7042
J. Phys. Chem. 1996, 100, 7042-7049
Effect of Temperature on the Dynamics of Benzophenone Anion Solvation in Alcohol† Zhang Xujia and Charles D. Jonah* Chemistry DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: October 2, 1995; In Final Form: January 2, 1996X
The solvation of the benzophenone anion in 1-propanol, 2-propanol, and 1-butanol has been measured over the temperature range -10 to -50 °C. The initial spectra of the benzophenone anion were very similar in all three alcohols. The final spectrum of the benzophenone anion in 2-propanol is less blue-shifted (17 nm) than the spectrum of the anion in 1-propanol and 1-butanol. The activation energies for solvation are 22 kJ/mol for 1 propanol and 1-butanol and 16 kJ/mol for 2-propanol, which are similar to the energy for hydrogen bond breakage in the pure solvents. This suggests that the solvent H-bond breakage plays an important role in anion solvation.
Introduction The solvent has always played an important role in chemistry and chemical reactivity.1 The solvent can dissipate or supply energy for reaction: it can inhibit the motion of chemical species, and it can rearrange to lower the energy of reacting states. Each of these roles has provoked considerable research. We are interested in the role of the solvent to modify the energy of states. These shifts will alter the potential reactivity in electron-transfer and proton-transfer reactions. One can probe these shifts in energy by measuring the shift of the absorption spectrum because as the energy states of a molecule are altered, there will be an alteration in the absorption spectrum. Early studies made use of the shift of the spectrum that occurred in different solvents.1 With experimental advances, it has been possible to measure solvation dynamics. The studies of the solvation of excited states have been recently reviewed (see for example recent reviews by Maroncelli and Barbara and Jarzeba),2,3 and we will not attempt a complete review here. There have also been studies of the solvation of anions4-7 and electrons.8-14 The experimental techniques were similar in all cases. A chemical species was created quickly that was not in equilibrium with the solvent. Excited-state solvation measurements make use of the fact that the excited state has different charge distribution than does the ground state. The anions were created via electron attachment. In the electron solvation experiments, an electron was injected into the solvent. In all three types of experiments, the solvent would rearrange around the solute, shifting the electronic state of the solute and leading to equilibration of the solvent-solute system. Excited state solvation has been measured by observing the fluorescence of the excited states, while the measurement of electron and anion solvation has been observed from the absorption of the ground electronic state. The two techniques have different capabilities. Emission measurements are inherently more sensitive and normally have a much wider dynamic range, while absorption measurements do not depend on excitedstate lifetimes. Both experiments are limited in the time range that can be observed by the lifetime of the probe state. The lifetimes of the fluorescent molecules are considerably shorter than the lifetimes of the anions. We have carried out experi† Work performed under the auspices of the Office of Basic Energy Sciences, Division of Chemical Science, US-DOE under Contract W-31109-ENG-38. X Abstract published in AdVance ACS Abstracts, April 1, 1996.
