Pulse Radiolysis of Liquid Ammonia
References and Notes (1) G. A. Mills and H. C. Urey, J. Am. Chem. SOC.,62, 1019 (1940). (2)D. J. Poulton and H. W. Baldwin, Can. J. Chem., 45, 1045 (1967). (3)D. M. Himmelblau and A. L. Babb. Am. inst. Chem. fng. J., 4, 143 (1958). (4)P. V. Danckwerts and K. A. Melkerson, Trans Faraday SOC.,58, 1832 (1962). ( 5 ) J. Koefoed and K. Engel, Acta Chem. Scand., 15, 1319 (1961).
1651 (6) R . H. Gerster, T. H. Maren, and D. N. Silverman, Proceedings of the First International Conference on Stable Isotopes in Chemistry, Biology and Medicine, May 9-1 1, 1973,Argonne National Laboratory, p 219. (7)R . Gerster, int. J. Appi. Radiat. isotopes, 22, 339 (1971). (8)D. M. Kern, J. Chem. fduc., 37, 14 (1960). (9)H. S. Harned and B. B. Owen, "The Physical Chemistry of Electrolytic Solutions", Reinhold, New York, N.Y., 1958 pp 690-694,755. (IO) B. H. Gibbons and J. T. Edsall, J. Bioi. Chem., 238, 3502 (1963). (1 1) D. N. Silverman, Mol. Pharmacoi., I O , 820 (1974). (12)D. N. Silverman and C. K. Tu, to be submitted for publication.
Absorption Spectrum, Yield, and Decay Kinetics of the Solvated Electron in Pulse Radiolysis of Liquid Ammonia at Various Temperatures Farhataziz"' and Lewis M. Perkey Radiation Laboratory,* University of Notre Dame, Notre Dame, indiana 46556 (Received February 12, 1975) Publication costs assisted by the U.S. Energy Research and Development Administration
The transient absorption spectrum of earn- induced in neat liquid ammonia by nanosecond pulse radiolysis shifts to higher energies as temperature decreases. The transition energy a t the absorption maximum increases from 0.67 to 0.88 eV as temperature decreases from 23 to -75'. The portion of bandwidth a t halfmaximum measured from the absorption maximum to the high-energy side of the spectrum is constant over the temperature range from 23 to -75'. For the same temperature range, the primary yield of earn- is constant with a value and average deviation of 3.1 f 0.3. The mechanism of decay of earn- in pulse radiolysis of neat liquid ammonia is complex.
Introduction Various investigator^^-^ have reported a measured 100eV yield of solvated electrons G (earn-) in pulse radiolysis of liquid ammonia under conditions for which G(earn-) can be identified with the primary yield g(earn-) of electrons that survive a period of nonhomogeneous kinetics3 At 23, -15, and -48', reported values of g(earn-) are 3.1,3 3.17,5 and 3.0,4 respectively. For liquid deuterated ammonia, Seddon et al.5 report that the primary yield is invariant over a temperature range from -15 to -50' with a value of g(earn-) = 3.6 f 0.2. An interpretation for the anomalously large g(earn-) is that the reaction of earn- with NH4+ is very S ~ O W ~ compared - ~ to a diffusion-controlled r e a ~ t i o nconse;~ quently, the loss of earn- by the geminate neutralization reaction during the period of nonhomogeneous kinetics is r e d u ~ e d . ~ . ~Such J O an interpretation is supported by the reported decrease of g(earn-) a t 23' with increase in pressure up to 6.7 kbars.ll A systematic determination of g(earn-) for a wide temperature range will contribute to development of a theoretical understanding of earn- and the radiolysis of liquid ammonia. Such studies require knowledge of the absorption spectrum of earn- as a function of temperature. Spectra are available for solutions of alkali metals in liquid ammonia.12-17 However, the absorption spectrum obtained for earn- in solutions of alkali metals in liquid ammonia can be c0mp1ex.l~Pulse radiolysis of neat liquid ammonia provides an alternative technique for determination of the absorption spectrum of earn-.ll In this paper we report determination of the absorption spectrum, yield, and decay ki-
netics of earn- for temperatures in the range from 23 to -75' by study of the pulse radiolysis of neat liquid ammonia.
