533
J . Phys. Chem. 1991, 95, 533-539
ARTICLES H/D Isotope Effects on Femtosecond Electron Reactivity in Aqueous Media Y. Gauduel,* S. Pommeret; A. Migus, and A. Antonetti Laboratoire d'Optique Appliquge-INSERM U275,Ecole Polytechnique- ENS Techniques Avancges, 91 120 Palaiseau, France (Received: February 9, 1990; In Final Form: June 6, 1990)
The effects of isotope substitution on the primary steps of electron reactivity in aqueous media have been investigated by using femtosecond near-infrared and visible spectroscopy. In neat deuterated water, a precursor of the hydrated electron has been identified. For both H 2 0 and D20, this localized state (eph d-) which absorbs in the infrared is distinct from the fully hydrated state (e,,,-). In neat D20 the dynamics of electron locafzation is slightly slower (9%) than in H 2 0 however, the lifetime of this transient electronic state (250 fs) remains similar to the analogue in light water. The absence of an H/D isotope effect on the electron hydration dynamics is confirmed in ionic aqueous media and in organized assemblies by using an anionic species (chloride ion) and a chromophore (phenothiazine), respectively, as the electron donor. In pure aqueous solutions, the changes due to isotope substitution are mainly observed during the early electron-radical pair recombination (ehN-+ X30+,ehN- + OX with X = H or D). The percentage of hydrated electrons involved in the fast recombination is increased in D20. This moderate H/D isotope effect can be linked to a change in the initial spatial distribution of the electron and prototropic radicals. The analysis of the femtosecond kinetics provides evidence that the primary electron-radical pairs (ehyd-"'XJO+, ehN-.-OX) execute a one-dimensional (1 D) walk before undergoing recombination. The jump rate of the neutralization processes is found to be significantly influenced by H/Disotope substitution (0.45 X 10l2s-I in D20 and 0.83 X 10l2s-I in HzO).Moreover, the recombination dynamics in H 2 0 and D20, which have a time scale comparable to the H/D bond lifetime, suggest the existence of a dynamical protic solvent reorganization in the vicinity of hydrated electron during the electron-radical pair's neutralization.
Introduction Knowledge of the early events immediately following the interaction of ionizing radiations with polar liquids has been greatly enhanced by the experimental and the theoretical analysis of energy deposition during pulse radiolysis and photolysis.'" The understanding of the primary reactions involving relaxed electrons with prototropic radicals represents a fundamental aspect of single electron transfer found in radiation chemistry and is an important step in the understanding of free-radical reactions occurring immediately after the energy deposition? In this respect, there has been a great deal of discussion over the relative role of the collisional and dielectric components solvent friction in the frequency factor for reaction."I0 The experimental investigation of ultrafast radical reactions in water therefore provides unique information which can test the validity of Monte Carlo calculations for modeling electron kinetics in liquid water and Onsager's diffusive model for electron-radical pair recombination.'I-l5 The various states of the electron can be used as a microprobe to test the molecular response of the solvent around a local charge. The femtosecond photoionization of water molecules or chromophores is a powerful tool in the investigation of the fast events that occur when an epithermal electron is ejected in a polar environment.16J7 Our previous studies in pure aqueous media at room temperature have demonstrated the existence of an electron localization process and have resolved the spectral evolution of an initially nonrelaxed state. One of our main conclusions was that the absence of a continuous spectral shift during the transition between the infrared and the visible spectral region clearly indicates that the final step of the hydration process cannot be understood as a unique dielectric response of the solvent. This suggests that an electronic response of the solvent influences the time dependence of the polarization in the localization and the solvation processes. More recent femtosecond photochemical studies have also permitted us to investigate the behavior of the *Author to whom correspondence should be addressed. 'Present address: Radiation Laboratory, University of Notre Dame, IN.