0022-3654/96/20100-7042$12.00/0
ments to greater than 50 ns when observing solvation effects in ionic solutions. Our studies on the solvation of anions arise from the results of Marignier et al.4 and Ichikawa et al.15 Marignier et al. observed that the spectrum of the benzophenone anion shifted as a function of time in low-temperature alcohols.4 They proposed that the solvation of the anion was responsible for the shift of the spectrum. Because charged species can be formed in many different electron transfer reactions, we felt it was important to see if solvation around anions was similar to solvation around dipoles or excited states. As was shown previously, there were considerable differences in the solvation behavior between excited states and ions.5-7 This is different than one would expect from continuum solvation, where the solvation time would be expected to be only slightly faster for ions than for dipoles.16 There have recently been experiments where the solvation of anion excited states were measured.17 In those experiments, it was found that the solvation times were quite similar to what had been measured for neutral excited states. Such experiments are unlike ours in that there is no change in molecular charge in those experiments. In our experiments, a high concentration of benzophenone is added to the solution. The high concentration of the benzophenone will react with the electron before the solvation of the electron. We are therefore not limited by the reaction time of the electron with the benzophenone. We showed previously that less than 5% of the electron remains at 0.05 M benzophenone at room temperature. From previous studies of electronprecursor reactions, the amount of a solvated electron decreases exponentially on concentration; this would suggest that the yield of the electron at 0.15 M relative to the yield of the electron in the absence of the benzophenone would be expected to be (0.05)3 (or 1.25 × 10-4).18,19 We found that the solvation of the anion became slower with longer alcohols. At room temperature, the solvation times were comparable to the solvation time of the electron in the same alcohol and were considerably faster than the solvation of a dipole in similar systems.5-7 The peak of the absorption spectrum of the anion appeared to shift continuously as a function of time until fully solvated, similar to what has been observed for the solvation of excited states and for the solvation of the electron in low-temperature glasses.20 The behavior is different than what has been observed in room temperature alcohols.8,9,11,12,14 The spectrum of the completely solvated anion was similar in all primary alcohols; © 1996 American Chemical Society
Benzophenone Anion Solvation in Alcohol however in secondary alcohols, the spectrum of the solvated anion is considerably red-shifted from that in primary alcohols.5-7 Because solvation times in alcohols are slower than in other solvents, alcohols make an excellent experimental medium for measurement. They are also desirable because they can mimic some of the characteristics of water solvation but with slower solvation processes. Because the dielectric processes are complex, however, it is difficult to isolate the microscopic processes that are occurring. The dielectric relaxation is often described as taking place with three lifetimes, traditionally called τ1, τ2, and τ3.21,22 This complexity has made it difficult to understand what physical processes in the alcohol will dominate the solvation process. However, because these processes have different activation energies, experiments as a function of temperature can distinguish between different processes. In preliminary work, we measured the solvation dynamics of the benzophenone anion in 1-propanol over the range -20 to -50 °C.23 In those measurements, we found that the activation energy could be correlated with the energy of hydrogen-bond breaking in the solvent. We also found that while the solvation time of the anion is much faster at room temperature than dipole solvation, it is slower at lower temperatures. These initial measurements were done with only one alcohol. We wanted to generalize these studies and see if similar effects occurred in different alcohols. In this work we report our measurements on the solvation of the benzophenone anion in 1-propanol, 2-propanol, and 1-butanol over the temperature range -10 to -50 °C. We also discuss the techniques that we have used for data analysis.
J. Phys. Chem., Vol. 100, No. 17, 1996 7043
Figure 1. Absorption spectrum of the benzophenone anion in n-butanol at -40 °C measured at three different times.
Experimental Section The experimental system has been previously described in detail.7,24 The accelerator gives an approximately 30 ps pulse. This pulse (pump pulse) is used for the generation of the electrons, which react with the benzophenone. The probe pulse is generated by the electrons in xenon gas via Cerenkov radiation and will have the same temporal profile as the electron pulse. The overall instrument response time was determined from the hydrated electron absorption at 600 nm. The initial hydrated electron concentration was approximately 20 µM. The time resolution of the measurements was always around 30-40 ps. Low temperatures were achieved by flowing the benzophenone solution through a coil immersed in a dry ice-acetone bath. The temperature was measured using a copper-constantin thermocouple located at the exit of the sample cell. The temperature was controlled in the range -55 to -10 °C by warming the solution before it entered the cell. The heating was controlled using an Omega CN 1000 TC temperature controller Benzophenone (specified as 99+%) and 1-propanol, 2-propanol, and 1-butanol (specified as 99.5+%) were purchased from Aldrich and were used as received. The samples were degassed by bubbling with argon. The concentration of the solutions was 0.15 mol/dm. No interference from the absorption of the solvated electron was observed. The initial yields of benzophenone anion at 800 nm are linearly dependent on dose and independent of temperatures. We can normalize the spectra for the comparisons at different experimental conditions. Results and Discussion Both time- and wavelength-dependent absorption spectra of benzophenone anion in different alcohols and at different temperatures were recorded. Figure 1 displays the absorption spectra in 1-butanol at three different times at -40 °C. The
Figure 2. Absorption spectrum of the benzophenone anion in n-butanol directly after the pulse at three different temperatures (the spectra at -20 and -40 °C were offset by plus and minus 0.02 OD units, respectively).
