Electron Spin Resonance of Amine Solutions of Alkali Metals

Abstract

Nuclear hyperfine structure has been observed in the electron spin resonance (ESR) absorption spectra of potassium and lithium dissolved in some alkylated amines. The four‐line spectrum displayed by the potassium solutions is attributed to ³⁹K‐electron contact interaction. This interaction increases with increasing temperature. The spin density at the potassium nucleus in ethylamine solution is 3.37±0.05×10²³ cm^—3 at —40°C and 1.21±0.05×10²⁴ cm^—3 at 50°C. The contact interaction also decreases when methylamine is added to an ethylamine solution. This behavior is interpreted in terms of a modified view of the monomer of the theory of Becker, Lindquist, and Alder. The monomer changes its structure as its environment changes. A statistical model which relates the experimental observations to these changes is constructed. The differences in the ESR absorption spectra observed for the three systems Na–NH₃, K—EtNH₂, and Li—EtNH₂ are discussed, and in part, attributed to differences among the three systems in the rates of spin exchange, electron exchange, and solvent exchange. Order‐of‐magnitude calculations relating measured line breadths to the rates of these processes are carried out in certain limiting cases. The results of these calculations lend support to the idea that the monomer includes remote solvent molecules within its structure. The rate constant for spin exchange is shown to increase with decreasing temperature in some potassium—amine solutions. No evidence for the solvated electron is found in the ESR spectra of K dissolved in ethylamine, or in a range of mixtures of ethylamine with methylamine. An upper limit of 10^—4 is placed on the dissociation constant of the monomer. For Li in ethylamine a composite spectrum is observed under some conditions. It consists of a nine‐line hyperfine pattern superimposed on a single relatively sharp line with the same g value. This sharp line is attributed to the solvated electron. The nine‐line pattern is attributed to hyperfine interaction between the unpaired electron and four equivalent ¹⁴N nuclei in the Li monomer. The spin density at each ¹⁴N is 1.4±0.1×10²³ cm^—3, independent of temperature. Similar spectra are observed for Li dissolved in two mixed amine solvents. The size of the monomer is discussed in terms of an expanded orbital approach. The K monomer is shown to vary between two and three times the size of atomic K. The trend toward lower percent atomic character with decreasing atomic number of the alkali metal is also discussed. Part of this trend may come about because the small atoms undergo a larger expansion in forming a monomer.