J . Phys. Chem. 1985,89, 1869-1872 troscopy at temperatures greater than -70 OC, presumably because it decomposes to (CH3)3GeC6H5and a (CH3)3Ge. radi~a1.l~ The fact that sterically unhindered alkoxy1 radicals (which are certainly electrophilic species) are able to add to C6F6 a t room temperature implies that they should also add to C6H6. The following experiment showed that this is indeed the case. When a sample containing benzene which had been saturated by bubbling with bis(trifluoromethy1) peroxide for 10 min at room temperature was irradiated with a 308-nm laser pulse in the laser flash photolysis system, a transient was observed which displayed a UVvisible spectrum identical with that recorded for other cyclohexadienyl radicals. There is a significant activation energy involved in radical additions to C6F6. For all systems for which measurements were
1869
made over a range of temperature the EPR spectrum of the RC6F6radical got stronger as the temperature increased. With R. = c-C,H,. (generated from cyclopropane and tert-butoxyl), for example, only the cyclopropyl radical could be detected at -100 OC, whereas only c-C3H,C6F6. was detectable at room temperature. Finally, we note that none of the RC6F6-radicals studied in this work are presistent, presumably because there is little or no steric hindrance to dimerization through the 4-position even for bulky R groups.
Acknowledgment. We thank Dr. D. Griller for a number of extremely helpful discussions and Dr. J. C. Scaiano for the use of the laser flash photolysis equipment.
D/H Fractionation in Sulfate Hydrate-Water Systems Trinetra M. Pradhananga* and Sadao Matsuo Department of Chemistry, Tokyo Institute of Technology, O-Okayama, Meguro-ku, Tokyo 152 (Received: October 16, 1984)
The D/H fractionation factors (a)of some of the sulfate hydrate-water systems were measured. A dependence of a on cationic parameters and M-H20 distance was found. The D/H fractionation factors of various hydrate-water systems were divided into two groups with respect to their a values: one having a < 1 and the other a > 1. There is almost no change in a for crystal-water systems having Sod2- as the anion of the first transition metallic ion series from Fez+to Zn2+except for Cu2+. The result is related to the common structure of the hydration sphere of cations and the common distance of M-H,O. The exceptional case for Cu2+is attributed to the distorted structure of octahedra surrounding Cuz+. A similar type of behavior of protons leading to the residual entropy has been found in some of the crystal-water systems having a > 1.
Introduction
Information on the fractionation of hydrogen and oxygen isotopes between a hydrate crystal and its saturated solution have been accumulated by, e.g., Day et al.,l Uusitalo: Teis? Watanabe: and H i g u ~ h i . These ~ studies, however, do not give quantitative information on the isotopic fractionation. Barrer and Denny6 found that enrichment of deuterium occurs in most cases in the mother liquors. ONeil' has determined the equilibrium fractionation factors of hydrogen and oxygen isotopes for the icewater system. The fractionation of hydrogen isotopes in the ice-water system and the effect of some of the salts on the fractionation have been studied by Johansson and HolmbergO8 Fractionations of hydrogen and oxygen isotopes in the gypsum-water system were studied by Fontes and Gonfiantini9 and Gonfiantini and Fontes,lo respectively. The D / H fractionation in the trona-water system and its dependence on temperature were studied by Matsuo et al." Stewart12 measured hydrogen and oxygen isotopic fractionation factors for the mirabilite (Na2S04.10H20)-water system. However, water of crystallization in a hydrate crystal is not always in the same geometric and/or energetic site, so that there should be an isotopic site preference in the hydrate crystal, as was pointed out by Heinzinger and Rao.13 The water molecules bound to Na ( 1 ) Day, J. N. E.; Hughes, E. D.; Ingold, C. K.; Wilson, C. L. J. Chem. Soe. 1934, 1593. (2) Uusitalo, E. Suom. Kemistil. B 1958, 31, 362. (3) Teis, R. V. Dokl. Akad. Nauk SSSR 1954,99, 585. (4) Watanabe, T. Japanese Patent, 1952/1963 (ORNL-TR-335). (5) Higuchi, I. J. Chem. SOC.Jpn. 1937, 58, 193. (6) Barrer, R. M.; Denny, A. F. J. Chem. SOC.1964, 4677. (7) ONeil, J. R. J. Phys. Chem. 1968, 72, 3683. (8) Johansson, M.; Holmberg, K. E. Acta Chim. Scand. 1969, 23, 765. (9) Fontes, J. C.; Gonfiantini, R. C. R. Acud. Sci., Paris 1967, 265, 4. (IO) Gonfiantini, R.; Fontes, J. C. Nature (London) 1963, 200, 644. ( 1 1) Matsuo, S.; Friedman, I.; Smith, G. I. Geochim. Cosmochim. Acta 1972, 36, 427. ( 1 2 ) Stewart, M. K. Geochim. Cosmochim. Acta 1974, 38, 167.
