A. H. Reddoch

The solvent effect on di‐tert‐butyl nitroxide. A dipole–dipole model for polar solutes in polar solvents

New measurements of the solvent effect on the nitrogen hyperfine coupling constant of di‐tert‐butyl nitroxide are reported. These, together with literature data, are used to test various models for the solvent effect. At the Huckel level of approximation, aN is a linear function of the applied electric field. Thus various reaction field theories may be considered. The widely used Onsager reaction field does not account for the effects of the more polar solvents or for the differences between polar and nonpolar solvents. The Wertheim and Block–Walker reaction fields are better, especially for very polar solvents. However none of these continuum reaction fields is entirely satisfactory theoretically or experimentally. We propose a dipole–dipole model for polar solvents which is superior to the continuum models. From the dipole–dipole model, we suggest that the quantity μρ/M is a convenient linear parameter for polar solvent effects, the factors being solvent dipole moment, density, and molecular weight. The...

Electrolytic Preparation and EPR Study of Crystalline Pyrene, Perylene, and Azulene Perchlorates

The preparation of crystalline pyrene, perylene, and azulene perchlorates is reported. Details of the novel method, growth on the anode in an electrolytic cell, are given. Crystalline samples of each compound give EPR resonances with very narrow linewidths, about 0.1 G. The resonances from the pyrene and perylene salts each consist of two components. One follows a Curie law and dominates at low temperature. It is attributed to “impurities,” probably lattice defects. The other component, dominating at high temperature, arises from thermally generated Wannier triplet excitons. The excitation energies are 0.18 and 0.27 eV for the pyrene and perylene salts, respectively. The azulene salt yielded only the “impurity” signal within the available temperature range.

Systematic Perturbations of the EPR Spectra of Anthracene and Azulene Anions in Solution

Experimental measurements of the hyperfine coupling constants of anthracene and azulene anions with various solvents and cations are presented. It is shown that the proton coupling constants depend on solvent, cation, concentration, and temperature and that variations of these constants of about 10% can occur. The variation of the proton coupling constants is shown to be systematic and is ascribed to the electrostatic perturbation of the cation. Calculations are presented to support this view. Most of the systems studied are classified as ion pairs, free anions, or equilibrium mixtures on the basis of qualitative features of their EPR spectra. The absence of hyperfine coupling from the counterion does not necessarily indicate that the anion is unassociated.

Effects of Ion Pairing on the EPR Spectrum of the Naphthalene Anion in Solution

Data are presented for the proton and cation hyperfine coupling constants and g values of the naphthalene anion as a function of solvent, cation, and temperature. The variation of the proton coupling with solvent and cation is comparatively small and somewhat irregular. The temperature coefficient of the proton coupling constants depends upon the cation. Cation coupling constants are reported for the five alkali ions, as well as the temperature dependence in some cases. The g value of ion pairs containing Rb or Cs differ considerably from that of the free anion, and is temperature dependent. These observations are discussed qualitatively in terms of the formation and structure of ion pairs, and of the effects of the electrostatic field and spin–orbit coupling of the cation.

An EPR study of triplet excitons in crystalline anthracene–tetracyanobenzene: Exciton dynamics, long‐range order parameter, and critical exponent

We report new measurements of the EPR spectra of triplet excitons in the charge‐transfer complex anthracene–tetracyanobenzene. The spectra show exchange phenomena arising from exciton motion over a lattice undergoing an order–disorder phase transition. The interpretation of these spectra requires a four‐site exchange model from which we estimate the rates of interchain and intrachain motion for the excitons, as well as the temperature dependence of the long‐range order parameter and the critical exponent β. This value β=0.34, the first for an order–disorder transition in a molecular crystal, is in agreement with the predictions of renormalization group theory.We report new measurements of the EPR spectra of triplet excitons in the charge‐transfer complex anthracene–tetracyanobenzene. The spectra show exchange phenomena arising from exciton motion over a lattice undergoing an order–disorder phase transition. The interpretation of these spectra requires a four‐site exchange model from which we estimate the rates of interchain and intrachain motion for the excitons, as well as the temperature dependence of the long‐range order parameter and the critical exponent β. This value β=0.34, the first for an order–disorder transition in a molecular crystal, is in agreement with the predictions of renormalization group theory.

A Raman study of the influence of isotopic substitution on the phase change in naphthalene-tetracyanobenzene

Abstract The temperature dependence of the low frequency modes of naphthalene- h 8 and naphthalene- d 8 tetracyanobenzene has been studied by Raman scattering. Abrupt changes in the band parameters occur, indicating the crystals undergo a phase change at 69 K and 62 K respectively, i.e. deuteration of the donor decreased the transition temperature.

EPR Spectrum and Heat of Dissociation of the Perylene Dimer Cation

The EPR spectrum of the perylene dimer cation, obtained by dissolving (perylene2)+ ClO4− in CH2Cl2 and in CH3CN, is reported. The proton coupling constants in CH2Cl2 are 1.913, 1.492, and 0.212 G, while the g value at − 80°C is 2.002601. The heat of dissociation in CH2Cl2 is estimated to be 8.6 kcal/mole.

Vibrational Effects on the Hyperfine Interactions of Aromatic Radicals in Solution

Measurements of the temperature dependence of the proton hyperfine coupling constants of a number of aromatic radicals in solution together with some measurements of the proton‐to‐deuteron coupling constant ratio are reported. These and data from the literature show the effects of out‐of‐plane CH vibrations. A relationship between the two types of measurements is demonstrated. Such vibrations lead to a small hyperfine coupling between the proton and spin densities on carbons adjacent to the one to which it is bonded. The implications of this effect are discussed.

Anomalous Effects in EPR Spectra of the Azulene Anion

The proton hyperfine coupling constants of the azulene anion in dimethoxyethane with lithium ions show systematic variations of the order of 10% as temperature and concentration are changed. No splitting is observed from the lithium ion. As the concentration decreases, the variations of the five proton coupling constants are linearly related and the coupling constants approach those obtained by Bernal, Rieger, and Fraenkel for azulene in dimethylformamide with tetra‐n‐propyl ammonium ions. As the temperature is lowered, the coupling constants change rapidly until by about —40°C they are close to those of Bernal et al. At lower temperatures the change is small. At —40°C the peak heights of the lines are those predicted by the degeneracies of the nuclear spin states. At higher temperatures the peak heights become anomalous and the widths of the lines vary from line to line.These effects are explained by a model involving a rapid chemical equilibrium between two states of the azulene—lithium ion system, havi...

Molecular dynamics and the phase transition in the naphthalene–tetracyanobenzene charge‐transfer complex as studied by 1H NMR and triplet state EPR

1H NMR second moments and static and rotating frame relaxation time measurements were performed on the crystalline naphthalene–tetracyanobenzene charge‐transfer complex between ∼50 and 150 K. The results are consistent with a model where the random jumping of naphthalene molecules between equivalent orientations becomes nonrandom below the phase transition at 72±1 K. Below the transition the two orientations must be inequivalent, with the molecules settling in the one of lower enery to yield a phase which ideally is completely ordered at 0 K. The barrier between the two orientations is 8.0 kJ/mol above Tc and 6.1 kJ/mol below Tc. The statistical average energy difference between the orientations increases with decreasing temperature from near zero at Tc and is 7.2 kJ/mol at 62 K. The EPR spectrum of the triplet naphthalene trap becomes weaker at higher temperature and disappears by 90 K. It shows no evidence of the phase transition. The loss of hfs at 55 K reveals a process, presumably thermal detrapping,...