Photolysis of Benzene in Cyclohexane at 2537 A (7) (a) C. S. Parmenter and M. W. Schuyler, Chem. Phys. Lett., 6, 339 (1970); (b) C. :S.Parmonter, M. W. Schuyler, W. R. Ware, and B. K. Sslinger, ibid., 6, 343 (1970). (8) G. M. Breuer and E. K. 6.Lee, J. Chem. Phys., 51,3130, 3615 (1969). (9) M. Nishikawa and P.R . Ludwig, J. Chem. Phys., 52, 107 (1970). (10) C. S. Burton and H. E.Iiunziker, J. Chem. Phys., 52, 3302 (1970). (11) B. K. Selinger and W. R . Ware, J. Chem. Phys., 52, 5482 (1970); 53, 3160 (1970). (12) C. L. Brawn, S. Kato, and 5.Lipsky, J. Chem. Phys., 39, 1645 (1963). (13) T. V. Ivanova, Q. A. Mokeeva, and B. Y. Sveshnikov. Opt. Spectroc., 12, 352 (1962). (14) I. B. Berlman, "Handbook of Fluorescence Spectra of Aromatic Molecubs," Academic Press, New York, N. Y., 1965. (15) J. W. Eastnian, J. Chem. Phys., 49, 4617 (1968). (16) P.A. Gregory and W. P. Helman, J. Chem. Phys., 56, 377 (1972).
1909 (17) M. Luria and G. Stein, Chem. Commun., 1650 (1970); J. Phys. Chem., 76. 165 (1972). (18) Y. ilan, d. Luria, and G. Stein, to be submitted for publica?ion. (19) H. Lutz and G. Stein, 78, 1909 (1974). (20) E. J. Bowen and A. H. Williams, Trans. Faraday Soc., 37, 765 (1039). (21) Th. Fbster, "Fluoreszenz Organischer Verbindungen," Gottingen, 1951. (22) M. Ofran and G. Stein, unpublished results. (23) M. J. Mantione and J. P.Dandey, Chem. Phys. Left., 6, 93 (1970). (24) D. G. Marketos, Anal. Chem., 41, 195 (1969). (25) A. Morikawa and R. J, Cvetanovic, J. Chem. Phys., 52, 3237 (1970). (26) R. B. Cundall and L. C. Pereira, J. Chem, Soc.. Faraday Trans. 2, 68, 1152(1972). (27) R. B. Cundall and D. A. Robinson, J. Chem. Soc., Faraday Trans. 2, 68, 1145 (1972). (28) Y. ilan, H. Lutz. and G. Stein, to be submitted for publication.
Photolysis of enzene in Cyclohexane at 2537 A enspeter Lutz and Gabriel Stein" Department of Physical Chemistry, The Hebrew University,Jerusalem, lsrael (Received December 29, 1972; Revised Manuscript Received June 3, 1974 Publica tion costs assisted by the lsrael Academy of Sciences
Initial quantum yields of benzvalene formation in the 2537-11 photolysis of benzene in dilute cyclohexane solutions in the absence of oxygen were determined using an analytical method based on the reaction of benzvslene with bromine. The rate of formation of benzvalene is discussed in terms of a kinetic model which involves the reaction of triplet benzene with benzvalene. The initial quantum yield of benzvalene is 0.18. This value is nearly independent of temperature in the 9-50' range. The results are consistent with the vkw that benzvalene arises from the spectroscopically prepared higher vibronic state prior to vibrational relaxation to the fluorescent level.
