Degradation of the cellulosic key chromophore 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ) under conditions of chlorine dioxide pulp bleaching: formation of rhodizonate as secondary chromophore—a combined experimental and theoretical study

Chlorine dioxide—although having some negative environmental repercussions—is still a major bleaching agent in the pulp and paper industries. It is a gas with good solubility and relatively high stability in acidic aqueous media, and thus it is applied this way at a pH around 3. ClO₂ is a strong oxidant for many organic compound classes, such as phenols, aldehydes, unsaturated structures, or amines. There are three typical initial reactions of ClO₂—hydrogen atom abstraction, one-electron transfer, and radical addition reactions to double bonds—which are typically followed by subsequent reactions of ClO₂-derived chlorine species (Leigh et al. 2014; Aguilar et al. 2014; Lehtimaa et al. 2010; Napolitano et al. 2005; Hull et al. 1967). Because of the different reaction modes and many side reactions, its application in synthetic organic chemistry is rather limited, but it is a favored oxidant in pulp bleaching because of its high selectivity: it mainly attacks residual lignin, with its aromatic and quinoid structures, while leaving the carbohydrate structures in cellulose and hemicelluloses largely unchanged. This is one advantage over oxygen-based bleaching chemicals, such as molecular oxygen (“O-stage” bleaching), hydrogen peroxide (“P-stage”) or ozone (“Z-stage”).

While being reasonably stable in acidic medium, ClO₂ offers a rich disproportionation and symproportionation chemistry in neutral and in particular alkaline aqueous media. Having an unstable oxidation state (+IV) of its central chlorine atom, the compound is a strong oxidant with high reactivity. It is a monoradical, having a doublet ground state. Computations as well as simple VSEPR (valence shell electron pair repulsion) considerations show that the molecule is V-shaped (i.e., it is not linear) with the spin density mainly located at the terminal oxygen atoms and only little at the central chlorine atom. Immediately disproportionating in alkaline medium, the pure substance degrades only slowly in acidic medium, giving rise to many chlorine species in different oxidation states, e.g. chloric acid (V), chlorous acid (III), hypochlorous acid (I), elemental chlorine (0), and chloride (−I). In the presence of redox-active reaction partners those conversions into other oxidation states are much faster, resulting in a very complex redox chemistry. The final, stable products from ClO₂ are hydrochloric acid (HCl) and chloric acid (HClO₃), having oxidation states of −I and +V, respectively. Recording kinetics of reactions involving ClO₂ is not reasonably possible by following ClO₂ itself or one of the chlorine-derived species because of the complexity of the underlying redox systems and the fast interconversion between the chlorine species. Kinetic studies are therefore reliant on the monitoring of the reaction partner, as was also applied in our case. But even then it is difficult to determine whether a compound was oxidized directly by ClO₂ or rather by one of the secondary oxidants derived from it.

The preliminary observations, as briefly described in the introduction, led to the general conclusion that oxidative damage to the pulps and metal ion management before/during bleaching were somehow involved in all cases where a poor bleachability in the final D-stage was observed. We thus performed experiments with highly bleached pulps available from previous work (Ahn et al. 2019), which had different degrees of oxidative damage. The pulps contained carbonyl groups (oxidized cellulosic hydroxyl groups) between 13 and 105 µmol/g and had an initial brightness of 88–91% ISO and above. In all bleaching experiments, bidistilled water and high-purity chemicals were used to make sure that any water-associated metal ion effects were at least largely limited, if not fully prevented. An aliquot of the starting pulps was aged under either dry or wet conditions according to standard procedures (ISO 2470-1 2009), which expectedly brought the brightness down to values between 82 and 88% ISO. For two arbitrarily selected aged pulps it was confirmed once more that the discoloration upon aging was mainly due to DHBQ formation, applying the CRI method to isolate the formed DHBQ. The DHBQ contents were between 0.4 and 2.8 ppm, sufficiently high to cause the observed brightness loss. As mentioned, it is known that the formation of DHBQ is a direct result of oxidative damage—to be precise: carbonyl group generation—in cellulosic pulps (Rosenau et al. 2005).

