302
J . Phys. Chem. 1990, 94, 302-308
for rotation_around the axis defined by the corresponding angular momenta J A = FCM,* X @ and J B = ?cM,B X @,respectively (@is the linear momentum of the fragments). xA and xBrefer to the angle between the vectors ?CM,A and F,-M,B, respectively, and @. Acknowledgment. This work was supported by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen
Forschung. We thank Dr. M. P. Docker for critically reading the manuscript and Mr. R. Pfister for synthesizing the alkyl nitrites. Registry No. IPN, 541-42-4; TBN, 540-80-7; ETN, 109-95-5; MEN, 624-91-9; EtO, 2154-50-9; i-Pro, 3958-66-5; t-BuO, 3141-58-0; NO, 10102-43-9; CH3CH0, 75-07-0; (CH3)2C0,67-64-1; methoxy radical, 2143-68-2; methyl radical, 2229-07-4.
Solvent Effects on the Reactivity of Solvated Electrons with Organic Solutes in MethanoVWater and EthanoVWater Mixed Solvents Charles C. Lai and Gordon R. Freeman* Department of Chemistry, University of Alberta, Edmonton, Canada T6G 2G2 (Received: May 4 , 1989; I n Final Form: July 10, 1989)
The present work extends the study of solvent effects ( J . Phys. Chem. 1988, 92, 1506 and 5142) to the binary solvents methanol/water and ethanol/water. The composition dependences of energies and entropies of activation for inefficient reactions (e; + toluene or phenol) are quite different from those for efficient reactions (e; + nitrobenzene or acetone). In ethanol/water, the inefficient reaction of e[ with toluene makes an interestingchange from being mainly entropy driven in pure water solvent (as,'= -51 J/(mol-K) and E z = 18 kJ/mol) to being mainly enthalpy driven in pure ethanol ( E 2 = 30 kJ/mol and AS2* = -1 1 J/(mol.K)). Correlations of e; reactivity with solvent viscosity and dielectric permittivity are discussed.
Introduction The solvent structure of alcohol/water mixtures can manifest itself in a large variety of physical including the optical absorption energy of solvated electrons.6 We have investigated the kinetic behavior of solvated electrons with polar and charged solutes in water mixtures with primary,',* seconda r ~ , ~and . ' tertiary ~ alcohols."J2 Differences of kinetic behavior within and between different alcohol/water series are due to differences in diffusion rates of the reactants, solvation energies of the electrons, and dielectric properties of the medium. The size and rigidity of the alkyl group on the alcohol molecule seem to have a direct influence on these factors. The present work extends the study of solvent structure effects on the reactivity of solvated electrons to the water mixtures of the two smallest alcohols, methanol and ethanol. Reaction rate constants of solvated electrons with scavengers of different efficiencies are reported as a function of solvent composition and temperature. Experimental Section Materials. Methanol (Aldrich, spectrophotometric grade, Gold Label, 99.9%) was treated for 3 h under argon (Linde, ultrahigh-purity grade, 99.999%) with sodium borohydride (Fisher ( I ) Symons, M. C. R.; Blandamer, M. J . In Hydrogen-Bonded Solvent Systems; Covington, A. K., Jones, P., Eds.; Taylor and Francis: London, 1968. (2) Berntolini, D.: Cassettori, M.: Salvetti, G. J . Chem. Phys. 1983. 78,
365. (3) Nakanishi, K.; Ikari, K.; Okazaki, S.; Touhara, H. J . Chem. Phys. 1984, 80, 1656; 1984, 81, 4065. (4) Perl, J . P.; Wasan, D. T.; Windsor, P., IV: Cole, R. H. J . Mol. Liq. 1984, 28, 103. (5) Madigosky, W. M.; Warfield, R. W. J . Chem. Phys. 1987, 86, 1491. (6) Leu, A. D.; Jha, K . N.; Freeman, G. R. Can. J . Chem. 1982,60,2342. ( 7 ) Mahan. Y . : Freeman, G . R. J . Phys. Chem. 1985,89,4347. (8) Mahan. Y.; Freeman, G. R. J . Phys. Chem. 1987, 91, 1561. (9) Mahan, Y.; Freeman, G. R. Can. J . Chem. 1988, 66, 1706. (10) Senanayake, P. C.; Freeman, G. R. J . Chem. Phys. 1987, 87, 7007. ( I I ) Senanayake, P. C.; Freeman, G. R. J . Phys. Chem. 1987, 91, 2123. On p 2125. the values of R , + Rs in zone c should read 1.2 nm for t BuOH/water and 1.0 for I-PrOH/water. These are larger than the 0.28-nm
radius of nitrobenzene, so the reaction occurs at essentially every encounter. (12) Senanayake. P.C.; Freeman, G. R. J . Phys. Chem. 1988, 92, 5142.
