J. Phys. Chem. 1985,89, 821-827
821
Absorption Spectra and Photochemical Rearrangements of CIOH,, Radical Cations in Solid Argon. Dicyclopentadiene, 1,3-Bishomocubane, Bicycle[6.2.0]deca-2,4,6-triene, and Decapentaene Lester Andrews,* Ian R. Dunkin, Benuel J. Kelsall, and Joseph T. Lurito Department of Chemistry, University of Virginia, Charlottesuille, Virginia 22901 (Received: January 31, 1984; In Final Form: November 12, 1984)
Matrix photoionization of CIOHI2 precursors endo-dicyclopentadiene, exo-dicyclopentadiene, 1,3-bishomocubane, bicyclo[6.2.0]deca-2,4,6-triene,and decapentaene has produced the parent radical cations for study in solid argon. The two dicyclopentadiene cations gave broad absorptions in the near-infrared that are similar to those for norbornadiene cation. Photolysis of the dicyclopentadiene radical cations with red light produced new bands near 25 000 cm-' due to a common cation species. Irradiation in the latter absorptions produced new structured band systems near 20000 cm-', which were interconverted by selective photolysis. These bands were observed in similar photolysis experiments with 1,3-bishomocubane and were produced directly from photoionization of bicyclo[6.2.0]deca-2,4,6-triene and all-trans-decapentaene during condensation, which identifies the 20 OOO-cm-' bands as decapentaene cation isomers. The successive photochemical ring opening of multicyclic CI&II2radical cations to give all-tram-decapentaenecation proceeds readily through a series of sigmatropic shifts and electrocyclic ring openings. These processes are predicted to be favorable for radical cation excited states.
Introduction The absorption spectroscopy of molecular cations in matrices has received considerable attention during the past decade.' With the exception of a few studies,*" the main focus of attention has been directed toward producing and characterizing molecular cations instead of examining the photochemical properties of the cations. The matrix-isolation technique is especially suited for studying the photochemistry of cations. With this technique, unrearranged radical cations of precursor molecules can be produced by matrix photoionization methods and isolated in a cold, inert environment before reaction or rearrangement can occur. Once isolated, the parent cation can be characterized spectroscopically and then subjected to selective irradiation to promote stepwise photochemical isomerization or decomposition. The neutral dicyclopentadiene molecule is a standard Diels-Alder condensation product, and its ready thermal decomposition to cyclopentadiene is well-known.' The dicyclopentadiene cation, however, photochemically rearranges to give a variety of cation products depending upon the matrix host. In Freon matrices at 77 K photolysis gives primarily cyclopentadiene monomer cation and a new CloH12isomer cation,* whereas in solid argon at 20 K, photolysis leads preferentially to decapentaene cation. In the present study, the rearrangement of three different dicyclopentadiene cations and bicyclodecatriene cation to decapentaene cation are explored. Experimental Section The cryogenic equipment and photolysis procedures have been described earlier.9J0 The endo-dicyclopentadiene and methyldicyclopentadiene were obtained from Aldrich Chemical Co. and samples of exo-dicyclopentadiene and 1,3-bishomocubane were prepared"J2 by the Shida group. Bicyclo[6.2.0]deca-2,4,6-triene Andrews, L. Ann. Rev. Phys. Chem. 1979, 30, 79. Andrews, L.; Keelan, B. W. J. Am. Chem. SOC.1981, 103, 99. Shida, T.; Kato, T.; Nosaka, Y. J . Phys. Chem. 1977, 81, 1095. Hays, J. D.; Dunbar, R. C. J. Phys. Chem. 1979,83, 3183. Kelsall, B. J.; Andrews, L. J . Am. Chem. SOC.1983, 105, 1413. (6) Kelsall, B. J.; Andrews, L.; Trindle, C. J. Phys. Chem. 1983,87,4898. ( 7 ) Wasserman, A. Monatsh. Chem. 1952, 83, 543. (8) Shida, T.; Momose, T.; Ono, N . J . Phys. Chem., preceding paper in this issue. In the final revision these authors suggest that the dicyclopentadiene parent radical cations may isomerize to bis-cyclopentaallylic radical cations, which retain the "endo"and =exo"characters of the parent cations. This does not alter the major thrust of the present paper, namely that the parent cations photochemically rearrange ultimately to decapentaene cations in solid argon. (9) Andrews, L. J . Chem. Phys. 1975,63,4465. (10) Kelsall, B. J.; Andrews, L. J . Chem. Phys. 1982, 76, 5005. (1 1) Nelson, G. L.; Kuo, C.-L. Synthesis 1975, 105. (12) Schenck, G. 0.; Steinmetz, R. Chem. Ber. 1963, 96, 520. (1) (2) (3) (4) (5)
was synthesized as described by two groups13J4and its identity was confirmed by proton NMR; decapentaene was synthesized by using the method outlined by the Christensen group and its identity was verified from the structured matrix UV s p e c t r ~ m . ' ~ All of the precursors were degassed by several freezethaw cycles, and gas samples were prepared by diluting room-temperature equilibrium vapor pressure (about 1-2 torr) precursor with argon containing CH2C12(Ar:CH2C12:precursori= 400: 1:1-2). The gas mixture containing precursor was condensed on a 20 K sapphire plate at about 1 mmol/h with a similar quantity of pure argon passing through a 3-mm quartz discharge tube16 excited by a 100-W microwave discharge for periods of 2-5 h. After condensation with simultaneous argon resonance photoionization, spectra were recorded between 300 and 2000 nm with a Cary 17 spectrophotometer. The matrix samples were next exposed to radiation from a high-pressure mercury arc lamp (1000 W, 11lumination Industries, Inc., BH-6-1) passing through a series of standard glass filters for 15-30-min periods and then more spectra were recorded. Spectra were digitized by using either an Apple I1 or Nicolet 1180 computer and converted to a linear wavenumber scale. For convenience, band positions and photolysis energies are given in dimensionlessquantities u / ( lo00 cm-I); band positions in the tables are rounded to the accuracy of the measurement.