Experimental Section The purification of ammonia, the irradiation cell, and the pulse-radiolysis system and split-beam technique for the measurement of transient absorption spectra are described el~ewhere.~Jl For irradiation, the cell containing the sample was held in the center hole of an aluminum cell holder that was placed inside a quartz dewar with three Suprasil windows (Figure 1). The aluminum cell holder was designed to accommodate five cells. Only the center hole could be used for irradiation. The four holes around the center hole were shallower than the center hole, and thereby were not in the path of electron beam from the Linac. In one of these holes, an irradiation cell filled with methanol was placed for a continuous monitoring of temperature during irradiation. The remaining three holes were used to cool extra samples. The temperature of the sample was fixed by continuous flow through the quartz dewar of a stream of nitrogen that was obtained by continuous boiling of liquid nitrogen with an immersion electric heater and was brought to the desired temperature by flow through a thermostatically controlled heater. Sample temperature was measured by a copper-constantan thermocouple and was read on a digital thermocouple thermometer (Model DS-100-T3 of Doric Scientific Corp.). The temperature was controlled within f0.2'. For determination of g(earn-), a liquid ammonia sample The Journal of Physical Chemistry, Voi. 79. No. 16, 1975
Farhataziz and Lewis M. Perkey
1652 QUARTZ WALL
TABLE I: Characteristics of the Spectrum a n d Decay of earn- Produced by Pulse Radiolysis of Liquid Ammonia at Various Temperatures
1
DEWAR
T , "C
E,,,,
eVa
W,eVb
k C
~
LINAC
I-
23 20.6
O LIGHT BEAM
0 .23d
27e 13 0.9 0.72 0.24 8.8 0.22 5 .O -25.7 0.79 0.82 0.24 3.5 -4 7 0.88 0.25 2 .o -75 a Transition energy at the absorption maximum. * That portion of the bandwidth at half-maximum from E,, to the high-energy side of the spectrum. Second-order specific rate (in units of 1010 M - l sec-l) for the decay of earn- (cf. text). From ref 11. eFrom ref 9.
1
ALL WINDOWS SUPRASIL QUARTZ
0.67d
?I= Figure 1. Quartz dewar for low-temperature pulse radiolysis. was irradiated a t room temperature (-23') with eight pulses of 5-nsec duration. The transient absorption of earn- was recorded at 1.35 y for pulses 1 and 2, 1.45 y for pulses 3 and 4, 1.75 for pulses 5 and 6, and 1.35 y for pulses 7 and 8. The procedure was repeated at 0.9, -25.7, -47, and -75' and again at room temperature. The dose per pulse was low enough to permit a linear extrapolation of the transient absorption signal to the beginning of the pulse and thereby obtain an initial signal that was used to calculate an initial optical density, D. At each temperature the average value of D obtained for the first two pulses agreed with that obtained for the last two pulses within i 5 % . Thus, it was assumed that the dose per pulse was constant for all eight pulses. For the experiments at room temperature that bracketed the lower-temperature experiments, average values of D a t the corresponding wavelengths agreed within i10%. Therefore, a constant dose per pulse was assumed at all temperatures. Decay of the earn- absorption at 1.75 and 1.35 y was studied a t all temperatures.
o
0.01.0 1.1
Results Absorption spectra of ea,- at various temperatures are shown in Figure 2. Characteristics of the spectra and earndecay are summarized in Table I in which W represents that portion of the bandwidth a t the half-maximum from the absorption maximum to the high-energy side of the spectrum, E,,, is the transition energy at the absorption maximum, and h is a second-order specific rate whose calculation is described subsequently. At a wavelength A, D and g(e,,-) are related by
D = dCpg(eam-1
(1)
in which d is a constant, p is the density of liquid ammonia, and c is the extinction coefficient of earn- a t wavelength A. Values o f t for various wavelengths and temperatures were calculated from the data in Figure 2 and a maximum extinction coefficient of 4.8 X lo4 M-l cm-l for earn- at all t e m p e r a t ~ r e s . ~The J ~ density of liquid ammonia was obtained for various temperatures by interpolation and exThe JourRal of Pbysical Chemisiry, Vol. 79,No. 16, 1975
1.2
1.3 1.4
1.5
1.6 1.7 1.8 1.9
XtP
Flgure
Absorption spectra of earn- at various temperatures: A, 230.11 , 0,0.9'; 0 ,-25.7'; A, -47'; 0 , -75'. The spectra have been displaced 0.2 units upward for each decrease in temperature. 2.