0022-3654/90/2094-0302$02.50/0
TABLE I: Symbols in Figures 1-4, Representing Mole Percent of Water in Methanol or Ethanol
-symbol
%H20
symbol
% HzO
symbol
% H20
X
0
v
45
0
IO
60
V
15 20
A A
0 0 0 0 0
95 97 98 99
+ +
0
30
70 80
90
100
Scientific, reagent grade; 1 g/L of methanol) at 328 K. It was then fractionally distilled under argon, through a 52 X 2.5 cm column packed with 6-mm glass beads, discarding the first 20% and last 35%. The middle fraction was collected and kept in an argon-pressurized syphon system. The water content determined by Karl-Fisher titration was 0.04 mol %. Absolute ethanol (reagent grade) was obtained from the U S . Industrial Chemical Co. Experience had shown this to be the best available,13having reported maximum impurity levels of 50 ppm of water, 5 ppm of methanol, and less than 1 ppm of benzene, halogen compounds, or carbonyl compounds. Treatment by the purification method used for methanol resulted in no improvement in purity. The water content measured by Karl-Fisher titration was 0.04 mol %. The solvated electron half-life after a 100-ns pulse of 1.9-MeV electrons (-2 X 10l6eV/g) at 298 K was 4 ~s for methanol and 6.5 p s for ethanol. Water was purified in a Barnstead Nanopure I1 ion-exchange system. The e; half-life after a 100-ns pulse of radiation was 20 PS.
Toluene (Aldrich, Gold Label) was distilled over sodium under argon. Phenol (Aldrich, 99+%) was sublimed three times under reduced pressure (100 Pa) at 308 K. Nitrobenzene (Aldrich, 99+%, Gold Label) and acetone (Aldrich 99+%, spectrophotometric grade, Gold Label) were used as received. Techniques. Sample preparation and spectrophotometry for solute concentration measurements are as described in ref 7 . Methods of irradiation, dosimetry, and optical measurements were (13) Akhtar, S. M. S.; Freeman, G. R. J . Am. Chem. SOC.1971, 75, 2756.
0 1990 American Chemical Society
Solvated Electrons in CH,OH/H,O and CH3CH20H/H20
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 303
105
1 06
1000/T (K) Figure 1. Arrhenius plots of &*(e; + S) in methanol/water mixed solvents (S = (a) nitrobenzene and (b) toluene). Symbols and solvent compositions are listed in Table 1.
1000/T (K) Figure 3. Arrhenius plots of &,(e; + S) in ethanol/water mixed solvents (S = (a) nitrobenzene and (b) phenol). Symbols and solvent compositions are listed in Table I.
't
I
yLx0.5 x0.25
105
l 0 i ! 8 '310'312 '314 316' 318 '410
I
'
' 310 '312 '314'3)6'318 '410 '
'
102
1000/T (K) Figure 2. Arrhenius plots of k2(e; + S) in methanol/water mixed solvents (S = (a) acetone and (b) phenol). Symbols and solvent compositions are listed in Table I.
1000/T (K) Figure 4. Arrhenius plots of kz(e; + S) in ethanol/water mixed solvents (S = (a) acetone and (b) toluene). Symbols and solvent compositions are listed in Table 1.
similar to those described in ref 14. Solvent Properties. Values of the viscosities and relative permittivities of the mixed solvents were obtained from ref 15.
mol), and AS2 (J/(mol.K)) in methanol/water and ethanol/water mixtures are listed in Tables I1 and 111, respectively.
Results The first-order decay rate constants kl of e[ were measured for at least five different solute concentrations in a given solvent. Second-order rate constants k2 were obtained from the slopes of plots of k , against solute concentration. Arrhenius plots of k 2 obtained in methanol/water mixtures for nitrobenzene, acetone, phenol, and toluene are given in Figures 1 and 2. (Tables of the k, values in each solvent are available as supplementary material.) Those obtained in ethanol/water mixtures are given in Figures 3 and 4. The Arrhenius parameters A2 (m3/(mol.s)), E2 (kJ/ (14) Bolton, G. L.; Freeman, G. R. J . Am. Chem. Soc. 1976, 98, 6825. ( 1 5) Timmermans, J. Physico-Chemical Constants of Binary Mixtures; Interscience: New York, 1960; Vol. 4.
k2 = A2 exp(-E,/RT) AS2* = 19(log A2 - 9.8), at 298 K
(1)
(2)
The values of AS2* are arbitrarily referred to a standard state co = 1 mol/dm3 for comparison with earlier A, = ( k e T / hco) ~ x P ( @ , */ R ) .
Discussion The rate constants for electron capture by inefficient scavengers display greater dependence on solvent composition than do those of the efficient scavengers (Figure 5). Efficient Scauengers. The reaction of solvated electrons with nitrobenzene is nearly diffusion ~ o n t r o l l e d At . ~ ~298 ~ ~K,~ the rate constants for nitrobenzene in ethanol/water mixtures are about 0.75 times those in methanol/water mixtures.