Introduction In recent years considerable experimental and theoretical effort has beon directed to the study of the radiationless processes occurring in benzene. For this molecule, excited in dilute hydrocarbon solution to its lBpu first excited singlet state, fluorescence and triplet state formation only account for about 30% of' the total energy input.l It has been suggested that part of the electronic energy in excited benzene is converted into chemical potential by way of valence bond isomerization. Direct evidence for such chemical relaxation of electronic energy has been provided by the observation of benivalene (I) in the 2537-11 photolysis of benzene in hexadecane ~ o l u t i o n Several .~ authors have tentativeiy related this isomerization reaction producing benzvalene to the thermally activated fluorescence quenching of difficultly with this interpretation is that no b e n z e ~ i e . ~A- ~ detailed studies of the kinetics of benzvalene formation and its temperature dependence have been reported. The quantum yield for formation of benzvalene in hexadecane solution a t 2537 -4 has been determined by gas chromatography to be 0.020.2z6 The yield of benzvalene was reported to increase with ~emperature,however, no quantitative results have been given. Another determination based on the photochemical rearrangement of benzene-1,3,5-d3 in hexagave a value of 0.051 for the decane solution a t 2537
quantum yield of the benzvalene intermediate assumed for this rearrangement. In order to learn more about the mechanism of benzvalene formation and its relation to the spectroscopic properties of benzene we have undertaken a detailed study of the initial quantum yield of benzvalene formation in cyclohexane solution and its dependence on temperature. The amount of benzvalene formed in irradiated benzene solutions in the present study was determined by an analytical method based on the specific uptake of bromine by the benzvalene molecule. The bromination of benzvalene has recently been studied in some detail by Roth and Katz7 These authors have observed that benzvalene (I) adds bromine, as shown in Scheme I, to form one single product, identified as 5,6-dibromobicyclo[2.l.l]hex-4-ene(11). The reaction has been shown to occur quantitatively. Interestingly, the olefinic Scheme I E3r /
'3r I
n
lt
The Journal of Physical Chemistry. Voi. 78. No. 79. 1974
1910
double bond, 1 he usual site of electrophilic attack, remains unaffected during the bromination. We used this reaction to determine benzvalene in irradiated benzene solutions by measuring the amount of bromine consumed by the photolysis mixture and assuming that one molecule of photoproduct consumes I molecule of Brz. In our determination we used a n ethanol solution of Br3- as brominating agent. The bromination was followed spectrophotometrically using the high absorption coefficient of the tribromide ion in the ultraviolet. In this way benzvalene concentrations as low as M could be determined. Benzvalene has been reported to be the only major photoproduct in the photolysis of benzene in dilute hydrocarbon solutions at 2b37 A.2 Fulvene, also reported to be formed, is a secondary product arising from benzvalene. Prolonged irradiation of very concentrated solutions of benzene in hexane results in a complex mixture of reaction products, most of them arising from reactions of the hexaFor the present study we accordingly astrienyl radi~cal.~ sumed that the first stable photoproduct is only benzvalene. We report further support for this assumption. In order to obtain initial quantum yields for the benzvalene formation, we carried out the irradiations at low light intensities (2 X lom6einstein 1.-l sec-I) and short irradiation times (up to 8 min). In addition, the benzene concentration was chosen sufficiently low (0.01 M)to avoid participation of singlet excimers in the photoreaction and prevent increase of triplet yield through excimer formation.9
~ x ~ e r iSeetion ~ e ~ t ~ ~ Benzene Fluka puriss (99.93%) thiophene free was used without further purification. The solvents, cyclohexane (Fluka) anid ethanol (Fluka), were of spectroscopic grade. The uv light source was a low-pressure mercury resonance lamp (Thermal Syndicate Ltd.). The 1849-A line of the lamp was eliminated by a filter containing KCl (75 g L-1) with an tcptical path length of 1 cm. The light intensity einstein I.-' sec-l a t 2537 A. was about 2 'r: In a typical experiment, 4 ml of a solution of 0.01 M benzene in cyclohexane was degassed by three freeze-pumpthaw cycles. The solution was irradiated at 2537 A in a 1em square silica cell. After completion of the irradiation, 4 ml of a solution of approximately 1-2 X lo-* M bromine in ethanol-KBr (KBr 1 g l.-l) was added to the photolysis mixture. A blank was simultaneously prepared by treating a nonirradiated benzene solution with an equal volume of the bromination agent. For experiments a t different temperatures, the photolysis mixture was brought to room temperature before adding bromine. The consumption of Br3- in the irradiated sample was followed against the blank as reference on a Cary 14 spectrophotometer with the recording wavelength fixed a t 300 nm. Results In irradiated solutions of benzene added bromine is consumed rapidly. Thlis is shown by Figure 1 (curve a) where the change in optical density a t 300 nm due to the Brj- disappearance is plotted us. the reaction time. In the range of the benzvalene concentrations obtained in our experiments the bromination went to completion within about 20 min of the reaction. Consequently, we have calculated the benzvalene Concentration from the amount of bromine consumed after 20 min of the bromination reaction. Since the exact value of the extinction coefficient of the tribromide complex at 300 nm in a cyclohexane-ethanol mixture has not been ireported, we calibrated our bromometric method The Journal oiPhysical Chemlstry. Vo/. 78. No. 79 7974
Hanspeter Lutz and Gabriel Stein '
O
r
'
'
'
-1
08
1
Figure 1. Change in optical density as a function of time for the reaction of brominating solution: (a) with 9 solution of 0.01 M benzene in cyclohexane irradiated at 2537 A with a dose of 1.74 X Mcycloeinstein I.-' at 23'; (b) with a solution of 9.64 X
pentene in cyclohexane.