0022-3654/85/2089-1869$01.50/0
polyhedra in borax were found to be much more depleted in deuterium compared with the OH radicals of the polyanionic group.I4 Similarly, in the case of CuS04.5H20, the water molecules bound to Cu2+were found to be depleted in deuterium relative to the water molecule bonded to SO?-through hydrogen bonding.I5 The study of the change in fractionation factor with different structure will shed light on the study of intracrystalline site preference. In this article, the D / H fractionation factor in some crystalline hydrate (mostly sulfate)-water systems and its dependence on various factors are discussed.
Experimental Section The crystalline hydrate was synthesized from its slightly supersaturated solution at 25 OC in a stoppered conical flask. Attainment of isotopic exchange equilibrium between the crystal hydrates and their saturated solutions has been claimedid but not proved. There is no difference in the D / H fractionation factor, a,with a change in the degree of supersaturation for the boraxwater system.I4 Inasmuch as it is not possible to see a change in a with a change in degree. of supersaturation for all the crystal hydrate-water systems studied, the crystals were grown from solutions with a low degree of supersaturation, Le., with a slow crystallization rate, so that equilibrium between the crystal hydrate-water system can be assumed. The crystalline hydrates synthesized in this way were not taken out of the flask before use in order to prevent the hydrates from decomposing and/or to avoid the chance of exchange between the water of crystallization and atmospheric water vapor. The method for the measurement of the bulk water of crystallization was the same as that of Pradhananga and Matsuo.14 (13) Heinzinger, K.; Rao, T. S. Z . Nuturforsch., A 1967, 2 2 4 2111. (14) Pradhananga, T. M.; Matsuo, S. J . Phys. Chem. 1985, 89, 72. (15) Kita, I.; Matsuo, S. J. Phys. Chem. 1981, 85, 792.
0 1985 American Chemical Society
1870 The Journal of Physical Chemistry, Vol. 89, No. 10, 1985
Pradhananga and Matsuo
TABLE I: D/H FractionationFactor, a, of Some of the Crystalline Hydrate (with SolG as Anion)-Water Systems crystal aO M source cationic radius,Is A BeS04.4H20 0.948 Barrer and Denny6 0.35 0.956 at 25 OC this study Na2S04.10H20 1.017 at 25 OC StewartI2 1.10 1.014 at 25.5-26.5 OC Johansson and Holmberg* this study 1.013 at 25 OC MgS04.7H20 0.999 at 25 OC this study 0.80 AI~(SO~),*IZH~O 0.966 Barrer and Denny6 0.61 CaS04.2H20 0.985 Fontes and Gonfiantini9 1.08 CuS04.5H20 0.979 at 25 OC Kita and Matsuo15 0.8 1 0.978 at 14.0-26.0 “C Johansson and Holmbergs NiSO4.7Hz0 0.995 at 25 OC this study 0.77 The amount of crystalline hydrate taken was such that the water given on complete dehydration was about 5 mg. The sample was heated under vacuum to ensure complete dehydration of the water of crystallization. The temperature was raised gradually to about 300 ‘C. A!l the hydrates examined in this study released their water of crystallization completely below 300 OC.I6 The dehydrated water was condensed at liquid nitrogen temperature, and then converted quantitatively into hydrogen gas by the method of Bigeleisen et al.,” Le., passing the water vapor through uranium metal at 750 ‘C. The amount of hydrogen gas thus obtained was measured and the D / H ratio was determined by a mass spectrometer (Hitachi-RMD). The relative deviation of the D / H ratio from that of the standard is given by the h o t a t i o n , which is defined as
The standard used is SMOW (standard mean Ocean water). The overall error is *1.5?4~. The fractionation factor, a,is defined as 6Dwc + 1000 a = (2) 6DML 1000
+
where 6D,, = 6D of the water of crystallization and bDML = 6D of the mother liquor. The value of a,obtained for each crystal-water system, was measured more than twice to check the reproducibility. The overall error included in a in this study is k2L.