The starting pulps as well as the dry aged and wet aged counterparts were subjected to a simulated D-stage bleaching (pH 3, temperature between 50 and 90 °C). Standard experiments were done at 50 °C, which is somewhat low in comparison to industrial conditions, but slowed the reactions down to time ranges in which they could be conveniently followed. Thus, this temperature seemed to be a good compromise. Upon ClO₂ treatment, the starting pulps showed no significant brightness gain due to the already high starting brightness. For all aged pulps, however, the brightness increase was quite significant, although—at 88–90% ISO—it remained slightly below the initial value (Table 1).

So far, the results had been rather commonplace and fully in line with expectations. However, the addition of Fe(III) cations (3 ppm) to the D-stage medium brought about a surprising twist: while the effect on the starting pulps was zero, the bleachability of all aged pulps was very negatively affected (74–88% ISO), Table 1. For the highly oxidized (C=O content > 62 µmol/g) and aged pulps, the brightness after the ClO₂-treatment was even lower than before! Spontaneously we tried to explain this by the “usual” transition-metal induced radical and degradation reactions, i.e. the often cited “Fenton chemistry”. But firstly, such reactions are typical of alkaline media and peroxide-based systems, but not of the conditions used, which differ greatly from classical Fenton systems. And secondly, the molecular weight of the pulps bleached with and without iron addition did not show any differences at all. This result was inconsistent with the assumption that Fe-based radical chemistry was involved, which surely would have affected cellulose integrity rather negatively. A very simple observation—admittedly purely coincidental—was the key at the time to steer our efforts in a better direction: the effect of iron was almost the same when it was added during the washing treatment, i.e. after the actual bleaching stage. It was now obvious that some iron-containing chromophores, which resulted from DHBQ, had to be the reason for the reduced bleachability, and that the loss of brightness was not caused simply by iron-induced, Fenton-type cellulose degradation. Moreover, these Fe-based chromophores were apparently quite potent (since the iron concentration was very low), formed quite fast (even during washing) and were quite stable on the pulp (because they were not simply washed away). They were formed not only when the metal ion was there during the D-stage, but also when it was present only during the short times of washing with its highly diluted regimes, and they were little affected by an alkaline extraction step.

In the following, the influence of metal ions other than Fe was tested. These results are summarized in Table 2. It was not unexpected that the effect of Cu(II) and Mn(II)—transition metals as well—on bleachability were the same as that of Fe(II/III); Cu(II) was even slightly more harmful than Fe, Mn(II) somewhat less. However, it was surprising that Zn(II), which is far less redox-active than Fe, Cu and Mn, behaved similarly, and it was even more puzzling that also the main group metals Mg(II) and Ca(II) had a significantly negative effective at higher degrees of oxidative pulp damage. The influence was less than in the cases of transition metals, but still very clear. At 20 times the transition metal concentration (60 ppm), Ca(II) or Mg(II) had an effect similar to Fe, Cu or Mn at a 3 ppm concentration (Table 2). Even very high concentrations of sodium and potassium (200 ppm) decreased the brightness, while Al(III) and ammonium had no negative effect at all (Table 2).

For all cations used, it did not matter with regard to final brightness whether they were already added during the D-stage bleaching or only afterwards in the washing water. There was even a slight tendency for the addition during washing to result in lower brightness, which indicated the fast formation of metal-based chromophores during the relatively short washing times. While the chromophores could be partially dissolved or degraded in the (relative long) D-stage, the washing was too short (or too ineffective) for such partial removal. The washing was usually performed with neutral bidistilled water (containing the metal salts to supply the respective metal ions). When subsequently an additional alkaline wash (pH 10) was applied, there was no or no significant improvement for the pulps containing Fe, Mn, Cu, Zn, Mg or Ca salts, but some improvement for pulps with Na or K (Table 2), suggesting that the negative effects of the latter two ions were at least partly reversible by alkaline washing.