trapolation of the data of Cragoe and Harper.lsa For the temperature range from 23 to -75O, a primary yield and average deviation of g(e,,-) = 3.1 & 0.3 was calculated from the average values of D a t 1.35, 1.45, and 1.75 y a t each temperature and g(e,,-) = 3.2 a t 23°.3 At no temperature studied does the decay of earn- (initial concentration -1.3 p M ) approximate first order. However, a plot of the reciprocal of optical density vs. time becomes increasingly linear with increase in temperature. Such a plot is linear up to 90 and 50% of the decay at 20.6 and -75', respectively. The slope of the linear part of such a plot is equated to k I ~ 1in which 1 is the optical path length (equal to cell width) and h is the specific rate for decay of
Pulse Radiolysis of Liquid Ammonia
1653
the absorption curve (plotted with t or optical density normalized to the maximum and units proportional t o energy on the abcissa) givesll f = c[9n/(n2
+ 2)2]€rnaxW
in which f is the oscillator strength, c is a constant, and n is the refractive index of liquid ammonia. Because tmax and W are temperature independent, the equation can be converted to
+
f = B[9n/(n2 2)2]
551 C
-80
0
I
I
I
I
-70
-60
-50
-40
I
I
I
I
I
I
-30
-20
-10
0
IO
20
A
I
30
I
I I 40
TEMPERATURE , 'C
Figure 3. Position of the absorption maximum (kmax-')of ea,vs. temperature: 0, present work; A , Quinn and Lagowski;" e, Blades and Hodgins;'* 8 , Douthit and Dye;'3 D, Gold and Jolly;'4 A,Corset and L e p o ~ t r e ; ' D, ~ Rubinstein, Tuttle and Golden;l7 X, Belloni and Fradin de la R e n a ~ d i e r e . 'The ~ disagreement among the data for alkali-metal ammonia solutions has been ascribed to concentration efwork [e,-] = [NH4+] = 2-3 p%f. f e c t ~ . ' ~In~present *~
earn- by a second-order reaction with a reactant a t equal concentration. For values of 1 obtained by the spectrophotometric technique and t a t various temperatures and wavelengths obtained as explained above, values of k were calculated from slopes of the linear plots and are summarized in Table I.
Discussion In Figure 3 values obtained in this work for the wave number (Xrnax-l) a t the absorption maximum of earn- are compared with literature values all of which are for alkalimetal solutions12-17 except that of Belloni and Fradin de la Renaudiere.lg The disagreement among the data for alkalimetal solutions has been ascribed to concentration eff e c t ~ .Present ~ ~ , ~values ~ of Xrnax-l agree best with those of Quinn and LagowskilG and of Douthit and Dye13 a t the lower temperature and with that of Corset and Lepoutrelj near room temperature. However, assuming that W is proportional to the bandwidth a t half-maximum the temperature independence of W shown in Table I is in accord with results of Blades and Hodgins12 and of Rubinstein et al.17 rather than with those of Quinn and Lagowski.16 In units of eV, reported bandwidths a t half-maximum of 0.40 a t -7Oo,l2 0.36 a t -71 and -78',12 0.39 at -70',16 and 0.40 a t -75' l 7 are in reasonable agreement with the value of 0.40 a t -75' obtained in this work. The value of E,,, increases with decreasing temperature and, therefore, with increasing static dielectric constant which increases with decreasing temperature.lab As previously noted,ll such a correlation is attributable to dErna,/ d T being determined largely by the effect of temperature on the local dielectric constant (via an effect on orientation of the dipoles in the first solvation shell) which is expected to change in the same direction as the static dielectric constant with change in temperature, both being affected in a similar manner by the same physical factors. Presumably, change in the cavity size of earn- also contributes, but to a smaller degree, to dErna,/dT. The assumption that W is proportional to the area under