continuous shift of the spectrum to the blue is similar to what was observed at room temperature and to what was observed previously for 1-propanol at low temperatures.5,6,7,23 The final broad absorption band (peak near 628 nm) has been assigned to the fully solvated benzophenone anion in n-alcohol while the spectra at early times are assigned to the anion before the rearrangement of the solvent. Figure 2 shows the absorption spectrum at early times at three different temperatures in 1-butanol. As was observed previously for 1-propanol,23 the spectra are the same at the three different temperatures despite differences in relaxation time. The benzophenone anion solvation in 2-propanol shows the continuous blue shift of the spectrum. The transient absorption spectra of the benzophenone anion in 2-propanol solution at three different times are shown in Figure 3. As seen in Figure 4, the absorption spectra at early times are also temperature independent. The absorption spectra for 1-propanol, 1-butanol, and 2-propanol are shown at early times in Figure 5a. The three spectra are very similar, although it appears as if the 2-propanol might be slightly red-shifted from the two primary alcohols. Because both the fast inertial motions that will solvate the benzophenone anion and the rotation of the OH bond are faster than our time resolution, we must assume that these processes have taken place.2,3 Thus it is clear that these early motions do not strongly depend on the alcohol structure or they do not strongly shift the spectrum. This conclusion can also be drawn from the
7044 J. Phys. Chem., Vol. 100, No. 17, 1996
Zhang and Jonah
Figure 3. Absorption spectrum of the benzophenone anion in 2-propanol at -30 °C at three different times.
Figure 4. Absorption spectrum of the benzophenone anion in 2-propanol at early times at three different temperatures. (the spectra at -20 and -40 °C were offset by plus and minus 0.02 OD units, respectively).
results in 1-octanol and 2-octanol at room temperature, where the initial spectra are very much the same.5,7 It has been observed that the amount of spectral shift on solvation is independent of the length of the alcohol at room temperaturesall primary alcohols show a similar final solvated spectrum. This feature remains within the present temperature range(25 to -50 °C). Figure 5b shows the similar final spectra between 1-propanol and 1-butanol at -40 °C. Similar results are obtained at other temperatures. Figure 5b shows that the spectrum of the fully solvated benzophenone anion in 2-propanol is considerably different from the spectra of the fully solvated benzophenone anion in either 1-propanol or 1-butanol. As one can see from the spectrum, the final spectral position is about 17 nm red-shifted from that in the 1-propanol and 1-butanol. Similar solvation behavior was also observed in octanol at room temperature, but the final spectral position in 2-octanol is about 35 nm red-shifted from that in 1-octanol.5,7 We thus have determined that while the length of the alcohol will not affect the final solvated spectrum, the structure (primary vs secondary) does affect the spectrum. However, the initial fast relaxation that must occur does not lead to differences in the initial measured spectrum. From this we conclude that the fast motions cannot make a major rearrangement of the solvent molecules, because a major rearrangement would lead to differences between our initially observed (after fast relaxation) spectrum as a function of alcohols similar to the differences observed in the final spectra for different alcohols. We will then be considering the observed spectral shift from the initial species to the final species, and no assumptions are necessary about what the form of the initial spectrum will be.
Figure 5. (a) Spectrum directly after the pulse for the benzophenone anion in 1-propanol, 1-butanol, and 2-propanol at -40 °C. Zero was offset by +0.02 OD units for 1-propanol and -0.02 OD units for 2-propanol. (b) Spectrum 2 ns after the pulse for the benzophenone anion in the same three alcohols.
