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12 The Photochemistry of Metal Solutions, III: Formation of Metal Solutions from Alkali Amides, in Ethylamine and Ammonia MICHAEL OTTOLENGHI and HENRY LINSCHITZ Department of Chemistry, Brandies University, Waltham,

Mass.

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch012

Ultraviolet (UV) irradiation of decomposed al kali metal solutions in ethylamine or ammonia regenerates the original species.

The reaction

has been studied using both steady and flash excitation.

In ammonia the reaction is as

cribed to electron transfer from excited amide ion to solvent, followed by competitive com bination between electron and amine radical, or two

amine

radicals.

In

ethylamine, it

seems that an electron is transferred from amide to cation within an excited ion pair. Flashing

bleached ammonia

solutions

with

light near 500 mμ produces a new short-lived transient, absorbing

at

510

mμ.

Photore-

generation of bleached ethylamine solutions is accompanied by irreversible formation of a band of 265 mμ. Extinction coefficients are given for the amides and for the metal mono mer and dimer species in ethylamine.

J h e formation of solvated or hydrated electrons by photoionization of various negative ions, i n suitable organic solvents or i n water, has been observed by several workers using either rigid solvent or flash technique (8, 11, 15, 20, 22). In developing a general understanding of electron behavior i n liquids, one must ultimately obtain data, either b y photoionization or by radiolysis (9,16,26), over the widest possible range of solvent media. Photoionization studies on simple ions i n liquid ammonia or amines are of special interest, particularly i n obtaining com parisons with the results i n water (15,16, 22), and i n connection with the closely related problem of the structure of metal solutions. Such studies on amides are described in this paper. W e have recently reported a new photochemical reaction, i n which faded alkali metal solutions i n ethylamine are regenerated by U V i r 149

Hart; Solvated Electron Advances in Chemistry; American Chemical Society: Washington, DC, 1965.

SOLVATED ELECTRON

150

radiation (23). Flash-photolysis experiments on such solutions i n ethylamine and ammonia were also described. W e present now a more complete account of this work, including measurements of the extinction coefficients of potassium amide and of monomer and dimer species i n alkali metal solutions.


Experimental Preparation of Samples. M e t a l solutions were prepared on the vacuum line as in our previous work (24), in thoroughly flamed-out pyrex ampoule assemblies, equipped as needed with various side arms and tubes for bulb-to-bulb distillation of metal, for absorption and flash-photolysis cells, for catalytic decomposition of solution, and for final freezing out of solutions before analysis. Preparations were sealed off under " s t i c k " vacuum. Ethylamine and ammonia were Matheson products and were degassed and dried over Hthium before distilling through glass wool plugs into the working ampoule. T h e concentrations of ethylamine solutions were readily controlled b y varying the contact time between solvent and metal mirror. Preparing very dilute metal-ammonia solutions is more difficult, owing to the rapid dissolution of metal. Accordingly, such solu tions were prepared by distilling small amounts of metal onto a glass bead containing a n iron core and transferring this bead magnetically into the ammonia bulb (28). Potassium-ammonia solutions were bleached by standing overnight at —78° C . over electrolytically blackened platinum foil i n a side bulb separated from the rest of the assembly b y a sintered disc. Ethylamine solutions were decomposed i n less than a n hour over blackened platinum at room temperature, or simply b y standing for several hours. P h o t o c h e m i c a l T e c h n i q u e . Flash-photolysis apparatus a n d procedures have been described previously (24). F o r work i n liquid ammonia, the photolysis cell was first cooled i n the flash Dewar and cold solution, then poured i n from the storage bulb. N o difficulty was experi enced i n the transfer through the short length of connecting tubing, and frosting of the cell windows was thus avoided. T h e spectral region of exciting light was controlled either b y dyed cellophane filters (Rosco Labs, Ν . Y . ) wrapped around the Dewar, or by interposing four 2 i n . X 2 i n . Corning filters between the four flash lamps (21, 24) and the pho tolysis cell assembly. These filters were supported i n a square frame, mounted symmetrically around the central focus of the four-leaved ellip tical reflector (21). Steady irradiations were carried out i n pyrex cells, about 10 cm. from a Hanovia type H S 150-watt mercury arc. Analysis of Solutions, (a) A B O D E S — T h e amide concentration i n decomposed metal-ethylamine solutions was determined b y titration, using bromcresol green-methyl orange indicator (2). T h e solution, i n a side arm of the ampoule, was cooled i n d r y ice-acetone, the side arm cut off and immediately connected to the vacuum line v i a a short length of rubber tubing, and the solvent distilled into another calibrated tube. T h e amount of solvent was determined either by volume or b y weighing the tube while cold. A known amount of degassed water was then dis tilled onto the amide residue and aliquots removed for titration with 500iV H C 1 , either directly or after gently boiling for five minutes i n a small Erlenmeyer. Blanks gave negligible corrections. Boiling reduced


