Dolores M. Holton

Nuclear spin‐lattice relaxation in the sodium anion, Na−

We report direct measurements of nuclear spin‐lattice relaxation times (T1n) for the sodium anion, Na−, in solutions containing both sodium and a heavier alkali metal in 1,4,7,10‐tetraoxacyclododecane (12‐crown‐4, 12C4). Nuclear spin‐lattice relaxation in Na− is found to be essentially independent of the alkali counterion and to depend only weakly upon the concentration of sodide ion in solution. The temperature dependence of T1n for Na− was used to determine an activation energy for the processes responsible for spin relaxation. The results are consistent with a dominant, but very inefficient, quadrupolar relaxation mechanism which involves the modulation of the electric field gradient at Na− via the reorientation and/or translational motion of surrounding 12C4 molecules in the liquid. Furthermore, we find that solvation of Na− in 12C4 as well described by a model in which there is neither preferential orientation of 12C4 molecules nor a clearly identifiable first solvation shell around the sodide ion. C...

N.m.r. spectrum of Na– in sodium–hexamethylphosphoric triamide solutions

The 23Na n.m.r. spectrum of the sodium anion in solutions of sodium in hexamethylphosphoric triamide is reported, and gives the first direct indication of a genuine sodium anion formed in a metal solution without added cation-complexing agents.

Spectroscopic Measurements on Solutions of Alkali Metals in 1,4,7,10,13- Pentaoxacyclopentadecane (15-Crown-5)

The alkali metals potassium, rubidium and caesium dissolve in the liquid crown ether 15-crown-5 to produce intensely coloured blue solutions. In this work we have employed a variety of spectroscopic techniques (optical spectroscopy, nuclear magnetic resonance (NMR) and electron spin resonance (ESR)) to investigate the nature of both the paramagnetic and diamagnetic solution species coexisting in equilibrium. The major paramagnetic species are the solvated electron, e-s, and the (metal-based) electron-cation complex M+se-s. The diamagnetic species is the alkalimetal anion, M-. Comparisons in characteristic properties for the two liquid macrocyclic ionophores, 12-crown-4 and 15-crown-5, suggest that these two solvents both complex the alkali-metal cation and solvate excess electrons to quite different extents. For the alkali metals in the title solvent, the various equilibria appear to favour the formation of e-s and M+se-s at the expense of the alkali anion.

Solvated electrons and electron–cation aggregates in solutions of the alkali metals in 12-crown-4

The alkali metals sodium, potassium, rubidium, and caesium dissolve in the cyclic ether 12-crown 4 (1,4,7,10-tetraoxacyclododecane) to give blue solutions containing metal cations, metal anions, electrons, and cation–electron pairs. The metal anions give rise to a characteristic optical absorption band; Na– and Rb– have also been detected by n.m.r. as ringlet resonances at high field. E.s.r. signals due to solvated electrons and electron–cation pairs are detected in frozen solutions; on melting the two resonances combine to give a single, time-averaged signal.

The radiation chemistry of organic amides I. A pulse radiolysis study of solvated electrons and alkali-metal-electron species in cyclic amides

Pulse radiolysis of the cyclic amides N-methylpyrrolidinone (NMP), N-ethylpyrrolidinone (NEP), 1,3-dimethyl-2-imidazolidinone (DMI) and 1,3-dimethyl-2-oxo-hexahydropyrimidine (DMH) and the non-cyclic amide tetramethylurea (TMU) yielded absorption spectra in the near infrared that are attributed to solvated electrons. Addition of a variety of alkali-metal salts caused no detectable change in the absorption spectrum of e-s and no new absorptions attributable to alkali-metal anions were detected. The effect of dose on the decay of e-s in NMP was studied in detail. The yields of e-s in these amides were estimated by using trans-stilbene as an electron scavenger. Absorption spectra, which are not removed by saturation with N2O and CO2, are observed in the wavelength range 300-500 nm.

NMR Studies of Alkali Anions in Non-Aqueous Solvents

Publisher Summary This chapter describes the progress that has been achieved to date in the study of alkali anions in liquid 12-crown-4 and 15-crown-5 (15C5). It reviews that the chemistry of the alkali elements is dominated by the occurrence of systems, involving the alkali cation M + . In the alkali anions, two electrons occupy the ns 2 valence orbital, the neutral atom having this orbital only singly occupied. Alkali metals dissolve in anhydrous liquid ammonia to yield, at very low metal concentration, solvated electrons, and solvated alkali cation. As the metal content is increased, these charged species aggregate to form a species of stoichiometry M whose spectral characteristics are remarkably insensitive to the particular alkali metal. Thus it has been inferred from electron spin resonance (ESR) and nuclear magnetic resonance (NMR) studies of metal–ammonia solutions that the electron (spin) density at the alkali nucleus in the species of stoichiometry M is as low as 1% of that in the corresponding gaseous ground-state atom. The chapter also focuses on the use of liquid crown–ether solvents for the preparation of alkali anions. It is known that these crown ethers form strong complexes with alkali cations. The formation of such strong cationic complexes provides the driving force for the dissolution of alkali metals in these liquid crowns. This, coupled with the use of binary alkali–metal alloys, has led to the production of stable metal solutions, containing high concentrations of alkali anions.

Magnetism in Lithium-Methylamine Solutions

— 300 K. The temperatureand concentration-dependence of the solutionphase magnetic susceptibilities are consistent with an equilibrium between diamagnetic spin-paired and paramagnetic spin-unpaired species. Enthalpy and entropy values for a single equilibrium of this type are derived from an analysis based on the susceptibility data for dilute solutions. Calculated susceptibilities based on this single equilibrium are in good agreement with the observed concentrationand temperature-dependence of lithium-methylamine solution-phase susceptibilities over nearly two orders of magnitude variation in concentration and over a 50° C temperature range. The derived spinunpairing enthalpy is 12.14 kJ/mol, or 0.125 eV. This value is considerably smaller in magnitude than the corresponding spin-unpairing enthalpy measured for dilute potassium-ammonia solutions (=; 0.40 eV), suggesting that the spin-pairing interaction is weaker in lithium-methylamine solutions.

The chemical dynamics of Na- in liquid 12-crown-4 solutions deduced from 23Na nuclear spin relaxation rates

Solutions containing the sodium anion (Na-) were prepared by dissolving either NaK, NaRb or NaCs alloys in liquid 12-crown-4, 12C4. The 23Na nuclear spin-lattice (T 1n ) and spin-spin (T 2n ) relaxation times of the N.M.R. signal originating from Na- have been measured as a function of temperature, concentration and counter-cation K+, Rb+ or Cs+. The details of the inequality between T 1n and T 2n shown that the lifetime of Na- is limited by a two-step process. In the first step a strong complex of an alkali cation and 12C4 molecules dissociates to yield an uncomplexed cation to which one 3s electron from Na- is transferred in the second step. The 23Na nuclear spin relaxation data are combined with the results of previous EPR and pulse radiolysis experiments to deduce both the reactions generating Na- as well as further equilibria involving both complexed and uncomplexed cations, solvated electrons and a variety of paramagnetic species containing an electron and a cation.

Alkali metal solutions in liquid 15-crown-5

Potassium, rubidium, caesium, and mixed alkali metals dissolve in the liquid crown ether, 1,4,7,10,13-pentaoxa-cyclopentadecane (15-crown-5), to give intensely coloured solutions; spectroscopic studies (optical, e.s.r., and n.m.r.) revealed alkali metal anions, solvated electrons, and various cation–electron aggregates in these systems.