hydrated electron in the bulk phase in the first 100 ps following the initial energy depition in the bulk phase.18J9 The preeminent feature was an early decay of the red-induced absorption which we assigned to the electron-radical pair recombination (H30+Or OH**'ehyd-). Although the dielectric properties of H20and D20are very similar,z0 numerous experimental investigations of the thermodynamic properties (melting point, viscosity, heat capacity) have ~~
~~
(1) Rentzepis, P. M.; Jones, R. P.; Jortner, J. J. J . Chem. Phys. 1973,59, 766. (2) Baxendale, J. H. Can. J . Chem. 1977, 78, 1996. (3) Huppert, D.; Kenney-Wallace,G. A.; Rentzepis, P. M. J . Chem. Phys. 1981, 75, 2265. (4) Kenney-Wallace, G. A.; Jonah, C. D. J . Phys. Chem. 1982,86,2572. ( 5 ) Nikogosyan, D. N.; Oraevsky, A. 0.;Rupasov, V. I. Chem. Phys. 1983, 77, 131. (6) Magee, J. L.; Chatterjee, A. In Radiation Chemistry; Farhataziz, Rodgers, Eds.; VCH: New York, 1987; p 173. (7) Klassen, N. V. In Radiation Chemistry; Farhataziz, Rodgers, Eds.; VCH: New York, 1987; p 29. (8) Razem, D.; Hamill, W. H. J . Phys. Chem. 1977,81, 1625. (9) Maroncelli, M.; Castner, E. W.; Bagchi, B.; Fleming, G. R. Faraday Discuss. Chem. SOC.1988, 85, 199. (10) Williams, R. J. P. Biochem. In?. 1989, 18, 475. (11) Onsager, L. Phys. Rev. 1938, 554. (12) Mozumder. A. Radial. Res. 1985, 33, 104 and references therein. (13) Clifford, P.; Green, N. J. B.; Pilling, M. J. R. J . Srar. Soc. B 1987, 49, 266. (14) Tachiya, M. J . Chem. Phys. 1987,87,4108. (15) Turner, J. E.; Magee, J. L.; Wright, H. A.; Chatterjee, A,; Hamm, R. N.; Ritchie, R. H. Rodiat. Res. 1983, 96, 437. (16) Gauduel, Y.;Martin, J. L.; Migus, A.; Antonetti, A. In Wltrafasr Phenomena V,Fleming, Siegman, Eds.; Springer Verlag: New York, 1986; p 308. Migus, A.; Gauduel, Y.; Martin, J. L.; Antonetti, A. Phys. Reu. Lrrt. 1987, 58, 1559. (17) Gauduel, Y.; Pommeret, S.; Migus, A.; Antonetti, A. Radial. Phys. Chem. 1989, 34, 5 and references therein. (18) Lu, H.; Long, F. H.;Bowman, R. M.; Eisenthal, K. B. J . Phys. Chem. 1989, 93, 27. (19) Gauduel, Y.;Pommeret, S.; Migus, A.; Antonetti, A. J . Phys. Chem. 1989, 93, 3880. (20) Collie, C. H.; Hasted, J. B.; Riston, D. M. Proc. Phys. Soc. 1948,60, 145.
0022-3654/9 1 /2095-0533%02.50/0 0 199 1 American Chemical Society
534 The Journal of Physical Chemistry, Vol. 9S, No. 2, 1991
Gauduel et al.
established that deuterated water is a "more structural, ordered liquid", exhibiting stronger hydrogen bonds than in normal water?I It is supposed that a change in diffusion coefficients and energetic vibrational modes may cause a solvent dependence in the primary processes including deposition of energy, thermalization, trapping, hydration, and finally electron-radical pair reaction^.^^-^^ For instance, information obtained by nanosecond pulse radiolysis of liquid water has demonstrated that isotope effects on recombination processes favor the formation of H2 over D2.2629 The goal of this present work is to experimentally examine the effect of isotope substitution on the primary steps of a fast photoinduced single electron transfer in aqueous media (neat liquid water, ionic aqueous solutions, and aqueous micellar solutions). In these polar environments, the isotope substitution is a selective method of investigating more precisely the role of the structural order of the solvent in the primary events important in electron localization or solvation and in the ultrafast reactivity of the fully relaxed electron with prototropic species. Experimental Section The experiments were performed at 194 K in a continuously vibrating fixed volume Suprasil cell (2 mm path length) so that using a laser pulse repetition rate of 10 Hz each laser pulse excites a new region of the sample. Bideionized water was doubly distilled with KMn04 (resistivity greater than 17 Mohms a t 294 K, pH 6.45) in a quartz still. Deuterated water (isotopic purity >99.95%) produced by Centre #Etudes Nuclkires of Saclay was used without further treatment. Sodium chloride (purity 99.999%) from Alfa Produkte was dissolved to give solutions of the concentration 0.4 M. The anionic aqueous micelles were prepared by dispersing the chromophore (phenothiazine 5 X lo-" M) in sodium lauryl sulfate solution (NaLS 0.1 M). The blend was gently stirred at 50 OC for 5 h to produce a clear solution which did not absorb the probe. The NaLS (from Serva) was purified by adding 50 g to 100 mL of warm methanol. This mixture was then cooled, filtered, and dried under vacuum. Phenothiazine (from Merck) was of purest quality and was used as received. Static spectral characterizations of the ionic aqueous solutions and the micellar samples were run on a dual-beam Varian 2300 spectrophotometer in the UV, visible, and IR spectral regions. Oxygen was removed from the samples by tonometry using pure nitrogen gas. The femtosecond absorption spectroscopy apparatus has been described in detail previou~ly.~*~' The photoionization of solvent molecules or the chromophore is induced by femtosecond ultraviolet pulses (& 310 nm; E, 4eV; I, 10" W cm-2) obtained by focusing an amplified 620-nm beam into a 1.5-mm KDP crystal. A continuum produced by the femtosecond laser provides the probe, tunable test, and reference beams. For a given wavelength the signal corresponding to the probe and reference pulse energies are transferred to a H P computer and stored on disk for further processing. Because of the large spectral region under investigation, the velocity dispersion of the continuum induces an important shift temporal overlap between the pump and the probe beam. Therefore, careful experiments were conducted at each wavelength to determine the zero time delay and the time response function of the apparatus. The precise procedures used to obtain the position of the zero time delay have been described in detail (21) Nemethy, G.; Sheraga, H. A. J . Chem. Phys. 1964, 4I,680. (22) Jonah, C. D.; Matheson. M. S.; Miller, J. R.; Hart, E. J. J . Phys. Chem. 1976.80, 1267. (23) Chernovitz. A. C.; Jonah, C. D. J . Phys. Chem. 1988, 92, 5946. (24) Long, F. H.; Lu, H.; Eisenthal, K. B. Chem. Phys. Lett. 1989, 160, 464. (25) Fielden. E. M.; Hart, E. J. Rudiut. Res. 1968, 33, 426. (26) Lifshitz, C. Con. J . Chem. 1963, 41, 2175. (27) Anbar, M.; Meyerstein, D. Trun.5. Furuduy Soc. 1966, 62, 2121. (28) Boyd, A. W.; Willis. C.; Lalor, G. Can. J. Chem. 1972, 50, 83. (29) Hart, E. J.; Anbar, M. In The Hydrurcd Electron; Wiley Interscience: New York, 1970. (30) Migus, A.; Antonetti, A.; Etchepare, J.; H u h , D.; Orszag, A. J . Opt. SOC.Am. 1985, B2, 584. (31) Gauduel, Y.; Migus. A,; Martin, J. L.; Lecarpentier, Y.; Antonetti, A. Ber. Bunsen-Ges. Phys. Chem. 1985, 89, 218.
