J . Phys. Chem. 1990, 94, 8396-8399
8396
quencies above 6 GHz,’ broadly confirms this cutoff frequency criterion and provides a nice consistency check on our results. In closing, we again stress that the determination of quadrupole coupling constants from an analysis of the frequencies of I4N double-quantum peaks according to eq 1 is justified, in general, only if the anisotropic hyperfine interaction is negligible compared to isotropic hyperfine and quadrupole interactions. An estimate of the magnitude of the anisotropic hyperfine interaction is directly available in the spectra, from the line widths of the doublequantum peak^.^^'^ This estimate may serve to confirm the applicability of eq 1 or, alternatively, to warn against its application. Although the evaluation of quadrupole coupling constants from the double-quantum frequencies is problematic in the latter sit-
uation, hyperfine couplings should typically be measurable with reasonable accuracy. Aside from the intrinsic importance of the accurate determination of hyperfine coupling constants, their values are also useful in the recovery of the quadrupolar information, as accomplished in the present study, by multifrequency ESEEM: they indicate the location of exact cancellation where the quadrupole couplings may be directly measured. Acknowledgment. This work is supported by the National Science Foundation (DMR 86- 14003) through the Harvard Material Research Laboratory. We thank D. van Ormondt and R. deBeer of the T. H. Delft for certain spectral analysis programs used in this work.
Picosecond Time-Resolved UV Resonance Raman Spectroscopy of the Photochemical Ring Opening of 1,3,5-CycIooctatriene and a-Phellandrene Philip J. Reid, Stephen J. Doig, and Richard A. Mathies* Department of Chemistry, University of California, Berkeley, California 94720 (Received: August 14, 19901
The kinetics of the photochemical ring opening of 1,3,5-cyclooctatrieneand a-phellandrene were determined by picosecond, time-resolved UV resonance Raman spectroscopy. An amplified, synchronously pumped dye laser provided a 275-nm pump pulse and probe pulses at either 284 or 298 nm with 2.6-ps time resolution. Transient Raman spectra were obtained from 0 ps to I O ns after photolysis. The time evolution of the ethylenic intensity demonstrated that the octatetraene photoproduct of 1,3,5-cyclooctatrieneappeared in 12 f 2 ps. The 3,7-dimethyl-1,3,5-octatrienephotoproduct produced from the photolysis of a-phellandreneappeared with a time constant of 1 1 f 2 ps. The similarity of these times to the 8-ps production of cis-hexatriene from 1,3-cyclohexadiene (Reid, P.; Doig, S.; Mathies, R. Chem. Phys. Left. 1989, 156, 163) indicates that the -IO-ps photoproduct appearance time is a general feature of photochemical electrocyclic rearrangements.
-
Introduction Despite the importance of electrocyclic ring-opening reactions in polyene photochemistry and in photobiology, little is known about the kinetics, molecular dynamics, and potential surfaces that determine the photochemistry.’-2 Resonance Raman intensity analysis has determined that, after photoexcitation, 1,3-cyclohexadiene (1,3-CHD) distorts rapidly along the predicted conrotatory reaction ~oordinate.’.~ This evolution appears to be followed by an equally fast (10-20 fs) depopulation of the initially prepared 1 B2 surface to a lower lying surface of AI symmetry.k6 Recent picosecond resonance Raman studies have demonstrated that I ,3-CHD photochemically converts to ground-state cishexatriene in 8 ps.’ However, it is not known whether the properties of 1,3-CHD are representative of other photochemical electrocyclic rearrangements. In this study, the rates of photoproduct formation for the photochemical ring openings of 1,3,5-cyclooctatriene and (R)-aphellandrene were determined by using picosecond, two-color, UV resonance Raman spectroscopy. 