J. Phys. Chem. 1992, 96, 2205-2210 The total pressure in their study was somewhat less than 1 Torr, and the gas consisted mainly of argon and nitrogen, as in the present experiment. David and Coombe7 report that the photon yield for reaction 1, defined as the ratio of Nz B3n, A3Z,+ photons to molecules of HN, consumed in the reactor, is approximately 20%. This value was based on their observed N2(B,v') population distribution for u ' L 2; the populations of the unobserved v' = 0 and 1 levels were estimated by extrapolation. This extrapolation must be viewed with some caution since our kinetic modeling indicates that a large population in v' = 0 is expected. Emission from this level must, however, be observed in the near-infrared region.I2 In preliminary experiments, Marinelli and Kessler9obtain a signifcantly higher photon yield than that derived by David and C ~ m b e The . ~ differing yields could be due, in part, to differences in the pressures and spectral regions scanned. While it was not possible with our kinetic model to use the observed steady-state N,(Bp') population distribution as a function of added argon to extract a nascent N2 product state distribution, it is still interesting to speculate on the implications of the observed pressure dependence. It is clear from the fact that the observed distribution becomes hotter with the addition of argon that nonemitting excited N 2 product molecules, as well as N2(B3n,), are directly formed in reaction 1. While it is not possible to assign the likely nonemitting states produced, it seems quite likely the excited N 2 product from reaction 1 is also produced in the
-
2205
close-lying W3A, state since the latter is known to be strongly collisionally coupled to the B3n, state. Excited molecular nitrogen in other nonemitting states, e.g., Bf3Z,,-, A%,+, could also be produced in reaction 1. The proportionally greater increase in the populations of N2(B,v'27) levels which we have observed with added argon could be due to the fact that there are more nonemitting N,(W,A,B') states with similar energies to these vibrational levels than to the lower N2(B) vibrational levels (see Figure 1 of ref 21) so that there is a greater chance for strong collisional coupling to funnel population into the higher v 'levels. This thus suggests that the Nz(A,B') electronic states may also be produced directly in reaction 1. A more satisfactory kinetic modeling of this and similar experiments to study this reaction requires the availability of state-to-state energy-transfer rate constants involving excited N z states other than just B3n, and W3A,.
Acknowledgment. This work has been supported by the Air Force Office of Scientific Research under contract F49620-88(2-0056 and by the National Science Foundation under grant CHE-9020727. Conversations and correspondence with S.Rosenwaks about this reaction and about collisional energy transfer in N 2 are gratefully acknowledged. Travel support by the US.-Israel Binational Science Foundation is gratefully acknowledged.
Novel Photochemical Reactions of CBH,, Radical Cations Y. Fujisaka, M. Makino, J. Takahashi, T. Shida,* Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
K. Roth, R. Straub, and T. Bally* Institut de Chimie Physique, UniuersitE de Fribourg, PErolles, CH- 1700 Fribourg, Switzerland (Received: September 25, 1991)
The radical cation of bicyclo[2.2.2]octa-2,5-dieneproduced in a y-irradiated Freon matrix at 77 K shows a broad chargeresonance = 680 nm. In contrast to earlier experiments in argon matrices, it was found that the primary absorption band with A, photoreaction of this species is a cleavage to yield the recently described complex cation between benzene and ethylene whose A, happens to coincide with that of the above precursor cation. This complex cation can in turn be converted eventually to 1,3,5,7-octatetraene radical cation. In the presence of an ex- of ethylene,the above complex cation undergoes a photoaddition whose spectrum indicates the presence of a linear conjugated triene chromophore. The electronic structure yielding a ClJ-Il4'+ of the different radical cations involved in this study is discussed and a reaction scheme is proposed.
Introduction Since the first discovery of the photoinduced ring-opening of the radical cation of 1,3-~yclohexadiene(CHD'+),I a considerable number of photoisomerizations of hydrocarbon radical cations have been studied by electronic absorption (EA) spectroscopy in cryogenic media., These include systems of the composition C6H8'+,13334C7HIO*+,5 CsH10*+,5-9CsH12*+r3 and CloH12'+,1*12 many of which undergo rearrangements which are unprecedented in neutral photochemistry and call for an entirely new theoretical framework for their description. In a recent study it wasshown that the radical cation of bicyclo[4.2.0]octa-2,4-diene(BCO'+, Scheme I) undergoes a cleavage upon photolysis in a Freon matrix to yield a novel type of *-complex cation between benzene and ethylene (BZ.ET'+).I3 In the course of this investigation, evidence began to accumulate which indicated that the isomeric radical cation of bicycle[2.2.2]octa-2,5-diene (dihydrobarrelene, DHB*+) also gives
* To whom correspondence should be addressed.