IO 700nm
IOio M-I s-I) can influence the early time behavior of fully hydrated electrons.' In agreement with previous data performed at 625 nm24 our femtosecond data shows the existence of a significant difference in the decay dynamics between heavy (X = D) and light water (X = H). To analyze the experimental time-resolved spectroscopic data, we have considered a model in which an initial electronradical pair (XO~+".eh,,d-or OX--eh d-) executes a geminate recombination following a ID walk (eq 3 and 4). This model
-
+ ehyd- x + x20 ox + ehyd- ox-
x30+
(3) (4)
includes the fact that at short time (?< 1O-Io s) a fraction of the photogenerated electrons is hydrated in the vicinity of the hydronium ion or the hydroxyl radical. A recombination controlled by 1D diffusion can be modeled by using the analytical expression: Ne-(h d)(t) = yNe-(hyd)erf (Td/1)lI2,in which y represents the fraction ofthe initial population of hydrated electrons (Ne-(hyd))that will recombine and Tdthe jump time of the recombination. This kinetical model implies that the recombination length represents the jump distance.I9 In pure deuterium oxide, the best computed fits of the experimental traces obtained in the first 30 ps give a jump rate of 0.45 X 10l2s-I and Td = 2.2 f 0.1 ps (Figure 1A). These kinetic data, when compared to values previously obtained in pure light Cjump rate 0.83 X 10l2s-I, Td = 1.2 ps), suggest the existence of a H / D isotope effect on the rates of the neutralization reactions involving hydrated electrons and the primary prototropic species (X30t or OX). Our analytical model of the neutralization process involving eh d- and the prototropic species satisfies the time dependence l / ( t ) l / 2 for the induced-absorption decay at longer time, Le., in the first 100 ps (Figure IB). This figure shows that the induced absorption decay is not complete during the first 100 ps after the the escape photoejection of electron. Indeed in pure D20and H20, can be attributed to a fraction of the excess electrons which are hydrated far from the radical OX or the cationic species X30+ for which the long-range Coulombtype interactions are negligible. Consequently, the total absorbance at the test wavelength (700 nm) includes the contribution of one population of ehyd- which recombines quickly [(ehyd-)r"b] (t < 100 ps) and a stable population [(eh d-)n-mb] which will recombine at longer time ( t >> 100 ps). T i e information describing the early electron-radical pair recombination in H20and D 2 0 is summarized in the Table I. The H/D isotope effect on the percentage of recombination is weak compared to that on the neutralization reaction dynamics. H I D Isotope Effect on Electron Trapping and Hydration Dynamics in Neat Water. The data on the dynamics of the earliest primary events occurring after the ejection of epithermal electron are summarized in the Figure 2. The time dependence of the induced absorption in the infrared and the red spectral regions
Figure 2. (Aj Transient infrared absorption of the epithermal electron in deuterium oxide (D20 > 99,95%) following femtosecond ionization with 100-fs ultraviolet laser pulses at 310 nm. The smooth lines represent
the computer best fits. At 1250 nm, the transient absorption due to a = A"[T2/(T2 - Tl)l[T2 exp(localized state population (Nc-(pre,,~)(!) t / T 2 )- T I exp(-t/T,)] is best fitted with a trapping time (TI)of 120 fs. This localized state relaxes towards the fully hydrated stated according to a monoexponential law with a time constant T2of 250 fs. The insert represents the rise time of the induced absorption assigned to the localized electron (trapped electron) in neat light water (TI = 1 IO fs and T2 = 240 fs). (e) Rise time of the induced absorption at 660 nm following femtosecond photoionization of heavy water by ultraviolet femtosecond pulses. The smooth line represents the computed best fits of the experimental traces assuming an appearance time of 120 fs and a final relaxation toward the hydrated electron in 250 fs as observed in the near-infrared region. The computed best fits of the initial give a random with Td = 2.2 ps. The insert shows the detail walk law (erf (Td/t)'I2) of the hydrated electron appearance in pure water assuming TI = 1 IO fs, T2 = 240 fs, Td = 1.2 ps. has been analyzed by a kinetic model which considers (i) the dynamics of the primary steps (electron trapping and solvation), (ii) the early behavior of the hydrated electron as previously investigated on the picosecond time scale (Figure 1). Starting from an initial virtual. species created instantaneously (an excited state of deuterium oxide molecules) we assume the existence of two separate species during the electron hydration process. One species, absorbing in the infrared (eprehyd-), appears with a time constant TI. This transient electronic state then relaxes toward the fully hydrated electron (ehyd-) following first-order kinetics with a time constant T2. For the alternative test wavelengths (660 and 1250 nm) the induced absorption rise time has been analyzed considering the convolution of the pumpprobe pulses with the expected signal rise dynamics due to electron trapping and solvation and the early electron-radical pair recombination. The total absorbance of the test pulse is described by an expression which includes the evolution of the different electron populations during the excitation and probe pulse (Ne-(prehyd) and Ne-(hyd)): = AAe-(prehyd)A(T) + AAe-(hyd)'(r)