Figure 6. Spectrum of the benzophenone anion in 1-butanol at three different temperatures at 2 ns after the pulse.
These results are not in conflict with what had been observed previously in room-temperature experiments. In those experiments, we found differences in the initial spectrum, where the shorter alcohols were blue-shifted relative to the longer alcohols. In those experiments, the kinetics observed were 20-50 ps, which are commensurate with the time resolution of the experiments. Thus the kinetics observed are sufficient to explain the shift. In the present experiments, the time resolution is faster than the kinetics that were observed, so that the measured kinetics will not influence the initial spectrum. Figure 6 shows that the spectra of the benzophenone anion at long times at different temperature in 1-butanol are the same. Similar results were also obtained in 2-propanol and 1-propanol, as we have described previously. The maximum of anion spectrum in 2-propanol is about 17 nm red-shifted from what
Benzophenone Anion Solvation in Alcohol
J. Phys. Chem., Vol. 100, No. 17, 1996 7045
TABLE 1: Width and Center of Spectral Bandsa
1-propanol
1-butanol
2-propanol
temp (°C)
W0 (nm)
Dg0 (nm)
Dl0 (nm)
W∞ (nm)
Dg∞ (nm)
Dl∞ (nm)
-10 -20 -30 -40 -50 -10 -20 -30 -40 -10 -20 -30 -40
692 703 695 706 693 703 695 707 692 692 696 708 709
129 121 132 134 132 122 114 120 113 105 158 142 132
85 109 148 132 142 109 84 89 83 78 86 103 93
630 628 627 628 630 629 630 628 632 645 644 647 648
67 62 67 65 69 64 63 64 62 68 63 69 67
91 94 100 90 106 94 106 89 82 108 91 89 83
a W0, Dg0, and Dl0 are the center wavelength, Gaussian width and Lorentzian width at t ) 0. W∞, Dg∞, and Dl∞, are the center wavelength, Gaussian width and Lorentzian width at t ) ∞.
is observed in the 1-propanol and 1-butanol. These spectra are consistent with the long-time spectra observed at room temperature and suggest that the fully solvated structure is independent of temperature from -50 to 20 °C. The results of the spectral fitting, which is discussed below, are given in Table 1. The error in the central wavelength is approximately (5 nm. The error in the Lorentzian width is larger than the others because of the overlap of the ketyl radical absorption. The amount of the shift of the spectrum depends on the fields at the central anion. These fields will be dominated by the first shell of the solvent rather than the dipole density of the solution. For example, the final spectrum of the solvated benzophenone anion is the same in 1-butanol and 1-decanol, despite the greater than a factor of 2 difference in dipole density. The OH dipole can point toward the anion, and the carbon chain will extend in the opposite direction so that the number of OH groups next to the anion will not be affected by the length of the hydrocarbon chain because structurally there will be little interference between alcohol molecules. The differences between the amount of spectral shift in primary and secondary alcohols have been attributed to steric factors, which prevent a close packing of secondary alcohols around an anion. For a primary alcohol, there is only a single carbon chain attached to the carbon atom bonded to the OH dipole. For a branched alcohol, the two carbon chains attached to the carbon atom bonded to the OH dipole will prevent close packing of the alcohols. Thus the number of OH dipoles that are close to the ion will be less in a secondary alcohol solution. With 2-propanol, one would expect less conflict between adjacent solvent molecules because the blocking chain is smaller. The solvation time, τs, measures the time for the solvent to relax around an entity in solution after a perturbation. The time is measured after