After D-stage bleaching, DHBQ was not detectable in any of the pulps. The action of chlorine dioxide obviously destroyed the DHBQ generated during accelerated aging, but the pulps now showed a general sensitivity towards metal ions causing poor bleachability—interestingly not limited to transition metal ions. This effect clearly increased with increasing degree of oxidation of the pulps. Since the degree of oxidation also runs parallel to the DHBQ content directly after aging, there was obviously also a correlation between this initial DHBQ content and the subsequent metal ion-induced bleachability problems, but it was impossible at this point to determine whether and how both factors depend on each other. Table 2 summarizes the initial bleaching trials with metal ion addition and illustrates the obvious negative effects of some metal ions. From this outcome, it seemed clear to us that conventional D-stage experiments with usual pulps would not allow more detailed explanations and that mechanistic studies with model pulps and model compounds were necessary for a thorough understanding of the underlying chemistry.

In the next set of experiments, we therefore used the starting pulps (carbonyl group content 28 and 45 µmol/g,¹ initial brightness > 90%ISO, see above) and enriched them with DHBQ. For this purpose, so much of an aqueous solution of DHBQ in distilled water was applied to the pulp that a DHBQ content of 3 ppm or 10 ppm, respectively, was reached after drying. Then the pulp was kneaded for a few minutes. The DHBQ is firmly adsorbed to the cellulose under these conditions and does not adhere to glassware or containers, thus ensuring that the amount of DHBQ transferred to the system with the water really remains in/on the cellulose and that the concentration is as intended within the error limits. For one case it was additionally confirmed that DHBQ was indeed only adsorptively (but not covalently) bound and can therefore be completely re-extracted by alkaline washing (0.1 mM NaOH, pH 10): three parallel extractions gave DHBQ recoveries of 96, 97 and 101%, respectively (UV/Vis determination). The ISO brightness of the DHBQ-enriched pulps was 83%ISO (3 ppm) and 79% ISO (10 ppm).

The pulps enriched with DHBQ were directly subjected to D-stage bleaching followed by washing to neutral pH with distilled water. In all cases, brightness returned nearly to the initial values (90–91% ISO). If an additional alkaline washing step was subsequently applied, the brightness gained was excellent with 91–92% ISO. It was obvious that the added DHBQ was efficiently destroyed in the D-stage (Table 3). When the DHBQ-enriched and bleached pulps were aged, the brightness was the same as that for the initial pulps, meaning that the two steps of DHBQ-enrichment and removal had no permanent negative effect to the pulp. The ClO₂ system had selectively attacked the additionally provided chromophore without causing any further damage to the cellulose, which would have shown up in additional chromophore formation and brightness decrease after aging. Our attempts at that time to isolate any reaction products of DHBQ from the pulps seemed to have failed. The NMR spectra of concentrated alkaline extracts (D₂O/NaOD) showed no signal in the ¹H domain, and a solitary, highly concentration and matrix dependent singlet (158 ppm) in the ¹³C spectra which—incorrectly, as it turned out later—was assigned to carbonate (see also the peak at 177 ppm in Fig. 1).

As soon as metal ions were involved, the situation changed once again fundamentally. In principle, all results were identical to those obtained with the above oxidized/aged pulps (which were not DHBQ-enriched, but contained DHBQ generated by aging). Thus, it was evident that the brightness-impairing effect of DHBQ—whatever its nature was—was independent of the DHBQ source, i.e. independent of whether DHBQ was generated “naturally” from oxidized functionalities upon accelerated aging or provided “artificially” by spiking the pulp with a DHBQ solution. The addition of 3 ppm of transition metal salt solutions (Fe(II), Cu(II), Mn(II) as the respective sulfates) during the D-step decreased the final brightness after bleaching to values between 72 and 78% ISO. Again, higher concentrations of Ca(II) and Mg(II) (60 ppm) as well as Na(I) and K(I) (300 ppm) had a similar effect, see Table 3. In all cases the point of metal ion addition played no role, i.e., the effect was the same when the metal ions were added during bleaching or thereafter together with the neutral washing solution. An additional alkaline washing step had only a minor effect for all metal ions except Na(I) and K(I), for both of which the brightness significantly improved (Table 3). It was evident that under D-stage conditions the combination of DHBQ (or its reaction/degradation products) and metal ions afforded some chromophores, which were not further degraded and thus decreased the final brightness, and which for most metal ions could not be removed by neutral or alkaline washing (Table 3).