against a solution of cyclopentene as standard. We obtained a value of 13,700 M-l cm-l for the extinction coefficient of Br3- a t 300 nm in a 1:1 mixture of cyclohexaneethanol. A typical curve showing the uptake of bromine by a solution of 9.64 X M cyclopentene in cyclohexane is given in Figure lb. On the basis of the findings of Roth and Katz7 that 1 mol of bromine adds quantitatively to I mol of benzvalene, we equated the loss of Br3- to the concentration of benzvalene initially present in the irradiated solution. The detailed kinetics in the bromination system are likely to be ~ o r n p l e xWe . ~ have not established the nature of the product, but use the method as one giving a reproducible end point. Figure 2 represents the concentration of benzvalene determined in the irradiated solutions by the above method as a function of light dose a t three different temperatures. It is seen that initially, a t low light doses, benzvalene is formed a t a high rate, which decreases at higher light doses. In addition, it is seen that the overall yield of benzvalene increases with temperature. There is a large experimental error a t low doses and much better reproducibility at higher ones. We found that the experimental observations could be interpreted by the following reaction scheme for the benzvalene formation 'B
4-
3B* ---+ Bv
+ %*
Izv
-
'B ---+
2'B
lB*
kl
= Ia
(1)
77 5
(5)
k6
(6)
Here, reaction 1 represents excitation to a higher vibrational level of the lBzu electronic state. I , is the absorbed light intensity. Process 2 represents all decay pathways, radiative and nonradiative, of the initially excited level other than benzvalene formation (reaction 3) and intersys-
Photolysis of 13enzene in Cyclohexane at 2537 A
1911
1
b ) 23°C
t
0 LIGHT DOSE
x +04
(einst
x
5
15
c'
Figure 2. Formation of benzvalene in an oxygen-free solution of 0.01 M benzene irradiated at 2537 A in cyclohexane as a function of light dose at 9, 23, and 50'. Curves are computer-drawn best fits to the experimental data using the kinetic scheme as described in the text.
tem crossing to 3B* (reaction 4). Expression 5 accounts for all decay processes of triplet benzene 3B* other than reaction 6, which rctpresents the triplet sensitized disappearance of benzvalene. Triplet sensiti~zeddisappearance of benzvalene has been observed by %laplanand Wilzbach in the gas-phase photolysis of benzene at 2637 A.I0 These authors reported a marked enhancement of the benzvalene production when benzene vapor is irradiated a t 2537 A in the presence of added gases which quench triplet benzene and facilitate the vibrational relaxation of "hot" benzvaiene. In the absence of triplet quencliers only traces of benzvalene were formed and no evidence for the formation of other photoproducts which may arise from the triplet sensitized destruction of benzvalene bas been reported. These observations are consisl ent with previous findings that benzene vapor is virtualhy inert to irradiation a t 2537 A.11 We assume that such triplet !sensitized disappearance of benzvalene also occurs in dilute hydrocarbon solutions. According to the above reaction scheme the differential equation describing the formation of benzvalene is
where $B" is the initial quantum yield of benzvalene formation according to reaction 3. Similarly, the kinetic equation expressing the benzene triplet state concentration takes the form
here, d~ represents t h quantum yield of the intersystem crossing. Since reaction 5 is fast (vide infra) we can use the stationary state hypothesitj for the benzene triplet concentration and put the right-hand side of eq 8 equal to zero. The stationary concentration of is then given by [3B*:lstationar~
= I,#~/(kg
-+ k,[Bv])
Introducing this result in eq 7 we obtain
(9)
This differential equation contains the two unknown parameters $B" and (h&5). In order to determine the values of these parameters eq 10 was solved analytically and the integrated function fitted to the experimental data on a 6400 CDC computer using a least-squares curve-fitting pro) the three gram. Values for the triplet quantum yield ( 4 ~at temperatures of our experiments have been taken from data published by Cundall and Robinson.12J3 The full line curves given in Figure 2 are the computer drawn representations of the integrated functions which fit best to the experimental data sets obtained at 9, 23, and 50°, respectively. The corresponding adjusted parameters &" and (h&5) are given in Table I. Figure 3 shows the quantum yields of benzvalene as a function of light dose calculated from the data in Table I. It shows that the initial quantum yield of benzvalene formation does not depend on temperature. It appears that the observed temperature enhancement of benzvalene formation a t finite doses can be fully ascribed to its triplet sensitized disappearance. The triplet sensitized destruction of benzvalene is expected to be reduced a t higher temperatures since the triplet state is decreased with increasing temperature.