Results and Discussion Inorganic crystalline hydrates can be classified into two groups with respect to their D / H fractionation factors between the hydrate and the mother liquor. One is the crystalline hydrates with higher D / H ratio than that of the mother liquor and the other with lower D / H ratio. In ice-water, mirabilite-water, natronwater and borax-water systems deuterium is enriched in the hydrate. In other systems studied, deuterium is depleted in the hydrate. In general, one can expect the fractionation factor of a crystal hydrate-water system to depend on various factors such as the temperature of crystallization, ionic parameters of cations, the structure of the hydrate crystallized, and the configuration of aquo ions in solution. The variation of D/H fractionation factors with these factors provides a basis for elucidating the behavior of hydrogen isotopes in crystal hydrates. The fractionation factors of some of the crystalline hydrates as the anion are given in Table I. The plot of the having Sod2ratio of the charge to the ionic radius, q / r , of the cation vs. the fractionation factor of the crystal-water systems given in Table I is shown in Figure 1. It is clear that a decreases with an increase in q / r , as seen in Figure 1. The dependence of a on q / r may be attributed to the change in bonding strength of water molecules (16) ‘Handbook of Chemistry and Physics”, 48th 4.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1967-1968; p B149. (17) Bigeleisen, J.; Perlman, M. L.; Prosser, H. C. A n d . Chem. 1952,24,
1356. (18) Whittaker, E. J. W.; Muntus, R. Geochim. Cosmochim. Acro 1970, 34, 945.
1.02
!.O1 6 L1 0
1.00
e
,
,
tk
,
I
,
.
,
I
.
,
.
--
2 0.99 -
.-5 0.98 0,97 .v 2 0.96 e
.
- -
u.
0.95
0
1.0
2.0
3.0
40
50
6.0
qr (%-I) Figure 1. Change in fractionation factor, a,of some sulfate hydratewater systems with the change in the ratio of ionic charge to ionic radius of the metallic ion: (1) BdO4.4H20, (2) A12(S04)3+zH20, (3) CuS046H20, (4) CaSO4.2H2O,( 5 ) NiS04-7H20,(6) FeS04.7H20, (7) CoS04.7H20, (8) ZnS04.7H20, (9) MgS04.7H20, (IO) Na2S04. 13H20. t!