In the case of Na(I) and K(I) being present, when chromophore extraction was possible, we tried to concentrate the extracts, keeping the pH at 10 (to avoid any condensations in the strongly alkaline medium that would form by removal of the water). Again, the ¹H NMR spectra did not show any signals, although the solution was deeply yellow. This was not surprising, since chromophore mixtures already at very low concentrations—far below the typical NMR-accessible range—might form strongly colored solutions. The ¹³C NMR spectra, in turn, provided only the alleged carbonate peak, as described above.²

To confirm the complete oxidation of DHBQ to carbonate in aqueous solution, we used the old and simple—yet quite reliable—acidification test, which was expected to cause some gas generation, and the purging of the formed CO₂ into a BaCl₂ solution that would turn white from the formation of precipitated BaCO₃. However, no gas development was observed during acidification, but a clearly noticeable brightening to light yellow was seen, which remained unchanged even after the acidified solution had been refluxed for some minutes. The initial dark yellow color returned after re-alkalization with NaOH. When the acidified sample was re-measured with NMR, the ¹³C NMR spectra surprisingly showed three peaks (at about 92, 141 and 190 ppm) which converged to a single signal at 177 ppm upon re-alkalization (Fig. 1).

This peculiar change between the solution structures in alkaline and acidic media together with the changes in the ¹³C resonances pointed to the structure of rhodizonic acid (RhA, 2), see Scheme 2. This was additionally confirmed by comparison and spiking with an authentic sample. Evidently, DHBQ had been converted into rhodizonic acid under the D-stage conditions, and the interaction of the formed RhA with metal ions caused the observed poor bleachability.

Rhodizonic acid is present as 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone (2) in the solid state and in non-aqueous solutions. In acidic and neutral aqueous media, the compound exists as the bis(ketohydrate), namely 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione (2a), cf. the observed characteristic ¹³C resonance of the ketohydrate at about 94 ppm. The compound is often (and wrongly) referred to as rhodizonic acid “dihydrate”, but it is in fact a bis(ketohydrate) and not a coordination compound with water in the hydrate shell. In alkaline medium, RhA forms a symmetric (space group S₆) rhodizonate dianion (2b), which in ¹³C NMR results in only a single resonance. The dianion is aromatic, which explains both its very high stability and the ease of formation.

Rhodizonic acid forms deeply colored salts with transition metal ions, such as Fe, Cu, Mn, Ag and Pb, which have extremely high extinction coefficients (see Fig. 2). The actual colors are well distinguishable in the low mM and µM range. At the very low concentrations present on the pulp, only a slight discoloration is observed, recognizable as yellowing effect, which was measured as decrease in ISO brightness (see Tables 2, 3). Moreover, the transition metal salts of RhA are almost insoluble, forming a so-called “lacquer” which readily precipitates and sticks tightly to surfaces. A simple color test for the presence of RhA is performed with Pb(II) ions, which give the deeply colored lead salts, the most stable and least soluble of all RhA salts (Chalmers and Telling 1967; Feigl and Suter 1942). Upon addition of Pb²⁺, main group salts of RhA (and also those of Zn²⁺ and ammonium) are converted into the Pb salt, which is accompanied with a significant color change. Therefore, the Pb(II) test works for both RhA itself and its main group salts, which are altogether converted into the black Pb-salt. In our experiments, the Pb(II) test had been clearly positive in all cases where decreased brightness and bleachability was observed, confirming that these effects were always due to rhodizonate salt formation.

