H. van Willigen

Fourier transform–electron paramagnetic resonance spectroscopy of correlated radical pairs

Utilizing the high time and phase resolution of Fourier transform electron paramagnetic resonance, the buildup and decay kinetics of the transient radical pair Zn tetraphenylporphyrin+/Duroquinone− in ethanol could be studied by detecting the dispersive component in the Fourier transformed free induction decays of the fully separated radical anions. The dispersive line components originate from the exchange coupling J of the radical pair in its metastable state trapped by an attractive temperature dependent Coulomb interaction U(rc,T) at an equilibrium distance rc. The decay rate constant kd into free ions was studied over the temperature range of 225–298 K and varied from kd=3.2×106 to 10×106 s−1. This change in kd could be fully rationalized in terms of the temperature dependence of the solvent dielectric constant (leading to a variation of the cage potential from 0.07 to 0.12 eV) and the diffusion constant. The buildup rate constants were consistent with the assumption of a diffusion‐controlled electro...

ESR Study of the Dimer Cation of Coronene

Treatment of coronene (c), dissolved in SO2, with BF3 affords a variety of ESR spectra depending on the BF3/c concentration ratio and the temperature. To account for the spectra, the formation of monomer and dimer cation species (c+ and c2+) is postulated. Possibly, a trimer cation (c3+) is also formed at relatively high concentrations of coronene. The spectral features are discussed in terms of current theories of ESR spectroscopy.

Quadrupole effects on the hyperfine structure of a gold (II) complex

Abstract Single crystal ESR spectra of Au(II) diethyldithiocarbamate show peculiar features ascribed to the effect of quadrupole interaction. Similar “anomalous” spectra are expected for other Au(II) compounds, since Au combines a small magnetogyric ratio with a relatively large quadrupole moment.

Fourier transform–electron spin resonance study of spin polarization generated by triplet‐ and radical‐pair mechanisms in a photoinduced electron transfer reaction

Fourier transform–electron spin resonance (FT–ESR) was utilized to measure the photoinduced charge transfer in the system Zn tetraphenylporphyrin/duroquinone with a time resolution of 10 ns. The separate observation of the intensity/time profile of different hyperfine lines in the spectrum, which was made possible by application of FT–ESR, enabled the independent determination of transient acceptor radical polarization originating from the triplet‐ and radical‐pair mechanism. The observed temperature dependence and the absolute value of the electron transfer rate lead to the conclusion that the process is diffusion limited with no indication of an activation barrier.

An E.S.R. study of the mono and dinegative ions of triphenylene. Evidence for the Jahn-Teller instability of the triplet dianion

The three types of fluid solution E.S.R. spectra generated by the mononegative ions of triphenylene could be assigned satisfactorily to the free anion and two different ion pairs, respectively. The available experimental data suggest that one of these ion pairs is solvent separated, whereas the other is a contact ion pair. These two different ion pairs are in rapid equilibrium with each other. The thermodynamic quantities characterizing this equilibrium have been derived using the observed temperature dependence of the alkali splitting. Rigid matrix E.S.R. spectra of the dinegative ions of triphenylene prove that the dianion has a triplet ground state in accordance with Hunds rule. In diglyme, a solvent with high solvating properties, the matrix spectra are characteristic for triplet species with trigonal spin distribution. These spectra could be assigned to the free dianion. In MTHF and mixtures of MTHF with more polar solvents the matrix spectra reveal the existence of two triplet species without trigo...

Alkali metal NMR experiments on alkali-aromatic radical ion pairs in solution

Abstract 23 Na, 85 Rb and 87 Rb NMR experiments prove the existence of positive and negative spin densities at alkali metal nuclei in alkali-aromatic radical ion pairs. Line-width measurements show that the width of the 85 Rb NMR line is determined by quadrupolar relaxation, while for 87 Rb and 23 Na also dipolar mechanisms play a significant role.

Interpretation of ESR Experiments on the Triplet Dianions of 1,3,5‐Triphenylbenzene and Triphenylene

A theoretical account is given for the ESR experiments on the triplet dianions of 1,3,5‐triphenylbenzene (Tpb) and triphenylene (Tp). The calculated ZFS parameters of the unperturbed dianions agree nicely with the experimental values obtained in strong solvating solvents, e.g., diglyme. The triplet spectra obtained in less solvating solvents like MTHF are attributed to the dianions perturbed by two counterions (Tpb2−·2Me+ and Tp2−·2Me+). The ZFS parameters and the energy of the lowest triplet and singlet states have been calculated as function of the counterion positions. Comparing the results of our calculations with experiments, we propose that in Tp2−·2Me+ the counterions are above and below the center of the same noncentral ring. In Tpb2−·2Me+, on the other hand, all experimental and theoretical results suggest strongly that the counterions are on the threefold axis of Tpb. In both cases the cation–aromatic plane distance is estimated between 2 and 3 A.