the creation of the perturbation. Because there may be different time scales for different relaxation processes, one would like to estimate the entire process and thus the spectrum directly after the perturbation is required. Fee and Maroncelli25 proposed a technique for determining the emission zero-time spectrum using the data for emission and absorption spectra in nonpolar media; unfortunately the benzophenone anion does not fluoresce and thus their treatment is not applicable. Solvation can take place via molecular rotations, internal rotations, and molecular translations. These different pathways would be expected to have different kinetics. As discussed above, one would expect that internal rotation (rotation of the OH bond around the C-O bond) and inertial rotation would be expected to be faster than our time resolution. Thus
we are observing only the final solvation processes. The fast, unobserved processes may be important in the energetics of solvation; however clearly the slower processes will dominate the solvation time. The dynamics of anion solvation, can be described as is dipole solution using the relaxation function C(t), which is defined by
C(t) )
ν(t) - ν(∞) ν(0) - ν(∞)
(1)
where ν(t) is the time-dependent maximum of the absorption spectrum. As has been seen above, the anion absorption spectrum gradually narrows and shifts as the solvent relaxes. As will be shown, the time response of the solvation energy of the system can be approximated by an exponential with a characteristic time response τs over a limited time range. To show this, we will construct a simple description of the experimental data. We can describe the center and width of the anion absorption by eqs 2 and 3 where ν0 and ν∞ represent the frequency of the
ν(t) ) ν∞ + (ν0 - ν∞)e-t/τs
(2)
∆(t) ) ∆∞ + (∆0 - ∆∞)e-t/τs
(3)
absorption maxima of the initial unsolvated anion and the final solvated anion, respectively, while ∆0 and ∆∞ are the corresponding absorption bandwidth. In our previous work at room temperature, we described the anion band as a function of wavelength and used a Gaussian to describe the band.7 In this work and our previous lowtemperature experiments, we have described the band in energy units rather than wavelength units. The low-energy side of the spectrum was given as a Gaussian and higher energy part as a Lorentzien.23 The Gaussian-Lorentzian form was previously used to describe the spectrum of the solvated electron in different solvents.26 These distributions are in energy; however, they are written in terms of wavelength to facilitate easy comparison with experimental data. We confirmed that the present calculation technique gave similar solvation times to the technique used by us previously. The low-energy, Gaussian, portion of the spectrum can be described by
∆l(t) ) ∆l∞ + (∆l0 - ∆l∞)e-t/τs
(4)
where ∆l0, ∆l∞, and ∆l(t) are the Gaussian widths (in reciprocal wavelength units) at time 0, infinity and time t, respectively. So the time- and wavelength-dependent anion absorption of the low-energy part can be written as
Il(λ,t) ) A exp(-(1/λ - 1/λ(t))2/∆l(t)2)
(5)
The high-energy, Lorentzian, portion of the spectrum can be described by
∆h(t) ) ∆h∞ + (∆h0 - ∆h∞)e-t/τs
(6)
where ∆h0, ∆h∞, and ∆h(t) are the widths of higher energy (Lorentzian) part (in reciprocal wavelength units) at time 0, infinity, and time t, respectively. So the time- and wavelengthdependent anion absorption of higher energy part can be written as
Ih(λ,t) ) A/(1 + (1/λ - 1/λ(t))2/∆h(t)2)
(7)
Hence, the time- and wavelength-dependent anion absorption
7046 J. Phys. Chem., Vol. 100, No. 17, 1996
Zhang and Jonah
Figure 7. Kinetics of the benzophenone anion in 1-butanol at -40 °C at different wavelengths.