(5)
with
In these expressions we assume the absorption constant does not to vary so that Figure 2 also represents the evolution of
536 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 transient electron populations (Ne-(prch ) and Ne-(hyd)). The kinetics obtained in the near-infrared spectraidregion (1250 nm) represent the evolution of a trapped electron (eprchfl-).The time dependence of this species fits perfectly to the convolution of the pulse profile and the expected signal rise defined in Figure 2A. In pure D20, the computed best fit gives an appearance time T Iof 120 fs and a lifetime T2of 250 fs. The incomplete recovery of the signal at 1250 nm is due to a weak contribution of the hydrated electron which still absorbs in the near-infrared region as previously shown in pure liquid water.I6 In the visible spectral region that contains the wide structureless band of the hydrated electron mainly, the rise time of the induced absorption is well fitted by the convolution of the pulse profile and the expected signal rise defined in Figure 2B. At the wavelength of 660 nm, the computed fit of the experimental trace also includes the contribution of an early recombination of the hydrated electron as previously shown in Figure 1. More precisely, the rise time of the normalized induced absorption at 660 nm consists of the linear combination of two signals: one assigned to a long-lived hydrated electron population and the other one to an hydrated electron population performing an early recombination according to a one-dimensional (1D)walk law [erf
Gauduel et al.
A
Simulation T d
-
1. 2pr. 100%
2. 2p-. 100%
TlME
I A
NTOT-
0
Simulation
Td
B
Simulation
Td
C
(Td/t)”*].
= ( 1 - r)q:&i))~(t-) + Yuf$hyd)).(tJ (9) In this expression for ut-hyd)J7) we employ the same values of TI and T2determined in the infrared region. If we consider the effect of refractive index on the position of the zero time delay, the computed best fits of the experimental traces demonstrate that H / D substitution does not modify the electron localization and hydration times ( T I ,T2). However, a careful analysis of the experimental traces obtained at 660 nm (Figure 2B) permits us to examine the influence of an early time one-dimensional walk dynamics (Td) on the global rise time of the induced absorption of the hydrated electron. The computations performed for light and heavy water are presented in the Figure 3. These calculations demonstrate that the rate (Td) and percentage (7)of the early electron-radical pair recombination influence the induced absorption rise time. The main point we emphasize here is that a significant H/D isotope effect on the rise time of 660 nm induced absorption is obtained without any change of the T , and T2values previously determined from the infrared data. HID Isotope Effect on Electron Hydration in Ionic Aqueous Solution and Aqueous Micelles. To completely understand the influence of isotope substitution on electron hydration dynamics, we have also investigated the H/D isotope effect on single electron transfer in ionic aqueous solutions and in aqueous organized assemblies. Electron photodetachment from the chloride anion through charge transfer to solvent spectra (CTTS) was achieved by pumping ionic aqueous solutions at 310 nm ( E = 4 eV). It has been suggested, from early flash photolysis experiments, that the femtosecond photolysis channel occurs through a highly excited state of the chloride anion which dissociates to give an epithermal electron and a chlorine a t ~ m . ~ ) . Starting ’~ from the transient absorption spectra obtained on the picosecond time scale, the primary steps occurring after the femtosecond energy deposition have been shown to be described by the reactions: CI- + hv (CI-)* CI + e,c (10) w:,(t)
e,{
+ n(X20)
--
1
T I M I
cc-a
x D20
Nror-
-
55%
2. 2pr. 49%
a
Figure 3. Computed H/D isotope influence of early electron-pair recombination on the time-resolved induced absorbance at 660 nm following femtosecond photoionization by ultraviolet pulses. In (A), the amount of recombination has been normalized to 100%to show the full effect of Td. In (B) the recombination percentages are 55% for H20and 49% for D20as measured at longer time (Figure 1). In (C), the simulation shows in detail the effect of Td and recombination rate on the appearance time of the signal assigned to the hydrated electron.
