Published: December 01, 2011 r2011 American Chemical Society 174 dx.doi.org/10.1021/jp211082t | J. Phys. Chem. A 2012, 116, 174–184 ARTICLE pubs.acs.org/JPCA EPR Analysis and DFT Computations of a Series of Polynitroxides M. Francesca Ottaviani,* ,† Alberto Modelli, ‡ Olaf Zeika, § Steffen Jockusch, § Alberto Moscatelli, § and Nicholas J. Turro § † Department of Earth, Life and Environment Sciences (DiSTeVA), University of Urbino, 61029 Urbino, Italy ‡ Department of Chemistry “G. Ciamician”, University of Bologna, 40126 Bologna, Italy, and Centro Interdipartimentale di Ricerca in Scienze Ambientali (CIRSA), 48123 Ravenna, Italy § Department of Chemistry, Columbia University, New York, New York 10027, United States b S Supporting Information ’ INTRODUCTION Organic polyradicals are of critical importance in organic magnetism, 13 molecular charge transfer, 4 and multiple spin labeling in structural biology. 5 The basis for bulk and molecular magnetic phenomena is the electron spinspin exchange cou- pling between unpaired electrons localized on different centers. The correlation of this exchange coupling with structural param- eters of the system of interest is a long-standing research topic. 13 Among organic radicals, nitroxides have the advantage that they are stable under ambient conditions and can be easily synthe- sized, manipulated, and derivatized. Polynitroxides have shown improved properties with respect to mononitroxides as organic ferromagnets, contrast agents in nuclear magnetic imaging, labels in electron magnetic resonance imaging, and radiation protectors during whole brain radiotherapy. 6 When used as electron spin agents for dynamic nuclear polarization (DNP), polynitroxides can enhance the sensitivity of nuclear magnetic resonance (NMR) signals by orders of magnitude. 7 The intramolecular spin exchange in polynitroxides has been investigated extensively, and a wide literature exists about the theory and analysis of their electron paramagnetic resonance (EPR) spectra. 13,812 Such exchange coupling can be either ferromagnetic or antiferromagnetic, depending upon the topol- ogy and conformation of the coupling pathway connecting the radicals. 13,13,14 Rigid polynitroxide scaffolds, which are too constrained to allow intramolecular nitroxide collisions, can show spin exchange coupling by through-bond and/or through-space mechanisms. 15 In general, for rigid polynitroxides with an aro- matic backbone, the exchange coupling is mediated more effec- tively by through-bond interactions, especially through a cross- conjugated π-system. 13,15 However, in solution, a through-space spinspin exchange may be promoted by the solvent and other molecules in the nitroxide microenvironment. 12 Most of the studies reported in the literature demonstrate the importance of coupling the EPR analysis of polynitroxides with structural and geometric analysis, such as X-ray diffraction analysis and computational studies. Recently, we reported a synthetic method based on nucleophilic aromatic substitution on electron-deficient aromatic compounds which yielded a series of polynitroxides exhibiting strong electron exchange between the radicals. 16 We also reported single crystal X-ray diffraction analysis and basic EPR spectra of these polynitroxides. In the present study we performed a computer-aided analysis of the EPR spectra, following a simplified procedure, for the series of polynitroxides shown in Scheme 1. These radicals were selected on the basis of different structural features, such as the Received: July 6, 2011 Revised: December 1, 2011 ABSTRACT: Polynitroxides with varying numbers of nitroxide groups (one to four) derived from different aromatic core structures show intramolecular electron spinspin coupling. The scope of this study is to establish an easy methodology for extracting structural, dynamical, and thermodynamical information from the EPR spectra of these polynitroxides which might find use as spin probes in complex systems, such as biological and host/guest systems, and as polarizing agents in dynamic nuclear polarization (DNP) applications. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level provided information on the structural details such as bond lengths and angles in the gas phase, which were compared with the single crystal X-ray diffraction data in the solid state. Polarizable continuum model (PCM) calculations were performed to account for solvent influences. The electron paramagnetic resonance (EPR) spectra of the polynitroxides in chloroform were analyzed in detail to extract information such as the percentages of different conformers, hyperfine coupling constants a, and rotational correlation times τ c . The temperature dependence on the line shape of the EPR spectra gave thermodynamic parameters ΔH and ΔS for the conformational transitions. These parameters were found to depend on the number and relative positions of the nitroxide and other polar groups.