can be written as
I(λ,t) ) Il(λ,t), (5), where λ greater than the peak from eq 2 (8) Ih(λ,t), (7) where λ is less than the peak from eq 2
{
We assume that the oscillator strength was constant for the absorption band as a function of time to allow us to determine the magnitude of the absorption as a function of time. With this assumption the coefficient A is
A)
C (∆h(t)‚π/2 + 2∆l(t)/xπ)
(9)
where C is the integrated intensity of the band. The kinetics at different wavelengths were evaluated as a function of temperature and alcohol structures using the above equations. Both time-dependent data at different wavelengths such as shown in Figure 7 and the spectra at different times, such as shown in Figure 1, were used to determine a value for τs. Because there appears to be no shift in the initial spectrum for the three alcohols as a function of temperature, we assume that the initial spectrum that we need concern ourselves with is known. This simplifies the analysis from previous work where we did not know the initial spectrum. Thus, all the parameters except τs for eq 8 are known, and we are fitting only observed spectral changes. The experimental data were compared with eq 10, which convolutes eq 8 with the instrument response
A(λ,t) ) ∫I(λ,t - τ) g(τ) dτ
(10)
function g(t) to give the experimentally expected values (note that for these measurements, the convolutions do not greatly alter the expected experimental curve). A(λ,t) was calculated for all systems, and the results are compared to time- and wavelength-dependent spectral data. Because of the overlap of the ketyl radical at the high-energy side of the band, the parameters for the Lorentzian width of the time zero and the final spectra are not well determined from our experiment. The agreement between the simulations and the experimental data are satisfactory for all three alcohols, as can be seen in Figures 8-10. The values for τs determined from these calculations are reported in Table 2. The error in the solvation time is approximately (10%. Figure 11 displays the Arrhenius plot for the 1-propanol, 1-butanol, and 2-propanol. As expected, the benzophenone anion solvation in 1-butanol is slower than in 1-propanol; however, they have similar activation energies, as shown by the similar slopes. At higher temperatures, the 2-propanol solvation is slower than 1-propanol but the solvation times
Figure 8. Experimental and fit results for the benzophenone anion in 1-propanol at -40 °C using eq 10: (a) kinetics at 800 nm; (b) kinetics at 750 nm; (c) spectrum at 100 ps after the pulse.
become more similar at lower temperatures. Thus, the benzophenone anion solvation energy in 2-propanol is smaller than in 1-propanol. (As seen in Figure 11, the slope of 2-propanol is smaller than that of 1-propanol.) The activation energies are given in the last column of the Table 2. They are approximately 22 kJ/mol for 1-propanol and 1-butanol and about 16 kJ/mol for 2-propanol. For similar systems, the activation energy for dipolar solvation is approximately 13 kJ/mol.2 If one examines the activation energy for solvation using the continuum model for 1-propanol (see below), one obtains a value of 27 kJ/mol.2,22 Marignier and Hickel did not measure the solvation time explicitly. They observed the decay of the benzophenone anion at 825 nm and measured the kinetics. They observed two kinetic processes. The activation for the faster process, which dominates the solvation process, was 25 ( 3 kJ/mole, and the activation energy of the slower process was 31 ( 3 kJ/mole. The activation energy is the same, within experimental error, as that which we have measured. The differences in activation energy lead to the result that solvation appears to be faster for anions than excited states at room temperature while it is slower at lower temperatures. These results suggest the importance of having temperature-dependent data to understand the solvation mechanisms. Solvation dynamics can be described using many different models. One of the simplest and most effective models has been a continuum dynamics model.16 It makes use of macroscopic properties of the solvent such as static and high-frequency dielectric constants and the microwave relaxation time. This model predicts the approximate time dependence of dipole solvation at room temperatures, as has been previously discussed, for a wide variety of solutes and solvents. There are differences between the experimental and predicted data, but these differences are not large. While the general time scale is
Benzophenone Anion Solvation in Alcohol
Figure 9. Experimental and fit results for the benzophenone anion in 1-butanol at -40 °C using eq 10: (a) kinetics at 800 nm; (b) kinetics at 750 nm; (c) spectrum at 100 ps after the pulse.
J. Phys. Chem., Vol. 100, No. 17, 1996 7047
Figure 10. Experimental and fit results for the benzophenone anion in 2-propanol at -30 °C using eq 10: (a) kinetics at 800 nm; (b) kinetics at 750 nm; (c) spectrum at 100 ps after the pulse.