c
e
4
-2
.o 1.028 U
1.3
1.5
1.7
1.9
Figure 2. Change in oxygen isotopic fractionation factor, a,of alkaline earth metal carbonatewater systems with the change in the ratio of ionic
charge to ionic radius of the metallic ion (cited from ONeil et aLzo):(1) BaCO,, (2) SrCO,, (3) CaCO,. to the metallic ion. It is known that cations of high q / r are bound to water that is enriched in I8O relative to water in sol~tion.’~ A quite good correlation between the q / r of alkaline earth metal carbonates and the 180/’60 fractionation factor in the carbonate-water system was found by O’Neil et alez0 They showed that a increases with an increase in q / r , as reproduced in Figure 2. If the analogy is also valid in the case of hydrogen isotopes for sulfate hydrate-water systems, a should increase with an increase in q / r of the cation. However, the observed trend is just the opposite as seen in Figure 1. The deviation of a of CuS04.5H20-water system from the regression line in Figure 1 may be due to the distorted structure of the octahedron around Cuz+caused by the Jahn-Teller effect. The anomalous IR spectra of Cuz+ aquo ion in the nearly saturated solution has also been attributed to the Jahn-Teller effect.21 On the other hand, the deviation of a for the CaS04.2H20-water system may be ascribed to the fact that tetrahedral or octahedral arrangements are not (19) Taube, H. J . Phys. Chem. 1954, 58, 523. (20) ONeil, J. R.; Clayton, R. N.; Mayeda, T. K. J . Chem. Phys. 1969, 51, 5547. (21) Hester, R. E.; Plane, R. A. Znorg. Chem. 1964, 3, 768.
The Journal of Physical Chemistry, Vol. 89, No. 10, 1985 1871
Sulfate Hydrate-Water Systems , - ,ZO. I
TABLE 11: D/H Fractionation Factor, a,of a Sequence of Transition Element Sulfate-Water Systems at 25 OC
t
I
d
6
I
0951
1.40
'
'
'
'
i
'
'
'
I
1.80 220 2.60 3.00
hydrates FeS04-7H20 CoS04.7H20 NiS04.7H20 CuS044H20 ZnSO4.7H20
a
0.992 0.994 0.995 0.979 0.992
source this study this study this study Kita and Matsuols this study
cationic radius.'* A
0.86 0.83 0.77 0.81 0.83
TABLE III: Some of the Crystal-Water Systems with D/H Fractionation Factor Greater Than Unitv
M-HzO,"A
Figure 3. Change in fractionation factor, a,of some sulfate hydratewater systems with the change in M-H20 bonding length: (1) BeS04e4H20,(2) A12(S04)3.nH20,(3) CuS04-5H20, (4) CaS04.2H20, (5) NiS04.7H20, (6) FeS04-7H20,(7) MgS04.7H20, (8) Na2S04-10H20.
possible for Ca2+in gypsum due to an insufficient number of water molecules.22-24 The bond energy between the metallic ion and HzO is dependent on the M-HzO distance.25 We can expect a to depend on the M-H20 distance or the bonding strength of water molecules to the metallic ion. The plot of the M-H20 distance of a hydrate with SOf as the anion against a is shown in Figure 3. A positive correlation can be seen in Figure 3. However, the hydrogen isotopic effect in water molecules bound to metallic ion can be regarded as a second-order effect with respect to M-H20 bond length. In other words, the oxygen isotopic effect should be the primary effect because the oxygen of water molecules is always bound directly to the cation. In addition, some of the water molecules are out of the hydration sphere of the cation except for BeS044H20. However, the scattered points from the regression line in Figure 3 seem to be related to the factors mentioned previously. Information on isotopic fractionation between water bound to metallic ion and the mother liquor is needed in order to obtain the effect of the crystal field on isotopic fractionation. In other words, it is important to obtain quantitative information on isotopic site preference to combine crystal field theory with the isotopic behavior of water molecules bound to metallic ions. In general, water molecules that are not bound to metallic ions have higher 6D values than those bound to the metallic i ~ n . ' ~ J ~ The effects of the size of cation and anion on oxygen isotopic behavior were studied