1,3,5-Cyclooctatriene is known to undergo ring opening with a disrotatory motion of the methylene portion of the ring.* a-Phellandrene is a substituted 1,3-cyclohexadiene in which one of the carbons undergoing conrotatory rotation is replaced by an isopropyl group9 We have determined that the time scales for photoproduct formation in these reactions are 12 f 2 and 1 1 f 2 ps, respectively. These time constants are quite similar to the 8-ps appearance time of cis-hexatriene from 1,3-CHD. It is evident that the =lO-ps time scale for photoproduct formation is a general feature of these reactions and does not depend strongly on the stereochemistry of the reaction or substitution of the bond being broken. Materials and Methods The two-color, UV picosecond Raman apparatus is depicted in Figure 1. An extended-cavity, Coherent 590 dye laser was
* Author to whom correspondence should be addressed
synchronously pumped with a extended-cavity, mode-locked Spectra Physics 2020 Ar+ laser. For 1,3,5-cyclooctatriene, the dye laser was tuned to 596 nm while 568 nm was utilized for a-phellandrene. The output of the dye laser was amplified with a four-stage amplifier pumped by a Continuum YG581C-50 Nd:YAG laser operating at 50 Hz. Rhodamine 6G was placed in the first stage of the amplifier while rhodamine 610 was used in the second and third stages. At 596 nm, kiton red was placed in the final stage of the amplifier while rhodamine 610 was utilized at 568 nm. The measured fwhm of the background-free autocorrelation trace of the amplified pulse was 2.8 ps and the ASE was 3% at both wavelengths. The output of the amplifier was split into two beams to generate the pump and probe. For the probe beam, 20% of the amplified pulse was directed through an optical delay line and then doubled with a I-mm piece of KDP to generate 298- or 284-nm light. The ( 1 ) (a) Hoffmann, R.; Woodward, R. B. Arc. Chem. Res. 1968, 17, 1. (b) Fukui, K. Acc. Chem. Res. 1971, 4 , 57. (2) (a) Dauben, W. G.; Kellogg, M. S.; Seeman, J . I.; Vietmeyer, N. D.; Wendschuh, P. H. Pure Appl. Chem. 1973, 33, 197. (b) Jacobs, H. J. C.; Havinga, E. Adu. Phorochem. 1979, 1 1 , 305. (c) Hudson, B. S.; Kohler, B. E.; Schulten, K. Excited States; Academic Press: New York, 1982; Vol. 6, P 1. (3) Trulson, M. 0.; Dollinger, G . D.; Mathies, R. A. J . Am. Chem. SOC. 1987, 109, 586. (4) Trulson, M . 0.;Dollinger, G. D.; Mathies, R. A. J . Chem. Phys. 1989, 90, 4274. (5) Share, P. E.;Kompa, K. L.; Peyerimhoff, S. D.; Van Hemert, M. C. Chem. Phys. 1988, 120, 411. (6) Jan Buma, W.; Kohler, 8. E.; Song,K. J . Chem. Phys. 1990,92,4622. (7) Reid, P. J.; Doig, S. J.; Mathies, R. A. Chem. Phys. Len. 1989, 156, 163. (8) (a) Goldfarb, T.; Lindquist, L. J . Am. Chem. Soc. 1%7,89,4588. (b) Dauben, W. G.; Mclnnis, E. L.; Michno, D. M. Rearrangements in Ground and Excited Stares; Academic Press: New York, 1980; Vol. 3, p 91. (9) (a) De Kock, R. J.; Minnard, N. G.; Havinga, E. Recl. Trau. Chim. Pays-Bas 1960, 79, 922. (b) Baldwin, J . E.; Kreuger, S. M. J . Am. Chem. SOC.1969, 91, 6444. (c) Crowley, K. J.; Erickson, K. L.; Eckell, A,; Meinwald, J . J . Chem. SOC.,Perkins Trans. 1 1973, 2671. (d) Spangler, C. W.; Hennis. R. D. J . Chem. Soc.. Chem. Commun. 1972. 24.
0022-3654/90/2094-8396$02.50/0 0 1990 American Chemical Society
Letters
The Journal of Physical Chemistry, Vol. 94, No. 22, 1990 8397 Amplifier 1
NdYAG m e r
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Figure 1. Schematic of the picosecond two-color UV Raman apparatus BP = band-pass filter, BS = beam splitter, DB = dichroic beam splitter, FJ = flowing jet, H20= water continuum cell, KDP = potassium dihydrogen phosphate doubling crystal, ML = mode locker, MLD = mode locker driver, OMA = optical multichannel analyzer, PR = polarization rotator, TB = timing box, TS = translation stage.