Lai and Freeman
304 The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990 TABLE 11: e; Reaction Rate Parameters in MeOH/Water at 298 K ni trobenzeneg mol 9c water lla C* k2C E,d log A2C 0 15 30 45 60 70 80 90 97 98 99 IO0
mol 9c water 0 15 30 45 60 70 80 90 97 100
0.557 0.760 1.000 1.265 1.525 1.655 1.585 1.340 1.040 0.995 0.940 0.895
32.6 37.0 42.0 48.0 54.5 59.5 65.5 71.5 76.5 77.3 78.0 78.5
E,d,k 143 140 139 139 139 139 139 137 134 132
End.' 10.7 13.6 16.2 18.6 20.8 20.9 19.9 19.0 17.0 15.4
2300 2100 1900 1800 1900 1900 2200 2500 3300 3300 3500 3700
acetoneh As2*/ -10 -3 2 7 12 13 13 14 14 14 13 12
9.30 9.66 9.92 10.17 10.41 10.50 10.50 10.51 10.54 10.55 10.47 10.42
11 13 15 17 18 18 18 18 17 17 17 16
k2C 490 530 520 530 520 5 20 530 640 760
log A2C 8.57 8.93 9.22 9.62 9.74 9.74 9.76 9.83 9.62
a52*/
11 12 14 17 17 17 17 17 16
770
15
9.45
-7
k2' 0.21 0.27 0.28 0.36 0.53 0.52
Ezd 28 27 26 24 23 21
log A2C 8.26 8.21 8 .OO 7.72 7.68 7.41
ASl*/ -29 -30 -34 -39 -40 -45
0.79
19
7.22
-49
1.1
18
7.14
-5 1
phenol' 1.1 1 .o 0.85 0.77
0.80 0.86 1.1 1.4 2.3 3.0
19 19 18 19 19 20 20 20 19 18
-1 1
-3 -1 -1 -1 1
-3
toluenej As2* -48 -49 -50 -50 -49 -46 -44 -40 -40 -42
log 7.25 7.23 7.15 7.17 7.24 7.38 7.48 7.72 7.69 7.58
klC
-23 -16
Viscosity, mPa.s (cP); ref 15. bStatic dielectric permittivity, ref 15. lo4 m3/(mol.s). dkJ/mol. e A 2 units, m3/(mol-s). /J/(mobK). mol/m3). hConcentrations,0.17-1.2 mol/m3. 'Concentrations, 5-30 mol/m3. 'Concentrations, 0.2-370 mol/m3. gConcentrations,6-42 kReference6: 96.5 kJ/mol 1.00 eV/entity. 'Calculated from data in ref 15.
-'?
3-
E -
m '
E
E
z 2 --
r-
Y
N
x
i
104
lo3;
' 2 b ' 4 0 ' Q O ' s b '
"
20 I
'
40 " 60 '
" 80
-
-
" 100
Mol % H,O
Figure 5. Dependence of k 2 at 298 K on solvent composition. S : 0,
nitrobenzene; A, acetone; 0, phenol; 0 , toluene. Solvent: (a) methanol/water; (b) ethanol/water. The diffusion-controlled rate constant kz(e; + S) for reaction of e; with a polar solute S is approximately related to the solvent viscosity 7 and relative permittivity 6 by the SmoluchowskiStokes-Debye e q ~ a t i o n ' ~ . ~ ~ J '
where N A is Avogrado's constant, kB is Boltzmann's constant, T is temperature, rx and R x are the effective radii of species X for diffusion and reaction, respectively, and f = [U(R)/kBT][eu(R)/kBT - I]-] (4) with
,,(~o-~P~s) Figure 6. Dependence of k2 for nitrobenzene at 298 K on the viscosity of mixed solvents: 0, methanol/water; A, ethanol/water. The mole percent of water is indicated beside each point.
is the angle of approach of e; to the dipole axis of S, to is the permittivity of vacuum, and R = Re + Rs is the center-to-center distance between es- and S at the instant just before reaction occurs. The positive end of the nitrobenzene molecule is the benzene ring, which would accept an electron. As e; and a nitrobenzene molecule diffuse together, the dipole would tend to align with the field of the electron, which favors 0 = 0 radians at the distance of closest approach R . Thus, eq 5 becomes eq 5', which givesf
>
1.
U(R)
U ( R ) = -p(
COS
0/41rtoeR'
(5)
where g is the dipole moment of S,Is ( is the electron charge, 0 (16) Debye, P. Trans. Electrochem. SOC.1942,82, 265. (17) Maham, Y.; Freeman, G.R . J . Phys. Chem. 1988, 92, 1506.
-p(/4i?€ocR2
(5')
Equation 3 suggests an approximately 7-l dependence of k2 for a diffusion-controlled reaction. For (e; + nitrobenzene), k 2 varies (18) Handbook of Chemistry and Physics, 59th ed.; Weast, R. C., Astle, M . J., Eds.; CRC Press: Boca Raton, FL, 1978.