In NaCl (0.4 M)/H20, an intermediate species, absorbing in the infrared region and identified as a localized electron, appears with a T Itime constant of.120 fs. Its relaxation toward a hydrated species follows a first-order kinetics with a T2 time constant of 250 fs (Figure 4A). In the red spectral region (700 nm), the signal assigned to the hydrated electron follows the kinetics described by the following equation:
hydrated electron
(X = H, D) (1 1) Our time-resolved spectroscopic data obtained in the infrared and visible spectral regions reveal that the photodetachment of an electron from CI-and its subsequent hydration proceed through at least one intermediate step (Figure 4). (33) Gauduel, Y.; Pommeret, S.;Yamada, N.; Migus, A.; Antonetti, A. J . Am. Chem. SOC.1989, 1 1 1 , 4974. Long, F. H.; Lu, H.; Eisenthal, K. B. J. Chem. Phys. 1989, 91.4413. (34) Matheson, M. S.; Mulac, W. A.; Rabani, J. J. fhys. Chem. 1%3,67, 2613.
a
II
+ H 2 0 -1. 2pr.
in which T I represents the dynamics of electron localization in the aqueous phase and T2 the dynamics of relaxation of the localized electron. These parameters have been previously measured in the infrared region. Frame B of the Figure 4 allows us to establish that when the electron donor is an anionic species (Cl-), the dynamics of electron trapping and hydration remain independent of an H/D isotope substitution of the solvent. Indeed, in D20, the localization and hydration steps are respectively characterized by the time constants T I = 120 fs and T2= 250 fs .
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 537
Electron Reactivity in Aqueous Media
I NaLSlPTH
@
I
[D20]
__.
, Figure 4. Kinetic results obtained at I250 and 700 nm after femtosecond UV photolysis of aqueous solution of sodium chloride (H20/NaCI (0.4 M) or D20/NaC1(0.4 M)). The smooth lines represent the computed best fits of the experimental curves. At 1250 nm, the fits assume a equals 260 trapping time ( T I )of 120 fs. The lifetime of the eVehyd-(T2) fs. At 700 nm, the fits of the experimental traces includes the combination of the appearance time of e,hfl- and ehyd-(eq 6) as measured in the infrared region. TABLE II: H/D Isotope Effect on the Transient Concentration of Hydrated Electron in Aqueous Media and Organized Assemblies Following the Femtosecond Ultraviolet Photoionization of Chloride Anion (K4 M ) or Phenothiazine (5 X lo4 M)"
transient concn of ehyd-lat t = 2 ps, uM
aaueous media 0.4 M NaCl
NaLS micelles (NaLS (0.1 M)/PTH) X
H,O
D,O
17 33
16 32
OThe transient concentrations are calculated taking Gc(ehyd)-')= 5 IO' in H 2 0 with G(ehd-) = 2.9.29*46
The influence of H/D isotope substitution on the transient concentration of the hydrated electron following femtosecond UV photoionization has been also investigated by considering the molecular extinction coefficient of the hydrated electron as defined by earlier pulse radiolysis and photolysis experiment^.^*^^ In light water, taking an absorption coefficient of 1.8 X 104 M-' cm-I for the hydrated electron at 700 r ~ mthe , ~ estimate of ehyd-concentration produced after each ultraviolet femtosecond pulse is 17 f 1 pM. Similar calculations performed with D20, using an absorption coefficient of 2.1 X IO4, give a comparable transient concentration (Table 11). Our conclusion is that H / D isotope substitution in dilute ionic solutions influences neither the photophysics of chloride ion in an aqueous environment nor the dynamics of formation of the fully relaxed radical (eh d-). To consider the early electron reactivity in organizedassemblies in which the polar phases (Stern and Gouy-Chapman layers) contain H 2 0 or D20, we have used NaLS micelles in which the electron donor (phenothiazine) is embedded in a hydrophobic core defined by the hydrophobic chains of surfactant molecules. On the picosecond time scale, it has been previously shown that the surface electric potential ($i) prevents an early electron-anion pair recombination." Therefore, we can now monitor the dynamics of electron hydration following femtosecond photoionization of phenothiazine in the absence of fast electron-radical pair recombination. The appearance of the regular hydrated electron spectrum demonstrates that the primary stages of photoionization include PTH+ + e,[) and intramicellar charge separation (PTH
-
w=D8',
L-_--_ 0
5
Figure 5. Time-resolved data showing the hydrated electron appearance in micellar systems (PTH/NaLS (0.1 M)/H20 or D20) following photoionization of the chromophore (PTH 5 X IO4 M) with a 100-fs laser pulse at 310 nm. In A and B, the smooth lines represent the computed best fit of experimental traces assuming an electron trapping time T I of 250 fs and its lifetime ( T 2 )of 260 fs. In the insert, the instantaneous
responses giving the zero time delay are also represented. subsequent electron escape into aqueous phase. Furthermore, if we assume that the excess electrons are produced via a biphotonic photoionization process we can deduce that the excess kinetic energy of photoelectrons is of the order of 2.5 eV considering that the ionization potential of PTH is around 5.6 eV.j5 For anionic micelles in solutions of H 2 0or D20, femtosecond absorption kinetics have been measured in the broad band peaking around 720 nm which corresponds to the well-known spectrum of the hydrated electron. Previous experiments performed with NaLS/PTH/H20 micelles have identified a transient infrared band assigned to a nonrelaxed hydrated electron. In comparison with pure water, the rate of formation of the precursor of the hydrated electron (evhy!-) is lengthened in the micellar aqueous phase (250 fs in NaLS micelles against 110 fs in pure water). This transient infrared species relaxes toward the fully hydrated state in 260 f 10 fs. In NaLS/PTH/D20 micelles, the rise time of the induced absorption (720 nm) is similar to the experimental trace obtained with H 2 0 (Figure 5 ) . More precisely, the induced absorption at 720 nm is well fitted with the convolution of the pumpprobe convolution and the expected signal rise defined by the eq 2. The best computed fit (Figure 5B) is obtained with the same TIand T2values as those determined with NaLS/PTH/H20 in the infrared ( T I = 250 fs, T2 = 260 fs). Previous structural investigations of sodium dodecyl sulfate micelles in aqueous solutions based on quasi-light-scattering techniques have shown that, under our experimental conditions (low ionic strength), (i) the intermicellar interactions and headgroup repulsions are the same for H 2 0 and D 2 0 and (ii) the isotopic substitution does not modify the hydrodynamic radius of micelles.36 Considering such structural information, our femtosecond investigationsdemonstrate that there is no measurable H / D isotope effect on photoionization and the hydration time of an electron for which the excess kinetic energy is higher than in pure liquid water (2.5 eV with phenothiazine against 1.5 eV with water molecules). Discussion The influence of H / D isotope substitution on the primary processes involved in the hydration of epithermal electron and on (35) Alkaitis, S. A.; Gratzel, M.;Henglein, A. Ber. Bunsen-Ges. Phys. Chem. 1975, 79, 541. (36) Chang, N. J.; Kaler, E. W.J . Phys. Chem. 1985, 89, 2996.