In conclusion, the action of ClO₂ on DHBQ upon D-stage bleaching produced rhodizonic acid as the primary, stable and detectable product. As a pure substance and in acidic aqueous solution RhA has a yellow color, but with many metal cations it forms deeply colored and hardly soluble salts. This is the reason for the observed loss of brightness, deteriorated bleachability, and the great influence of metal ions. It also explains why the bleachability changes with the transition metal content in the pulp, which shows for instance seasonal or inter-species variations.

Commercial chemicals from Sigma-Aldrich (Schnelldorf, Germany) were of the highest grade available and were used without further purification. Distilled water was used for all aqueous solutions.

For NMR analysis, a Bruker Avance II 400 instrument (¹H resonance at 400.13 MHz, ¹³C resonance at 100.61 MHz) with a 5 mm broadband probe head (BBFO) equipped with z-gradient with standard Bruker pulse programs and temperature unit were used. ¹H NMR data were collected with 32 k complex data points and apodized with a Gaussian window function (lb = − 0.3 Hz, gb = 0.3 Hz) prior to Fourier transformation. ¹³C-jmod spectra with WALTZ16 ¹H decoupling were acquired using 64 k data points. Signal-to-noise enhancement was achieved by multiplication of the FID with an exponential window function (lb = 1 Hz). Bruker TopSpin 3.5 (3.0) was used for the acquisition and processing of the NMR data. Chemical shifts are given in ppm, referenced to residual solvent signals (7.26 ppm for ¹H, 77.0 ppm for ¹³C in case of CDCl₃).

An Agilent 6560 QTOF mass spectrometer equipped with an Agilent G1607A dual Jetstream coaxial ESI interface was used, injection volume: 5 µL, sheath gas temperature: 150 °C, sheath gas flow rate: 12 L/min, nebulizer gas pressure: 20 psi, MS capillary voltage: 4 kV, nozzle voltage: 2 kV, fragmentor: 275 V, scanning mass range: 50–1700 m/z with a TOF acquisition rate of 2.8 spectra/s.

For paperspray MS, the paper tips (isosceles triangle, a = 10 mm) were cut and connected to 4 kV capillary voltage. Whatman filter paper No 1 (Wagner & Munz GmbH, Vienna, Austria) was used as the model sample matrix. The aqueous sample solution was used as a spray solvent. After applying 30 µL of the spray solvent onto the paper surface, mass spectra were recorded in positive or negative ion mode with a Thermo LTQ-MS (LTQ XL™ Linear Ion Trap Mass Spectrometer, Thermo Fisher Scientific, Waltham, Massachusetts, USA) equipped with an ESI ion source. Injection volume: 5 µL. Where applicable, a splitter with the ratio of 1:2 was used to divert the flow for better ESI spray quality. MS settings: Spray voltage: 6 kV, sheath gas pressure: 5 psi, auxiliary gas: 2 a.u., transfer capillary temperature: 275 °C, scan range: m/z 300 to 1000. In case of tandem-MS investigations, an isolation width of ± 0.5 m/z was selected. Thermo Fisher Scientific Xcalibur software (Thermo Xcalibur 2.2 SP1 build 48) was used for operating the MS instrument.

A 30 wt% aqueous sulfuric acid solution (7.5 mL) was added dropwise to 17.5 mL of an aqueous sodium chlorite solution (300 g/L) in a 100 mL multi-necked flask at an addition rate of 0.5 mL/min at room temperature under a nitrogen gas flow through the reaction solution. The gaseous chlorine dioxide produced was introduced with the nitrogen flow into 100 mL of deionized water in another flask pre-cooled at 0 °C along. After sulfuric acid addition was complete, the reaction solution was allowed to remain under nitrogen flushing for another 30 min at room temperature, to give 100 mL of a yellow-colored aqueous solution of chlorine dioxide in the second flask.