remaining 80% of the amplified pulse was focused into a 7-cm cell of H,O for continuum generation. A band-pass filter (Omega Optics) was used to select a 2-nm bandwidth centered at 550 nm from the white-light continuum. The light at 550 nm was directed to a two-stage amplifier pumped by the same Nd:YAG laser used for the first amplifier. Rhodamine 560 was utilized in the first stage of this amplifier while fluorescein 548 in basic solution was placed in the second stage. The fwhm of the background-free autocorrelation trace of the 550-nm beam was 2.6 ps and the ASE was 5%. The 550-nm pulse was then doubled with a 1-mm piece of KDP and the resulting radiation at 275 nm was utilized as the actinic pulse. Cross-correlation of the 275-nm beam with either the 298-nm or the 284-nm beam and the determination of zero time was accomplished by two-photon ionization of triethylamine. The fwhm of the cross-correlation was 2.6 ps. The polarization of the pump beam was rotated to 5 5 O relative to the probe beam to minimize the contribution of rotational effects to the observed kinetics. Both beams were monitored by photodiodes and found to be stable to *lo%. 1,3,5-CycIooctatriene (COT) was purchased (Organometallics Inc.) and further purification to eliminate bicyclo-[4.2.0]-2,4octadiene was performed as outlined by Cope et a1.I0 The resultant C O T was -98% pure as determined by I3C N M R and off-resonance Raman spectroscopies. Solutions of COT (40 mM) and solutions of a-phellandrene (60 mM) [(R)-S-isopropyl-2methyl-I ,3-cyclohexadiene] (Fluka, 99%) in spectrophotometric grade cyclohexane were used in this study. The pump and probe beams were directed toward a free-flowing solution of the sample of interest with a dichroic beam splitter and spherically focused with a 100 mm focal length lens to a -150 pm diameter spot. The solution reservoir was replenished repeatedly to minimize the accumulation of photoproduct and absorption spectra of the solution after the experiment demonstrated no evidence of significant bulk photolysis. Raman scattering was collected with UV grade lenses and delivered to a Spex 500m spectrograph (f/4) equipped with a 1200 groove/mm classically ruled grating blazed at 500 nm operating in second order and detected with a PAR Model 1421 intensified photodiode array. The entrance slit width was 120 pm yielding a band-pass of I2 cm-I. Vibrational frequencies are accurate to *2 cm-I. The Raman spectra at a given time delay were collected (IO) Cope, A. C.; Haven, A. C., Jr.; Ramp, F. L.; Trumbull, E. R. J . Am. Chem. Soc. 1952, 74, 4867.
1
hk
400
800
1200
1600
2000
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Figure 2. Resonance Raman difference spectra of the conversion of 1,3,5-~yclooctatriene to cis&-octatetraene. The spectra were obtained with a 275-nm pump (1.6 mW) and a probe at 298 nm (500 pW). Positive peaks are due to the appearance of photoproduct. A ground-state spectrum of COT excited at 298 nm in cyclohexane is given at the bottom for comparison. Lines due to the cyclohexane solvent are marked with
an asterisk. with the probe beam only, the pump and probe beams, and the pump beam only incident on the sample. To remove scattering contributions from the pump beam, the pump-only scan was subtracted from the pump-probe scan resulting in the “photolysis spectrum”. The probe-only spectrum was then subtracted from the photolysis spectrum to produce the difference spectra reported here. The Raman scattering of the photoproduct was determined to be linearly dependent on the pump power up to 2.5 mW and on the probe power up to 700 pW. To correct for time-dependent changes in the optical absorbance of the sample, the intensity of the product scattering was ratioed to the intensity of the 801-cm-I solvent line in the photolysis spectrum.