Results Argon resonance photoionization experiments, performed with five different CloH12precursors, will be described in turn. endeDicyclopentadiene. Ten experiments were performed with the endo-dicyclopentadiene isomer; all of these yielded similar results with differences due to the particular sequence of photolysis energies used. The spectra for a typical experiment are shown in Figure 1. Trace a illustrates the initial spectrum after sample preparation, trace b represents the spectrum following 20-10 photolysis, trace c shows the spectrum aCter 34-10 irradiation, and trace d gives the spectrum recorded after 42-25 irradiation for 30-min periods. Trace a shows two broad absorptions near 28 and 14 with full width at half-maximum = fwhm = 4000 and 2000 cm-I, respectively, and three moderately sharp bands near 25.6, 24.5, and 22.9 with absorbances of about 0.02; no absorption was observed below the 14 band. The very broad 28 band is due to CH2C12+formed by photoionization1' of some of the methylene ~
~
~~~~~
(13) Staley, S . W.; Henry, T. J. J . Am. Chem. SOC.1970, 92, 7612. (14) Kaupp, G.;Jostkleigreiue, E.; Riisch, K. Chem. Eer. 1977, 110, 2394. (15) D'Amico, K. L.; Manos, C.; Christensen, R. L. J. Am. Chem. SOC. 1980 102 1777 1980, 102, 1177. (16) Andrews, L.; Tevault, D. W.; Smardzewski, R. R. Appl. Specfrosc. 1978, 1Y78, 32, 157.
0022-3654/85/2089-0821$01.50/00 1985 American Chemical Society
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985
822 l
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1 TABLE I: Absorptions (v/(lOOO cm-I)) and Assignments for New Species Produced in Matrix Photoionization Experiments with endoand exo-Dicyclopentadienes and 1,3-Bishomocubane
u/(1000
cm-')
11.92 14.0
11.58 12.55 18.32 18.62 18.76 19.05 19.195 19.495 19.795 19.460 19.735 20.015 20.290 20.73 21.27 21.77 22.86 24.52 25.55 26.57 24.93 25.15 30.47 31.67 32.11 33.73 J
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assignment" exo parent cation endo parent cation dpt*, s dp+., t dpt*, o dpt*, P dp+., q dp+., r dpt*, s
dpt., s dpt*, s dpt*, t dpt., t dpt., t dp'., t
(tetraene cation) (tetraene cation) (tetraene cation) intermediate cation intermediate cation intermediate cation intermediate cation (triene cation fragment) (triene cation fragment) dP dP dP dP
" dpt. denotes decapentaene radical cation; the following letter denotes structural isomers where t is the all-trans species. The dp'. bands were produced in greater yields from decapentaene and bicyclodecatriene precursors. Parentheses denote tentative assignments.
Figure 1. Absorption spectra in the 800Q-34000.cm-1 spectral region for
a sample of endo-dicyclopentadieneand methylene chloride in argon subjected to argon resonance photoionization during condensation at 20 K: (a) spectrum after sample preparation for 4 h, (b) spectrum after 30-min photolysis with 20-10 radiation, (c) spectrum after 34-10 photolysis, (d) spectrum after 42-25 irradiation. chloride added to serve as an electron trap. The first photolysis using 20-10 radiation, shown in Figure lb, destroyed the 14 and 22.9 bands, and no new absorptions were observed. The second photolysis, which irradiated the visible region and the 24.5 and 25.6 bands, destroyed the 24.5 and 25.6 bands and produced an intense band system between 18 and 24 and weak sharp bands at 12.5 and 11.6, which are shown in Figure IC; the 19.5 and 12.5 peaks tracked together on photolysis and are labeled t, and the 19.2 and 11.6 peaks tracked together and are labeled s. The final irradiation in the UV region changed the relative intensities of the s and t bands and produced a sharp 24.9, 25.1 doublet and a weak 30.5 band. Band positions are given in Table I. A second study gave an identical initial spectrum. Irradiation with 17-10 light for 15 min destroyed the 14 and 22.9 bands, increased the 24.5 and 25.5 bands by 50%, and revealed a third component at 26.5. A 26-10 photolysis destroyed the 24.5 band system and produced a new band series beginning at 19.20 ( A = 0.10) with components at 19.50 and 19.80 and low-frequency shoulders at 19.0, 18.8, 18.4, and weak new bands at 20.7 and 30.5; subsequent 34-10 photolysis continued this band growth with 19.20 reaching A = 0.32 and 19.50 at A = 0.22. Weak bands appeared at 11.6 and 12.5, and a sharp doublet was observed at 24.9,25.1. A final 45-10 irradiation increased the t bands relative to s, shifted t to 19.46, increased the sharp doublet, and produced strong UV bands at 30.5, 32.1, and 33.7. The latter UV bands are due to decapentaene based on comparison with matrix, gasphase, and solution spectra.15 In another experiment using the endo-dicyclopentadiene precursor, the initial irradiation was performed with 42-25 energy. (17) Andrews, L.; Prochaska, F. T.; Ault, 101, 9.