in which B is a new constant. Using values of n a t various temperatures calculated from the Lorentz-Lorenz function as described previously,ll the function 9n/(n2 2)2 and, therefore, f decreases by 6% as temperature decreases from 23 to -75'. Such a change probably falls within the error in calculation o f f contributed by errors in the measured W and emax and the calculated n. I t is possible, therefore, that f is constant over the temperature range from 23 to -75'. Invariance of oscillator strength, uncorrected for the change in refractive index, has been reported by Quinn and LagowskilGfor the temperature range from -70 t o -33'. The value of g(earn-) = 3.1 f 0.3 obtained in this work for the temperature range -75 to 23' agrees with values of 3.17 f 0.15 a t -15' and 3 a t -48°.4 Also, the observed temperature independence of g(ea,-) in liquid NH3 is in accord with the results reported for ND3.j The temperature independence of g(e,,-) suggests that the relative roles of various factors involved in the nonhomogeneous kinetics are not affected by a change in temperature from -75 to 23'. A theoretical study and a picosecond experimental study would be of interest. The measured k = 1.3 X 1011 M-l sec-l a t 20.6' in the present work agrees poorly with our previously measured valueg of 2.7 X 10l1 M-l sec-' a t 23'. In the previous study a 3 X 7 mm irradiation cell was used instead of the 1-cm square cell used in the present work. This difference in cell dimensions could be a cause of the poor agreement between the values of k if there were a difference in focusing of the Linac beam that caused the effective optical length of an irradiation cell to be shorter than the corresponding dimension. Comparison of above given values of h a t 20.6 and 23' with the calculated values 1.4 X 10l2 and 2.9 X 10l1 M-l sec-l a t 25' (by application of Smoluchowski-Debye equation) for the specific rates of diffusion-controlled reactions of earn- with a univalent positive ion and a neutral species respectivelyg indicates that a t 20.6 and 23' the observed decay of earn- may be due to a diffusion-controlled reaction of earn- with a neutral species. Present results agree poorly with the following reported values of k in units of 1O1O M-l sec-l and for the temperatures given in parentheses: 2.5(-15°);21 1.1(20'), 0.76(-16') and 0.58(-48°);22 and 1.6(-60°).23 However, h = 3 X 1O1O M-l sec-I a t -45' is in reasonable agreement with the present result. The assumptions involved in calculation of k probably are not valid. A plot of In k vs. 1/T is not a straight line and, as noted previously, a smaller percentage of earndecay conforms to a simple second-order plot as temperature decreases. Such results suggest that the mechanism of decay is complex. The temperature dependence of conformity of earn- decay to a second-order plot could be explained by the reactions
+
earn- + NH2
-
"2-
NHz + NH2 .- NzH4
(2) (3)
The Journal of Physical Chemistry, Vol. 79, No. 16, 1975
P. L. Huyskens and J. J. Tack
1854
if activation energy of reaction 3 were smaller than that of reaction 2 and specific rates of both reactions were comparable a t low temperatures.
Acknowledgments. The authors thank Dr. W. P. Helman for his assistance in computer programming and are grateful to Dr. R. R. Hentz for his continuous guidance during this work. References and Notes (1) 6 n leave from the Pakistan Atomic Energy Commission. (2) The Radiation Laboratory of the University of Notre Dame is operated under contract with the U.S. Energy Research and Development Administration. This Is ERDA Document No. COO-38-984. (3) Farhataziz, L. M. Perkey, and R. R. Hentz, J. Chem. Phys., 60, 717 (1974). (4) J. Belloni, P. Cordier, and J. Delaire, Chem. Phys. Letf., 27, 241 (1974). (5) W. A. Seddon, J. W. Fletcher, F. C. Sopchyshyn, and J. Jevcak, Can. J. Chem., 52, 3269 (1974). (6) J. M. Brooks, Diss. Abstr., 833, 4213 (1973). (7) G. I. Khaikin, V. A. Zhigunov, and P. I. Doiin. High Energy Chem. (Engi. Trans/.),5, 72 (1971).