12.

OTTOLENGHI AND LINSCHITZ

Photochemistry of Metal Solutions

151

the titer of the aliquots by half ( ± 2%), corresponding to the expected amount of free ammonia or amine produced on amide hydrolysis. S a m ples which were immediately acidified and then back-titrated with sodium hydroxide gave results agreeing with the direct titrations. (b) F R E E M E T A L — E t h y l a m i n e solutions were analyzed for alkali metal b y measuring hydrogen gas evolution upon adding water or stand ing over platinum catalyst at room temperature. After decomposition, samples were degassed by thoroughly agitating, freezing, and opening the sample to the M c C l e o d gauge. F o u r or five such cycles were gener ally required to remove the hydrogen completely. Results


Ethylamine Solutions,

(a) P H O T O R E G E N E R A T I O N O F B L E A C H E D

S O L U T I O N S — T h e absorption spectra of alkali metals i n ethytemine have been discussed i n previous publications (24,25). Briefly, three character istic bands are found, absorbing at about 650 (V), 850 (R) and 1300 (IR) πΐμ, and attributed respectively to metal monomers, dimers, and solvated electrons. In potassium solutions, the equilibria favor the V - b a n d , while in rubidium the R - b a n d is most prominent, except at extreme dilutions.

0.12 > α: UJ

ο 0.08F (Λ ω N H " k

161 (20

2

k'%


NH

2

+ NH

( N H ) or other products

2

2

2

(3')

Reaction 1' represents formation of the initial far red transient, Reaction 2' the initial fast decay stage and 3' accounts for the residual far red absorption as those electrons which escape recombination with amine radicals. Figure 8 shows that the residual solvated electron concentration at the end of the fast decay process is about 3 0 % of the initial yield. O n the time scale of our flash observations, cage and geminate recombination effects play no role, and the measured ratio of long- to short-lived product thus is fixed by the ratio of k to k%. Without solving the above kinetic scheme i n detail, it is evident that k ' and k% are of the same order of magnitude. T h i s result is i n general agreement with previous measure ments on the rate of electron capture i n solution (11). T h e solvated electrons i n the flash transient may be present either as free or paired ions. T h e extent of pairing may be estimated i n a given experiment from the extinction coefficient of the solvated electron (taken to be 1.0 Χ 10 at 980 πιμ) (10), from the potassium amide dissociation constant and from the ion pair dissociation constant for the process [ M e~] — M + + e. T a k i n g this to be about 3 X 1 0 M (1,12,14), we find for a typical flashed solution i n which AD 8o ~ 0.02 and D335 = 0.9, that the ratio (unpaired/paired) solvated electrons is about 30. T h i s result is difficult to confirm, at least by spectroscopic methods, owing to the obedience to Beer's L a w i n the pairing region (13). There remains the question of the 510 πιμ transient. W e have seen that the photoregeneration reaction produces the thermodynamically stable species appropriate to each solvent, the cation-centered monomer (650 πιμ band) i n potassium-ethylamine and the far red solvated electron in potassium-ammonia. T h e as yet unidentified cation-centered mono mer i n ammonia (5) should have a high-energy absorption and short lifetime, and it is tempting to ascribe the 510 πιμ intermediate to this species. Following its presumed formation by charge-transfer within an ion pair (long-wave tail of the amide absorption band?), this species might be expected to decay to the stable solvated electron. However, repeated and careful search gave no evidence for a growing-in of the far red tran sient i n flash regenerated potassium amide solutions. It is clear that further spectroscopic and photochemical studies are needed over a wider concentration range. 2

2

4

+

_ 3

9

Acknowledgment We wish to thank D r . K . B a r - E l i and Prof. T . R . Tuttle for their as sistance at various stages of this research.