Discussion Previous studies on the mechanism of benzvalene formation have indicated that benzvalene is not derived directly from the fluorescent state but that it must come, on a very short time scale, from upper vibrational levels of the lBzu state.6J0 This conclusion has been drawn from the fact that the yield of benzvalene in the gas-phase photolysis of benzene increases with increasing energy of the exciting radiation while an opposite relationship is found for the fluorescence yield. That benzvalene does not arise from the lowest triplet state of benzene has been demonstrated by the observation of an increased benzvalene production in the presence of triplet quenchers.1° These conclusions derived from studies in the gas phase are consistent with our results in solution. Our experimental observation that the initial quantum yield of benzvalene is independent of temperature makes the possibility that the fluorescent level is responsible for the "nnzvalene formation less likely since the fluorescence :yield decreases The Journal of Physical Chemistry. Vol. 78 No. 19. 1974
Hanspeter Lutz arid Gabriel Stein
1912
0.35 0.24 0 .I3
9
23 50
0.184 0.180 0.174
x 104 2.92 X IO* 5.07 X 104 2.06
1
n
I
Figure 3. Quantum yields of benzvalene formation at 9, 23, and 50’
as a func:tioii of light dose. Data calculated from computer-drawn
best fit curves in Figure 2. markedly with t e r n p e r a t ~ r e . l , j - ~ ,In l ~addition, ,l~ it appears that the or ern11 benzvalene formation in cyclohexane is enhanced as the triplet formation 1s decreased at higher temperatures. T h e results derived from the bromination exper2 and 3 are subject to large experiiments s h ~ in w Figures ~ mental errors at low light doses and thus low bromine uptake. They arc’ more accurate a t higher doses. It is the good fit of the theoretical curves over the entire range and a t three temperatures, using the same parameters, which gives support to rhe overall picture, though the extrapolated yields at zero dose may be subject to error. The result$ of the present study correlate with the photochemical re letions occurring in oxygenated aqueous benzene solutions. The main photochemical product of the phol ooxidatic n of bmzene in water,16 identified in alkaline solution as hl7droxycyclopentadiene carboxaldehyde,17 has been shown to arise from addition of 0 2 to a reactive intermediate whic n originates in the energy-rich state preceding thermal equilibrrum and preceding the formation of the fluorescent level.18a Evidence for the latter conclusion has becn drawn fiom the fact that excitation a t 229 and 214 nm gives the photooxidation product in slightly higher yield than excitation a t 2537 or a t 2650 A.18d,19Yet the primary state resulting on absorption a t 229 or 214 nm does not efficiently convert into the fluorescent leve1.18a,20The reactive Intermediate w ~ i 5thought to be an isomer of ground-state beniene, e g bw-izvalene>or the precursor from which this may arise. hterestmgly, the maximum quantum yield of the photooxidation preduct obtained in water solutions sufficiently saturated with oxygen is 0.19,1sh a figure very close to our value for the extrapolated initial quantum yield of benzvalen? formation in cyclohexane. I t appears that in water saturated wil h oxygen the precursor is efficiently trapped forming ultimately the oxidation product.lsa Similar quan~
The Jourqal s f Physical Chemistry. Vol. 78
No. 19
1974