SCHEME I
BCOf
k,: 405nm
BZ.ET:
DHBt
COT?
680 nm
680 nm
505 nm
/ O F (different rotamers)
k W :455, 470 nm BZ-ET" upon visible irradiation. However, earlier experiments in argon matrices had shown that DHB" photorearranges to ( 1 ) Shida, T.; Kato, T.; Nosaka, Y. J. Phys. Chem. 1977, 81, 1095.
0022-3654/92/2096-2205$03.00/00 1992 American Chemical Society
2206 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
I
Fujisaka et al.
I
400 600 800 nm Figure 1. (A) EA spectrum of X-irradiated DHB in Ar doped with CH2CI2(Ar:DHB:CH2CI2= 1000:1:2). (B) Same as (A) after photolysis at X > 650 nm. (C) Same as b after photolysis at X > 550 nm.
W \ . . \ .
I
1,3,5,7-~tatetraeneradical cation (OT'+),a reaction which was postulated to proceed via one of two possible paths, both of which involved 1,3,5-~yclooctatrieneradical cation (COT'+)as an intermediate.5 To resolve this apparent contradiction, a more thorough investigation was needed whose results are presented in this paper. Thus, the formation of BZ.ET'+ from DHB'+ was confirmed, and OT" was shown to arise as a final product of BZ.ET'+ photolysis in a process which was shown to involve at least two intermediates. Finally, it was found that BZ.ET'+ or an intermediate rearrangement product thereof undergoes photoaddition of a second molecule of ethylene to yield a CloHI4'+species containing the chromophore of a linear conjugated triene radical cation.
Results and Discussion (a) Bicycl~2.2.2]octa-2,5-diene(Dihydrobarrelene, DHB). In the above-mentioned argon matrix experiment^,^ DHB was expased to intense VUV radiation during deposition, and apparently the only products which could be detected were various rotamers of OT+.Unfortunately, the region where DHB'+ is expected to absorb was not shown in the ~ p e c t r a .On ~ the other hand, if the experiment is carried out using X-rays to ionize matrix-isolated DHB, a broad band peaking at A,, = 672 nm is detected next to several of the typical OF+ rotamer peaks between 500 and (2) (a) For a review on radical ion photochemistry,see: Bally, T. In Ionic Molecular Radicals; Lund, A., Shiotani, M., Eds.; Kluwer: Doordrecht, 1990. (b) For a comprehensive catalogue of radical ion EA spectra, see: Shida, T. Electronic Absorption Spectra of Radical Ions;Elsevier: Amsterdam, 1988. (c) For a review of the Freon matrix technique, see: Shida, T.; Haselbach, E.; Bally, T. Acc. Chem. Res. 1984, 17, 180. (3) Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1984, 88, 2723. (4) Bally, T.; Nitsche, S.; Roth, K.; Haselbach, E. J . Phys. Chem. 1985, 89, 2528. ( 5 ) Dunkin, I. R.; Andrews, L.; Lurito, J. T.; Kelsall, B. J. J . Phys. Chem. 1985,89, 1701. (6) Bally, T.; Nitsche, S.;Roth, K. J . Chem. Phys. 1986, 84, 2577. (7) Andrews, L.; Lurito, J. T. Tetrahedron 1986, 42, 6343. (8) Bally, T.; Haselbach, E.; Nitsche, S.; Roth, K. Tetrahedron 1986, 42, 6325. (9) Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1984, 88, 5893. (10) Shida, T.; Momose, T.; Ono,N. J . Phys. Chem. 1985, 89, 815. (1 1) Andrews, L.; Dunkin, I. R.;Kelsall, B. J.; Lurito, J. T.J . Phys. Chem. 1985, 89, 821. (12) Momose, T.; Shida, T.; Kobayashi, T.Tetrahedron 1986, 42, 6337. (13) Bally, T.; Roth, K.; Straub, R. J . Am. Chem. SOC.1988, 110, 1639.
