) are the main leucochromophores (chromophores) formed during photoyellowing (Gellerstedt, Pettersson 1977; cf.