tum yields in polar and nonpolar solvents indicate that the nature of the solvent has little or no effect on the formation of benzvalene. This result gives further support for the proposed mechanism in which benzvalene is formed in a preequilibrium state before solute-solvent interactions may occur. Since we postulate that benzvalene arises before interaction with the solvent, similar limiting initial yields would be expected in the gas phase. Noyes and Harter21 showed that, in the gas phase a t pressures higher than 10 Torr the sum of ( 4 ~ (PT), the quantum yields of fluorescence and triplet formation is about 0.80. This would then leave open the possibility of the quantum yield &v approaching 0.20. As in our present case of the photoisomerization of benzene in cyclohexane the photooxidation of benzene in water has been found to be independent of ternperature.18aJgThe temperature independent product yields observed in both cases indicate that for the highly symmetrical benzene molecule the energy-rich states responsible for the photoisomerization are less readily accessible by reversible thermal population from the fluorescent state. This would mean that benzvalene formation is not the result of the thermally activated quenching of benzene fluorescence. Other nonradiative relaxation processes must therefore be invoked to explain the inverse temperature dependence of the benzene fluorescence. A possible nonradiative channel from the lBzu state is the thermally activated transition to a state such as that suggested by Callomon, Parkin, and LopezDelgado.22 For the formation of benzvalene out of plane distortion of the benzene molecule is required. Benzene is planar in both the ground and fluorescent state.23 The out of plane distortion in benzene appears to occur from electronic excitation into nontotally symmetrical vibrational levels above the fluorescent state, before relaxation. However, when a fluorine or methyl substituent is introduced in the benzene molecule, thus decreasing the synimetry, the photooxidation product in water appears to be accessible through thermally promoted vibrational states from the fluorescent leveL2* Table I gives the adjusted values for the ratio between the bimolecular rate constant for the triplet sensitized disappearance of benzvalene ( k s )and the rate constant for the unimolecular triplet decay (h5). Assuming the reaction of triplet benzene with benzvalene to be diffusion controlled, the parameters in Table I allow an estimate of the lifetime of the benzene triplet in dilute cyclohexane solutions at the given temperatures. Using a value of 6.7 x I O 9 M-1 sec-1 for the diffusion-controlled rate constant,25 and the rate constant ratio, we get a triplet lifetime of about 4 psec a t 23’. This figure is in reasonable agreement with other estimates given in the literature. Sandrot;”; estimated a triplet lifetime of 2 psec and LipskyZ7a value of 1 psec in dilute cyclohexane solution. Recently, Curidall and Robinson13 presented evidence that the true triplet lifetime is in fact much shorter than these previously given estimates. If so, a longer lived species derived from triplet benzene would be responsible for the sensitized disappearance of benzvalene. However, further experimental details would be needed to explore this possibility.
+
Acknowledgment. We thank Professor W. J. G. Hayman for valuable advice and discussions. This work was supported by the Israel Academy of Sciences.
llltrasonic Absorption in Aqueous Calcium Acetate
(1) K. Sandros, Acta Chern. Scand., 25, 3651 (1971). (2) K. E. Wilzbach, J. S. Ritscher, and L. Kaplan, J. Amer. Chem. SOC., 89, 1031 (1967). (3) J. W. Eastman, J. Chem. Phys., 49, 4617 (1968). (4) W. P. Helman, J. Cheni. Phys., 51, 354 (1969). (5) T. A. Gregory and W. P. Helman, J. Chem. Phys., 56, 377 (1972). (6) K. E. Wilzbach, A. I. liarkness, and L. Kaplan, J. Amer. Chem. SOC., 90, 1116(1968). (7) R. J. Roth and 'T, J. Katz, J. Amer. Chem. Soc., 94, 4770 (1972). (8) K. H, Grellmanii and W. Kuhnle, Tetrahedron Lett., 1537 (1969). (9) R. 9.Cundall, I.. 6.Pereira, and D. A. Robinson, Chem. Phys. Lett., 13, 253 11972). > - -, (IO) i-kaplan and K. E. Mlilzbach, J. Amer. Chem. SOC., 90, 3291 (1968). (11) J. N. Pitts, ,Jr.. ,J. K. Fade. and J. K. S. Wan, Photochem. Photobiol., 4, 323 (1965). (12) R. E. Cundall and D. A. Robinson, Chen?. Phys. Left., 14, 438 (1972). (13) R. B. Cundall aqd D.A. Robinson. J. Chem. SOC.,Faraday Trans. 2, 68, 1145 (1972).