R
1
400 500 600 700800 lOaOnm Figure 2. (A) EA spectrum of y-irradiated DHB (1.6 X IOI9 eV/g) in CFCI,/CF2BrCF2Br (1:l) at 77K;(B) same as (A) after 2-min photolysis at X > 690 nm; (C) same as (B) after additional 37-min photolysis at X > 690 nm; (D) same as (C) after subsequent 5 h photolysis through a 520-nm interference filter; (D) same as (D) after 10-min photolysis at X > 600 nm. 300
450 nm (Figure 1A). A very similar spectrum is obtained after y-irradiation of solutions of DHB in a Freon mixture (e = lo3), is shifted to 680 nm and the OT'+ peaks are except that A, absent (Figure 2A). The above spectra are similar to that of the related radical cation of norbomadiene (NBD'+),14 which has A-, = 635 nm. In both cases the electronic absorption (EA) band can be interpreted in terms of intramolecular charge resonance between the two ethylenic 'K units. The energy difference corresponding to this transition is also reflected in the separation between the first two peaks in the photoelectron (PE) spectra of NBD and DHB.I5 This value decreases from 0.86 eV in NBD to 0.58 eV in DHB, and hence a lowering of the electronic excitation energy in the radical cations is not unexpected.16 Thus, the species with the broad 680-nm band in the Freon matrix can be identified as DHB'+. In the Ar matrix experiments, photolysis at >600 nm led to an increase in the OT'+ absorptions which was, however, not accompanied by any measurable decrease in the intensity of the 680-nm band (Figure 1B). On the other hand, we were suprised to find that in the Freon glass experiments, photolysis at 700 nm initially led to a strong increase of the 680 nm band (Figure 2B). Concomitantly, BZ and ET appeared in the gas chromatograms of the thawed samples. Thus it seems that BZ-ET+, which happens to show an EA band of very similar shape peaking at the same A- as that of DHB'+,I3 is initially formed from DHW+ in a process which may be formally classified as the reverse of (14) Haselbach, E.; Bally, T.; Lanyiova, Z.; Birtschi, P. Hela Chim. Acta
--.
1979. .. . - , 62. 5 8 5 .
(15) Bischof,
P.;Hashmall, J. A.; Heilbronner, E.; Hornung, V. Helu.
Chim.Acta 1969, 52, 1745. (16) The energy gap between the first two PE bands of NBD and DHB does not translate directly into the energy differencebetween the ground state and the first excited state of the corresponding radical cations because ionization results in a significant shortening of the distance between the double bonds and hence to an increase in the b,/a, orbital energy splitting, as outlined previou~ly.'~
Novel Photochemical Reactions of C8HloRadical Cations
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2207
680
Figure 3. Plot of the absorbance changes at 680 nm (BZ.ET'+ and/or DHB'+) and 455 nm (OT+) as a function of X < 690 nm photolysis time. Initial and final conditions are represented by spectra
B and D in Figure
2.
the well-known radical cation Diels-Alder reaction." Although no evidence was obtained for the above cleavage of DHB'+ in Ar matrices, it may be that the formation of BZ-ET'+ is approximately balanced by the rearrangement of BZ*ET'+and/or DHB'+ to OT'+,which would explain the apparent insensitivity of the 680-nm band to photolysis in Ar. In contrast to the Ar matrix results, continued photolysis at 700 nm in Freon glasses yielded only small amounts of OT" (A,= 455 nad 470 nm) next to a weak, broad band peaking at -380 nm (Figure 2C). Only if higher energy light (520-nm interference filter) was used for irradiation did the above-mentioned pair of OT'+peab begin to grow significantly while their long-wavelength (see Figure 2D). counterparts at 700-750 nm began to In addition, it becomes apparent from Figure 3 that during the first 20 min of photolysis at 700 nm, the decay of the 6 8 0 . ~ 1band is not paralleled by a concomitant increase of the OT'+absorptions, although c(OT'+) is some 2 orders of magnitude higher than c(DHB'+). Conversely, the latter peaks continue to grow even after the decay of the BZ-ET" absorption has nearly leveled off. Thus, there must be at least one intermediate between BZ*ET'+ (or DHB'+) and OT*+,an intermediate which is slightly photolabile at 700 nm and more so at 500 nm. Since the above intermediate yields OT'+ upon photolysis, it is reasonable to assume that it is a valence isomeric C8Hl{+ specie8 which must furthermore be accessible from BZ-ET'+ (or DHB'+) in a limited number of reaction steps. The most obvious choices are BCO'+ and COT'+, but their characteristic absorptions at 405" and 505 nm,a8respectively, were not clearly detected during the isomerizations to the final product, OT'+.Therefore we sought other C8HI{+ isomers which might yield OT'+ upon photolysis. For example, 5-vinyl- 1,3cyclohexadiene (SVCH) could undergo ring-opening to OT according to