in some of the aqueous solutions of alkali metal salts by Bopp et alez6 Their results show that there is a decrease in oxygen isotopic fractionation with an increase in ionic radii of the cations and anions in aqueous solution. The D / H fractionation factors of the crystal hydrate-water systems including sulfates of transition metals from Fez+ to Zn2+ were measured to see the effect of the crystal field stabilization energy of metallic ions. The results are given in Table 11. There is no significant change in a from Fe2+ to Zn2+except for Cuz+. The ionic radius of the metallic ions are also given in the Table 11. There is not much change in the ionic radius of these divalent ions in harmony with the constant (within the experimental error) a values. The bond distances between ions of Co2+,Ni2+, and Zn2+and water molecules in the hydration shells were measured by Shapovalov et al.27*28 to be the same, Le., 2.15 A. Although hydrogen isotopic effects may be secondary with respect to bond (22) Atoji, M.; Rundle, R. E. J . Chem. Phys. 1958, 29, 1306. (23) Cole, W. F.; Lancucki, C. J. Nature (London), Phys. Sci. 1972,238, 95. (24) Cole, W. F.; Lancucki, C. J. Nature (London),Phys. Sci. 1973,242, 104. (25) Basolo, F.; Pearson, R. G. 'Mechanisms of Inorganic Reactions", 2nd ed.;Wiley: New York, 1967; p 62. (26) Bopp, P.; Heinzinger, K.; Klemm, A. Z . Naturforsch., A 1977,32A, 1419. (27) Shapovalov, I. M.; Radchenko, I. V.Rum. J . Struct. Chem. 1971,12, 769. (28) Shapovalov, I. M.; Radchenko, I. V.; Lesoristskaya, M. K. Russ. J . Slruct. Chem. 1972, 13, 140.
ice
1.019 at 1.019 at 1.021 at Na2S04.10H20 1.017 a t 1.019 at 1.013 at Na2COJ.10H20 1.015 at 1.017 at Na2B407.10H20 1.005 at
0 "C 0 OC 0 OC 25 OC 0 OC 25 OC 5 *C 10 OC 25 OC
O'Neil' WestonJs Posey and SmithJ6 Stewadz StewartI2 this study this study this study Pradhananga and MatsuoI4
length, as mentioned previously, the constant a values from Fe2+ to Zn2+except for Cu2+in the first transition metallic ion series can be related to the similar ionic radii of the cations as well as the common structure of their aquo complexes in the solution. On the other hand, the singular value of a for the CuS04. 5HzO-water system may be attributed to the distorted structure of octahedra around Cu2+ and also the distorted shape of [Cu(HzO),]2+ in the s o l ~ t i o n . ~It~ should - ~ ~ be mentioned that all these sulfate hydrates have similar structure~,~O-~~ Le., the metallic ions are surrounded octahedrally by six water molecules and the seventh water molecule is not coordinated with the metallic ion, resembling that of the fifth water molecule of C U S O ~ ~ ~ H ~ O . ~ ~ The heptahydrates of nickel sulfate, magnesium sulfate, and zinc sulfate are i s o ~ t r u c t u r a l . ~The ~ ~cell ~ ~ dimension of ferrous sulfate heptahydrate is different from those of these three isostructural heptahydrates," whereas the M-H20 distances in all four of these crystalline hydrates lie in the range of 2.05-2.12 A.30-32,34 Table I11 gives some of the crystalline hydrates in which the solid phase is enriched in deuterium compared with the mother liquor; Le., the fractionation factor is greater than unity. Substances of this type are ice, Na2SO4-10H20,Na2CO3-10H20,and Na2B407*10H20.The fractionation factors, a , are in the range of 1.013-1.021 except for borax. There is residual entropy in ice due to the disordered proton or the alternative arrangement of hydrogen atoms in the struct ~ r e . ~Residual ~ r ~ ~ entropy has also been observed in sodium sulfate d e ~ a h y d r a t e . ~The ~ , ~structure ~ of sodium sulfate decahydrate was given by Ruben et a1.4' and Levy and Lisensky!2 The presence of residual entropy in this compound has been explained by an alternating arrangement of hydrogen a t ~ r n s . ~ In ' , ~this ~ compound, a sodium ion is surrounded by water molecules oc(29) Ohtaki, H.; Yamaguchi, T.; Maeda, M. Bull. Chem. SOC.Jpn. 1976, 49, 701.