Solvated Electrons in C H 3 0 H / H z 0 and CH3CHz0H/Hz0
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 305
TABLE 111: e; Reaction Rate Parameters in EtOH/Water at 298 K
nitrobenzeneg E2d log A2C
mol %
va
cb
k2C
1.10 1.23 1.38 1.56 1.87 2.18 2.34 2.34 1.91 1.44 0.895
24.3 25.5 27.0 29.5 34.5 42.5 49.5 57.0 67.0 72.5 78.5
1600 1400 1500 1600 1500 1600 1400 1700 2200 2800 3700
Epk
E$’ 14.1 15.6 16.7 18.2 20.4 23.0 25.5 27.3 26.3 21.3 15.4
kzC
water 0
IO 20 30 45 60 70 80 90 95 100 mol % water
0 10 20 30 45 60 70 80 90 95
126 122 124 128 130 139 139 139 138 136 132
100
13 14 15 15 17 18 20 21 20 18 16
9.40 9.52 9.78 9.88 10.11 10.40 10.62 10.87 10.78 10.62 10.42
phenol‘ log A2C 20 16 16 17 19 20 21 22 21 20 18
5.9 2.7 1.5 1.1 0.77 0.65 0.64 0.70 1.3 1.6 3.0
8.35 7.29 7.11 6.96 7.13 7.40 7.54 7.67 7.77 7.7 1 7.58
As2*/ -8 -5 0 2 6 11 16 20 19 16 12
k2‘
Ezd
630 580 520 500 500 500 470 490 550 660 770
13 14 16 17 19 21 22 21 19 17 15
acetonel’ log 9.00 9.27 9.44 9.74 10.08 10.43 10.47 10.36 10.00 9.72 9.45
A&*( -15 -10 -8 -1 5 12 13 11 4 -2 -7
toluene1 a 2 * /
-27 -48 -5 1 -54 -5 1 -45 -43 -4 1 -39 -40 -42
k2C 0.92 0.84 0.66 0.54 0.50 0.40 0.42 0.48
30 23 21 20 20 19 18 18
log A2C 9.20 7.91 7.53 7.31 7.10 6.83 6.83 6.85
As2*/ -1 1 -36 -4 3 -47 -5 1 -56 -56 -56
1.1
18
7.14
-5 1
“Viscosity, mPa.s (cP): ref 15. bStatic dielectric permittivity, ref 15. IO4 m3/(mol-s). dkJ/mol. C A 2 units, m3/(mol.s). fJ/(mol.K). gconcentrations, 6-62 ( mol/m3). Concentrations, 0.17-0.93 mol/m3. ‘Concentrations, 5-57 mol/m3. jConcentrations, 0.2-450 mol/m3. Ir Reference 6, 96.5 kJ/mol 9 1 .OO eV/entity. ‘Calculated from data in ref 15. TABLE IV: Stokes Region of Alcohol/Water, k(e;
alcohol
+ nitrobenzene), 298 K
Stokes range of mol % water
c
loop
(10) Pa”/mol)
Rlr
-97-75 -97-80 97-80 97-80 97-75
(78.5) 77-63 75-57 73-40 72-45 70-3 1
(103) IO4 F 1 104 F 1 105 ‘F 2 105 T 2 106 F 2
(33) 34 40 40 42 47
(19) 20 23 23 24 27
(water)b
methanol ethanol 1-propanol
2-propanold
2-methyl-2-propanolc = 1.41 X
Reference 7.
C.m (ref 18); assumed R = 1.0 nm; Reference 9. Reference 1 1.
= values i
indicate the range off values for the range of
approximately as 7-I in only a small range of solvent compositions; these are on the water-rich side, in methanol between 70 and 100 mol % water and in ethanol between 80 and 95 mol % water (Figure 6). In other alcohol/water systems, the “Stokes region” is between 75 f 5 and 97 mol %water and has been labelzed “zone c ” . ~ - The ’ ~ viscosity increases rapidly upon adding alcohol to water and reaches a maximum at 20-35% alcohol, depending on the alcohol. This is approximately the Stokes region of kz, where R / r = constant, with r-l = r;’ rs-’. The solvation of the reactants and transition state apparently remains relatively constant, dominated by water, in this range of solvent compositions. For reaction of e; with nitrobenzene in water mixtures with methanol or ethanol, the reaction radius R = 1 nm was determined.I9 In water, the “Stokes radius” of e; for diffusion is very small, 0.05 nm,” because e; is easily distortable and is not a rigid sphere. Thus, the value of the ratio R / r = 20 reflects the special properties of e; large Re and small re. For nitrobenzene, we take Rs = rs = molecular radius = 0.28 nm,I8 whence r = (0.05-1 0.28-’)-’ = 0.04 nm, R = Re + Rs = 0.8 nm, and Re = 0.5 nm. The effective radius of e; for reaction with nitrobenzene is large because nitrobenzene has a positive electron affinity ( 1 6 7 kJ/ relatively little wave function overlap is needed for the electron to transfer from the solvated state to the anion state. The values of R / r in the Stokes region of water mixtures with C,-C4alcohols are slightly larger than those in water. They tend
+
+
(19) MilosavljeviE, B. H.; MiEiE, 0. I. J . Phys. Chem. 1978, 82, 1359. (20) Lifshitz, C.; Tiernan, T. D.; Hughes, B. M. J . Chem. Phys. 1973, 59, 3 182.
c.