Figure 4. (A) Spectrum of y-irradiated 5-vinyl-1,3-cyclohexadiene(5VCH). Conditions, see caption to Figure 2A, dotted line spectrum of y-irradiated 1,3-cyclohexadiene (CHD); (B) same as (A) after 20-min photolysis a t X > 500 nm (compare to spectrum 2E). The spectrum of SVCH'+ in a Freon mixture at 77 K (Figure 4A) consists of three bands, two of which are nearly coincident with those of 1,3-cyclohexadieneradical cation (CHD", dotted line in Figure 4A), This is to be expected because the vinyl group is not in conjugation with the diene part of SVCH'+ and will hence not perturb the CHD'+ chromophore to any great extent. In addition we find a weak broad band peaking at =700 nm which can be attributed to an electronic transition involving partial charge transfer from the diene to the olefin moiety?l Upon photolysis at 2500 nm, the spectrum of SVCH" changed to that shown in Figure 4B, which is reminiscent of that observed after photolysis of BZ*ET'+obtained from DHB'+ (Figure 2E). In particular, the same two OT'+ rotamers appear in similar proportion which suggests that the photochemical precursor could be the same. Figure 2C does indeed show a shoulder at -380 nm which emerges upon >690 nm photolysis of DHB'+ (or BZ-ET'+), and it is tempting to attribute this to SVCH'+. The next question is therefore how SVCH'+ could form from BZ.ET'+, but before going into this discussion we wish to present the results obtained from mixtures of BZ and ET, results which deviate from those presented above is an intriguing fashion. (b) Benzene Ethylene. Figure SA shows the spectrum of a y-irradiated Freon matrix containing 50 mM BZ and ET (near saturation). As shown previ~usly,'~ it consists mainly of the 680-nm band of BZ-ET" and a contribution from the benzene dimer cation (BZ,'+) peaking at 900 nm. Upon irradiation at >700 nm, the bands of OT'+at 455 and 470 nm began to emerge, and after exhaustive photolysis the spectrum shown in Figure 5B resulted, which is again very similar to that in Figures 2E and 4B. Under these conditions, the sequence of photoreactions seems
+
5vcH
OT
(17) Bauld, N. L.;Bellville, D. J.; Harirchian, B.; Lorenz, K.T.; Pabon, R. A,; Reynolds, D. W.; Wirth, D. D.; Chiou, H.-S.; Marsh, B. K. Acc. Chem. Res. 1987, 20, 377 and references cited therein. (18) The pair of OT+ rotamers absorbing at 455 and 470 nm can be reversibly interconverted wbereby two additional rotamers (A, = 480 and 493 nm) appear. In analogy to the correspondin argon matrix experiments6** 8 all rotamers can be converted to all-trans-OT' (A, = 455 nm in FM) by careful photolysis at X > 460 nm. (19) Similar experiments were conducted with BCO" as a precursor to BZ.ET'+. The course of the photoreactions is virtually the same except that the spectra are slightly more-complicateddue to the concomitant formation of C O T + upon photolysis of BCO'+. (20) Bally, T.; Roth, K.;Straub, R. Helu. Chim.Acta 1989, 72, 7 3 .
(21) A lower limit for this transition is given by the energy AH for the charge transfer between CHD'+ and propene which is 1.47 eV (I:[CHD] = 8.25 eV and I:[propene] = 9.73 eV from: Bieri, G.; Burger, F.;Heilbronner, E.;Maier, J. P. Helu. Chim.Acta 1977, 60, 2213). The fact that the CT absorption in SVCH'+ actually occurs at 4 . 3 eV higher energy indicates some interaction between the two moieties leading to a widening of the gap between the diene and the olefin T levels.
2208
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 420 j.
D
420
Fujisaka et al. unlikely that any new rotamers, should they ever be detected, have A,, < 455 nm for their second EA bands. (c) The position of the first, weak transition of OT'+ is rather insensitive to the conformation and lies between 780 and 660 nm. Again, we can see no reason why it should fall completely outside this range for any of the hitherto unobserved OT'+ rotamers. On the other hand, the general features of the new spectra are indicative of a conjugated triene'+ chromophore, possibly involving a cyclic link. Species of this type which may be imagined to arise from BZ.ET'+ include the radical cations of 5-ethylidene-l,3cyclohexadiene (ECH") as well as 1- and 2-vinyl-l,3-cyclohexadiene (lVCH'+ and 2VCH'+).