B.S. J . Am. Chem. SOC.1979,
This photolysis destroyed the moderately sharp 25.6, 24.5, and 22.9 bands and the broad CH2C12+band and produced only a small yield (maximum absorbance, A = 0.03) of the group of bands absorbing between 18 and 24. The weak broad 14 band was unaffected by this photolysis. A 15-min illumination with 34-10 radiation destroyed the broad 14 band, increased the 18 to 22 bands markedly (19.2 and 19.5 to A = 0.46 and 0.26, respectively), and produced weak UV bands at 30.5 ( A = 0.01) and 32.2. In addition, sharp weak bands were revealed at 12.5 and 11.6 with A = 0.02 and 0.01, respectively. A 10-min exposure to 45-10 radiation increased the 19.5 band to A = 0.48 with similar increases for the associated bands, and it increased the UV bands 7-fold and left the 19.2 band essentially unchanged. A similar sample was deposited without argon resonance photoionization and then irradiated with 45-10 light for 35 min; no product absorptions were observed. A final experiment was conducted with the endo precursor and no CH2C12. After argon resonance radiation for 3 h, the product spectrum was a weaker version of Figure Id with sharp 19.2 ( A = 0.08) and 19.5 ( A = 0.06) bands and weaker 18.4, 18.8, and 19.1 bands. Photolysis at 34-10 increased both sharp bands to A = 0.10 and decreased the broad bands, and 45-10 irradiation decreased 19.2 to A = 0.6 and 19.5 to A = 0.08. exo-Dicyclopentadien. Three argon resonance photoionization experiments were performed with exo-dicyclopentadiene using methylene chloride as an electron-trapping molecule. In the first experiment, the sample, following argon resonance irradiation, exhibited a broad absorption peaking near 12 ( A = 0.05, fwhm = 2000 cm-') and very weak ( A = 0.01), moderately sharp bands at 24.5 and 25.5 in addition to the broad CH2C12+band. A 15-min 17-10 photolysis destroyed the CH2C12t band, reduced the 12 band by 25%, increased the moderately sharp 24.5 ( A = 0.02) and 25.5 bands, and revealed a new band near 26.5. Exposure to 20-10 radiation virtually destroyed the 12 band and doubled the intensity of the moderately sharp bands. The third photolysis, using 26-10 radiation, destroyed all of the moderately sharp absorptions and produced weak bands between 18 and 24. The
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 823
CloHlzRadical Cations in Solid Argon
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Figure 2. Absorption spectra in the 8000-34000-~m-~region for ar-
gon/CH2C12/exo-dicyclopentadienesample: (a) spectrum after argon resonance photoionization during condensation at 20 K for 4.5 h, (b) spectrum after 15-minphotolysis with 21-10 radiation, (c) spectrum after 15-min irradiation at 34-10, (d) spectrum after 30-min photolysis with 45-10 radiation.
three most intense bands were 19.0, 19.2, and 19.5 with absorbances of 0.01,0.06, and 0.03. The final photolysis using the full output of the mercury arc lamp, substantially increased the absorptions between 18 and 24 and produced strong decapentaene absorptions. Spectra from the experiment with the highest yield of absorbing species are shown in Figure 2. The initial spectrum, trace a, shows that broad 12 ( A = 0.05) and 28 bands are the dominant absorptions. The first photolysis at 21-10 destroyed the 28 methylene chloride cation band, nearly destroyed the 12 band, and produced a prominent series of new absorptions a t 24.5 ( A = 0.05), 25.5 ( A = 0.04), and 26.5 ( A = 0.01) and two very weak bands at 19.2 ( A = 0.02) and 19.5 ( A = 0.01), labeled s and t in Figure 2b. The next photolysis at 34-10 completely destroyed the 12, 24.5, 25.5, and 26.5 bands and produced strong new absorptions in the 24-18 region that are listed in Table I. A comparison between Figures I C and 2c shows that these absorptions are the same. In addition to these intense new bands, sharp weak new s and t bands were detected at 11.6 ( A = 0.01) and 12.5 ( A = 0.02). As with the endo-dicyclopentadiene precursor, irradiation with the full output of the mercury arc, trace d, changed the relative intensities of the s and t bands in the 24-18 and 11-13 regions and markedly increased UV bands at 30.5 ( A = 0.22), 32.1, and 33.7 due to decapentaene (labeled dp). Bishomocubane. Two argon resonance photoionization experiments were performed on 1,3-bishomocubane with virtually the same results. Figure 3 shows spectra for one of these experiments: scan a represents the initial spectrum, and the following scans illustrate spectra after 30-min photolysis periods. In contrast with the two dicyclopentadienes, the 1,3-bishomocubane sample
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20
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Figure 3. Absorption spectra in the 18 000-28 000-cm-' region for argon/bishomocubanesample with methylene chloride added: (a) spectrum after argon resonance photoionization during condensation at 20 K for 4 h, (b) spectrum after 30-min photolysis with 24-10 radiation, (c) spectrum after irradiation at 34-10, (d) spectrum after photolysis with