*Pure water, for comparison.
to increase with increasing size of the alkyl group (Table IV). Judging from the few reported values of electron mobilities in alcohols,21re does does not vary monotonically in the alcohol series. No conclusion can yet be drawn about individual variations or r and R . In solvents with water content less than that at the viscosity maximum, formerly labeled zones a and b,7-12kz(e; nitrobenzene) is nearly independent of 7 for ethanol and for methanol at between 70 and 30 mol % water (Figure 6). In methanol containing 530 mol % water, there is a weak viscosity dependence, k z a 74,3 (Figure 6). The rate constants remain below those in the solvents with >90 mol % water because the Stokes radius re in pure methanol or ethanol is larger, 0.25 nm, estimated from the reported mobility value^.'^-^' The values of Re in pure methanol and ethanol, estimated from eq 3-5’ and data in Tables I1 and 111, are 0.6 and 1.0 nm, respectively. The structure of the solvent about the reactants is apparently different on the two sides of the viscosity maximum. On the high-water-content side of the viscosity maximum, water dominates the solvation’ofthe reactants and bulk viscosity has a major effect on the reaction rate. On the low-water-content side of the viscosity maximum, at 580 mol % water, the value of log (&) decreases approximately linearly with increasing e-l (Figure 7). This dependence on e-I is in the opposite direction to that expected from
+
(21) Freeman, G. R. In Kinetics of Nonhomogeneous Processes: A Practical Introduction for Chemists, Biologists, Physicists and Materials Scientists; Freeman, G . R., Ed.; Wiley-Interscience: New York, 1987; Chapter 2.
306
Lai and Freeman
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 I
3t
I
1
2
\ 0
I
I
3
I
1
i I
10-4
I
I
130
120
4
1OOIE Figure 7. Viscosity-normalized rate constants for reaction of e; with nitrobenzene (circles) and acetone (triangles) as functions of 8 in methanol/water (filled points) and ethanol/water (open points) mixed solvents at 298 K. The numbers labeling the points indicate the mole percent of water (see Tables 11 and 111).
the value off if the electron approaches the positive end of the dipole. We suggest that nitrobenzene is solvated by alcohol molecules oriented such that alkyl groups are adjacent to the benzene ring and hydroxyl groups adjacent to the nitro group. This would facilitate electron approach to the latter, favor 0 = s in eq 5, and produce f < 1. Similar behavior would be expected (CH3)2CO)and was observed (Figure 7 ) . for k2(e; However, the magnitudes of the decreases of 7k2with decreasing water content are too large to be explained by f values alone. The overall change in f would be 10% whereas 7k2decreases by about 60% (Figure 7). The main effect is due to a rapid decrease in electron diffusion c ~ e f f i c i e n t ,which '~ means an increase in re. The approximately linear decrease of log (7k2)with increasing t-l at C80 mol 7% water (formerly designated zones a and b7-I2) suggests a relationship between changes of the free energy of activation of e; diffusion and changes of the solvation energy of the electron in these mixed solvents. The latter may be approximated as the change in polarization energy of the solvent around the ion, AP,, on going from solvent 1 to solvent 22'
I
140
Er(kJ1mol) Figure 8. Encounter efficiency k l / k Nof reaction e; with phenol (circles) and toluene (triangles) plotted against e; optical absorption energy E,. T = 298 K. Methanol/water (filled points, mole percent of water below points); ethanol/water (open points, mole percent of water above pints). 100 kJ/mol = 1.04 eV/entity. Dashed lines join p i n t s for pure methanol and pure ethanol, to guide the eye.
(Figure 5 ) . The electron is more stable in the solvation potential well than on a phenol or toluene molecule in these solvents. The transient anion S; dissociates unless it becomes protonated by a solvent molecule
+
where d is the distance from the center of the ion beyond which the polarization of the medium is in equilibrium with thermal agitation of the dipoles. Polarization is saturated at distances Cd; the value of d is about 0.3 nm or roughly a water molecule diameter. Although eq 6 neglects the specific structure of the liquid, which is different from one type of pure compound to another and has a large influence on the e; optical absorption energy,22 structure effects seem to cancel out for changes in this composition range of alcohol/water mixed solvents. The correlation between log (&) and e-' is in the opposite direction to that expected. It indicates that the nonpolar alkyl groups of the alcohol molecules inhibit electron transport, perhaps by increasing the distance between the electron localization sites, which are the OH groups oriented with the H toward the center of the electron charge density distribution. IneSficient Scavengers. The rate constants for phenol and toluene are 3 orders of magnitude lower than that of nitrobenzene (22) Freeman, G.R . J . Phys. Chem. 1973, 7 7 , I
(7)
[e;,S] + S; S;
+ ROH
+
SH
(8)
+ RO;
(9)
where the square brackets indicate an encounter pair. The probability of reaction 8 per encounter tends to be smaller when the e; trap depth is greater. The encounter efficiency is approximately equal to k 2 / k Nwhere , k 2 is the net rate constant for disappearance of e; by reactions 7-9 and k, is the rate constant for reaction with nitrobenzene, which has an encounter efficiency near unity. The electron trap depth in the solvent is similar to the e; optical absorption energy.23 The broad optical absorption band indicates that electrons are in traps of different depths. The electrons that have the lowest excitation energies have the greatest reactivity with inefficient scavenger^.^^,^^ For kinetics purposes, the e; trap depth is taken as the optical absorption energy E,, which is half-way up the low-energy side of the The variation of E, with water content is much smaller for methanol and ethanol (Figure 8) than it was for tert-butyl alcohol (70-140 kJ/mol)." While there is a rough tendency for k 2 / k Nto increase with decreasing e; trap depth in methanol and ethanol mixtures with water (Figure 8), the correlation is not as clear as it was in the t-butyl alcohol systems." Other factors contribute, which might have to do with specific liquid structures in the mixed solvents. For example, solvent rearrangement around the charge site is required for reaction 9. The ease of rearrangement is probably affected by the initial intermolecular arrangement of the solvent about the site. (23) Jortner, J.; Noyes, R. M. J . Phys. Chem. 1966, 70, 770. (24) Okazaki, K.; Freeman, G. R. Can.J . Chem. 1978, 56, 2313. (25) Afanassiev, A. M.; Okazaki, K.; Freeman, G. R. (a) J . Phys. Chem. 1979, 83, 1244; (b) Can. J . Chem. 1979, 57, 839.