ECH
I
400 600 800 lOOOnm Figure 5. (A) EA spectrum of y-irradiated benzene (50 mM) plus ethylene (400 mM). Conditions, see caption to Figure 2A. (B) Same as (A) after 137-min photolysis at h > 600 nm. (C) Same as (A) after 5-h photolysis through a 520-nm interference filter. (D) Same as (C) after subsequent 5-h photolysis through a 720-nm interference filter (subsequent photolysis with the 520-nm interference filter reformed spectrum C in a reversible fashion). to be the same, irrespective of the mode of formation of BZ.ET'+. However, if light of shorter wavelength (520-nm interference filter) was used for the bleaching of BZ.ET'+, a different spectrum (Figure 5C) resulted. Surprisingly, it lacks the peaks of OT'+ but shows instead a set of new sharp peaks at 420,432,450, and 464 nm which are accompanied by weak, structured features between 580 and 660 nm. Detailed narrow-band photolysis experiments revealed that these bands are due to at least three different species which can be reversibly interconverted. An example of such a reaction is shown in spectrum 5D which resulted from photolysis of the above sample (Figure 5C) with a 720-nm interference filter. Subsequent photolysis through the 520-nm filter quantitatively reformed spectrum 5C. At first sight the above finding seems to be inconsistent with the previous results since we suppose that BZ.ET'+, as manifested by its 680-nm band, is involved in all cases. In the hope of being able to shed some light on this enigma, we engaged in a search for possible candidates which might account for the above set of new spectra. In view of their similarity to that of OT'+, it is tempting to assign these peaks in terms of a new, hitherto unobserved set of OT'+ rotamers. However, this interpretation must be discounted for the following reasons: (a) In previous experiments6v8the same set of six OT" rotamers was obtained from three different precursors (BCO", COT'+, and ionized OT) and no evidence for the formation of additional rotamers was obtained in any of the numerous selective photolysis experiments conducted on these samples. (b) The second, strong EA band of all-trans-OT'+ peaks at 455 nm in FM while all other rotamers observed so far absorb at longer wavelengths, as is to be expected if cis configurations are introduced. Hence it is
1VCH
2VCH
Of these, ECHO+ has been previously observed in an argon matrix where it was prepared by photolysis of ionized ethylb e n ~ e n e . ~We , ~ ~repeated the latter experiment in the presently used Freon mixture and found A, = 445 and 434 nm for the two ethylidene rotamers of ECH" which can be reversibly interconverted but otherwise undergo no further photoreactions. On the other hand, lVCH'+ appears in the form of a single species with A, = 450 nm, but does not yield any of the other bands (420, 432, 464 nm) upon photolysis. Instead, it is transformed to what appears to be the radical cation of 2-vinyl-1,3,5-hexatriene.23 Finally, 2VCH'+, being a cross-conjugated rather than a linear polyene cation, gives an entirely different spectrum consisting essentially of two band systems of similar intensity peaking at 770 and 448 nm, respectively. Upon photolysis, the spectrum changes into that of 3-vinyl-1,3,5-hexatrienecation.23 Neither of the two above cross-conjugated tetraene cations show any absorptions which correspond to those of the expected set of trienic species. However, a clue was obtained when we found that the relative intensities of the 4551470 and the 42014321450 nm peaks depend on the composition of the BZ + E T samples in that the former set dominates at small and the latter at high ET concentrations. Thus, the presence of excess E T seems to be prerequisite for the formation of the presumed triene radical cations and the possibility of a photoaddition of ET to some C8Hlo'+ was taken into consideration. Indeed it turned out that the 42014321450 nm set of peaks appears also when ionized samples of DHB or BCO saturated with E T are photolyzed at >500 nm. Since the initial formation of BZ.ET'+ from either of the two bicyclic precursors is not influenced by the presence of ET, the addition of a second ET molecule must occur at the stage of BZ-ET" or later. To bracket this event, the photolysis of 5VCH'+ was also reinvestigated in the presence of ET, but no differenceto the experiments without ET could be discerned. This proves that the ET photoaddition takes place before the formation of SVCH", the immediate OT'+ precursor.
Mechanistic Conclusions The data presented above prove unambiguously that BZ.ET'+ may be formed either by cleavage of DHB'+ or BCO" as well as by addition of ET to BZ". They also suggest that 5VCH'+ is the precursor of OT'+ obtained as a final photoproduct from any of the above starting materials, although its presence could not be proven directly. On the other hand, no direct evidence was obtained concerning the pathway for formation of 5VCH'+, and we will therefore have to use some chemical intuition to propose a mechanistic scheme which is compatible with our findings. (22) Compare also the spectrum of parent 5-methyIene-l,3-~yclohexadiene: Bally, T.; Hasselmann, D.; Loosen, K. Helu. Chim. Acta 1985, 68, 345. (23) Bally, T.; Purro, P.; Roth, K..unpublished.
Novel Photochemical Reactions of CsHlo Radical Cations SCHEME I1
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2209 SCHEME IV
Pn? ........................
w BZ.ET?
SVCH ?