(30) Beevers, C. A.; Schwartz, C. M. Z.Kristallogr. 1935, 91, 157. (31) Baur. W. H. Acta Crvstallopr. 1964. 17. 1167. (32) Wyckoff, R. W. G. "Crystal Structures", 2nd ed.; Interscience: New York, 1965; Vol. 3, p 837. (33) Beevers, C. A.; Lipson, H. Proc. R . SOC.London, Ser. A 1934,146, 570. . .
(34) Baur, W. H. Acta Crystallogr. 1964, 17, 1361. (35) Weston, Jr., R. E. Geochim. Cosmochim. Acta 1955, 8, 281. (36) Posey, J. C.; Smith, H. A. J. Am. Chem. SOC.1957, 79, 5 5 5 . (37) Pauling, L. J . Am. Chem. SOC.1935, 57, 2680. (38) Giauque, W. F.; Stout, J. W. J . Am. Chem. SOC.1936, 58, 1144. (39) Pitzer, K. S.;Coulter, L. V. J . Am. Chem. SOC.1938, 60, 1310. (40) Brodale, G.; Giauque, W. F. J . Am. Chem. SOC.1958, 80, 2042. (41) Ruben, H. W.; Templeton, D. H.; Rosenstein, R. D.; Olovsson, I. J . Am. Chem. SOC.1960, 83, 820. (42) Levy, H. A,; Lisensky, G. C. Acta Crystallogr., Sect. B 1978, 8 3 4 , 3502.
1872
J . Phys. Chem. 1985,89, 1872-1875
tahedrally, and these octahedra share edges making a chain of N a polyhedra as in borax.43 The other two water molecules are held by hydrogen bonding to S042-as in the fifth water molecule of copper sulfate ~ e n t a h y d r a t e The . ~ ~ crystal structure of sodium carbonate decahydrate may be taken as the distorted NaCl type structure with [Na2(H20)lo]2+and C 0 3 2 -ions.44 The [Na2(Hz0)lo]2' ion is formed by sharing one edge of two Na polyhedra. A similar type of dielectric dispersion behavior was found in sodium sulfate decahydrate and s 4 i u m carbonate decah~drate,"~ so that disorder in the configuration may also be possible in sodium carbonate decahydrate." Hence, one of the factors for the higher (43) Morimoto, N. Min. J. 1956, 2, 1. (44) Taga, T. Acta Crystallogr., Secr. B 1969, B25,2656. (45) Kiriyama, R.; Saito, Y . Bull. G e m . SOC.Jpn. 1953, 26, 531.
value of cy, i.e., greater than unity, may be related to the disordered protons in these inorganic crystalline hydrates. The dielectric dispersion behavior found in sodium sulfate decahydrate and sodium carbonate decahydrate has not yet been found in borax.45 It should be mentioned that the average Na-H20 distances in these three crystalline hydrates are in the range 2.42-2.44 The deviation of the a value in borax from those of the other three may be explained by the absence of residual entropy or by the strong isotopic site preference between Na2.8H20 and B,O,(OH),, giving an a value accidentally quite close to unity, the latter being suggested by Pradhananga and Matsuo.14 Registry NO.Al2(S04)ynH20, 17927-65-0;CuS04.5H20, 7758-99-8; CaS04.2H20, 10 101-4 1-4; NiS04.7H20, 10 101-98-1; FeSO4.7H20, 7782-63-0; CoS04.7H20, 10026-24-1; ZnS04-7H20,7446-20-0; MgSO4.7H,O, 10034-99-8; Na2S0,.10H20, 7727-73-3; H2, 1333-74-0.