Solvated Electrons in C H 3 0 H / H 2 0 and C H 3 C H 2 0 H / H 2 0 I
I
20
The Journal of Physical Chemistry, Vol. 94, No. I, 1990 307
L I 20
Mol
40
60
80
100
Mol % H 2 0
H20
Figure 9. Solvent composition dependence of activation energies E2of e; reactions and E,, of the solvent viscosity, near 298 K. Solute: 0, nitrobenzene; A, acetone; 0, phenol; 9 , toluene, E,,. Solvent: (a) methanol/water; (b) ethanol/water.
Figure 10. Solvent composition dependence of entropies of activation pS2' of e; reactions, near 298 K. Symbols as in Figure 9.
e-,
It is interesting to join the pure methanol and ethanol solvent points in Figure 8 with a straight line. For nonpolar toluene, this line passes through the pure water solvent point, and the total curve is sort of a figure 8. For hydrogen-bonded phenol, the line is to the right of the rest of the points; in the pure alcohols, the overall reaction is faster than a correlation with E, would indicate. We suspect this is due to the relative enhancement of the protonation reaction (9). The addition of 10 mol % water to an alcohol produces hydrogen-bonded structures that make the protons of the -OH groups less accessible to solvate the electrons: which decreases E,, and less accessible to protonate the anion, which decreases the rate of reaction 9.7,8 These effects are greater in ethanol than in methanol (Figure 8). Energies and Entropies of Activation. The solvent composition dependence of E , for the efficient scavengers is similar to that for viscous flow, but for most of the mixed solvent range, Ez C E,, (Figure 9). The stiffening of the liquid structure that occurs when an alcohol and water are mixed decreases the diffusivity of the deformable e; less than it decreases the diffusivity of the hydrogen-bonded solvent molecules. Solvent dipole reorientation is required during the diffusion of a charge through a liquid. The activation energy of dielectric relaxation in the single-component liquids is larger than E,, by 4 f 1 kJ/mol in water, methanol, and ethanoLZ6 Unfortunately, activation energies of dielectric relaxation in the mixed solvents are not known. The solvent composition dependence of E , for the inefficient scavengers is quite different. The most different behavior is that of the least efficient scavenger, toluene (Figure 9). The values of E2 for phenol and toluene in water are only 2-3 kJ/mol larger than those for nitrobenzene and acetone. However, the value of E , for toluene increases monotonically with increasing alcohol content in the solvent. In pure methanol or ethanol solvent, E , for toluene is 17 kJ/mol larger than those of nitrobenzene and acetone. The larger E, in alcohol solvent is attributed to a larger activation energy of reaction 8; it is accompanied by a less negative entropy of activation (Figure 10). The large negative entropy of activation of electron capture by toluene and phenol in the liquid phase is related to their negative electron affinities in the gas phase.27 Electron capture by these molecules requires stabilization of the anion by a favorable orientation of solvent dipoles around it. The large negative entropies of activation indicate that the solvent dipole reorientation around
-
(26) Buckley, F.;Maryott, A. A. Tables of Dielectric Dispersion Dam for Pure Liquids and Dilute Solutions; National Bureau of Standards Circular (US.)589; US. Department of Commerce: Washington, DC, 1958. (27) Blaunstein, R. P.; Christophorou, L. G . Radial. Res. Rev. 1971, 3,
69.
