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A new and easy method for preparing blue sodalite pigments which involves high-temperature calcination of sodalite samples synthesized with aluminum sulfate and an organic template, is presented. Calcination generated the S(3)(-) and S(2)(-) radicals, and the effects of the Al/Si ratio and the calcination temperature on the nature and amounts of the radicals were examined. The radicals were characterized in detail by continuous wave and pulsed EPR at X- and W-band frequencies (approximately 9 and 95 GHz, respectively) complemented by UV-vis measurements. The high-field electron-paramagnetic resonance (EPR) measurements allowed us to clearly resolve the g anisotropy of S(3)(-) and W-band electron nuclear double resonance (ENDOR) measurements detected strong coupling with extra-framework (23)Na cations and weak coupling with framework (27)Al. On the basis of the spectroscopic results and density functional theory (DFT) calculations of the g-tensors of S(3)(-) and S(2)(-) radicals, the EPR signals were attributed to three different radicals, all with the open structure C(2v), that are located within the sodalite beta cages. While two of these radicals are well isolated, the third one is associated with an exchange-narrowed signal originating from S(3)(-) radicals in nearby sodalite cages.

Abstract:The complexes of copper with histidine exhibit a wide variety of coordination modes in aqueous solution. This stems from the three potential coordination sites of the histidine molecule and the existence of mono-and bis-complexes. The present work concentrates on the determination of the carboxylate binding mode, via the 13 C hyperfine coupling of the carboxyl, in a number of copper complexes in frozen solutions. These are then used as references for the determination of the coordination mode of two zeolite encapsulated complexes. The 13 C hyperfine coupling (sign and magnitude) was determined by a variety of advanced pulsed EPR and electron-nuclear double resonance (ENDOR) techniques carried out at conventional and high magnetic fields. These showed that while the carboxyl 13 C isotropic hyperfine coupling of an equatorially coordinated carboxylate is negative with a magnitude of 3-4 MHz, that of a free carboxylate is small (∼1 MHz) and positive. To rationalize the experimentally determined ligand hyperfine couplings ( 1 H and 13 C) and further understand their dependence on the coordination mode and degree of protonation, density functional theory (DFT) calculations were carried out on a number of model complexes, representing the different Cu-histidine complexes studied experimentally. The exchange-correlation functional used for the calculation of the EPR parameters was B3LYP with triple-plus polarization (TZP) quality basis sets. While the polarization agreement between the magnitudes of the calculated and experimental values varied among the various nuclei, sometimes exhibiting deviations of up to 40%, an excellent agreement was found for the sign prediction. This shows the unique advantage of combining high field ENDOR, by which the sign of the hyperfine can often be determined, with DFT predictions for structure determination.

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