SCHEME 111
H
B
We first note that 5VCH'+ cannot be formed in a single step from BZ.ET'+ (or one of the two bicyclic precursors) because this reaction involves both the formation (or cleavage in the case of DHB'+ or BCO'+) of a C-C bond as well as a [1,3] hydrogen shift. While it is difficult to conceive intermediates corresponding to a hydrogen shift preceding C-C bond formation (or cleavage), the inverse sequence of events results in a species where spin and charge are formally separated, i.e., a distonic ion (DI'+) consisting of an ethyl radical attached to a cyclohexadienyl cation moiety, as a primary product. This may undergo two different [1,3] hydrogen shifts (a and b in Scheme IIZ4),one leading to the radical cation of ethylbenzene (which is not involved in the sequence of events discussed above) and the other to 5VCH'+. Although neither the cyclohexadienyl cation (protonated benzene25a)nor the ethyl radical chromophorezsbof DI'+ absorb in the visible region, this cation is expected to have a weak charge-transfer transition similar in nature to that observed in 5VCH" (Amx = 700 nm; see above). Its exact position in DI'+ is difficult to predict, but a lower limit may be obtained similarly to the above case of 5VCH'+ from the energy change AH for the charge-transfer reaction C6H7+ CzH5' C6H; C2H5+,which is 39.5 kcal/mo1,26 corresponding to 725 nm.2' Due to the strongly nonvertical nature of the CT excitation, the corresponding EA band is expected to be very broad, and hence one may assume that DI absorbs throughout the region where photolyses were effected in our experiments. Thus it may be that DI'+-although involved as an intermediate-cannot be detected either because its C T band is
+
-
+
(24) A third possible [1,3] H shift leads to another distonic cation (ethyl radical attached to the 2 position of the cyclohexadienyl cation), but since this species does not relate to any of the observed products its involvement is unlikely. (25) (a) Freiser, B. S.;Beauchamp, J. D. J . Am. Chem. SOC.1976, 98, 3136. (b) Wendt, H. R.; Hunziker, H. E. J. Chem. Phys. 1984, 81, 717. (26) AHHp(C6H7*) = 204 kcal/mol (Lias, S.G.; Liebman, J. F.; Levin, R. D. J . Phys. Chem. ReJ Dafa 1984,13,695);AHf"(C,Hs') = 28.4 kcal/mol (Brouard, M.; Lightfoot, P. D.; Pilling, M.J. J. Phys. Chem. 1986, 90,445); AHHp(C,H,') = 50.0 kcal/mol (Tang, W. [bid. 1986,90,452); AHfo(ClH5+) = 222 kcal/mol (from IWfn(C,H,') plus I:(C2Hs') = 8.39 eV: Houle, F. A,; Beauchamp, J. D. J. Am. Chem. SOC.1979, 101, 4067). (27) Note that the splitting between the two states involved in the CT transition of DI" will be augmented by the interaction of its two ?r moieties which can occur through space as well as via the intervening three C C bonds. Both contributions are expected to be small,Z8 and since they tend to cancel their negelect in a rough estimate of X,,,(CT) appears justified. (28) See, e.g., the reports on the PE spectra of 1,4-hexadiene (Bunzli, J. C.; Burak, A. J.; Frost, D. C. Tetrahedron, 1973, 2, 3735) or 1,2-divinylcyclobutane (Bischof, P.; Gleiter, R.; Gubernator, K.; Haider, R.; Musso, H.; Schwarz, W.; Trautmann, W.; Hopf, H. Chem. Eer. 1981,114,994), where the interaction of two ?r systems via three C-C bonds is discussed. Note, however, that in certain favorable cases, such an interaction can be quite substantial (for example, in the anfi dimer of cyclobutadiene: Gleiter, R.; Heilbronner, E.; Heckmann, M.; Martin, H.-D. Chem. Eer. 1973, 106, 28).