TABLE 2: Solvation Times and Activation Energy for Benzophenone Solvation in Alcohols
predicted correctly by this simple model, the nonexponential character is not predicted. However, as we noted above, the description appears to break down at low temperatures because the activation energy for solvation using the continuum model is different from the experimental value. As discussed above, the situation is far more complex in alcohols, where there is not a single solvation mode but where the dielectric relaxation data suggest multiple processes. It is then not possible to simply estimate what solvation kinetics would be like More sophisticated models of solvation have also been proposed. These include the mean spherical approximation (MSA) and molecular dynamics. As has been shown (see, for example, ref 27), solvation would be faster for ions than for dipoles in the MSA approximation. The difference between the predictions of the MSA approximation and the continuum approximation depends on the relative size of the ion and the solvent. While the charge may be localized on the benzophenone molecule, the ion will be fairly large. For that reason we do not expect that the MSA approximation will be a major improvement over the continuum approximation. Recently there have been several molecular dynamics simulations of solvation in hydrogen-bonding solutes.28-30 While the linear response approximation has been found to follow quite well for acetonitrile,30 it has found to be less satisfactory for methanol.28-30 In similar studies, we have found that the linear response approximation does not appear to work well for our simulations of benzophenone in ethanol.31 Kumar and Maroncelli were able to get good correspondence between experiment and theory for the solvation of C153 in acetonitrile but considerably less satisfactory results for the solvation in
solvation time (ps)
1-propanol 2-propanol 1-butanol
25 °C
-10 °C
-20 °C
-30 °C
-40 °C
-50 °C
activation energy (kJ/M)
30
120 165 160
155 250 270
220 320 400
380 470 600
500
22 16 22
40
Figure 11. Solvation time as a function of temperature + 1-propanol: (b) 1-butanol; (O) 2 propanol.
methanol. This they ascribe to a failure in the model of the solvent molecule. Solvent viscosity is also a measure of the dynamics within a solvent. Figure 12 displays the correspondence between the viscosity32 and the solvation time. For n-propanol and n-
7048 J. Phys. Chem., Vol. 100, No. 17, 1996
Figure 12. Solvation time of benzophenone anion as a function of solvent viscosity: (O) 1-propanol; (b) 1-butanol; (+) 2-propanol. The solid line is a fit to the combined 1-propanol-1-butanol data; the dashed line is the fit to the 2-propanol.
butanol, we find that ln(1/τ) vs ln(viscosity) can be approximately described by a single straight line with a slope of -1. Thus, for these two alcohols, the viscosity and the solvation time are most likely controlled by similar microscopic processes. For 2-propanol, however, we find that the slope is not one so that the viscosity is not related to the solvation time simply. The consequences of this fact need to be investigated further. Because the effect of hydrogen bonds will influence the dielectric relaxation times, continuum predictions implicitly contain information about the dynamics of solvent-solvent hydrogen bonding. The difference of hydrogen bond between 1-propanol and 2-propanol will be reflected in Figure 11. If we compare the anion solvation energies to the corresponding H-bond energies, we observed that the anion solvation energies are close to the H-bond energies (anion solvation energies 22, 22, and 16 kJ/mol for 1-propanol, 1-butanol, and 2-propanol, respectively; H-bond energies 25, 25, and 17 kJ/mol for 1-propanol, 1-butanol, and 2-propanol, respectively33). These data suggest that the hydrogen-bond breakage may be the dominant process during anion solvation process. However, the correlation between τ2 and τs suggested that almost freely rotating molecules govern the final stages of electron solvation and not hydrogen-bond breakage.8,9 There can also be hydrogen bonding to the solute as well.34-36 In a recent study of dipole solvation in 1-propanol, Maroncelli studied a wide range of solutes and observed that there were two distinct classes in the solvation dynamics.34 Most of the solutes showed similar dynamics, while a subgroup showed a different behavior, where the solvation times were at least 2-fold faster than those of normal solutes. They concluded that the abnormal behavior is due to the solute hydrogen bond with solvent. Yu and Berg studied the solvation of