Solvent Isotope Eftect on Electron-Transfer Processes J. Lee and G . Wilse Robinson* Picosecond and Quantum Radiation Laboratory, Texas Tech University, Lubbock, Texas 79409 (Received: October 25, 1984)
Intermolecular electron ejection from indole is investigated in solvent mixtures of H 2 0 and D20. The activation energy is found to remain constant, 10.8 f 0.6 kcal/mol, for all mixtures. However, the frequency factor decreases linearly as a function of DzOconcentration and is 3 times smaller in pure DzO than in pure HzO.This isotope factor is attributed to a Franck-Condon effect accompanying the transition from neutral to ionized forms, the electron in the latter case being strongly coupled to a cluster of 4 1 water molecules.
*
Introduction
Electron ejection has been determined to be a major radiationless deactivation process in the excited state of indole in aqueous solutions. This process has been shown to be sensitive to environmental solvent structure and temperature.',2 The associated large activation energy has been attributed2' to the energy required to reorganize the hydrogen bonding in adjacent water molecules so as to facilitate electron ejection from indole. The importance of a specific water structure, acting as electron acceptor, has been demonstrated quantitatively in water/methanol solvent mixture^.^ Addition of alcohol, which has a different molecular structure and physical properties from water, was found to break the water structure and thus to inhibit the electron ejection process. Theoretical analysis of the decay kinetics using a Markov random walk scheme has successfully been applied to the experimental data. A water cluster of 4 f 1 members was determined to be the effective electron acceptor. In contrast to alcohol, deuterium oxide (D20) has a molecular structure and physical properties close to those of ordinary water. Hence, it is of interest to see how the heavier isotope in the solvent effects the electron ejection process in indole and how the Markov random walk theory applies to this system. Experimental Section
Indole was purchased from Eastman Kodak. HPLC grade water and gold label grade deuterium oxide (99.8%) were purchased from Fisher and Aldrich, respectively. All chemicals were used without further purification. H 2 0 / D 2 0 mixtures of 0, 25, (1) Kirby, E. P.; Steiner, R. F. J . Phys. Cfiem. 1970, 74, 4480. (2) Klein, R.; Tatischeff, I. Cfiem. Phys. Leu. 1977, 51, 333. (3) Lee, J.; Robinson,G . W. J . Cfiem. Phys. 1984, 81, 1203.
0022-3654/85/2089-1872$01.50/0
50, 75, and 100% were prepared by volume fraction. The concentration of indole in all mixtures was maintained at 5 X M. Details of the apparatus for measuring fluorescence quantum yields, a, and emission lifetimes, 7,were discussed previ~usly.~ Quantum yields at temperatures ranging from 0 to 65 OC were measured for each solvent mixture. Since the radiative rate constant (k,) is insensitive to temperature and to the solvent isotope effect,] only lifetimes in the two pure solvents at room temperature (20 "C) are needed in order to obtain the nonradiative rate constants from the quantum yield data over the whole range of solvent concentrations.
Results Neither the absorption nor the emission spectrum of indole shows a frequency shift upon addition of D20. Fluorescence quantum yields, using indole in H 2 0 (0.3 1) as a reference, are plotted for three different temperatures as a function of D 2 0 concentration in Figure 1. At each temperature, the quantum yield increases as the D 2 0 concentration increases, exhibiting its greatest value in the pure D 2 0 solvent. On the other hand, in all of the solvent mixtures, the quantum yield decreases as the temperature increases. Lifetime measurements at 20 "C give 7 = 5.6 ns for indole in pure D20. The radiative rate (k, = @.?I) is determined to be 8.21 X lo7 s-I, which is comparable to the value 8.29 X lo7 s-I already reported for indole dissolved in H20.3 Hence, a constant value of k, = 8.29 X lo7 s-I was assumed for all H 2 0 / D z 0mixtures. The overall decay rate constant k( 7') H 7-l for indole in solvent mixtures can be expressed by k ( T ) = k, + k,,' + k, exp(-pAE) (1) where k, and k,,' are temperature-independent radiative and 0 1985 American Chemical Society