The current concentration of chlorine dioxide in the solution was determined by titration. To 1.0 mL of the chlorine dioxide solution, 2.0 mL of a 30 wt% aqueous sulfuric acid solution and 2.0 mL of a 10 wt% aqueous potassium iodide solution were added. The color of the resulting solution turned to dark brown (oxidation of iodide to elemental iodine). The iodine produced was titrated with a 0.1 M sodium thiosulfate solution against starch as an indicator. The concentration according to the above procedure ranged between 50 and 60 mmol/L. The chlorine dioxide solution was diluted with deionized water whenever necessary (UV/Vis measurements).

All measurements were performed on a Lambda 35 UV/VIS Spectrometer (PerkinElmer^®, Waltham, MA, U.S.A.) controlled by the UV WinLab software (version: 6.0.3).

A 50 µM solution of DHBQ (50 mL) in water at pH 3 (set by sulfuric acid or 0.01 M phosphate buffer) was prepared in a stirred 100 mL Erlenmeyer flask and heated to 50 °C. Freshly prepared aqueous chlorine dioxide solution (concentration recently determined) was added in amounts corresponding to target DHBQ/ClO₂ molar ratios, the volumes being usually between 15 and 50 mL. After certain reaction times, a 0.5 mL aliquot of the reaction mixture was taken and introduced in a stirred UV cuvette containing 1 mL of concentrated aqueous sodium sulfite solution in 0.1 M aqueous NaOH. The concentration of DHBQ and rhodizonate was determined by UV/Vis measurements from calibration curves recorded with the pure substances and its mixtures (molar ratios 100/1, 50/1 10/1, 5/1, 2/1, 1/1, 1/2, 1/5, 1/10, 1/50, 1/100) in neat 0.1 M aqueous NaOH.

Two cellulose samples (industrial pulps) were used: (1) a bleached beech sulfite pulp (kappa number 0.22, brightness 91.2% ISO, viscosity [cuen] 565 mL/g, pentosan 0.93%, DCM extract 0.18%, ash 0.05%, bleaching sequence E-O-Z-P), and (2) a bleached Eucalyptus pre-hydrolysis kraft pulp (kappa number 0.37, brightness 90.9% ISO, viscosity [cuen] 530 mL/g, pentosan 1.73%, DCM extract 0.13%, ash 0.05%, bleaching sequence D₀-EOP-D). The samples were thoroughly washed with HPLC-grade acetone (to remove extractives) and then with distilled water, followed by air-drying. Initial brightness of the dry samples was 94% ISO.

The detailed procedure is given in Röhrling et al. (2002a, b) and Potthast et al. (2003). The GPC system is in detail discussed in Potthast et al. (2015). In short, a stock solution of the CCOA label was prepared by dissolving the fluorescence label (62.50 mg) in 50 mL of 20 mM zinc acetate buffer (pH 4.0). Wet pulp (corresponding to 20–25 mg of dry pulp) was suspended in the acetate buffer containing the dissolved labelling compound (4 mL). The suspension was agitated in a water bath for 7 days at 40 °C. The pulp was isolated by filtration, then activated and dissolved in 2 mL of DMAc/LiCl (9%, m/V) overnight at room temperature. Then, the samples of the solution were diluted with DMAc, filtered through 0.45 µm syringe filters, and analyzed by GPC. Calibration of the system was done with the pure CCOA label and by means of reference pulps, as described previously. For the determination of the overall carbonyl content, the carbonyl peak area was normalized with regard to the injected mass.

