S. I. Weissman

Isotropic Hyperfine Interactions in Aromatic Free Radicals

It is shown, by a procedure similar to one used by Abragam, Horowitz, and Pryce in their studies of hyperfine structure in paramagnetic ions and atoms, that configurational interaction may contribute to the isotropic hyperfine interaction in free radicals. For a configurational admixture of the type (σB)2(π)+λ(σB) (σA) (π), the hyperfine interaction is 32π/3√6μημeλσB(rη)σA(rη) where μe and μη are magnetic moments of the electron and the nucleus η, and σ(rη) is the magnitude of the orbital σ at the position of the nucleus. Since the hyperfine coupling is linear in λ, only small admixtures of excited configurations are required to account for the proton hyperfine splittings observed in aromatic free radicals.

Effects of Perturbations on Phosphorescence: Luminescence of Metal Organic Complexes

Fluorescence and phosphorescence spectra, phosphorescence lifetimes, and relative ratios of intensities of fluorescence to phosphorescence of the dibenzoylmethane derivatives of the trivalent ions of Al, Sc, Y, La, Gd, and Lu have been measured. The lifetimes and intensity ratios of the various compounds differ widely, while the spectra are very similar. For instance, although the Lu and Gd compounds have practically indistinguishable phosphorescent spectra, the lifetimes of their phosphorescences differ by a factor of fifty. The results are interpreted in terms of Lewiss triplet theory of the phosphorescent state and spin‐orbit perturbation of the levels. The largest effect is produced by the paramagnetic gadolinium ion. The effects produced by the non‐magnetic ions depend on their nuclear charge and on their electronic structure.

Spin Exchange in Biradicals

The electron spin resonance spectra of four isotopically labeled biradicals have been observed. The hyperfine splittings demonstrate independence of the halves.

Hyperfine Splittings in Polyatomic Free Radicals

The hyperfine interaction in polyatomic molecules in doublet states is investigated. It is demonstrated that for molecules undergoing rapid and jittery tumbling the dipolar part of the interaction vanishes.

Symmetry Classification of the Energy Levels of Some Triarylmethyl Free Radicals and Their Cations

The absorption and luminescence spectra of triphenylmethyl, tri‐p‐xenylmethyl, phenyl‐di‐p‐xenylmethyl, diphenyl‐p‐xenylmethyl, and their cations have been determined. Polarization of the luminescence of each substance under excitation by plane polarized light has been examined. Assignment of symmetry classifications to the energy levels of these substances has been attempted.

Triplet State ESR of Metal Chelate Compounds

The electron spin resonance spectra of a series of metal chelate compounds are reported. The metals are alkaline earths; the ligands are bidentate and tridentate derivatives of 2,2′‐bipyridine and 1,10‐phenanthroline and related compounds. The ESR spectra are interpreted as due to randomly oriented molecular triplets. Most spectra can be computer‐simulated with a spin‐Hamiltonian corresponding to a magnetic dipole—dipole interaction of the unpaired electrons of D≅0.01 cm−1 and E=0. The vanishing value of E is interpreted to mean that the complexes have S4 symmetry with the ligands on opposite sides of the cation and having mutually perpendicular planes. The simulation of some spectra requires a distribution of D and E values which we attribute to interaction with the glassy environment.The spectra of some potassium chelate dimers are also reported. These characteristically appear to have nonvanishing E values.

Spin Density in Pyrene Negative Ion

Improved resolution has permitted assignment of all proton hyperfine coupling constants in pyrene negative ion. The results are accounted for by a simple calculation of configurational interaction.

Polarization of the Singlet‐Singlet and Triplet‐Singlet Luminescence of Chelate Compounds

The polarizations of the luminescence of the dibenzoylmethane complexes of lithium, potassium, aluminum, lanthanum, and gadolinium have been determined. The first excited singlet state of the univalent chelates has symmetry B1. The triplet states have not been uniquely classified. The nature of the perturbation of the triplet state introduced by the magnetic gadolinium ion seems qualitatively different from the perturbations introduced by non‐magnetic ions.

Nonexponential Decay of Luminescence of Rare‐Earth Chelates

Studies were made on the nonexponential decay of luminescence of europium complexes of picrate, dibenzoylmethide (DB), and thenoyltrifluoracetonate (TTA) and the DB and TTA compounds of terbium. Solutions in methylcyclohexane, ethanol, and toluene were studied in the temperature range 77-300 deg K. The terbium compounds and the europium picrate yielded simple exponential decays under all conditions, with decay constants of the order 500 x 10/sup -6/ sec in rigid glasses and 30 x 10/sup -6/ sec in fluids at room temperature. The decays of the luminescences of the DB and TTA compounds of europium were not exponential at low temperatures in rigid media. (P.C.H.)