the site is a random event that precedes the electron jumping from its site in the solvent onto the molecule, reaction 8. Reaction 9 also involves a translation of the charge sites and concomitant rotational diffusion of the associated solvent dipoles, which could have a negative entropy of activation. In terms of equilibrium constants K7 and Ks of reactions 7 and 8, respectively, and the first-order rate constant k( = k9[ROH] for the protonation of the transient anion by a solvent R O H molecule, the overall second-order rate constant k2 for the consumption of e; is k2 = K7Ksk9'
(10)
Thus, for inefficient scavengers
AS,' = AS10 + A S S O + AS9' = (-6 J/(mol.K))
+ ASSo+ AS9*
(1 1)
and
+ +
E, = AHTo AHso + AH9* = A H S O AH9'
(12)
The value = -6 J/(mol-K) corresponds to the decrease of randomness of reactant distribution, and -AH70 < 1 kJ/mol corresponds to U(R)for toluene ( p = 1.2 X lWMC.m)'* or phenol ( p = 4.8 X C.m).I8 The values of AHl' = E , and AS7* = AS,' for the reactions of efficient scavengers such as nitrobenzene are different from AH: and AS,", because the former are mainly due to the diffusion of the e; and S through the liquid. Furthermore, AH7* = AH-7*, and ASl' = AS-7'. In pure water, the value of E , for toluene is only 2 kJ/mol greater than that for nitrobenzene (Figure 9). This value of Ez for toluene in water is attributed to the enthalpy of activation of dipole reorientation during reaction 8 or 9, whichever dominates the limitation of the overall reaction rate. By contrast, the value of E, for toluene in methanol or ethanol is 17 kJ/mol greater than that for nitrobenzene (Figure 9), and the values of AS2* for toluene are much closer to those for nitrobenzene than they were in water (Figure 10). In pure ethanol solvent, AS2* for toluene is only 3 J/(mol.K) more negative than that for nitrobenzene. In this case, the electron is transferred from the solvent site to the toluene molecule mainly by elevation of the energy state by thermal excitation. The reason for the shift of driving force from entropy in water to enthalpy in alcohol is not known. An extra source of information about molecular dynamics in these quaternary systems is needed. Acknowledgment. We thank the Natural Sciences and Engineering Research Council of Canada for financial assistance
J . Phys. Chem. 1990. 94, 308-315
308
and the staff of the Radiation Research Center for technical assistance. Registry No. Nitrobenzene, 98-95-3;acetone, 67-64-1; phenol, 10895-2: toluene, 108-88-3.
Supplementary Material Available: Tables of rate constants as functions of solute, solvent composition, and temperature and of rate constant parameters for solvated electrons in alcohol/water solvents (16 pages). Ordering information is given on any current masthead page.
Quenching of Porphyrin Excited States by Silver(1) Ions and Charge Separation in Bimolecular Systems and in Macropolycyclic Coreceptors Michel Gubelmann,+.’ Anthony Harriman,*’*,ZJean-Marie Lehn,*.+ and Jonathan L. S e ~ s l e r t - ~ Institut Le Bel, UniuersitP Louis Pasteur, 4 rue Blaise Pascal, 67000 Strasbourg, France, and Center for Fast Kinetics Research, The University of Texas at Austin, Austin, Texas 7871 2 (Received: June 15, 1989)
Silver(1) ions quench the excited states of free-base octaethylporphyrin (H,OEP) and its zinc complex (ZnOEP) in acetonitrile. Redox products are observed only from the singlet excited state of ZnOEP in all other cases the quenching mechanism involves catalyzed intersystem crossing due to spin-orbit coupling. Silver( I) ions complex readily with an [ 18]-N20, macrocycle in acetonitrile, and the complex quenches the porphyrin excited states predominantly through an electron-transfer mechanism, forming separated redox products. Two [ 18]-N204receptor molecules were covalently bound to opposite sides of the porphyrin ring and connected together via a biphenyl strap to give a macrotetracyclic ligand (1) capable of binding Ag’ at each of the lateral receptors, with cooperatiuity, in addition to forming a zinc porphyrin complex (2). With Ag+ ions in each [18]-N20, receptor, fluorescence from the porphyrin subunits in I and 2 was quenched (>85%), and flash photolysis studies showed the intermediate formation of redox products. The rate of intramolecular charge separation ( k > 6 X lo8 s-I) depended upon the reactants and solvent while charge recombination was slow ( k < IO7 s-l). The triplet excited states of the porphyrin subunits in 1 and 2 are hardly quenched by bound Ag’ ions due to poor thermodynamic driving forces. Replacing the biphenyl strap by a second porphyrin, forming a macropentacyclic ligand (4), induces extensive exciton coupling between the cofacial porphyrin rings. Again, with Ag’ ions in the two lateral receptors available in 4 and its zinc porphyrin complex ( 5 ) , rapid charge separation is followed by relatively slow recombination.
