The structure and bonding of the azo dye Orange II (Acid Orange 7) in parent and reduced forms have been studied using NMR, infrared, Raman, UV-visible, and electron paramagnetic resonance (EPR) spectroscopy, allied with density functional theory (DFT) calculations on three hydrazone models (no sulfonate, anionic sulfonate, and protonated sulfonate) and one azo model (protonated sulfonate). The calculated structures of the three hydrazone models are similar to each other and that of the model without a sulfonate group (Solvent Yellow 14) closely matches its reported crystal structure. The 1H and 13C NMR resonances of Orange II, assigned directly from 1D and 2D experimental data, indicate that it is present as > or = 95% hydrazone in aqueous solution, and as a ca. 70:30 hydrazone:azo mixture in dimethyl sulfoxide at 300 K. Overall, the experimental data from Orange II are matched well by calculations on the hydrazone model with a protonated sulfonate group; the IR, Raman, and UV-visible spectra of Orange II are assigned to specific vibrational modes and electronic transitions calculated for this model. The EPR spectrum obtained on one-electron reduction of Orange II by the 2-hydroxy-2-propyl radical (*CMe2OH) at pH 4 is attributed to the hydrazyl radical produced on protonation of the radical anion. Calculations on reduced forms of the model dyes support this assignment, with electron spin density on the two nitrogen atoms and the naphthyl ring; in addition, they provide estimates of the structures, vibrational spectra, and electronic transitions of the radicals.

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PFG NMR results are reported on H 2 O, PEG200, PEG1500, PEG8000, and PEG20000 in wet cotton fibers and H 2 O in wet cotton linters. The data are analyzed in terms of a two-site exchange model (water/cotton) and show that the probe molecules in fibers are trapped in cages. The cage size decreases from 10 to 2 µm as the probes' size increases from 0.23 to 9.2 nm, although the overall accessible volume only decreases from 50 to 20-30%. This behavior may be explained by size-exclusion effects on the connectivity and accessibility. Analysis of the diffusion coefficients at short diffusion times indicates that fiber cages are water pools held between the growth rings of the fiber. In linters these cages do not occur, and a freely diffusing signal from H 2 O in the amorphous region is observed with D ) (4.5-8.9) × 10 -11 m 2 s -1 , which when compared to the predicted value from the microviscosity of the amorphous regions gives a tortuosity of 2-4. Exit of all probes from linters and fibers takes 0.2 s and requires hydrogen bonds to be broken with an activation energy of 50 kJ mol -1 .

To understand the factors controlling the motion of molecules contained within the nanopores of dry cotton, an electron paramagnetic resonance spin probe study of TEMPOL (4-hydroxy-2,2,6,6-tetramethyl-1piperidinyloxy) nitroxide radicals in dry cotton was conducted. Spectra were recorded between 10 and 70 °C with TEMPOL loadings from 6 × 10 -5 to 5 × 10 -2 mol kg -1 . At low loadings (<3 × 10 -3 mol kg -1 ), the radicals are in an equilibrium between a mobile state in the amorphous pores and adsorbed onto the crystallites. The temperature dependence of the equilibrium constant, K, was measured, and from this, the enthalpy and entropy of the adsorption sites were obtained. These show a remarkable loading dependence, ∆H 0 dropping from 75 to 50 kJ mol -1 and ∆S 0 from 0.25 to 0.20 kJ mol -1 K -1 between loadings of 2 × 10 -4 and 3 × 10 -3 mol kg -1 . The viscosity of the amorphous region, obtained from the rotational correlation time of the mobile radicals, shows an analogous dependence on loading rising from 20 to 35 cP between 2 × 10 -4 and 3 × 10 -3 mol kg -1 at 298 K. The effects are indicative of microdomains of different character with the preferred microdomains being those with pores of equivalent size to TEMPOL (d ≈ 1 nm). There are approximately 4 × 10 -4 mol kg -1 of these pores. The absolute values of the viscosity, ∆H 0 and ∆S 0 , are discussed in terms of the structure and interactions in cotton. At high loadings (>3 × 10 -3 mol kg -1 ), aggregation of the radicals occurs, which is well-modeled using the Poisson distribution and the assumption that aggregation occurs when the number of radicals in a microdomain is double the number of pores.

The intermolecular interactions of the bis-azo dye Direct Blue 1 (Chicago Sky Blue 6B) have been studied as a function of concentration in aqueous solution and in cellophane using UV-visible absorption, NMR, and resonance Raman spectroscopy. UV-visible spectroscopy indicates that dimerization occurs in aqueous solution (K dim ≈ 77 000 dm 3 mol -1 at I ) 0.01) and that it occurs more readily at higher ionic strength, where trimerization also occurs (K dim ≈ 580 000 dm 3 mol -1 and K trim ≈ 2700 dm 3 mol -1 at I ) 0.1); the driving force is enthalpic rather than entropic (∆H dim ≈ -53 kJ mol -1 and ∆S dim ≈ -90 J K -1 mol -1 at I ) 0.01). Dimerization occurs much less readily in cellophane (K dim ≈ 42 dm 3 mol -1 ) than in aqueous solution, indicating that strong dye-cellulose interactions compete effectively with dye-dye interactions. NMR spectroscopy indicates that Direct Blue 1 molecules interact by π-stacking at the central biphenyl group, while resonance Raman spectroscopy indicates that the internal structure and bonding of the monomers is essentially retained on stacking. The UV-visible spectra are consistent with this interpretation, and the application of exciton theory indicates that stacking results in angles between adjacent molecules which are different in the dimer (θ ≈ 84°) and trimer (θ ≈ 58°); they are attributed to geometries which minimize the repulsion between charged naphthylsulfonate groups.

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Spin probe EPR experiments using TEMPO and TEMPOL are reported on dry cotton. The spectra are in the slow tumbling region with τrot ∼ 1 ns, the line width is found to be affected by Heisenberg spin exchange and aggregation of the radical. To probe the entire amorphous zone of cotton, ethanol and water are used to deposit the radicals; to probe the fiber surface specifically toluene was used. Analysis shows that in both regions the viscosity is 30 cP and that the fiber surface is an amorphous layer with a depth of approximately 2 nm. The radical motion follows normal liquid theory with a diffusion rate constant of 2 × 108 mol-1 L s-1. A small proportion of the radicals deposit to an area of much higher viscosity. The classical spin probe method is extended via a simple electron spin polarization experiment. This involved irradiation of the sample and analysis of the radical−triplet mechanism polarization of TEMPOL/O2, which indicates 1O2 diffuses with a similar rate constant.

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