Results I ,3,5-Cyciooctatriene. UV resonance Raman difference spectra of the photoconversion of COT to octatetraene are presented in Figure 2. At 0 ps, a small negative peak at 1610 cm-’ is observed and assigned to the symmetric ethylenic stretch of COT. This negative peak represents partial depletion of the ground state by the 275-nm actinic pulse. At 4 ps, positive peaks at 1590, 1288, and 1248 cm-’ are observed. Since these lines have positive intensity in the difference spectrum, they are assigned as Raman peaks of the octatetraene photoproduct. A negative line at 801 cm-’ is clearly evident in the 4-ps spectrum and is assigned to the cyclohexane solvent. The observation that this line is negative is an indication that the optical absorbance of the sample has increased due to photolysis of COT. The 801-cm-’ line remains negative going and loses intensity at longer times. Between 4 and 64 ps, the line at 1590 cm-’ gains intensity and shifts up to 1615 cm-’. Based on group frequency arguments, we assign this line as the symmetric ethylenic stretch of the photoproduct. The positive lines at 1288 and 1248 cm-’ also increase in intensity and upshift to 1294 and 1255 cm-I, respectively. At 16 ps, the delayed appearance of a low-frequency photoproduct peak at 351 cm-l is observed. This peak increases in intensity relative to the other modes present at higher frequency until 64
Letters
8398 The Journal of Physical Chemistry, Vol. 94, No. 22, I990
J I
d
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-
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20
-
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- - - -- - - -- - - - -- 60
80
-
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100
Time (ps)
Figure 3. Intensity of the ethylenic stretch of (A) cis&-octatetraene and (B) 3,7-dimethyl-1,3,5-octatriene as a function of time. Data were fit to a single exponential resulting in formation time constants of 12 f 2 ps for ci5.ci.v-octatetraeneand 1 1 f 2 ps for 3,7-dimethyl-l,3,5-octatriene.
ps. No further evolution in the photoproduct peaks is observed between 64 ps and 12 ns. The peak height of the photoproduct ethylenic line was corrected for changes in the optical absorbance of the sample and changes in pump and probe powers between time points. The corrected intensity is plotted as a function of time in the upper half of Figure 3. The best fit to a single exponential gave a time constant of 12 f 2 ps for formation of cis,cis-octatetraene from COT. a-Phellandrene. UV Raman difference spectra of the formation (OT) from a-phellandrene ( a of 3,7-dimethyl-l,3,5-octatriene PHE) are given in Figure 4. At 0 ps, the small, negative line at 1588 cm-' corresponds to the ethylenic stretch of a-PHE and is due to ground-state depletion. At 3 ps, a positive peak at 1613 cm-l is observed and is assigned as the ethylenic stretch of the OT photoproduct. The intensity of this line grows and the frequency upshifts to 1630 cm-l at 100 ps. At 7 ps, two positive peaks are observed at 1313 and 1 145 cm-l. These lines also gain intensity and upshift in frequency to 1320 and 11 50 cm-I, respectively, at 100 ps. Since these peaks are positive in the difference spectrum, they are assigned to the photoproduct. No further changes in the spectra are observed between 100 ps and 3.8 ns. Unlike the COT photoconversion described above, a negative-going solvent line at 801 cm-' is not observed, indicating that under these conditions there is little change in the optical absorbance of the sample at the probe wavelength. The peak height of the photoproduct ethylenic line was corrected for power fluctuations and optical absorbance changes between time points. The resulting intensity was plotted as a function of time as depicted in the bottom half of Figure 3. The best fit to a single exponential gave a time constant of 1 1 f 2 ps for the formation of OT from a-PHE.
Discussion The time-resolved resonance Raman spectra reported here contain both kinetic and structural information on these photochemical electrocyclic ring-opening reactions. The time evolution of the photoproduct ethylenic intensity demonstrates that the ring opening of COT to form octatetraene is complete in 12 2 ps. This time scale is similar to the 8-ps time constant previously reported for the photochemical production of cis-hexatriene from 1,3-CHD.' Therefore, the disrotatory rearrangement of COT and the conrotatory ring opening of 1,3-CHD are complete on the same time scale. Also, the 1 1 f 2 ps formation time of O T from the
*
Figure 4. Resonance Raman difference spectra of the conversion of a-phellandrene to 3,7-dimethyl-l,3,5-octatriene.Spectra were obtained with a 275 nm (1.6mW) pump and a probe at 284 nm (500 pW). A spectrum of a-PHE in cyclohexane obtained at 284 nm is given at the bottom for comparison. Lines due to the cyclohexane solvent are marked with an asterisk.
