12691
J. Phys. Chem. 1993,97, 12691-12699
Resonance Raman Intensity Analysis of the Photochemical Hydrogen Migration in 1,3,5-Cycloheptatriene Philip J. Reid,? Andrew P. Shreve, and Richard A. Mathies’ Department of Chemistry, University of California, Berkeley, California 94720 Received: June 8, 1993; In Final Form: August 31, 19930
Absolute resonance Raman scattering excitation profiles are presented for 1,3,5-~ycloheptatriene(CHT) excited in resonance with the lA’-lA” photochemically active electronic transition at 260 nm. The resonance Raman cross sectionsand the absorption spectrum are modeled to quantitate the geometric evolutionoccurring immediately after excitation. The vibrational scattering intensities are consistent with planarization of the C H T ring, while no evidence for evolution along the methylene C H stretch coordinate is observed. This suggests that the hydrogen-shift reaction proceeds sequentially with planarization of the ring occurring before any changes in C H bonding. Analysis of the absolute resonance Raman intensities, yielding an excited-state homogeneous line width of -100 cm-l, as well as the low fluorescence quantum yield of C H T (1.4 X 10-6) shows that the excited-state population decay occurs on a 25-50-fs time scale. These results indicate that evolution along ring planarization coordinates leads to rapid internal conversion of the initially prepared excited state to a lowerenergy surface (not previously considered in orbital symmetry models) upon which the sigmatropic shift occurs.
Introduction Elucidating the mechanism of pericyclic photochemical rearrangementsis of interest due to the importantrole these reactions have played in the development of our understanding of chemical reactivity.’-’ Recent studies examining the photochemistry of electrocyclic ring-opening reactions have demonstrated that femtosecond excited-state nuclear dynamics and picosecond ground-state photoproduct formation times are characteristic features of these rearrangements.4-10 Sigmatropic shifts are a class of pericyclic reactions in which structural changes occur by themigration of u-bonds. The photochemical hydrogen migration of 1,3,5-~ycloheptatriene(CHT) is a classic example of this reaction class (Scheme I). Early gas-phaseexperimentson CHT determined that photolysis leads to the formation of toluene with a quantum yield of 0.97.l1JZ This gas-phase reaction is believed to proceed through a vibrationally hot, ground-stateintermediate. In contrast, the solution-phase photochemistry of CHT is dominated by the migration of a hydrogen atom from the 7- to 1-position of the ring (denoted as [1,7]).13 This was verified by Ter Borg and Kloosterziel in studies of deuterated CHT.14J5A unimolecular ring closure also occurs, but at a rate 500 times slower than that of hydrogen migrati0n.1~ Two distinct models have been proposed to explain the stereospecific nature of this reaction. In the theory of Paulick et al., excitation of CHT leads to a near-planar intermediate from which the sigmatropic shift takes place as dictated by orbital symmetry conservation.’J6J7 This mechanism has been questioned recently by Tezuka et al., who propose that twisting about one of the terminal ethylenic bonds results in polarization of the bond.’* The charge separation associated with this “sudden polarization”would then direct the hydrogen migration. Although photochemical sigmatropicshift reactionshave been widely studied, relatively little is known about their mechanism and excited-state dynamics. The only photochemical quantum yield measurement for a solution-phase sigmatropic shift is that of 7-aryl-CHT (I#J= 0.5).19 Multiphoton ionization studies on CHT indicatethat the initially-prepared excited state depopulates
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t Present addreac: Department of Chemistry, University of Minnesota, Minneapolis, MN 55455. * To whom correspondence should be addrcased. Abstract published in Aduancc ACS Absrracts. October 15, 1993.
0022-365419312097-12691$04.00/0
13Scyclohcptatricnc
in -60 fse20 This rapid decay as well as the low fluorescence quantum yield (X10-4) suggests that the photochemistryofCHT is characterized by rapid excited-state evolution.16 Recent picosecond time-resolved UV resonance Raman experiments on CHT have shown that ground-state CHT appears 26 ps after photolysis, providing the first measurement of the reaction time for a [ 1,7]-sigmatropic shift.*’ Although information on the kinetics of photoproduct formation has been obtained, the nature of the excited-state evolution responsible for the reaction dynamics remains unknown. To explore the initial excited-state photochemical reaction dynamics of CHT, we have performed an absolute resonance Raman intensity analysis. Resonance Raman intensity analysis has proven valuable in elucidating the femtosecond nuclear dynamics of a variety of photochemically reactive systems.22-M In this study, the absolute resonance Raman scattering profiles of CHT are measured throughout the lowest-energy r-r* transition. The Franck-Condon displacements that reproduce the absorption spectrum and the resonance Raman scattering intensities are determined. The enhanced modes provide a quantitative measure of the structural evolution of the initiallyprepared excited state. The Raman intensities demonstrate that, upon excitation, CHT distorts along ring planarization coordinates in agreement with the proposed reaction mechanism of Paulick et a1.16 Also, the absence of resonance enhancement of the methylene CH stretches demonstrates that the initial nuclear motion does not involve significant hydrogen atom migration. The fluorescence quantum yield of CHT (1.4 X lod), which correspondsto an excited-state decay time of 25 fs, demonstrates that nuclear motion out of the Franck-Condon region is followed by rapid internal conversion. This observation, coupled with the picosecond ground-state recovery of photoexcited-CHT,suggests that a second, lower-lying excited state, previously unconsidered N
0 1993 American Chemical Society
Reid and Mathies
12692 The Journal of Physical Chemistry, Vol. 97, No. 49, 1993
in orbital symmetry analyses, plays a principal role in pericyclic photochemical rearrangements.
Materials and Experimental Methods Resonance Raman Spectroscopy. Resonance Raman spectra of 1,3,5-cycloheptatriene (CHT) were obtained at 319.9,299.1, 282.4,266.0,252.7,239.6,228.7,217.8,and 208.9 nmusing the H2-shifted532- and 266-nm output from an amplified Nd:YAG laser (Spectra Physics DCR-2A) operating at 20 Hz. Spectra were also obtained at 257.3 nm utilizing the frequency-doubled output from a mode-locked Ar+ ion laser (Spectra Physics 2040E). Dilute solutions (3-50 mM) of CHT (Wiley Organic, 95%) dissolved in HPLC grade cyclohexane (Fisher) were employed. The Raman spectra of commercially available and freshly distilled CHT were identical; therefore, CHT was used without further purification. The absorption spectrum of the sample was measured before and after each experiment to monitor the concentrationwith the change kept to