resorufin as a probe of hydrogen bonding by observing a new spectral band.35 Similarly Decle´my and Rullie´re observed the formation of a new emission band for the molecule MPQB in alcohols.36 This band is not seen in aprotic solvents. If the benzophenone were already H bonded with an alcohol before the solvation, the formation of a negative ion would only strengthen and shorten the bond. This process might be too fast to be observed. In our benzophenone anion solvation study, we have no direct evidence against hydrogen bonding between the solute and the solvent; however, there are data that suggest that initial hydrogen bonding is not dominant. If there were considerable hydrogen bonding present before the creation of the anion, one would have expected that the amount of this hydrogen bonding as a function alcohol structure would vary. Because of the similar
Zhang and Jonah absorption spectra at early times, even for 2-propanol, this does not seem to be the case. It has been suggested that the solvation of the benzophenone occurs via the formation of a hydrogen bond.15 From the experiments cited here, there is no evidence whether the solvation process is indeed due to the formation of a hydrogen bond or due to electrostatic effects. We have shown that there are similar shifts of the benzophenone spectrum in salt solutions in acetonitrile or in propylene carbonate solutions.37 In both of these experimental systems, there is no possibility of hydrogen-bond formation. At lower temperatures, our results may be compared to the dipole solvation results. It was found that the solvation time in dipole solvation became longer with larger spectral shifts. This might suggest that the larger energies would lead to a greater rearrangement of the solvent structure and this greater rearrangement took a longer time.2 The benzophenone anion would be expected to put a larger force on the solvent than dipole; subsequently the solvation process is slower. In our earlier work, it was found that the solvation time of the anion was comparable to the solvation time of the electron in the same alcohol at room temperature.5-7 Using the data of Chase and Hunt, we find that the activation energy of the electron solvation is lower than the activation energy for anion solvation in 1-propanol and 1-butanol.8 The activation energy for electron solvation is not known in 2-propanol. It would be interesting to determine whether the low energy seen for anion solvation in 2-propanol is also the case for electron solvation. We are pursuing these measurements. Summary Benzophenone anion solvation was studied in 1-propanol, 1-butanol, and 2-propanol from -50 to 20 °C. The solvation time was obtained by simultaneously fitting time- and wavelengthdependent spectrum. These data showed that the final spectrum is independent of the temperature over this range and the carbon chain length but depends on alcohol structure. Also the spectral shift on solvation is smaller for longer branched alcohols. The initial spectrum is independent of both the temperature and alcohol structure. This reflects the newly created ion species in random solvent configurations. Similar to room-temperature experiments, the solvation time is longer for longer carbon chains. However, at lower temperature the solvation process in 2-propanol is faster than those in 1-propanol. The kinetics show the same form as was seen at room temperature. The rates are different at different wavelengths, but these can be rationalized by assuming a shifting spectrum as has been done previously. The similarity of the spectrum and the similarity of the form of the kinetics suggest that the solvation mechanism has not changed at lower temperatures. The activation energy was found to be similar to that for the H-bond breakage. We believe that a change in the solvent H-bond plays an important role in anion solvation process. References and Notes (1) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry; VCH: Weinheim, Germany, 1988. (2) Maroncelli, M. J. Mol. Liq. 1993, 57, 1. (3) Barbara, P. F.; Jarzeba, W. AdV. Photochem. 1990, 15, 1-68. (4) Marignier, J. L.; Hickel, B. J. Phys. Chem. 1984, 88, 5375. Marignier, J. L.; Hickel, B. Chem. Phys. Lett . 1982, 86, 95. (5) Lin, Y.; Jonah, C. D. J. Phys. Chem. 1992, 96, 10119. (6) Lin, Y.; Jonah, C. D. J. Phys. Chem. 1993, 97, 295. (7) Lin, Y.; Jonah, C. D. Ultrafast Dynamics of Chemical Systems; Simon, J. D., Ed.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994; pp 137-162. (8) Chase, W. J.; Hunt, J. W J. Phys. Chem. 1975, 79, 2835.
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