Abstract. The very interesting magnetic properties of frustrated magnetic molecules are often hardly accessible due to the prohibitive size of the related Hilbert spaces. The finite-temperature Lanczos method is able to treat spin systems for Hilbert space sizes up to 10 9 . Here we first demonstrate for exactly solvable systems that the method is indeed accurate. Then we discuss the thermal properties of one of the biggest magnetic molecules synthesized to date, the icosidodecahedron with antiferromagnetically coupled spins of s = 1/2. We show how genuine quantum features such as the magnetization plateau behave as a function of temperature.

The effect of water content on the physicochemical properties of the amorphous regions in cotton were investigated by measuring the electron paramagnetic resonance (EPR) of TEMPOL nitroxide radicals, deposited in cotton at different loadings, as a function of the relative humidity (RH) and temperature. Three different components contribute differently to the experimental EPR spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous region, (b) slow moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. These components were analyzed by means of computer-aided simulations of the line shapes and simplified line width methods. Polarity and mobility parameters were extracted from the analysis of the spectra. For all loadings and temperatures, the polarity suddenly dropped when the water content fell below approximately 3 wt %, i.e., when water was removed from the bulk amorphous regions. At the lowest loading (2 x 10(-5) mol kg(-1)), the spectra were independent of the RH, and only mobile radicals were observed. At intermediate loading (10(-4)-10(-3) mol kg(-1)) both mobile (fast) and adsorbed (slow) moving radicals were present, the fraction of which depended on the RH. The mobility of the adsorbed and mobile radical signals was smaller at higher loadings, indicating microdomains of different character. The temperature dependence of the rotational correlation times provided the activation energies, which were much lower than in liquids. An equilibrium exists between the mobile and the adsorbed radicals. The temperature dependence of the equilibrium constant, K, gave the enthalpy and the entropy of the adsorption process. At low RH, the enthalpy and the entropy values indicated a simple adsorption process. At 10(-3) mol kg(-1), the values were independent of the RH, but at low loadings the values increased with the increase in the RH, which suggested a displacement of adsorbed water by the radicals at high water content. At loadings above 10(-3) mol kg(-1), signals from radicals strongly interacting via spin exchange were observed, which are assigned to aggregated radicals; simulation of the spectra gave an activation energy of 13 kJ mol(-1) for the spin exchange process. These effects are rationalized on the basis of microdomains of different character within cotton, reflecting the variation in pore sizes (0.5-8 nm) and the relaxation behavior of the cellulose chains.

To understand the effect of charge on spin probe interactions and dynamics in the nanopores of cotton, an electron paramagnetic resonance (EPR) study of 4-amino-TEMPO (T-NH 2 ) and 4-carboxy-TEMPO (T-COOH) nitroxide radicals deposited on dry cotton has been conducted. Spectra are recorded as a function of probe loading (2 × 10 -5 to 10 -2 mol kg -1 ), temperature (293-333 K), and relative humidity (RH, 6 and 75%) and compared to results from the neutral probe 4-hydroxy TEMPO (T-OH). The effective internal pH of the cotton was measured using codeposited indicator dyes, which showed that both probes were charged, T-COO -and T-NH 3 + , at loadings up to and including 10 -3 mol kg -1 . Three components contribute to the spectra, corresponding to (a) mobile radicals absorbed in the bulk amorphous regions, (b) slowly moving radicals adsorbed on the crystallite surfaces in cotton, and (c) aggregated radicals. Independent of loading, the mobile T-NH 3 + experience a highly polar aqueous-like environment at 75% RH, which became much less polar at 6% RH as water was removed from the cotton. An analogous but much smaller drop in polarity was found for T-COO -, ascribed to the higher hydrogen-bonding ability of the probe, binding water more effectively. The rotational correlation time of mobile T-NH 3 + and T-COO -radicals had much larger activation energies than T-OH and different loading dependencies. This was ascribed to interactions with the 5.3 mmol kg -1 of carboxylic acid groups in cotton, so that the charged probes monitor different aspects of the cellulose chain relaxation. The enthalpy and entropy of the adsorption equilibrium between mobile and adsorbed radicals was extracted at 10 -3 mol kg -1 . Values were independent of humidity except for T-COO -, because of its stronger interactions with water. Both the T-NH 3 + and the T-COO -probes preferentially adsorb on less constricted binding sites than T-OH. At 10 -2 mol kg -1 , the carboxy probe becomes predominately protonated, T-COOH, radically changing the spectra observed, with a much greater fraction of mobile radicals. For the amino probe, the spectra at 10 -2 mol kg -1 are dominated by aggregated radicals.