45-10 light. exhibited only the CH2ClZ+band and three moderately sharp bands near 22.9 ( A = 0.02), 24.5 ( A = OM), and 25.5 ( A = 0.03). The first 19-10 photolysis (not shown) destroyed the methylene chloride cation and 22.9 bands, doubled the intensity of the 24.5 and 25.5 bands, and revealed a new band at 26.5 ( A = 0.02). In addition, weak new bands appeared at 18.8, 19.2 ( A = 0.03) and 19.5. The second 24-10 photolysis produced a stronger visible band system, labeled s in Figure 3b and Table I, and a weak 30.5 band. The third photolysis (not shown) using more energetic 29-17 radiation that could excite the species responsible for the 24.5,25.5, and 26.5 absorptions destroyed those bands and further increased the visible and UV bands. A 34-10 irradiation, Figure 3c, continued these trends and produced a sharp weak 24.9 band and weak 11.6 and 12.5 bands. A final 45-10 irradiation had the same effect as it had on the dicyclopentadiene product species; the s bands were reduced relative to the t bands, Figure 3d, and the UV bands due to decapentaene increased substantially. Bicyclo[6.2.0]deca-2,4,6-triene. Five argon resonance photoionization experiments were done with the bicyclodecatriene precursor using different photolysis sequences, and Figure 4 illustrates a typical spectrum. After argon resonance photoionization during condensation, the initial scan (Figure 4a) revealed sharp bands between 18.3 and 19.5 (labeled 0,p, q, r, s, and t, respectively), which are common to the dicyclopentadiene experiments, but these bands exhibited markedly different relative intensities in the bicyclodecatriene studies. Also present are new bands at 17.57 and 17.86 (labeled m and n), which were not present in the dicyclopentadiene experiments, the CH2CI2+band at 28, and weak decapentaene absorptions at 30.5 and 32.1, which were not present in unphotoionized samples. Photolysis at 17-10 doubled the major s and t absorptions, without detectable changes in the other sharp bands, and destroyed the broad CH2C12+band (not shown). A subsequent 19-10 irradiation destroyed the sharp o and p absorptions, increased the s and t absorptions 4-fold, as
824 The Journal of Physical Chemistry, Vol. 89, No. 5. 1985 I
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Figure 4. Absorption spectra in the visible region for argon/CH,Cl?/
bicyclodecatriene using nonlinear wavenumber scale: (a) spectrum after argon rmonance photoionization during condensation at 20 K for 4 h, (b) spectrum after 15-min photolysis with l e 1 0 radiation, (c) spectrum after 24-10 irradiation, (d) spectrum after photolysis at 34-10. shown in Figure 4b, and produced weak new s and t absorptions a t 11.56 and 12.55. Irradiation with 24-10 light (Figure 4c) further increased the q, r, s, and t bands and additional absorptions between 21.7 and 20.7. Photolysis at 34-10 (Figure 4d) increased the t bands relative to the s bands and increased the decapentaene bands 3-fold. A final 45-10 photolysis (not shown) decreased the s and t bands slightly and markedly increased the decapentaene band system. One experiment was done with bicyclodecatriene, CH2C12,and only the high-pressure mercury arc source. Irradiation for 30 min produced very strong decapentaene absorptions, in accord with solution ph~tochemistry,'~ sharp t bands with half the intensity of Figure 4a, and substantially weaker s, r, and q peaks. Decapentaene. Six experiments were done with decapentaene using different photolysis sequences, discharge powers, and sample concentrations. The initial scan after sample photoionization, Figure 5a, exhibited very strong t bands that were produced only after secondary photolysis in experiments with the other precursors. The weak q and o bands were also observed, with similar intensities as in Figure 4a. Photolysis with 19-10 light (Figure 5b) destroyed the weak o and p bands and increased the s and t absorptions by 10%. Irradiation through a dielectric filter (transmits 19.3e19.84) decreased the s and t bands by 10% and restored the p band, as shown in Figure 5c. Photolysis at 34-10 (Figure 5d) destroyed the weak p band, markedly increased the r absorption, slightly decreased the s bands, and increased the t bands by 10%; additional t bands were identified at 20.77 and 21.04 and in the red region as listed in Table 11. A final 45-10 irradiation (not shown)
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Figure 5. Absorption spectra in the 11 000-22 OOO-cm-' region for de-
capentaene, methylene chloride sample diluted in argon: (a) spectrum after photoionization with open resonance lamp for 5 h, (b) spectrum after 15-min photolysis at 19-10, (c) spectrum after 2-h irradiation through 19.30-19.84 transmitting (30% at 19.57) dielectric filter, (d) spectrum after 15-min 34-10 photolysis. Note more sensitive absorbance scale for the 11 000-15000-cm-1 region. TABLE II: Vibronic Absorptions (cm-I), Assignments, and Vibrational Intewals for Deeapentaene Cation in Matrix Photoionization Experiments with Decapentaene
absorption 11 595 11 765 11 890 12 185 12 560' 12730 12850 13010 13 150
(I
assignment
assignment
s, origin s (170)
absorption 13810 14 120 s (295) 19 180 s (2 X 295) 19475O t, origin 19 750 t (170) 20 025 t (290) 20 300 t (450) 20 770 t (2 X 285) 21 040 Full width at half-maximum is 80 cm-'.