(8) J. L. Dye, M . G. DeBacker, and L. M. Dorfman, J. Chem. Phys., 52, 6251 (1970). (9) L. M. Perkey and Farhataziz, int. J. Radiat. Phys. Chem., in press. (IO) J. Belloni and E. Saito in "Electrons in Fluids", J. Jortner and N. R. Kestner, Ed., Springer-Verlag, New York, N.Y., 1973, p 461. (11) Farhataziz, L. M. Perkey, and R. R. Hentz, J. Chem. Phys., 60, 4383 (1974). (12) H. Bladesand J. W. Hodgins, Can. J. Chem., 33, 411 (1955). (13) R. C. Douthit and J. L. Dye, J. Am. Chem. Soc., 82, 4472 (1960). (14) M. Gold and W. L. Jolly, inorg. Chem., 1, 818 (1962). (15) J. Corse! and G. Lepoutre in "Solutions Metal Ammoniac", G. Lepoutre and M. J. Sienko, Ed., W. A. Benjamin, New York, N.Y., 1964, p 187. (16) R. K. Quinn and J. J. Lagowski, J. Phys. Chem., 73, 2326 (1969). (17) G. Rubinstein, T. R. Tuttle, Jr., and S. Golden, J. Phys. Chem., 77, 2872 (1973). (18) W. L. Jolly and C. J. Hallada in "Non-Aqueous Solvent Systems", T. C. Waddington, Ed., Academic Press, New York, N.Y., 1965, (a) p 4, (b) p 5. (19) J. Belloni and J. Fradin de la Renaudiere, Nature (London), 232, 173 (1971). (20) I. Hurley, S. Golden, and T. R. Tuttle, Jr., in "Metal-Ammonia Solutions", J. J. Lagowski and M. J. Sienko, Ed., Butterworths, London, 1970, p 503. (21) WA : . Seddon, J. W. Fletcher, J. Jevcak, and F. C. Sopchyshyn, Can. J. Chem., 51, 3653 (1973). (22) V. N. Shubin, V. A. Zhigunov, G. I. Khalkin, L. P. Beruchashvili, and P. I. Doiin, Adv. Chem. Ser., No. 81, 95 (1968). (23) G. I. Khaikin, V. A. Zhigunov, and P. I. Doh, High Energy Chem. (Engi. Trans/.),5, 44 (1971).
Specific Interactions of Phenols with Water Pierre L. Huyskens" and Joris J. Tack University of Louvain (KUL), Department of Chemistry, Ceiest/jnenlaan200-F, 3030-Heveriee, Belgium (Received March 25, 1974; Revised Manuscript Received April 7, 1975) Publication costs assisted by F.C.F.O. and KUL (Belgium)
The extrapolated distribution coefficients P1 a t infinite dilution of phenol derivatives between cyclohexane and water and their transfer enthalpies were determined. The data are compared with those of other kinds of molecules. When the specific interactions with water do not exist or remain constant the influence of inon the transfer free energy AGIO is nearly the same for all substances, creasing the molar volume 6AG1°/6q5 being of the order of 0.04 kcal mol-' cm3. I t is then possible to compute the value AGIO* that a given substance would present if its molar volume would be equal to a given reference value. Differences in AGIO* are mainly due to the specific interactions of the dissolved monomolecules with water. The stabilization through "basic" hydrogen bonds with water increases AGIO* with respect to hydrocarbons without specific bonds of some 2.8 kcal mol-' in anisole, while in diethyl ether this increase is of the order of 4.2 kcal mol-l. For phenols the 0-H bond forms "acidic" hydrogen bonds with water which increases the stabilization; the difference of AGIO* with respect to the hydrocarbons rises to the order of 5.8 kcal. In alcohols the supplementary effect of the acidic 0-H bonds is weaker. Analogous estimations of the effects of the specific bonds on AGIO* were made for amines and anilines. For phenols without ortho substituents the value of AGIO rises as the pK, approaches that of water. Methyl groups in ortho position clearly lower the value of AGlO*. A good approximation for the value of P1 a t 25' for the phenols can be computed from the relation log P1 = -1.06 0.034 - 0.25pKa 0.44north0.The transfer enthalpies AH1 depend in an analogous manner on the molar volume, the pK,, and the presence of ortho substituents.
+
+
Introduction In a previous work1 we showed from the distribution coefficients of anilines between cyclohexane and water that these substances not only engage in hydrogen bonds as proton acceptors but also that the N-H links act as proton donors in hydrogen bonds with the neighboring water moleThe Journal of Physical Chemistry, Vol. 79, No. 16, 1975
+
cules. These deductions were based on the influence of methyl and chloro substituents on the distribution coefficient a t high dilution P1 and on the enthalpy of transfer AH1 of the monomolecules from water to cyclohexane. In the present paper we apply this method to phenol derivatives which are compared with compounds belonging to other groups.