1913 (14) R. B. Cundall and L. C. Pereira, J. Chem. Soc., Faraday Trans. 2, 68, 1152 (1972). (15) R. B. Cundall and D, A. Robinson, J. Chem. Soc., Faraday Trans. 2, 68, 1133 (1972). (16) E. Farenhorst, Tetrahedron Lett., 4835 (1968). (17) M. Luria and G. Stein, Chem. Commun., 1650 (1970). (18) (a) M. Luria and G. Stein, J. Phys. Chem,, 76, 165 (1972); (b) submitted for publication. (19) Y. Ilan, M. Luria, and G. Stein, to be submitted for publication. (20) (a) C. L. Braun,s. Kato, and S. Lipsky, J. Chem, Phys., 39, 1645 (1963); (b) M. D. Lumb, C. L. Braga, and L. C. Pereira, Trans. Faraday Soc., 65, 1992 11969). ..., (21) W. A. Noyes, Jr., and D. A. Harter, J. Chem. Phys., 46, 674 (1967). (22) J. H. Callomon, J. E. Parkin, and R. Lopez-Delgado, Chem. Phys. Lett., 13, 125 (1972). (23) T. M. Dunn in "Studies in Chemical Structure and Reactivity," J. M. Ridd, Ed., Methuen, London, 1966, p 125. (24) H. Lutz, Y. Ilan, and G. Stein, to be submitted for publication, (25) P. J. Wagner and I. Kochevar, J. Amer. Chem. SOC., 90,2232 (1968) (26) K. Sandros, Acta Chem. Scand., 23,2815 (1969). (27) S. Lipsky, J. Chem. Phys., 38, 2786 (1963). _ _ _ \
Ultrasonic Absorption in Aqueous Solutions of Calcium Acetate and Other Biwalent Metal Acetates ordon Atkinson,"' Mostafa M. Emara, and R. Fernander-Prini De)epat?rnentof Chemistry, University of Oklahoma, Norman, Oklahoma 73069 (Received July 27, 1973;Revised Manuscript Received April 11, 1974) Publication costs assisted by the National Science Foundation
Ultrasonic absorption measurements in aqueous solutions of calcium acetate in the concentration range 0.14-1.09 M , a t various temperatures, show one relaxation for the low-concentration solutions and two for the high concentrations. On the basis of these measurements and analogous ones on Mg2 b, Ba", and Ni2+ solutions, we conclude that the relaxations are due to the substitution of one or two waters in the inner coordination sphere of the Ca2+ ion by acetate. The scanty thermodynamic data available and the high concentration of some of the solutions hampered a thorough quantitative interpretation of the data. In magnesium and calcium acetate solutions more concentrated than 1.4 M , the ultrasonic absorption is not explicable in terms of discrete relaxation processes.
Introduction Since Bazulin" measured ultrasonic absorption in aqueous zinc acetate iolutions in 1939, many other acetate solutions have been studied by this technique. Stuehr and Yeager3 have (wrnmarized the results of ultrasonic investigations of aqueous acetate solutions. It may be concluded that (1) transption metal acetate solutions show an ultrasonic relaxation in the megahertz region due to a chemical process, (2) alkali metad acetate solutions show no significant excess sound absorption, (3) 0.05 M calcium acetate solutions were found by Kurtze and Tamm* to show significant excess absorption with a relaxation frequency above 40 MHz, and (4) other processes such as acetate ion hydrolysis and acetic acid dimerization do not contribute significantly to sound a bsorptdion in metal acetate aqueous solutions. We were partrcularly interested in studying alkaline earth acetate solutions, especially those of Ca2+,so that the
results could be compared with those obtained in previous measurements of calcium and magnesium polycarboxylates5 Furthermore, we felt that previous measurements in alkaline earth solutions had been affected by experimental artifact^.^ After the measurements reported here were completed, Purdie and Barlow6 published the results of ultrasonic measurements in aqueous magnesium and calcium acetate solutions. Their data, in general, agree with those presented here but are restricted to a narrower concentration range (
