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Vol. GG
SOLUTIONS OF ALKALI n!tETALS IN,ETHYLE;\;EDIAMIXEl BY STAXLEY WIXDVERAND BENSONR. SUNDHEIM Department of Chemist~g,Yew York University, Yew York 3, Y e w York Received November $4, 1961
The electron spin resonance spectra, optical absorption spectra, and electrical conductivities of solutions of alkali metals in ethylenediamine are reported. I t is concluded that the species responsible for thz spin resonance is tlhe electron (rather than the monomer), and that the light absorption is due to the monomer and dimer. Variations in line width of the spin resonance absorption reveal tha,t the electron is not completely independent of t’heionic core. The opt’icalahsorpt,ion spectrum of riibidium is dist)inctly different from that of sodium or potassium.
The nature of solutions of alkali metals in various liquids, such as ammonia, amines, and ethers has been a sub,ject of continuing interest.2 The data obtained from these systems are currently interpreted within the general framework of the Kaplaii and Kittel,a and the Becker, Lindquist, and Alder4 theories. Specifically, the properties of metal solutions are interpreted in terms of dynamic equilibria among various species : e- (solvated electron), e?= (solvated electron pair), N+ (solvated metal ion), M (metal atom, ion pair, “monomer”), h12 (metal dimer), and A I z (higher polymers). It has been reported that solutions of potassium in ethylenediamine give perceptible electron spin resonance, whereas sodium solutions do not .5 This suggests that the solvated electron is a more significant species in one solution than the other. The present work was undertaken in order to elucidate the structure of solutioiis of alkali metals in ethylenediamine.
I. Experimental Methods
.
These metal solutions resemble solutions of allrali metals in ammonia and in other amines in their metastability and sensitivity to impnrities.6 All of the steps in the preparation, handling, and measurement were carried out in scrupulously clean glass and quartz equipment under varuum conditions. Breakseals were used in place of stopcocks and seal-offs were avoided wherever possible since the slightest outgassing has a noticeable effect in promoting the decomposition of the solutions. Even under the best of conditions, the solutions usually faded within a day or so. A. Preparation of Materials.-The ethylenediamine (Eastman Kodak Co., 98% pure) was dried over CaO, filtered in a cellophane bag under nitrogen, refluxed with sodium overnight, and then vacuum distilled through a 10ft. column. The middle fraction was collrcted, S a / K alloy introduced under vacuum conditions, and the vessel repeatedly put through a cycle of shaking, freezing, evacuating, and then remelting until a permanent blue color developed. This permanent blue color was taken as a sign of purity. The ethylenediamine was then vacuum distilled into capsules for storage. Rubidium (MacKap Company) was received in capsules. Sodium and potassium ( J . T. Baker Chemical Company) were cut in the form of cubes from the centers of large (1) This article is based upon a dissertation submitted b y Stanley Windwer t o t h e Graduate School of Arts a n d Sciences of New York University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, October, 1960. (2) (a) W. L. Jolly, “Metal Ammonia Solutions” in “Progress in Inorganic Chemistry” I. Interscience Publ., New York, N. P., 1959, P. 235-281; (b) M. C. R . Symons, Quart. Rev. (London), 18,99 (1969); (e) E. C. Evers a n d R . L. Kay, “Solutions of Eleotrolyt,es” in “Annual Reviews of Physical Chemistry,” Annual Reviews, Inc., Palo Alto, Cal., Vol. 11, 1960, p. 21. (3) J. Iiaplan a n d C . Kittel, J . Chem. Phgs., 26, 1429 (1953). (4) E. Beaker, R. Lindquist, a n d B. Alder, < b X 826, 971 (1956). ( 5 ) G. W. Fowles, W. R . McGregor, a n d hl. C . R. Symons, J . Chem. Sac., 3323 (1957). (61 C. A. Kraus, .I. Chem. Educ., 80, 83 (3053).
chunks. In each case the metal was melted in vacuo, passed through a series of capillaries to remove adhering oxide fihns, and then distilled into the reaction vessel for use. Lithium metal was purified using a procedure similar to that developed by Evers, Young, and Panson.’ Reagent grade sodium iodide (Fisher Scientific Company) was dried a t 55” in vacuo, and used as such. B. Measurements.-For electron paramagnetic resonance measurements, each freshly prepared solution was filtered through a sintered glass disk to assure the absence of metallic particles and into a capillary which then was inserted into the cavity of a Varian Associates >lode1 V-4500 spectrometer. The spectrum was obtained a t a fixed frequency of 9.5 kMc. by scanning the magnetic field. The magnetic field strength was determined by calibration with solid DPPH and dipotassium peroxylamine disulfonate solutions. Because of the difficulties in handling conducting solutions, the spin densities were determined by affixing a weighed crystal of DPPH to the outside of the capillary. The signal from this crystal appeared on the same tracing as the solutions and the (automatically) integrated peaks then could be compared directly. As few as 2 X 10’3 spins could be detected in DPPH. In order to measure the optical absorption spectra, the solutions were prepared as above, filtered into 0.1 or 1.0 mm. quartz optical cells, and observed in a specially fabricated thermostated compartment mounted in a Cary Model 14 spectrophotometer. An electrical conductivity cell was connected to the absorption cell so that the solutions could be passed from one to the other merely by tilting the whole assembly. Gold plated electrodes were used, since they appeared to catalyze the decomposition reaction less than tungsten or platinum. The cell vias mounted in a thermostated oil bath and the conductivity determined with a Jones conductivity bridge. No effect of frequency was detected. The readings of conductivity and absorptivity were carried out as functions of time and, vr-hen necessary, adjusted to the same time by linear extrapolation. C. Analysis.-At the conclusion of each measurement, samples were isolated in previously prepared capsules. The contents of these were mixed with water and the volume of gas evolved determined with a gas buret and manometer. By determining the initial weight of the sample, the concentration of oxidizable alkali metal in the solution could be determined. A comprehensive description of the expcrimental procedures can be found in the dissertation of Windwer.*
11. Experimental Results A. Electron Spin Resonance Spectra.-The sodium, potassium, and rubidium solutions gave distinct electron spin resoiiance absorption. Lithium and cesium solutioiis decomposed too rapidly to be studied. The spin resonance spectra mere followed as the solutions decomposed in order t o examine the effect of changing concentration. No change in the g-value or line widths were found as the spin densities decreased. The experimental results are summarized in Table I. (7) E. C. Evers, A. E. Young, a n d A. J . Panson, J . A m . Chem. SOC. 79, 5118 (1957). (8) See $EC Document No. 2183; see also L. C. Card No. hIIC 61707 University AIicrofilms, I n r , , Ann Arbor, Michigan.
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TABLEI E.P.R.RESULTS Li Na K Rb
1Lp.r.
8-value
Line width,
pos pos pos
2.0015 f 0.0002 2 0016 & ,0002 2.0013 f ,0002
gauss
0 75