photolysis of a - P H E is close to the appearance time of cis-hexatriene. Although one may expect the conrotatory motion of the methylene portion of the ring to be slowed by the presence of an isopropyl group, the ground-state photoproduct formation rate does not appear to be sensitive to the presence of alkyl substituents. The COT photoproduct line at 1255 cm-' is close in frequency to the value of 1260 cm-' reported by Kohler and co-workers for the single-bond stretch of cis,cis-octatetraene isolated in a lowtemperature matrix." Although the barrier for double bond isomerization on the 2Al surface in configurationally unrelaxed octatetraenes is small,12,the similarity in frequency suggests that no excited-state double-bond isomerizations have occurred and that the ground-state photoproduct is cis&-octatetraene. The delayed appearance of the COT photoproduct mode at 351 cm-' until -16 ps may be the result of thermal relaxation on the ground-state surface. Since polyatomic including CHD'4bare known to vibrationally cool on the IO-ps time scale, at the earliest times studied here cis&-octatetraene is probably vibrationally hot. The intensity of this low-frequency mode in the 4- and 8-ps spectra may be reduced relative to high-frequency modes due to inhomogeneous broadening. The initially hot photoproduct ensemble would contain a number of different conformers each having a different torsional frequency. Cooling would reduce the number of torsional conformers and explain the sudden appearance of this line in the 16-ps spectrum. The temporal evolution of the cis,cis-octatetraene photoproduct spectra provides information on the conformation of the photoproduct on the ground-state surface. The observed upshift in the ( 1 1 ) Hossain,
M. H.; Kohler, B. E.; West, P. J . Pfiys. Chem. 1982, 86,
4918.
(12) Ackerman, J . R.; Kohler, B. E. J . Am. Chem. SOC.1984, 106,3681. (13) Yoshida. H.; Furukawa, Y.; Tasumi, M. J . Mol. Strucl. 1989, 194, 219. ( I 4) (a) Wild, W.; Seilmeier, A,; Gottfried, N. H.; Kaiser, W. Cfiem.Pfiys. Left. 1985, ( 1 9 , 259. (b) Doig, S . J.; Reid, P. J.; Mathies, R. A. Manuscript in preparation.
J . Phys. Chem. 1990, 94, 8399-8401
photoproduct ethylenic line from 1590 to 1615 cm-I may be one indication of single-bond conformational relaxation. This frequency upshift is very similar to the upshift observed for the cis-hexatriene photoproduct of 1,3-CHDe7QCFF-pi calculations performed in this laboratory and ab initio calculations by Tasumi and co-workersi3 predict that, in cis-hexatriene, an upshift in ethylenic frequency will accompany single-bond conformational relaxation to the more extended s-trans,cis,s-trans form and a similar effect is expected for cis,cis-octatetraene. QCFF-pi calculations performed in this laboratory show that s-trans,cis,s-cis,cis,s-trans-octatetraene still has significant steric interaction at the ring-opened end of the molecule, resulting in distortion of one of the central double bonds away from planarity. This torsional strain would be consistent with the presence of an intense double-bond torsional mode a t 351 cm-I. It is also possible that the observed frequency upshifts are due to anharmonicity of the ground-state potential surface. At earliest times, higher vibrational levels may contribute significantly to the observed Stokes spectrum resulting in vibrational frequencies which are lower than those observed from a thermally relaxed photoproduct. An upshift in frequency would accompany vibrational relaxation on the ground-state surface. The ethylenic frequency of the OT photoproduct also upshifts as a function of time. The ring opening of a-PHE proceeds along the direction of conrotatory rotation that minimizes steric interactions of the isopropyl group with the remainder of the molecule.9b The ring-opened all-cis photoproduct would still be the high-energy conformer due to the steric interaction of the terminal hydrogens similar to all-cis-he~atriene.~~ Therefore, (15) Tai, J. C.; Allinger, N. L. J . Am. Chem. SOC.1976, 98, 7928.
8399
it is reasonable to suggest that the observed upshift in ethylenic frequency of the 3,7-dimethyl-l,3,5-octatrienephotoproduct represents ground-state conformational relaxation to s-cis-cis,strans-OT and/or s-trans,cis,s-trans-OT, although vibrational cooling may also contribute to this evolution. To further elucidate the conformational composition of the OT photoproduct, more detailed information about the vibrational spectra of the various conformers is needed. In conclusion, the similar photoproduct formation times for COT, a-PHE, and I,3-CHD indica.te that the IO-ps time scale for the appearance of photoproduct on the ground-state surface is a general feature of pericyclic photochemical ring-opening reactions and this time does not depend on the stereochemistry of the reaction or chemical substitution. The fact that the appearance time does not depend on the stereochemistry or substitution of the reactant argues that the structural products of electrocyclic ring-opening reactions are determined predominantly by the excited states which participate in the photochemistry. These reactions are characterized by a rapid excited-state evolution which dictates the product composition, followed by a 10-ps relaxation to the ground-state surface. Resonance Raman intensity analysis should prove useful in comparing the femtosecond excited-state dynamics of COT and a-PHE with 1,3-CHD.I6 It will also be important to examine the long-lived excited states of these systems to understand the cause of the unusually strong coupling to the ground-state surface.