too weak to stand out against the absorptions of the other involved species or because DI'+ is directly converted to 5VCH" under the conditions of our photochemical experiments. Finally, we wish to propose a hypothesis with regard to the identity of the trienic species formed by photoaddition of excess ET to BZ.ET'+ or DI'+. Since the latter species is presumably more reactive than the former, it is likely that DI'+ rather than BZ-ET" attacks a nearby ET. Within DI'+, the most reactive positions are the radical center and the termini of the pentadienyl cation (or, conversely, the -CH2+ center and the termini of the cyclohexadienyl radical moiety after CT excitation). Hence the primary products of E T addition to DI'+ are expected to be the distonic ions A and B in Scheme I11 which are expected to undergo ring closure to form bicyclic ion C. A secondary photoreaction could lead to the radical cation of cyclcdecatriene (CDT'+) which may in fact exist in the form of several rotamers showing a set of EA peaks in analogy to various open-chain polyene radical ion^.^-'^ Experiments to test this hypothesis are planned. Conclusions Three different access paths to the complex cation between benzene and ethylene (BZ.ET'+) have been identified. Two of them involve a photoinduced cycloreversion reaction of a bicyclic precursor radical cation, while the third corresponds to its formation from the components, Le., BZ'+ and ET. Upon photolysis, BZ-ET" rearranges to the radical cation of 1,3,5,7-octatetraene (OT'+), a reaction which proceeds through at least two intermediates. The first of these is presumably a distonic ion (DI'+) arising through formation of a single C-C bond between BZ" and ET and could not be identified due to its lack of a palpable absorption in the visible. The second (a tautomer of the former) is assigned tentatively to the radical cation of 5-vinyl-l,3-cyclohexadiene (5VCH'+), which was shown in an independent experiment to yield the same proportion of two OT'+ rotamers as BZ-ET" upon photolysis. In the presence of excess ET, BZ-ET'+ (or DI'+) undergoes photoaddition of a second molecule of ethylene to yield a Cl&II4'+ species. The EA spectra indicate the presence of four different rotamers of a cation carrying a conjugated triene chromophore. However, no conclusion could be reached with regard to the exact identity of this species. Scheme IV summarizes our interpretation of the above results in mechanistic terms. Several of the depicted reaction pathways must be considered hypothetical at this point of time, and clearly more work is needed to fully elucidate the intriguing photochemistry on the multifaceted CsHlo'+ hypersurface. However, the above described results serve to illustrate the usefulness of the Freon glass technique for the identification of metastable intermediates arising in the course of complex photochemical transformations of organic radical cations. Experimental Section Bicyclo[2.2.2]octa-2,5-diene (Dihydrobarrelene, DHB). This compound was synthesized according to De Lucchi et aLZ9and separated from the accompanying monoene by elution with n(29) De Lucchi, 0.;Lucchini, V.; Pasquato, L.; Modena, G. J. Org. Chem. 1985, 89, 815.
2210
J . Phys. Chem. 1992, 96, 2210-2216
pentane from an 80-cm column packed with silica gel (2-250. B i c y ~ 4 . 2 . 0 ~ - 2 , 4 d i e n(Bo). e The synthesis and isolation of this compound have been described recently.20 Samples were purified by gas chromatography (GC) on &9'-oxydiproprionitrile (ODPN) before each experiment. !bVinyl-1,3-cyclohexadlene(SVCH). 5VCH was prepared by the method of von Doering and Roth'O and purified by GC. 1- and 2-Vinyl-1,fcyclohexadiene(1VCH and 2VCH). A mixture of the two isomers was obtained according to S ~ a n g l e r . ~ ' Their separation was achieved by GC on a column containing 0-xylyl dicyanide impregnated with Procedures. The techniques used to obtain radical ion spectra in Freon glasses and argon matrices have been outlined previO U S ~ Optical ~ . ~ spectra ~ ~ ~ were ~ ~recorded ~ using a Cary 171 (Kyoto) or a Perkin-Elmer Lambda 9 spectrometer (Fribourg). (30) von Doering, W.; Roth, W. R. Tetrahedron 1963, 19,715. (31) Spangler, C. W. Tetrahedron 1976, 32, 2681.
Background absorptions before X- or y-irradiation were subtracted unless otherwise noted.
Acknowledgment. Y.F. and T.S. wish to thank Dr. K. Tanaka (Kyoto University) for various suggestions and assistance in synthesizing DHB and the three VCHs. The Kyoto work was supported by subsidies from Scientific Research of the Ministry of Education in Japan, Grants No. 62606006 and 63606005. T.B. and K.R. express their gratitude to Prof. M. Gross and Dr. C. Warner (University of Nebraska) for their gift of a sample of 5VCH. The efforts of Mr. Philippe Wrro (University of Fribourg) in the separation of lVCH and 2VCH are gratefully acknowledged. We also thank Prof. E. Haselbach for his continuing support and encouragement. The work in Fribourg was supported by the Swiss National Science Foundation, Grant No. 2028842.90. Registry NO. DHB", 95589-50-7; OT", 72257-39-7; BZ, 71-43-2; ET, 74-85-1.
