The structure and interactions of water species in hydrated Nafion membranes as a function of water content were investigated on the basis of medium-infrared spectral analysis and molecular dynamics (MD) simulations. The spectral decomposition of the FT-IR data in the stretching OH region was performed on different levels of hydration of the sulfate functional groups (lambdaH2O/RSO3- = 2-22). Quantum mechanical calculations of two model systems [perfluoroethanesulfonic acid/(H2O)6 cluster] and a [perfluorobutanesulfonic acid/(H2O)6 crystal] were carried out in order to account for the band assignments of Nafion in the stretching OH region (2500-4000 cm-1). Our findings indicated that the secondary structure of water species in Nafion can be accurately explained in terms of our reactive force field for water. The distinction between "surface" and "bulk" water contributions in Nafion membrane pores is proposed along with a quantitative estimate of the different types of OH groups present in the system. The average pore size was calculated and supported by the spectral results.

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A series of C60 derivatives substituted with the nitroxide group 2,2,6,6-tetramethylpiperidine-1-oxyl have been synthesized. A detailed EPR and ENDOR study of the neutral radicals and of their reduction products is reported. All the neutral species give EPR spectra consisting of a main triplet of lines with ∼15 G splitting by the 14N nucleus, typical of nitroxide radicals. Some of them show additional well-resolved splittings by methyl and methylene protons, which are discussed in relation to the conformation of the nitroxide ring. When the compounds are progressively reduced under vacuum by contact with an alkali metal mirror, new EPR lines are observed; a 1:1:1 triplet with a splitting constant half that of a nitroxide radical and g = 2.0030 is attributed to biradical anions where one electron is located in the fullerene moiety and experiences a strong exchange coupling with the nitroxide unpaired electron. Frozen solution spectra of these species allow the determination of the electron−electron dipolar interaction parameters which are compared with theoretical MO calculations. The experimental values agree with a spin distribution of one unpaired electron mostly in the equatorial plane of the fullerene. Another single line which appears in the spectrum as the reduction proceeds further on (g = 1.9999) is attributed to a new species:  possibly a substituted fullerene radical anion in which the nitroxide group is irreversibly reduced.

Poly(2,5-benzimidazole) (AB-PBI) membranes are investigated by studying the FT-Raman signals due to the benzimidazole ring vibration together with the C-C and C-H out-of-and in-plane ring deformations. By immersion in aqueous ortho-phosphoric acid for different time periods, membranes with various doping degrees, i.e. different molar fractions of acid, are prepared. The chemical-physical interactions between polymer and acid are studied through band shifting and intensity change of diagnostic peaks in the 500-2000 cm À1 spectral range. The formation of hydrogen bonding networks surrounding the polymer seems to be the main reason for the observed interactions. Only if the AB-PBI polymer is highly doped, the Raman spectra show an additional signal, which can be attributed to the presence of free phosphoric acid molecules in the polymer network. For low and intermediate doping degrees no evidence for free phosphoric acid molecules can be seen in the spectra. The extent of the polymer-phosphoric acid interactions in the doped membrane material is reinvestigated after a period of one month and the stability discussed. Our results provide insight into the role of phosphoric acid as a medium in the conductivity mechanism in polybenzimidazole.

The sign of the exchange interaction J in a series of radical triplet pairs (RTPs), formed by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together by covalent bonds in different geometries. It is shown that the sign of J depends on the overlap between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly occupied in the excited triplet state. When the overlap does not vanish, a negative contribution to J arises from the admixing of a charge transfer structure in the wave function of the excited doublet state D of the RTP, which does not take place in the excited quartet state Q. The mixing of D* and Q' states lowers the energy of the former spin state and gives antiferromagnetic coupling.