t (1 250) t (1560) s, origin t, origin t (275) t (2 X 275) t (3 X 275) t (1295) t (1565)
decreased the t bands by 20% and the s bands by 40%. In other decapentaene experiments with higher discharge power and lamp output, the yields of p, q, r, and s bands were higher relative to t, and the photolysis sequence increased the t bands relatively more at the expense of the p, q, r, and s bands. All photolyses including the 18-20 region increased the t bands more than the other sharp absorptions. Methylcyclopentadiene Dimers. As a complement to the cyclopentadiene dimer experiments, several argon resonance photoionization experiments were performed with a sample containing a mixture of methylcyclopentadiene dimers. These experiments gave spectra which were similar to those obtained with the
The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 825
CIOHI2 Radical Cations in Solid Argon endo-dicyclopentadiene. Broad 28 and 14 bands ( A = 0.06, fwhm = 2000 cm-I) were observed in the initial spectrum. Photolysis with 24-10 radiation for 30 min destroyed the broad bands and produced a weak broad 26.3 band. A final 45-10 irradiation produced a strong 19.1 band ( A = 0.25) with partially resolved structure and a similar weaker 20.9 satellite.
Discussion Product absorptions will be identified and the photochemical rearrangements will be discussed. Identification. The major new absorptions produced in this study are assigned to molecular cations. Since the endo- and exo-dicyclopentadienes have structures that are similar to norbomadiene and norbornylene with first ionization energies of 8.70 and 8.97 eV,'* it is reasonable to assume that the dicyclopentadienes will have similar first ionization energies. Likewise bicyclodecatriene and decapentaene are expected to have first ionization energies near or below hexatriene at 8.30 eV.19 These precursors can be photoionized readily by the 11.6-1 1.8-eV output of the argon resonance lamp. Furthermore, it has been shown in several other studies,s,6,20+21 that the addition of good electron-capturing molecules, such as methylene chloride, enhances the yield of isolated cations in matrix experiments. The 5-fold increase in the yield of visible absorbing cation products with methylene chloride added attests to this point. Assignments. The broad 14 and 12 bands are attributed to the endo- and exo-dicyclopentadiene radical cations, respectively. These weak bands were the major absorptions produced by argon resonance radiation, which can ionize the parent molecule with little or no rearrangement, and they are in good agreement with broad 13.4 and 11.7 bands produced by y-radiolysis of the precursors in glassy matrices.* The positions of these absorptions, in the near-infrared, are in the same region as the 15.4 absorption for norbornadiene cation isolated in solid argon.s The norbornadiene cation absorption has been assigned to a A A transition involving the two nonconjugated double bonds.22 Since the endo-dicyclopentadiene molecule has its two double bonds farther apart than norbornadiene, there should be a weaker interaction between the two r systems and consequently a lower energy transition for the endo-dicyclopentadiene cation than for the norbornadiene cation. Using a similar argument, the even lower energy transition of the exo-dicyclopentadiene cation can be attributed to the even wider separation between the two double-bond systems. The substantially larger yield of the exo isomer in these experiments (Figure l a vs. 2a) is attributed to the greater stability of the exo form compared to the endo form, as judged from the likely nonbonded interactions. The failure to observe a spectrum for the 1,3-bishomocubane parent cation in this region is due to the absence of A systems. The sharp absorptions between 18 and 20 can be assigned to at least two different species based upon the differences in relative intensities following the 34-10 and 45-10 photolyses. The two major absorptions, t and s at 19.5 and 19.2, fall below the origins of the two major octatetraene cation absorptions at 22.4 and 21.6 in solid argon.23 In like fashion, the weak red t and s bands at 12.5 and 11.6 are lower than the 13.5 octatetraene cation red band origin in solid argon23and the gas phase.24 A straightforward extrapolation from the electronic absorptions of trans-hexatriene cation (26.4, 15.5) and trans-octatetraene cation (22.4, 13.4) in solid a r g ~ n ~provides ' . ~ ~ strong evidence in support of assignment of the 19.5 and 12.5 bands to a decapentaene cation. The direct
-
(18) Haselbach, E.; Rossi, M. Helu. Chim. Acta 1976, 59, 2635. (19) Allan, M.; Dannacher, H.; Maier, J. P. J. Chem. Phys. 1980, 73, 3114. (20) Kelsall, B. J.; Andrews, L.; McGarvey, G.J. J . Phys. Chem. 1983, 87, 1788. (21) Kelsall, B. J.; Andrews, L. J . Phys. Chem. 1984, 88, 2723. (22) Haselbach, E.; Bally, T.; Lanyiova, Z.; Baertschi, P. Helu. Chim. Acta 1979, 62, 583. (23) Dunkin, I. R.; Andrews, L.; Lurito, J. T.;Kelsall, B. J. J . Phys. Chem., to be published. (24) Jones, T. B.; Maier, J. P. Inf. J. Mass Spectrom. Ion Phys. 1979, 31, 287.
