Solvated Electron

10 Spectroscopy of Dilute Metal-Deuteroammonia Solutions D. F. BUROW and J. J. LAGOWSKI

Downloaded by KTH ROYAL INST OF TECHNOLOGY on December 4, 2015 | http://pubs.acs.org Publication Date: January 1, 1965 | doi: 10.1021/ba-1965-0050.ch010

Department of Chemistry, The University of Texas, Austin, Tex.

The spectra of dilute solutions of lithium, so dium,

potassium,

calcium, and

barium

in

liquid deutero-ammonia indicate that the ab sorbing species is the same in each case. The dependence of the shape, intensity, and energy of the absorption band on temperature was investigated for sodium-ND

3

solutions.

The

data are discussed in terms of the electron-in-a– cavity

model.

No

spectral evidence

was

found for the presence of new species in ND

3

solutions containing mixtures of sodium and sodium iodide.

J h e properties of solutions of the active metals i n liquid ammonia depend strikingly upon concentration. T h e more concentrated sys tems have a bronze color and have been described as pseudo-metals; on the other hand, the more dilute solutions have a very intense blue color. There seems to be little doubt that the properties of the more dilute solutions arise, not from the dissolved metal species, but from some ill-defined species which is best described (18) as a "solvated electron." Numerous investigators have attempted to describe the properties of these dilute solutions on the basis of models of the solvated electron which fall into two general categories: the cavity model (15,21) and the expanded metal model (1). Spectroscopic methods appear to offer a relatively unambiguous means of studying these solutions since many substances (—e.g., metallic electrode surfaces) appear to catalyze the decomposition of metal-ammonia solutions into the corresponding amide. T h e intense blue color associated with the dilute solutions immediately suggests that they absorb radiation i n the red or infrared region of the spectrum. Jolly and his co-workers (9, 10) and Douthit and D y e (6), who have made quantitative measurements on dilute solutions i n this spectral region, report that solutions i n the concentration range 10 " to 10~ Af possess an intense (e~4 χ 10 ) broad absorption band with a maximum near 15,000 A . A shift in the band maximum to higher energy 1

8

4

125

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

126

SOLVATED ELECTRON 4r


3h

Φ

ο

.ο

for H 0 is 1.779 at 2 0 ° C . and that for D 0 is 1.764 at 2 0 ° C . (13). Hence, the assumption that D p for N H is equal to D for N D should not introduce a large error into any calculations for the N D system. T h e static d i electric constant of liquid N D as a function of temperature was measured using the heterodyne beat method (4) at 500 kilocycles; the values obtained are given i n Table I. Temperature control was accomplished by using standard slush baths, and the temperature of the system was obtained from the vapor pressure of N D . It was essential that the apparatus be cleaned by the procedure described i n the experimental section before quantitative optical measurements could be made on the metal-ammonia solutions. Significant decomposition arises from reaction of the metal solution with water a d sorbed on the surface of the glass (23); a similar phenomenon was observed for solutions of strongly basic species in liquid ammonia (5). T h e rinsing and aging procedure apparently replaces adsorbed water a n d / or hydroxyl species on the surface of the glass with ammonia. Removing the rinse-ammonia from the cell by distillation rather than as liquid was a noticeably less effective procedure. Since the surface of the cell became contaminated quickly on exposure to the atmosphere with species that react with the metal solutions, it would appear that removing water molecules by displacement with less polar ammonia molecules proceeds more slowly than the reverse process. After a cell had been cleaned b y this procedure, the decomposition rate of a 10 ~ M m e t a l - N D solution was reduced to such a level that no detectable decomposition occurred within 20 hours. Since the aging solutions ( 1 0 M metal i n N H ) retained their deep blue color over a period of 48 hours, this clearly indicates that the N H solutions possess a similar stability. op

8

ol

2

2

0

op

8

8

3


8

8

b

8

_ 8

8

8

Table I. T, -74 -72 -63 -50 -45 -37

Dielectric Constants of Liquid NH and ND 8

8

Static dielectric constant NH? ND> ... 26.0 ± 25.5 d=0.2 24.5 =fc 0.2 25.0 ± 23.3 =fc 0.2 23.8 =fc 22.7 =h 0.2 23.1 =fc 21.8 =h 0.2 22.1 ±