Gauduel et al,
538 The Journal of Physical Chemistry, Vol. 95, No. 2, I991
electron-radical pair recombination have been investigated in neat liquid water and ionic aqueous solutions by using visible and infrared femtosecond spectroscopy. In light and heavy water, the hydration process of epithermal electrons having an excess kinetic energy around 1.5 eV occurs through an intermediate state absorbing in the infrared which is attributed to a localized or prehydrated state. The dynamics of electron trapping in pure D 2 0 correspond very closely to those in light water. As shown in the Table I, the trapping time ratio between D 2 0 and H 2 0 (Tl(D2o)/TI(H2O))does not exceed 1.09. This value is lower than (i) the ratio of energetic vibrational mode (OH vs OD) which is 2’12 times greater in H 2 0 than in D20,27 and (ii) the estimate of the ratio of the energy loss rate.37 If the rate of energy loss for the photoejected electron during the thermalization and trapping steps is dependent on the coupling with the most energetic vibrational mode of the OX bond (antisymetric stretch), the time necessary for complete energy dissipation during localization would increase in the same proportion as the ratio of energetic vibrational mode (OH/OD). As the H/D isotope effect on trapping time is lower than this value (Table I), it can be concluded that the thermalization step in D 2 0 remains very fast and is not the limiting factor in the prehydration process. Moreover, for electrons with low kinetic energy, the localization time is dependent neither on number of collisions which are needed for the energy dissipation of the electron nor on the thermalization length. The second important point to draw from this study is that H/D isotope substitution does not affect the relaxation dynamics of prehydrated electrons in neat liquid water. The present data establish that hydration dynamics of subexcitation electron remain largely independent of the physical properties of the polar solvent, such as viscosity, Debye time, vibrational energy of antisymmetric stretch (OH,OD). Although the Debye relaxation time (TD) is longer in D 2 0 than in H20(10.2 ps vs 8.2 ps at 294 K,38339there is a discrepancy between the estimate of isotope effect on the hydration time defined by the continuum theory (TL = TD*f,/to)” and the experimental values (Table I). This discrepancy is in agreement with previous data obtained for light water where the absence of a continuous shift between the infrared band assigned to a trapped state of electron (eprchyd-1 and the fully hydrated state (ehyd-) supports the conclusion that the relaxation of water molecules in the vicinity of an excess electron involves extremely small water motions. These two experimental facts lead to the conclusion that an epithermal electron reaches its hydration state without a dominant action of the dielectric response of the polar medium.I6 This conclusion is contrary to the recent results of Long et who report the existence of an isotope substitution effect on the rate of appearance of the hydrated electron in neat water. The discrepancy between our results and those of Long et al.“ is likely due, in part, to the fact that these authors do not consider the effect of the refractive index and the fast recombination at short times on the risetime for the appearance of the hydrated electron at 625 nm. In the present study, additional data obtained in ionic aqueous media allow us to conclude that there is no H / D isotope effect on the global hydration time and on the photoionization yield for epithermal electrons whatever the mode of electron photoejection: multiphotoionization of solvent molecules or electron photodetachment by CTTS (Tables I and 11). It is interesting to compare our femtosecond data on electron hydration in neat water with several aspects of recent theoretical quantum calculations on ultrafast molecular motions describing Adiabatic the configurational disorder of the “frozen” (37) Konokalov, V. V.; Raitsiimring, A. M.; Tsvetkov, Y. D. Rudiut. Phys. rhem -.._ .... I=. - - -_, -32. - 623 - -_. (38) Grant, E. H.; Shack, R. 1968, 1519. (39) Calef, D. F.; Wolynes. P. G. J . Phys. Chem. 1983, 87, 3387. (40) Schnitker, J.; Rouky, P. J.: Kenney-Wallace, P. J. J . Chem. Phys. 1986,85, 2926. (41) Wallqvist, A.; Martyna, G.; Berne, B. J. J. Phys. Chem. 1988, 92, 1721.