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Acknowledgment. This work was supported by a grant from the N S F (CHE 86-15093). (16) Lawless, M. K.; Mathies, R. A. Manuscript in preparation.
Photoelectron Spectrum of the Ptopargyl Radical in a Supersonic Beam David W. Minsek and Peter Chen*.' Mallinckrodt Chemical Laboratories, Harvard University, Cambridge, Massachusetts 021 38 (Received: August 16, 1990)
The laser photoionization mass and photoelectron spectra of propargyl radical ( K 3 H 3 )and two partially deuterated isotopomers are measured in a supersonic molecular beam. Radicals were produced by flash pyrolysis in a heated supersonic tube. nozzle. The PES exhibits an intense origin band at 8.67 f 0.02 eV, which is assigned to the adiabatic ionization potential previously seen by electron impact mass spectroscopy. A second photoelectron band at higher energy is assigned to the previously unobserved first excited state of the propargyl cation. One highly anharmonic vibrational progression in the cation is also observed.
Introduction We report' preliminary results of the vibrationally resolved photoelectron spectrum of the hydrocarbon free radical propargyl (C3H3)in a supersonic beam. Propargyl has attracted considerable attention as one of the simplest conjugated organic free radicals. It has been discussed as a possible precursor to benzene and other aromatic hydrocarbons in flames2 The propargyl cation appears commonly in mass spectrometry and also appears in abundance in flames.3 Despite its importance, gas-phase spectroscopic and thermochemical data on propargyl remain scarce. Lossing measured an ionization potential (IP) of 8.68 eV for propargyl by electron i m p a ~ t . ~Ramsay and Thistlethwaite reported an ultraviolet absorption spectrum of propargyl in 1968, but the ( I ) David and Lucile Packard Fellow, NSF Presidential Young Investigator, Camille and Henry Dreyfus Distinguished New Faculty Fellow. ( 2 ) Westmoreland, P. R.; Dean, A. M.; Howard, J. B.; Longwell, J. P. J . Phys. Chem. 1989, 93, 8171, and references therein. (3) Goodings, J. M.; Bohme, D. K.; Ng,C.-W. Combust. Flame 1979, 36, 21. (4) Lossing, F. P. Can. J . Chem. 1972, 50, 3973.
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observed lines were diffuse, and definite assignments could not the be madeus In matrix isolation studies of propargyl infrared and ESR spectra have been reported. Flash pyrolytic production of reactive free radicals followed by supersonic expansion has been shown to be a useful technique for the isolation of radicals under stabilizing conditions, Le., jet cooled and collision free.*-" We have applied this technique for the production and isolation of the propargyl radical and obtained the first photoelectron spectra of this important species. From these spectra we have (i) confirmed the previous IP measurement; (ii) found spectroscopic evidence for a low-lying excited state of the corresponding cation, predicted by theory12but never observed; ( 5 ) Ramsay, D. A.; Thistlethwaite, P. Can. J . Phys. 1966, 44, 1381. (6) Jacox, M.E.; Milligan, D. E. Chem. Phys. 1974, 4, 45. (7) Fessenden, R. W.; Schuler, R. H. J . Chem. Phys. 1963, 39, 2147. (8) Blush, J. A.; Park, J.; Chen, P. J . Am. Chem. SOC.1989, I l l , 8951. (9) Chen, P.; Colson, S. D.; Chupka, W. A.; Berson, J. A. J. Phys. Chem. 1986, 90, 2319. (IO) Chen, P.; Colson, S.D.; Chupka, W. A. Chem. Phys. Lett. 1988,147, 466. ( 1 I ) Dunlop, J. R.; Karolczak, J.; Clouthier, D. J. Chem. Phys. Len. 1988, 151, 362. Clouthier, D. J.; Karolczak, J. J. Phys. Chem. 1989, 93, 7542.
0 1990 American Chemical Society
