Harden M. McConnell

Indirect Hyperfine Interactions in the Paramagnetic Resonance Spectra of Aromatic Free Radicals

It is shown that an electron spin polarization associated with an odd electron in a π‐molecular orbital in an aromatic free radical can, through an atomic exchange coupling mechanism, produce an appreciable resultant electron spin polarization in s‐atomic orbitals at the aromatic protons. This resultant spin polarization gives rise to hyperfine interactions with aromatic protons which are of the same order of magnitude as those observed in the paramagnetic resonance spectra of aromatic free radicals. An equation is developed which relates (proton‐spin)‐(electron‐spin) hyperfine coupling constants to the π distribution of the odd electron in the aromatic free radicals.

Analysis of Spin‐Spin Multiplets in Nuclear Magnetic Resonance Spectra

A systematic procedure is developed for the analysis of the spin‐spin multiplets that are observed in the high‐resolution NMR (nuclear magnetic resonance) spectra of liquids. It is shown how the symmetry properties of nuclear spin functions can be used to simplify the interpretation of NMR multiplets in terms of the empirical quantities, chemical shifts, and spin‐spin coupling constants. It is also pointed out that when NMR multiplets have been properly interpreted in terms of these empirical quantities, it is then possible to predict theoretically relative intensities of multiplets to the same degree of precision as that involved in the measurement of multiplet frequency separations.As an example the multiplet splittings of the H1 and F19 resonances in 1,1‐difluoroethylene are analyzed using the procedure proposed in the present paper.

Analysis of High‐Resolution NMR Spectra

Van Vleck has shown the usefulness of moment calculations in the interpretation of magnetic interactions found in crystals. This approach has been extended to include liquids where separate resonances may be well resolved. In a simple chemical system containing only two groups of nuclei, this procedure allows the direct calculation of the spin coupling constant and the chemical shift in terms of directly measured quantities; in more complex systems, it allows some information to be obtained without finding the complete solution to a particular problem.

Rate of Electron Exchange between Cuprous and Cupric Ions in Hydrochloric Acid Solutions by Nuclear Magnetic Resonance

A determination of the Cu63 nuclear resonance line width in concentrated hydrochloric acid solutions containing both cuprous and cupric chloride leads to an evaluation of the bimolecular rate constant k for the electron exchange process Cu+++Cu+→+Cu++Cu++. The experimentally determined value for k is k = 0.5×108 liter/mole‐second.

Ultraviolet Absorption Spectra of Methylnaphthalenes at 77°K

A simple and reliable apparatus has been developed for the determination of low‐temperature ultraviolet absorption spectra in rigid glass media. This apparatus is particularly suitable for the measurement of accurate absorption intensities and has been used for the determination of the ultraviolet absorption spectra of naphthalene, 2‐methylnaphthalene, and 2,6‐dimethylnaphthalene at the temperature of boiling nitrogen (77°K). A comparison of these low‐temperature spectra provides evidence that the electronic transition of naphthalene in the 2900—3200 A range is symmetry allowed.