.
simulations of electron solvation in water have determined a repartition of favorable sites for the initial electron lacalization.~ These sites (preexisting shallow traps) may be due to fluctuations of the electronic density and solvent molecules orientation yielding an absorption line shape which is in agreement with the observed early IR absorption. Recent simulation of Messmer et aLu suggest that the time evolution of this transient IR band would correspond to a relaxation of an excited state of a hydrated electron toward a fully hydrated ground state involving solvent fluctuation, translational, or librational motions. The third point of this study concerns the H / D isotope effect on the early electron-radical pair reaction in pure liquid water. In the understanding on the evolution of the coupling between the electron, prototropic species, and water molecules, we must wonder if the recombination process involves an intrinsic structural response of the protic solvent as the recombination occurs on a time scale during which proton transfer becomes appreciable. The investigation of hydrated electron behavior in the range 0-100 ps has permitted us to extend the understanding of ultrafast reactions involving primary radical species (eh d-9 prototropic species). Under our experimental conditions where the excess kinetic energy of the electron is estimated to be around 1.5 eV, we assume that the behavior of the hydrated electron is defined by an electron-radical pair which executes a onedimensional walk (1 D) before undergoing neutralization process (ehfl- OX or ehdX30+, with X = H or D). The linear combination of two populations of hydrated electron in the computed fits (1 - Y(ehyd-)nrecomb + T(ehyd-)rmmb)allows us to estimate the percentage of electron that escape from electron-radical pair recombination. In pure liquid water, the fraction of the initial population of hydrated electrons which is not involved in this fast recombination (t > 100 ps) is weakly modified by an isotope substitution. From our computed fits, this fraction increased from 45 f 2% for H 2 0 to 51 f 2% in D20. These values are in agreement with recent femtosecond data obtained at 625 nm in H 2 0 and D20.18 The simultaneous weak H/D isotope effect on the trapping time (T,) and the percentage of recombination indicates that the initial spatial distribution of ionized species including ehyd-9 OX, and X30+is slightly broader in D 2 0 because the distance traveled by electron prior to hydration in D 2 0 is greater than in H20.The initial spatial distribution of electron and prototropic species can be influenced by the rate of energy deposition through coupling with the vibrational modes of the solvent as previously suggested.27 An estimate of the initial distribution from our femtosecond experiments cannot be obtained with any accuracy as the treatment of this ultrafast recombination reaction by a continuum theory including Coulombic interactions and diffusion processes must be questioned since Tdis much shorter than the longitudinal relaxation time TL and comparable to the lifetime of prototropic species (Table I). Considering the high mobility of the hydronium ion compared to ehfl-(DH,?+ = 9 x cm2 s-’, Dc-(hfl)= 4.75 x io-’ cm2 PI),” our analytical solution emphasizes the role of the dynamical structure of the hydrated X30+during the electron-prototropic pair recombination including the time dependence of hydrogen bonds polarization. Let us consider, in particular, NMR studies on the H/D isotope effect on the prototropic species temporal characteristics.& It is interesting to notice (Table I) that in pure liquid water the jump rate ratio ( Td-’/(D20)/Td-’/(H20)) is inside in the range of values defined by the ratio of prototropic lifetimes. The existence of H/D isotope effects on both the dynamics of recombination and the lifetimes of the prototropic species would suggest that one of the limiting step in the recombination process of the hydrated electron
+
+
(42) Motakabbir, K. A.; Rossky, P. J. Chem. Phys. 1989, 129, 253 and references therein. (43) Barnett, R. B.; Ladman, U.;Nitzan, A. J . Chem. Phys. 1989, 90,
A. A..-. l7
(44) Messmer, M. C.; Simon, J . D. J . Phys. Chem. 1990, 91, 1220. (45) Zaider, M.; Brenner, D. J. Rudiut. Res. 1984, 100,245. Schmidt, K. H.; Buck, W. L. Science 1966, 151, 70. (46) Halle, B.; Karlstrom, G. J. Chem. Soc.,Furuduy Trans. I 1983, 70, 1047.
J . Phys. Chem. 1991,95, 539-550 is the lifetime of hydrated X30+ or OX. This interpretation supports our analytical model of the recombination as a process in which the jump distance equals the recombination length. The model implies that in a (x,o+:ox)hyd'"(ehyd-) pair a proton transfer from X30+ executes a I D walk in the vicinity of the hydrated electron before undergoing recombination. We think it reasonable to suggest that ultrafast electron-radical pair recombination is governed by specific molecular motions of the protic solvent around the electron and radical species. Following this hypothesis, if a hydrated prototropic species is in the vicinity of the hydrated electron, the proton-transfer dynamics from the prototropic species to the electron one will limit the recombination dynamics and the relaxation of the hydrated electron will be dependent either on the H-bond dynamics between H30+ and water molecules, or on the proton migration from hydronium to neighboring water molecules. This being the case, the limiting factor in the deactivation dynamics corresponds to the activation energy of the hydrated electron-cation bond cleavage reaction, including proton jump or the local polarization effect on H bonds.