production of these bands by photoionization of decapentaene and their ready formation in bicyclodecatriene experiments confirms this assignment. The greater stability of the 19.5 and 12.5 bands (labeled t in the figures) under full arc photolysis and their being the major product with the all-trans-de~apentaenel~ precursor indicate that the 19.5 and 12.5 bands are due to the all-transdecapentaene cation. The 19.2 and 11.6 bands (labeled s) are due to another isomer slightly less stable than all-trans but considerably more stable than other configurations; this may be a terminal-cis arrangement with all remaining bonds in trans configurations. The 0,p, q, and r bands are assigned to decapentaene cations with varying degrees of trans arrangements, and the m and n bands observed in the bicyclodecatriene experiments are probably due to decapentaene cation isomers with more cis than trans configurations. The small differences between band measurements for the t and s decapentaene cation bands using different precursors (Tables I vs. 11) reflect the photochemical history of the sample in the matrix cage. In the former, considerable rearrangement within the matrix cage is required, and in the latter, argon solvates the ion after photoionization of the neutral molecule. Methyl substitution in the final photolysis product gave a small red shift as found for other radical cations; 2o unfortunately the methyl-substituted product bands were broadened by the random position of the methyl group in these experiments. Although there is some overlap between the strong t and s band systems, the latter band system is produced first by red visible photolysis and three distinct bands at 19.195, 19.495, and 19.795 were observed (see Figure 3b). These bands exhibit a spacing of 300 f 10 cm-'. After full arc photolysis, three bands were observed at 19.460, 19.735, and 20.015 with identical substructure due to site splittings; these bands exhibit a 275 f 10 cm-l spacing. First, the separation is distinctly different for the two decapentaene isomers, and these values are smaller than similar 340 f 10 and 360 f 10 cm-' separations in the blue band systems for trans~ t a t e t r a e n and e ~ ~tram- 1,3,5-heptatriene cations.21 This interval is due to a skeletal deformation mode observed at 347 cm-' for hexatriene itself.25 The red absorption band systems (Table 11) also exhibit a similar vibrational interval. In addition the intervals of 1250 and 1560 cm-' in the red band system and 1295 and 1565 cm-' in the green band are appropriate for the "single and double bond" stretching modes, which have been assigned to 1200 and 1600 cm-' intervals in the solution absorption spectrum of de~ a p e n t a e n eand ' ~ 1220 and 1630 f 10 cm-' intervals in the argon matrix absorption spectrum of decapentaene. The 19.5 and 12.5 bands assigned here to decapentane cation are probably due to transitions from the ground state ( ~ 1 ~ ~ 2 ~to~upper 3 ~ states ~ 4 mixed ~ ~ 5by~configuration ) interaction, as was the case for hexatriene ~ a t i o n .The ~ upper state for the red transition is probably dominated by the ( A ~ ~ A ~configuration, ~ T ~ ~ and A the ~ ~upper ~ state ~ ~ for ) the green transition is probably dominated by the ( ~ 1 ~ ~ 2configu~ ~ 3 ~ ~ 6 ~ ) ration. The bands at 20.7 and 21.3 grow on photolysis; the sharper 20.7 band does not track with either the 19.2 or 19.5 bands, but the broader 21.3 and 21.8 bands may be associated with the 19.5 band system. An alternative possibility is for the 20.7 and 21.3 bands to be due to an intermediate cyclic conjugated tetraene species, which is consistent with their position in the spectrum. The 24.5, 25.5, and 26.5 bands observed with both dicyclopentadiene isomers and bishomocubane can be assigned to a common species. The 24.5 and 25.5 bands were always produced and destroyed in unison and the weaker 26.5 band was only detected when the absorbances for the 24.5 and 25.5 bands were high. These bands are clearly due to intermediate cations in the photochemical pathway between dicyclopentadiene cation and decapentaene cation. They occur in the region of cyclopentadienes and cyclohexadiene absorptions,21and they are probably due to cyclic intermediate species with conjugated diene cation subunits. The band system and 1030 10 cm-l interval are strikingly similar
*
(25) Lippincott, E. R.; Kenny, T.E. J . Am. Chem. SOC.1962,84, 3641.
826 The Journal of Physical Chemistry, Vol. 89, No. 5, 1985 Scheme I
Andrews et al. Scheme I1
2
2
1
1
1
3 ( 1 , 5 ) H shif
4
3
I
(1,3) H shift
5
1-
t 1.
t.