An Experimental Estimate of the Threshold Barrier for the 1,P-Fluorine Atom Migration in l,l,l-Trifluoromethylcarbene Bert E.Holmes* Department of Chemistry, Arkansas College, Batesville, Arkansas 72501
and David J. Rakestraw? Department of Chemistry, Ohio Northern University, Ada, Ohio 4581 0 (Received: October 29, 1991)
A threshold energy barrier of 29 f 4 kcal/mol was estimated for the 1,2-fluorine migration reaction converting l,l,l-trifluoromethylcarbene, CF3CH, into CF2==CHF in the gas phase. The CF3CH was formed by the 1,l-eliminationof HCl from chemically activated CF3CH2C1containing 97.5 kcal/mol of internal energy. RRKM theory was used to calculate rate constants for the 1,2-fluorineshift that were fitted to the experimental pressure dependence to determine the threshold barrier.
Introduction Intramolecular 1,2-atom migrations converting carbenes to alkenes have been of interest to theoreticians and experimentalist for over three Carbene precursors' and new detection techniques monitoring short-lived speciese6 have recently been developed so that the richness of carbene reaction chemistry is now being unraveled at a remarkable pace. Current theoretical methodologies complement experiment by providing accurate 1,2-migration threshold energies for small, undetectable carbenes that cannot be probed spectroscopically and for carbenes whose 1,Zatom shift is too fast for current diagnostic techniques.' Little is known about methylcarbene, the simplest carbene that can rearrange, because the very small threshold barrier for 1,2-hydrogen shift, Eo( 1,2-H), leads to intramolecular reaction before the carbene can be detected or intercepted. The most sophisticated calculations predict an E,( 1,2-H) of 0.6 kcal/mol for CH3CH.' Recent work has shown that halogen substituents attached to the carbene carbon increase the Eo( 1,2-H) so that bimolecular reactions become competitive with intramolecularrearrangement.8 Solution-phase Arrhenius parameters for the 1,Zhydrogen shift in methylchlorocarbene, CH3CCl, are E, = 4.9 f 0.5 kcal/mol and the A factor = (6.0 f 4) X lo9 s-I. Determinations of the Arrhenius parameters were based on time-resolved photoacoustic calorimetry in solution,8a and laser flash photolysis has confirmed the overall rate constant.8b The laser flash photolysis technique
'
Present address: Sandia National Laboratory, Combustion Research Division-8362, Livermore, CA 94551.
gave Arrhenius activation barriers of 4.5-4.8 and 4.7 kcal/mol for benzylchlorocarbene and benzylbromocarbene, respectively.8b*c Arrhenius A factors are between 0.8 X lo1*and 1.2 X 10l2s-' for the benzylhalocarbenes. A 2 order of magnitude difference between the Arrhenius A factors for CH3CCl and benzylhalocarbenes is unusual for similar reactions; a value of 6 X lo9 s-l is anomalously low and implies an extremely rigid transition state. If the A factor is closer to lo1*s-I for CH3CCl then the E, = 10 kcal/mol. Competitive rates for 1,Zhydrogen migration and addition to alkenes have been reported for difluoromethylfluorocarbene, CF,HCF, in the gas phases9 The Arrhenius activation energy (1) Kistiakowsky,G. B.; Mahan, B. H. J . Am. Chem. Soc. 1957,79,2412. (2) Chong, D. P.; Kistiakowsky, G. B. J . Phys. Chem. 1964, 68, 1793. (3) Moss, R. A. Acc. Chem. Res. 1989, 22, IS. (4) Platz, M. S.;Maloney, V. M. In Kinetics and Spectroscopy of Carbenes and Eiradicals; Platz, M. S.,Ed.; Plenum: New York, 1990; p 239 f. (5) Bley, U.; Koch, M.; Temps, F.; Wagner, H. Gg. Eer. Eunsenges. Phys. Chem. 1989, 93, 833. (6) Petek, H.; Nesbitt, D. J.; Moore, C. B. J . Chem. Phys. 1987,86, 1189. (7) Evanseck, K. D.; Houk, K. N. J . Phys. Chem. 1990, 94, 5518. (8) (a) LaVilla, J. A.; Goodman, J. L. J . Am. Chem. Soc. 1989,111,6877. (b) Liu, M. T. H.; Bonneau, R. J . Am. Chem. Soc. 1990, 112, 3915 and references therein. (c) Liu, M. T. H.; Subramanian, R. J. Phys. Chem. 1986, 90, 75. (d) Moss, R. A,; Mamantov, A. J . Am. Chem. Soc. 1970,92,6951. ( e ) Liu, M. T. H.; Subramanian, R. J . Chem. Soc., Chem. Commun. 1984, 1062. (9) Haszeldine, R. N.; Parkinson, C.; Robinson, P. J.; Williams, W. J. J . Chem. Soc., Perkin Trans. 2 1979, 954.
0022-3654/92/2096-22 10$03.OO/O 0 1992 American Chemical Society