162

SOLVATED ELECTRON


Literature

Cited

(1) Arnold, E., Patterson, Α., J. Chem. Phys. 4 1 , 3089 (1964). (2) Bar-Eli, K . , personal communication. (3) Bar-Eli, K . , P h . D . Thesis, Rehovoth, Israel (1962). (4) Bar-Eli, K . , Tuttle, T . R., J. Chem. Phys. 4 0 , 2508 (1964). (5) Becker, E., Lindquist, R. H . , Alder, B . J., J. Chem. Phys. 2 5 , 971 (1956). (6) Cleaver, D . , Colfinson, E., Dainton, F . S., Trans. Faraday Soc. 5 6 , 1640 (1960). (7) Dobson, G . , Grossweiner, L . I., Radiation Research 2 3 , 290 (1964). (8) Dobson, G . , Grossweiner, L . I., Trans. Faraday Soc. 6 1 , 708 (1965). (9) Dorfman, L . M., Science 1 4 1 , 493 (1963). (10) Douthit, R. C., Dye, J. L., J. Am. Chem. Soc. 82, 4472 (1960). (11) Eloranta, J., Linschitz, H . , J. Chem. Phys. 3 8 , 2214 (1963). (12) Evers, E. C., Frank, P. W., J. Chem. Phys. 3 0 , 61 (1959). (13) Gold, M., Jolly, W . L . , Pitzer, K . W., J. Am. Chem. Soc. 8 4 , 2264 (1962). (14) Golden, S., Guttman, C., Tuttle, T. R., J. Am. Chem. Soc. 8 7 , 135 (1965). (15) Grossweiner, L . I., Swenson, G . W., Zwicker, E. F., Science 1 4 1 , 805, 1180 (1963). (16) Hart, E. J., Boag, J. W., J. Am. Chem. Soc. 8 4 , 4090 (1962). (17) Hawes, W . H . , J. Am. Chem. Soc. 5 5 , 4422 (1933). (18) Higginson, W . C . E., Wooding, N . S., J. Chem. Soc. 1952, 766. (19) Jortner, J., Ottolenghi, M., Stein, G . , J. Phys. Chem. 6 8 , 274 (1964). (20) Linschitz, H., Berry, M., Schweitzer, D . , J. Am. Chem. Soc. 76, 5833 (1954). (21) Linschitz, H., Sarkanen, K . , J. Am. Chem. Soc. 8 0 , 4826 (1958). (22) Matheson, M., Mulac, W . Α., Rabani, J., J. Phys. Chem. 6 7 , 2613 (1963). (23) Ottolenghi, M., Bar-Eli, K . , Linschitz, H . , J. Am. Chem. Soc. 8 7 , 1809 (1965). (24) Ottolenghi, M., Bar-Eli, K . , Linschitz, H . , J. Chem. Phys. (in press). (25) Ottolenghi, M., Bar-Eli, K . , Linschitz, H., Tuttle, T. R., J. Chem. Phys. 4 0 , 3729 (1964). (26) Ronayne, M. R., Guarino, J. P., Ha mill, W . H . , J. Am. Chem. Soc. 8 4 , 4230 (1962). (27) Stein, G . , Treinin, Α., Trans. Faraday Soc. 5 5 , 1091 (1959). (28) Warshawsky, I., "Metal Ammonia Solutions," ed. Lepoutre and Sienko, p. 167, W . A . Benjamin, New York, 1964. R E C E I V E D May 14, 1965. Supported by a grant from the U . S. Atomic Energy Commission to Brandeis University (No. AT-30-1-2003).


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