6
to 401-, 393-, and 378-nm bands in the radiolysis of endo- but not exedicyclopentadiene and assigned to a new dicyclopentadiene isomer cation! but the associated 1800-nm band was not detected in the argon matrix experiments. Possible explanations include the following: (a) the similar Freon and argon matrix bands are in fact due to different intermediate species; (b) they are due to the same species but the associated broad near-infrared band was not detected in the lower yield argon matrix experiments. The 22.9 band, which was produced in the endedicyclopentadiene and 1,3-bishomocubane experiments, was more photosensitive than the 24.5 series of absorptions, and this band is clearly due to a different intermediate cation not produced with the exo precursor. The sharp weak 24.9,25.1 doublet appeared on 34-10 photolysis and changed little on 45-10 photolysis in these experiments. It is unlikely that this doublet is associated with either the t or s decapentaene cation species. Since this weak absorption falls in the hexatriene cation region of the spectrum,21it could be due to a fragment containing a conjugated hexatriene cation subunit, but this is not a definitive identification. Comparison with Freon Matrix. The parallel experiments of Shida et a1.* subject a Freon matrix (FM) containing dicyclopentadienes at 77 K to y-rays. Since the final step in the ionization sequence is charge transfer from guest molecule to solvent hole, the FM method is softer than direct argon resonance (AR) vacuum-UV photoionization, and the FM technique traps a substantially larger yield of dicyclopentadiene parent cations than the present experiments. The Freon matrix interacts more strongly with guest cations than the argon matrix, as demonstrated by red shifts in band positions (Ar to Freon) and this stronger interaction might lead to faster vibrational relaxation and stabilization of radical cations with less rearrangement in the FM than AR experiments. This is consistent with the major photolysis products being a dicyclopentadiene cation isomer and CSH,+.with FM and decapentaene cation with AR. Photochemistry. The parent radical cations were produced here by argon resonance photoionization during condensation with excess argon. The role of the solid argon matrix environment is to quench any excess internal energy remaining in the newly formed cations before they can rearrange or dissociate. The purpose of methylene chloride is to serve as an efficient electron-capturing agent to prevent neutralization of the cations with electrons produced by photoionization or by photodetachment from less stable electron traps during subsequent p h o t o l y s i ~ . ~ ~ ~ ~ Once the parent cations have been produced, visible photolysis initiates the rearrangement process. Scheme I shows two possible pathways for this rearrangement. For the endo- and exo-di-
cyclopentadiene cations, 1, low-energy visible light is readily absorbed through the broad 14 or 12 bands to initiate rearrangement to more stable conjugated polyene cation products. In both pathways, the first step is migration of one C-C bond of the methylene bridge to give a substituted norcarene cation product, 2. This type of rearrangement has been inferred in earlier s t u d i e ~ using ~ - ~ ~norbornadiene and 7-chloronorbornadiene; a similar intermediate was involved in the rearrangement of 5methylenenorbornylene cation to S-methylenecyclohexa-1,3-diene cation?' the same conjugated triene cation that was formed from p-xylene catiomZ0As the scheme shows, two different products are formed depending on which of the methylene C-C bonds is broken. The substituted norcarene cation products are nonconjugated dienes and should have weak spectra very similar to the parent cations which are also nonconjugated dienes. The failure to observe these intermediates is probably due to low absorption coefficients. The next step in the proposed rearrangement sequence is ring expansion by reorganization of electrons to give 1,3,6,8-cyclodecatetraene or 1,3,5,8-cyclodecatetraenecations, 3. The former contains two conjugated diene cation subunits, which may absorb in the 25 region, while the latter contains a triene cation subunit; each rearranges by simple hydrogen shifts to give the conjugated cyclodecatetraene cation 4, which may spontaneously ring open. Note that 34-25 light most efficiently produces the decapentaene cation bands, and this is the region of conjugated diene and triene cation absorption^.^*^' Finally, opening of the cyclic decatetraene cation ring is expected to give decapentaene cations with a number of subunits in cis configurations, 5. Continued photolysis aids the unfolding process to the more stable all-trans-decapentaene cation, 6. Similar ring openings with subsequent rearrangement to all-trans isomer cations have been observed for 1,3-cyclohexadiene2' and 1,3,5-cyclooctatrienez3 cations in solid argon. The photochemical production of neutral decapentaene in dicyclopentadiene photoionization experiments provides support for the decapentaene cation rearrangement product. Recall that 45-10 irradiation of dicyclopentadiene did not produce decapentaene, but final 45-10 irradiation of samples containing decapentaene cation and chloride anion electron traps (Figure 2d) gave good yields of decapentaene. This UV radiation is sufficient to photodetach electrons from chloride anions,28which are then free to neutralize decapentaene cations. Thus, the growth of decapentaene in dicyclopentadiene cation experiments is due to photoneutralization of the decapentaene cation rearrangement product. The only difference in the photochemical rearrangement sequence for the endo and exo precursors is the lack of the photosensitive 22.9 intermediate absorption for the exo precursor. The 1,3-bishomocubane precursor gave the same photochemical products as the precursor, indicating the formation of a parent radical cation, although no parent cation absorption was detected. The rearrangement of the cubane parent cation is shown in Scheme 11. In this reaction scheme the bonds designated x are photolytically cleaved; with appropriate pairing of the four electrons and unfolding of the product species, the endo form of the dicyclopentadiene cation is produced. Once the endo form is produced, the rearrangement paths illustrated in Scheme I become available, which readily accounts for the identical products that are generated by these notably different forms of dicyclopentadiene. Because of the extreme photosensitivity of the (26) Andrews, L.; Kelsall, B. J.; Payne, C. K.; Rodig, 0. R.; Schwartz, H. J . Phys. Chem. 1982, 86, 3714. (27) Andrews, L.; Kelsall, B. J. unpublished results, 1982. (28) Berry, R. S.; Reimann, D. W. J. Chem. Phys. 1963, 38, 1540.