539
In conclusion, photochemical investigations of the H/D isotope effect on epithermal electron reactivity in aqueous media emphasize that changes in the macroscopic physical properties of the polar solvent (viscosity, Debye time) do not influence significantly the hydration dynamics of excess electron whatever is the mode of photoejection (two photoionization process of water molecules or photoionization of a chromophore). The femtosecond experiments on early recombination processes provide fundamental information about the dynamics of fast radical reactions in aqueous media. Indeed, the early reactivity of hydrated electron with prototropic species would involve fast local reorganization of water molecules in the vicinity of reactive species without the intervention of a signifiant diffusion process and this suggests a preeminent role for the local molecular or electronic structure of the solvent.
Acknowledgmenf. S . P. is the recipient of a grant in aid from DRET, Paris. We thank Dr.J. Belloni and S. Pimblott for stimulating discussions and critical reading of the manuscript and gratefully acknowledge the technical assistance of N. Yamada.
Far Infrared Spectrum, Conformational Stability, Barriers to Internal Rotation, ab Initio Calculations, and Vibrational Assignment of Propionyi Chloride J. R. Durig,* A. Q.McArver, H.V. Phan? and C. A. Guirgist Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received: April 26, 1990)
The far infrared spectrum (375 to 30 cm-l) of gaseous propionyl chloride has been recorded at a resolution of 0.10 cm-l. A substantial number of bands have been assigned to both the symmetric and asymmetric torsional madm for both the s-tronr (oxygen atom eclipsing the methyl group) and the higher energy gauche conformers. From these data both the symmetric and asymmetric torsional potential functions have been calculated. The potential function coefficients for the asymmetric torsion are calculated to be (in cm-I) VI = 872 f 40, V, = -164 f 31, V3 = 644 f 10, and V4 = 18 f 6, with an enthalpy difference between the more stable s-trans and the gauche conformers of 491 f 81 cm-' (1.40 f 0.23 kcal/mol). This function gives values of 760 f 6 cm-' (2.17 f 0.02 kcal/mol), 1026 f 39 cm-I (2.93 f 0.11 kcal/mol), and 269 f 11 cm-' (0.77 f 0.03 kcal/mol) for the s-trans to gauche, gauche to gauche, and gauche to s-trans barriers, respectively. From this potential function, the dihedral angle of the gauche conformer is calculated to be 108.5 f 0 . 5 O . From the studies of the Raman spectra at different temperatures, the enthalpy difference was determined to be 596 f 45 cm-' (1.70 f 0.13 kcal/mol) and 607 f 50 cm-l (1.74 f 0.14 kcal/mol) for the gas and liquid, respectively. The barrier to methyl rotation for the s-trans conformer is found to be 950 f 2 cm-'(2.72 kcal/mol). The r, structural parameters for the heavy atom skeleton of the s-trans conformer have been calculated from the previously reported rotational constants of the 3sCl and 37CIisotopic species. The conformational stability, barriers to internal rotation, fundamental vibrational frequencies,and structural parameters that have been determined experimentally are compared to those obtained from ab initio Hartree-Fcck gradient calculations employing both the 3-21G* and 6-31G* basis sets and to the corresponding quantities for some similar molecules.
Earlier, we completed low frequency vibrational studies of propanal,' and propionyl fluoride* in the gas phase. These molecules gave infrared spectra very rich in transitions for the symmetric and asymmetric torsions for two conformations. The observed Q branches for these torsions and excited states of them allowed the calculation of the symmetric and asymmetric potential functions from which the barriers to internal rotation were obtained. Both of these molecules have the s-trans conformer (oxygen atom eclipsing the methyl group) more stable than the gauche conformer. Several studies have been carried out on the conformational equilibrium of propionyl chloride in the gas. These studies utilized 'Prcscnt address: Ethyl Corporation, P.O. Box 1028, Orangeburg, SC 29115
tPermanent address: Analytical Research Laboratory, Dyes and Pigments Division. Mobay Corporation. Bushy Park Plant, Charleston, SC 2941 1 .
0022-3654/91/2095-0539$02.50/0
m i c r ~ w a v eand ~ , ~vibrationals spectroscopy, as well as the electron diffraction technique.6 In all of these studies it was concluded that propionyl chloride exists as a mixture of two conformers, the s-frans and the gauche rotamers, with the s-trans conformer being the more stable form. For the s-frans conformer, all of the heavy atoms are in the plane of symmetry and the C-CI bond is trans to the CH3 group. In the two equivalent gauche conformers, the C - 0 bond is essentially eclipsing one of the hydrogen atoms. Karlsson3 from a microwave study predicted the asymmetric torsional mode of the s-trans conformer to be a t 86 f 10 cm-I (1) Durig. J. R.; Compton, D. A. C.; McArver, A. Q.J. Chem. Phys. 1980,
73, 719.
(2) Guirgis, G. A.; Barton, B. A.; Durig,J. R. J . Chem. Phys. 1983, 79, 5918. ( 3 ) Karlsson, H. J . Mol. Srrucr. 1976, 33, 227. (4) Mata, F.; Alonso, J. L. J . Mol. Srrucr. 1979, 56, 199. (5) Frankiss. S. G.; Kynaston, W. Specrrochim. Acta 1975, 31A, 661. (6) Dyngcscth, S.;Schei, S.H.; Hagen, K. J . Mol. Srrucr. 1984,116,257.
0 1991 American Chemical Society