The preparation of cellulosic pulps with increased contents of functional groups has been described in detail before, and the procedures described there have been followed throughout (Siller et al. 2015; Ahn et al. 2019). In short, sodium hypochlorite (HOCl) oxidation was performed with 2 g of dry pulp in total. To improve accessibility, the pulp was suspended in water and shortly disintegrated. The excess of water was removed by vacuum filtration and the wet pulp transferred into a 2 L beaker to suspend it in sodium acetate buffer (1 L, 1 M, adjustment of pH with glacial acetic acid to pH 6.5). The suspension of pulp in the buffer was continuously stirred with a magnetic stirrer. Next, different volumes (between 1 and 120 mL) of HOCl (active chlorine 10–13%, Sigma Aldrich, Schnelldorf, Germany) were added to effect the oxidation. The oxidation was stopped after 45 min by addition of ethanol and was followed by thoroughly washing with water.

For periodate oxidation, different volumes (between 1 and 150 mL) of 0.2 M aqueous sodium metaperiodate (NaIO₄) solutions were added to the cellulose sample (2 g suspended in 100 mL of distilled water). The suspension was stirred at room temperature for 30 min, filtered, and then resuspended in 100 mL of distilled water. A total of 5 mL glycol was added, and the stirring was continued for 1 h. The pulp was separated by filtration and washed thoroughly on a Büchner funnel.

Handsheets were prepared from 2 g of pulp suspended in distilled water (500 mL) on a Büchner funnel, followed by pressing. In some experiments, the pH of water was modified using either sulfuric acid (1 mM) or sodium hydroxide (1 mM). The handsheets were dried at 92 °C for 5 min. Brightness was measured before and after aging according to ISO 2470 (2009), following the remission of UV/Vis light at 457 nm. Aging was carried out continuously under dry conditions following the TAPPI method UM 200 (105 °C, 40% rel. humidity, 4 h), and under humid conditions according to Paptac E.4P (100 °C, 100% rel. humidity, 1 h). The progress was continuously followed by UV/Vis (brightness reversion) measurements to record the kinetics of chromophore formation.

Chlorine dioxide bleaching (1% ClO₂, 50 °C if not otherwise stated, 1 h) was run in double-sealed plastic bags in water baths. The impregnate pulps (50 g) were mixed with water and ClO₂ solution to 10% stock consistency and the suspension was kneaded before incubation of the sealed bags in the water bath. In some cases, the added water contained certain amounts of metal ions (3 ppm for transition metal ions and up to 200 ppm for main group metal ions, see main text). After bleaching, a pulp aliquot (5 g) was taken for handsheet production, washed with distilled water or with water containing certain amounts of metal ions (3 ppm for transition metal ions and up to 200 ppm for main group metal ions, see main text). The pulp was then disintegrated with water to 1% stock consistency and filtered directly or after setting the pH of the suspension to pH 6 by addition of 10 mM NaOH. The mixtures were filtered through a Büchner funnel and the remainder dried to obtain the handsheets.

An aliquot of a saturated solution of DHBQ in distilled water containing 150 µg or 500 µg of DHBQ was mixed with 100 mL of water and added to the dry pulp, so that a content of 3 and 10 ppm, respectively, was set. The pulp was kneaded for about 5 min, in some cases distilled water was added to facilitate intimate mixing. A pulp aliquot of 5 g was taken and the ISO brightness determined, affording 81% ISO (3 ppm DHBQ added) and 75% ISO (10 ppm DHBQ added). The remaining pulp was used for the ClO₂ bleaching experiments as given above.

In a 500 mL round-bottom flask, freshly prepared chlorine dioxide solution (concentration recently determined) was added at once to 100 mL of an aqueous DHBQ solution (pH 3, set by sulfuric acid or 0.1 M phosphate buffer), and the mixture was stirred at 50 °C in the dark. Reaction temperatures were varied for kinetic measurements as described in the main text. The amount of chlorine dioxide solution was set in a way that a target DHBQ/ClO₂ molar ratio was reached. At certain time intervals, 5 mL aliquots were taken and extracted twice with 1 mL of CDCl₃. The two extracts were combined and analyzed by NMR either directly or after filtration through a pipette (5 cm path) filled with solid anhydrous MgSO₄. Care was taken that the time between taking the aliquot and NMR analysis was less than 30 s (45 s when filtered). The filtered extract was also used for GC/MS analysis. In some cases, a drop of N,O-bis(trimethylsilyl)-trifluoroacetamide (BSTFA) was added prior to NMR and GC/MS runs.