Introduction In recent years there has been a considerable upsurge in interest in the study of intramolecular electron- and energy-transfer processes, due partly to the availability of ultrafast detection systems capable of resolving kinetic parameters associated with the transition. Such studies have led to observations of the “Marcus inverted effectw4and to quantitative data describing the ,~ rate of transfer as a function of separation d i ~ t a n c emutual orientation: energy gap,’ and type of bridging unit.8 In addition, it is becoming clear that rapid electron transfer can proceed through the u-bonded framework of a nonconjugated spacer group.5 In this respect, it is important to employ model systems in which the donor and acceptor groups are held a t reasonably well-defined geometries rather than being linked through a single, flexible bridge that permits multiple conformations to be adopted. Although several different types of model systems have been described, there is a genuine need to enlarge the scope and versatility of the subject. As such, macropolycyclic ligands which are capable of assembling several substrates within the same superstructural unit appear to be particularly attractive reagents for constructing novel photoactive units? The design and synthesis of such units have been described recently,’&I4 and preliminary results have been presented”*I3 that confirm their ability to function as molecular photodiodes. In this paper, we corroborate our earlier studiesI3 made with the zinc porphyrin 2 in which each lateral [ 1 8]-N204macrocycle contains a silver(1) ion and extend the work to include related superstructural units 1, 4, and 5. To facilitate interpretation of the results, the corresponding separated molecular systems have been studied. Experimental Section Samples of octaethylporphyrin ( H 2 0 E P ) and zinc octaethylporphyrin (ZnOEP) were purchased from Midcentury Chemical Universite Louis Pasteur. *The University of Texas at Austin
+
0022-3654/90/2094-0308$02.50/0
Co. and chromatographed on silica gel with chloroform used as eluant. Small samples were further purified by thin-layer chromatography (TLC) on silica gel with benzene as solvent imme(1) Present address: Rhone-Poulenc Recherches, B. P. 62, Centre de Recherches de Saint-Fons, F-69190 Saint-Fons, France. (2) Much of the experimental work described in this article was performed at the Davy Faraday Research Laboratory, The Royal Institution, 21 A l k marle St., London W1X 4BS, U.K. (3) Present address: Department of Chemistry, University of Texas at
Austin, Austin, TX 78712. (4) (a) Miller, J. R.; Calcaterra, L. T.; Closs, G . L. J . Am. Chem. SOC. 1984, 106, 3047. (b) Wasielewski, M. R.; Niemczyk, M. P.; Svec, W. A,; Pewitt, E. B. J . Am. Chem. SOC.1985,10, 1080. (c) Imine, M. P.; Harrison, R. J.; Beddard, G.S.; Leighton, P.; Sanders,J. K. M. Chem. Phys. 1986, 104, 315. (5) (a) Joran, A. D.; Leland, B. A.; Geller, G.G.;Hopfield, J. J.; Derven, P. B. J . Am. Chem. SOC.1984,106, 1090. (b) Wasielewski, M. R.;Niemczyk, M. P.; Svec, W. A.; Pewitt, E. B. J . Am. Chem. SOC.1985, 107, 5562. (c) Warman, J. M.; de Haas, M. P.; Oevering, H.; Verhoeven, J. W.; PaddonRow, M. N.; Oliver, M. N.; Hush, N. S. Chem. Phys. Lett. 1986, 128, 95. (d) Heitele, H.; Michel-Beyerle, M. E.; Finckh, P. Chem. Phys. Lett. 1987, 134, 273. (6) (a) Cowan, J. A.; Sanders, J. K. M.; Beddard, G. S.; Harrison, R. J. J . Chem. Soc., Chem. Commun. 1987, 55. (b) Lindsey, J. S.; Delaney, J. K.; Mauzerall, D. C.; Linschitz, H. J . Am. Chem. SOC.1988, 110, 3610. (c) Sessler, J. L.; Johnson, M. R.; Lin, T.-Y.; Creager, S. E. J . Am. Chem. SOC. 1988, 110, 3659. (7) Kakitani, T.; Mataga, N. J . Phys. Chem. 1985, 89, 8. (8) Schmidt, J. A.; McIntosh, A. R.; Weedon, A. C.; Bolton, J. R.; Connolly, J. s.;Hurley, J. K.; Wasielewski, M. R. J . Am. Chem. SOC.1988, 110. 1733. (9) (a) Lehn, J. M. In Biomimetic Chemistry; Yoshida, Z., Ise, N., Eds.; Kodansha and Elsevier: Tokyo and Amsterdam, 1983; p 163. (b) Lehn, J. M. In Frontiers of Chemistry (IUPAC);Laidler, K. J., Ed.; Pergamon: New York, 1982; p 265. (10) Thanabal, V.; Krishnan, V. J . Am. Chem. SOC.1982, 104, 3643. ( 1 I ) Blondeel, G.; Harriman, A,; Porter, G.;Wilowska, A. J . Chem. SOC., Faraday Trans. 2 1984, 80, 867. (12) Hamilton, A. D.; Lehn, J. M.; Sessler, J. L. J . Chem. SOC.,Chem. Commun. 1984, 311. J . Am. Chem. SOC.1986, 108, 5158. (13) Gubelmann, M.; Harriman, A.; Lehn, J. M.; Sessler, J . L. J . Chem. SOC.,Chem. Commun. 1988, 77. (14) Kobayashi, N.; Lever, A. B. P. J . Am. Chem. SOC.1987, 109,7433.
1990 American Chemical Society