°C. ± 1 =fc 1 ± 1 ± 1 ± 1 ± 1

0.2 0.2 0.2 0.2 0.2

« * ^ïfo o ^ ^ r & ï * * Î £ l ^ ^ii? reported for N H t : 26 at - 7 7 . 7 ° C . (22), 23.8 at - 5 0 ° C . (S), 22.5 at - 4 0 ° C . (8), 22.4 at - 3 3 . 4 ° C . ( i l j / a n d 22 at - 3 3 ° C . (13). a

0

t r i

t a n t e

1 ) 6 6 1 1

Ordinarily, high temperature and ultraviolet radiation seem to promote the decomposition of metal-ammonia solutions. However there is evidence that these factors merely accelerate decomposition which was initially begun by catalytic amounts of contaminants on the walls of the containers. A silica tube was rinsed with liquid ammonia, the rinseammonia removed by evaporation, and the tube evacuated; a concentrated blue metal-ammonia solution (~1M) was prepared i n the tube,


SOLVATED ELECTRON

132

and the tube sealed. After four weeks irradiation under intense ultra violet light at 3 5 ° C . the solution faded; i n similar tubes which had not been rinsed with ammonia the solutions faded within a few days. It appears that metal-ammonia solutions are more stable than previously reported. T h e spectra of solutions of Uthium, sodium, potassium, calcium, and barium i n liquid N D (Figure 4) i n the concentration range 5 — 50 X 1 0 ~ M are identical within experimental error, indicating that the ab sorbing species is the same in each case. T h e only absorption band pres ent occurs at 13,800 A . (7246/cm.) at —70° C . for the concentration range investigated (Figure 4). A s with N H solutions, the bands for N D solutions are symmetrical when absorbance is plotted against wave 3


6

8

3

2.0r-

12

14

9

16

,

18

Wavelength ( A X 10") Figure 4. A typical spectrum of a metal-ND solution at -70°C. (3.52 X 10~ M sodium): (a) absorbance vs. wave number ( P m * x = 7245/cm.); (b) absorbance vs. wave length ( X m a x = 13,800 Α.). z

4


10.

BUROW AND LAGOWSKI

Metal-Deuteroammonia Solutions

133


3r

Figure 5.

Absorbance vs. concentration for metal-NDi solutions at 13,800 A. at —70°C: φ lithium; Q sodium; Ο barium.

number. T h e wavelength of the absorbance maximum is independent of concentration within the given range at a given temperature. Douthit and D y e (6) and Jolly et al. (14) report that for more concentrated metalammonia solutions the maximum absorbance varies with concentration [7100/cm. (14,085 A . ) for 1 0 ~ M solutions to 6500/cm. (15,380 A . ) for 1 0 A f solutions] at —65° C . Preliminary measurements made i n this laboratory indicate that the absorbance maximum for metal-ammonia solutions of ca. 1 0 ~ M is between 14,000 A . (7143/cm.) and 14,500 A . (6896/cm.). Unfortunately these measurements on N H solutions are ambiguous because solvent bands are present. In any event, the ab sorbance maximum observed for the N D solutions occurs at higher energy than that observed in N H solutions. Preliminary calculations using Jortner's model, appear to indicate that the difference in the energy of transition between N H and N D solutions may be accounted for solely by the difference i n the dielectric constants of the two solvents. T h e absorbance at the band maximum for solutions of lithium, sodium, and barium in N D is a linear function of the concentration, and extrapolation of the linear function to zero concentration predicts zero absorbance (Figure 5 and Table II). T h e molar extinction coefficients as calculated by the least squares method for solutions of lithium, sodium, and barium i n N D at —70° C . are given i n Table II. If it is assumed that barium loses two electrons upon solvation, the molar extinction coefficients of Uthium, sodium, and barium solutions are the same to within the estimated error of measurement (4%). T h e residual ab sorbance, as calculated from the least squares analysis, i n each case is 8

- 1

4

3

3

8

3

3

3

3


SOLVATED ELECTRON

134

zero within experimental error. These observations suggest that the species giving rise to the absorption band is not significantly involved in equilibrium processes at these concentrations.


Table II.

Molar Extinction Coefficients at - 7 0 ° C. for ND, Solutions of Lithium, Sodium, and Barium 0

13,800 A.) Absorbance at zero concentration 6

Ba*

Li

Na

4.84 X 10*

4.99 Χ 10

0.040

0.001

5.00 X 10*

4

-0.050

α Calculated by the method of least squares. b Assumes barium loses two electrons. e Calculated from the least square intercept.

2

Ta

Q>

α c ο •ei.o| ο (Λ J Q

Solvated Electron

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