After a reaction time of 2 h, a 50 mL aliquot was analyzed for CO₂ according to standard procedures (precipitation as BaCO₃). The remaining solution was extracted and analyzed (GC/MS, NMR) for remaining organic compounds according to a procedure previously optimized for carboxylic acid and hydroxyacids in complex matrices (Liftinger et al. 2015).

¹H NMR (CDCl₃): δ 6.12 (s, 1H), 7.20 ppm (s, br, OH). ¹³C NMR (CDCl₃): δ 112.2 (CH), 144.8 (d.i., C-1, C-5), 146.5 (d.i., C-2, C-4), 154.1 ppm (C-3). Analysis as per-trimethylsilyl derivative: ¹H NMR (CDCl₃): δ 0.18 (s, 9H, 1 × SiMe₃), 0.21 (s, 18H, 2 × SiMe₃), 0.25 (s, 18H, 2 × SiMe₃), 6.08 ppm (s, 1H). MS data given in Table 4.

¹H NMR (CDCl₃): δ 5.94 (s, 1H), 9.8 ppm (s, br, 3H, OH). ¹³C NMR (CD₃OD): δ 108.3 (C-6), 142.4 (C-2), 148.8 (C-3), 152.0 (C-5), 180.4 (C-1), 182.9 ppm (C-4).

Measurement in D₂O/NaOD, pD = 10, assignment confirmed by spiking with authentic samples. ¹³C NMR: δ 144.3 (br, carbonate), 162.0 ppm (oxalate).

The GAUSSIAN 09 program package was used for all calculations (Frisch et al. 2016). The geometry optimization was carried out at the DFT(M06-2X) level of theory (Zhao and Truhlar 2008). The 6-31G(d) basis sets were employed for H, C, O and Cl, where a diffuse function was added to each of O and a p-polarization function was added to H (these basis sets being named BS-I). It was ascertained that each equilibrium geometry exhibited no imaginary frequency and each transition state exhibited one imaginary frequency. Enthalpy, entropy, and Gibbs energy changes were evaluated at 323.15 K (50 °C). Zero-point energy, thermal energy, and entropy change were evaluated at the DFT(M06-2X)/BS-I level. For the water molecule, experimental entropy of water at 298.15 K and 1 atm (16.7 cal/mol K) was employed for the estimation of Gibbs energy.

For single point calculations, either CCSD(T), MP4(SDQ), or DFT(M06-2X) method was employed. The CCDS(T) and MP4(SDQ) methods were mainly employed for the radical and ionic species, respectively. In the CCSD(T) calculations for radicals, restricted open-shell wave functions (ROHF functions) were selected as the reference wave functions, as the unrestricted wave functions (UHF functions) were significantly spin-contaminated. For large species, such as DHNQ-ClO₂ complexes, we employed the DFT(M06-2X) method. In the single point calculations at the CCSD(T) and the DFT(M06-2X) levels, 6-311G(d) basis sets (BS-II) were employed for H, C, O and Cl, where a diffuse function was added to each of O and Cl and a p-polarization function was added to H. For the MP4(SDQ) calculations, we employed other basis sets: the aug-cc-pVTZ basis sets were used for O and Cl and the cc-pVTZ basis sets were selected for C and H, where f-type and d-type functions were omitted from the heavy atoms and hydrogen, respectively, to reduce computational costs (BS-III).

In all calculations, the solvation energy in water was evaluated according to the PCM method. For the determination of the cavity size in the PCM calculations, the UFF parameters and the united atom topological model optimized at the HF/6-31G(d) level of theory were used for geometry optimization and energy evaluation, respectively.