J . Phys. Chem. 1985,89, 827-830
827
for spectroscopic observation. Finally, rearrangement among the many decapentaene cation isomers proceeds following activation by absorption in the green band system; the near-infrared absorptions do not provide sufficient internal energy to initiate the rearrangement.
Scheme I11
/I+. endo-dicyclopentadiene cation, it was not observed as an intermedate in the rearrangement process of the cubane cation. The parent radical cation of bicyclodecatriene was surely produced in this study on the basis of the photochemical appearance of decapentaene cations in bicyclodecatriene experiments as suggested by Scheme 111, which has at least two intermediate cation species in common with Scheme I. The photochemical rearrangement of the bicyclodecatriene molecule to decapentaene has been documented in s o l ~ t i o n ~and ~ * in ' ~ the present matrix studies, and it is no surprise that the radical cation undergoes the same rearrangements. The production of a small amount of decapentaene cation by 45-10 irradiation of the bicyclodecatriene sample is explained by resonance two-photon ionizationlo of the decapentaene photolysis product. The photochemical conversion of decapentaene cations to the all-trans isomer was partially reversed by irradiation in only the t bands using a dielectric filter. Continued photolysis in all of the decapentaene cation band systems favored the all-trans isomer. Similar behavior has been found for hexatriene and octatetraene cations in solid a r g ~ n . ~ Activation ',~~ of a particular isomer by absorption of light unique to that isomer established a dynamic equilibrium among isomeric structures, and those that are not irradiated may then be deactivated by the matrix cage and trapped
Conclusions This investigation has produced dicyclopentadiene cations by matrix photoionization methods and observed weak bands for the endo and ex0 forms. Subsequent photolyses of these parent cations have led to the formation of intermediate cyclic-polyene cations. Additional photolyses produced ring opening to decapentaene cation isomers, with band systems at 19.5,12.5 and 19.2, 11.6 which are lower in energy than similar band systems for octatetraene and hexatriene cations. The decapentaene cation absorption bands were also produced by photochemical rearrangement of bicyclodecatriene cations and by direct photoionization of decapentaene. Prolonged irradiation favored rearrangement to the all-trans-decapentaene cation, the isomer prepared in greatest yield from this neutral molecule. These experiments are complementary to radiolysis in a Freon matrix for the study of cation spectroscopy and photochemistry. Acknowledgment. We gratefully acknowledge financial support from National Science Foundation Grant CHE 82-17749,the gift of samples from and helpful discussions with T. Shida prior to publication of his results, information from R. L. Christensen on decapentaene synthesis, and the loan of apparatus by D. F. Hunt. I.R.D. acknowledges financial support from the University of Strathclyde. Registry No. endo-dicyclopentadiene, 1755-01-7; exo-dicyclopentadiene, 933-60-8; methyldicyclopentadiene, 26472-00-4; 1,3-bishomocubane, 6707-86-4; bicyclo[6.2.0]deca-2,4,6-triene, 36093-14-8; decapentaene, 2423-91-8.
Impurity Perturbations of the Fundamental Band of Carbon Monoxide in Solid Nitrogen Bengt Nelander Thermochemistry Laboratory, University of Lund, Chemical Center, P.O. Box 740, S-220 07 Lund, Sweden (Received: May 23, 1984; In Final Form: October 5, 1984)
The fundamental band of CO in solid nitrogen has been studied at 0.05-cm-' resolution with an FTIR spectrometer. The result indicates that CO is trapped in a substitutional site. Impurities which do not appear to form well-defined complexes with CO (Ar, 02,C02) produce a symmetric pattern of satellites around the CO fundamental, in contrast to impurities which form a well-defined binary complex (H20,HCl). Deposition at 20 K give the sharpest CO band; deposition at lower temperatures gives broader bands. Annealing at 25 K can remove the effects of a low deposition temperature. Surface diffusion during deposition controls the amount of binary complex formed with H 2 0 or HCl.
Introduction Ever since the pioneering work of Pimentel and co-workers,'S2 the matrix isolation technique has been used to study molecular interactions. In this type of work there are one, two, or more impurities in the matrix simultaneously; the presence of binary and higher complexes between these is revealed by the appearance of new bands that cannot be attributed to the monomeric impurities themselves. From the observed spectrum of one such complex, conclusions are then drawn about its shape and binding strength. (1) Thiel, Mathias van; Becker, Edwin D.; Pimentel, George C. J . Chem. Phys. 1957, 27, 95. (2) Thiel, Mathias van; Becker, Edwin D.; Pimentel, George C . J . Chem. Phys. 1957, 27, 486.
However, the presence of impurities in the matrix, in concentrations large enough to give observable amounts of binary complexes, will also influence the spectra of the monomeric molecules. It is common experience in this type of work that the spectrum of a matrix-isolated compound looks better when the compound is alone in the matrix compared to when another compound is also present. There are several mechanisms whereby impurities in the matrix will inhomogeneously broaden absorption bands of matrix-isolated molecules. For instance, if the matrix contains molecules with dipole or quadrupole moments, these will give rise to electric fields which will vary randomly in the matrix and influence the absorption bands of molecules in the matrix. Most molecules differ considerably in size and shape from the matrix-forming atoms or molecules. They will therefore disrupt the matrix sturcture and shift the spectra of nearby molecules to some
0022-3654/85/2089-0827$01.50/00 1985 American Chemical Society
