J. Phys. Chem. 1991,95,4935-4939 5, and 6 cluster ions are almost identical. However, at stagnation pressure above 2 atm, the intensity of the n = 6 ion is significantly enhanced. We believe that the (C&J6+ ion represents a maximum in the polymeric ion distribution generated, under conditions of extensive clustering, via the addition reactions described in Figure 4. Recently, a similar observation of a magic number for the (C2H4)4+ion in the ethene cluster ion distribution has been reported and attributed to a single molecular ion.I3 The exothermicity of the condensation reactionsIbfj resulting in the formation of a polymeric ion within the cluster can be efficiently dissipated through evaporative loss of vdW bonded monomers in the cluster. For smaller clusters this mechanism may not be very efficient, and considerable amounts of energy may be deposited within the polymeric ions which can cause their characteristic fragmentations. The abundance of the polymeric ions is therefore expected to depend on the exothermicity of the addition reactions, the rate constants of the individual reactions, and the size of the cluster together with the time scale for observation and the critical energies and frequency factors of competing fragmentations. The factors that control the product ion distributions in cationic polymerization within vdW clusters are of continued interest in our laboratory, and other experimental techniques are currently being employed in order to fully understand these processes.I4 In summary, the experimental observations that support the conclusion that the (C,H,),,+ and C4H5+(C5H8),I ions are condensation polymeric ions formed via intracluster ion-molecule reactions are as follows: (1) The fragment ions observed from (C5H8)2+ are identical with those from E1 ionization of limonene, CIOHI6,which is the major product of Diels-Alder reaction of isoprene radical cation with its neutral molecule. (2) The fragment ions are identical with the products of the ion-molecule reactions observed in an ion trap after 300-ms reaction time. This is consistent with their formation of fragmentation of an internally excited limonene ion. (3) The relative intensities of the fragments C9HI3+,CsHI2+,C8HII+,C7Hll+,C8Hlo+,C7H9+,C6H9+, and C7H8+observed in the cluster beam and those reported from the ion-molecule reactions were found to be in excellent agreement. (4) The magic number observed for CI4H21+ ion is consistent with (13) (a) Cmlbaugh, M. T.; Peifcr, W. R.; Garvey, J. F. Chem. Phys. Lelr. 1990, 168, 337. (b) Garvey, J. F.; Peifer, W. R.; Coolbaugh, M. T. Acc. Chem. Res. 1991, 21, 48. (14) El-Shall, M. S.;Meot-Ner (Mautner), M. To be. published.
4935
a stable cyclic structure which can be derived from a series of ion-molecule consecutive addition-eliiination reactions. ( 5 ) The magic number observed for (CSH8)6+under experimental conditions where larger clusters are formed is consistent with a polymeric ion distribution characterized by a maximum which appears at n = 6 under our experimental conditions. (6) The observed ions generated within the clusters cannot be explained by simple monomer fragment ion solvation. Addition and elimination reactions within the clusters according to the suggested scheme in Figure 4 seem to account for all the observed ions.
Conclusions a d Outlook The main conclusions that can be drawn from this study are as follows: 1. Isoprene clusters provide a good example for demonstrating the feasibility of studying cationic polymerization within vdW clusters. The system seems to be sensitive to both the kinetic and internal energies of the reacting ions. The greater reactivity of the molecular ion leads to its consumption in a rapid Diels-Alder reaction. 2. The cationic polymerization of isoprene proceeds via the formation of the stable dimer C1oH16+ which, most likely, has a structure similar to limonene and can undergo further addition reactions to generate larger ions. 3. The formation of stable ionic species (cyclic structure) interrupts the general pattern of successive addition reactions. A clear example in isoprene study is the observation of a magic number corresponding to CI4H2'+ion. It is interesting that similar cyclic structures have been found in polyisoprene formed by bulk cationic polymerization of isoprene. 4. Clusters provide a feasible and valuable approach to understand both the mechanism of ionic polymerization and how the size of polymer chains is controlled in such a process. Work is under way in our laboratory to study ionic polymerization initiated by photoionization within the clusters.
Acknowledgment. The authors thank Professor R. M. Ottenbrite for helpful discussions regarding Diels-Alder reactions of isoprene. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, to the Thomas F. and Kate Miller Jeffress Memorial Trust, and to the Grants-In-Aid program for faculty of Virginia Commonwealth University for the partial support of this research.
Structure of the Fluorene-Benzene Aromatic Dimer from Rotational Coherence Spectroscopy P. W. Joireman, L. L. Connell, S. M. Ohline, and P. M. Feker**+ Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90024- 1549 (Received: March 26, 1991; In Final Form: May 9, 1991) Rotational coherence spectroscopy has been used to measure rotational constants for four isotopomers of the ammaticammatic dimer fluorentbenzene. Possibilities for the vibrationally averaged dimer geometry have been deduced from the measured values. In the geometries the benzene moiety is above ths five-membered ring of fluorene, displaced away from the apex of that ring, and tilted into the plane of the fluorene. The geometries are discussed in terms of the intermolecular forces between the species and the theoretical and experimental results on other aromatic dimer structures.
Introduction Spectroscopic studies of binary molecular complexes have proved to very useful in efforts to characterize intermolecular forces.' Such studies have been especially successful in expanding and refining knowledge of hydrogen-bonding and moleculerare
'NSF Presidential Young Investigator 1987-92.
gas atom interactions. In large part, this success can be attributed to the fact that rotationally resolved spectra leading to structural information have been obtained for numerous hydrogen-bonded (1) For reviews, see: (a) Strucrure a d Dynamics of Weakry Bound Complexes; Weber, A., Ed.;Reidel: Dortrecht, 1987. (b) Legon, A. C.; Millen, D. J . Chem. Rev. 1986.86,635.
0022-365419112095-4935502.50/0 0 1991 American Chemical Society
4936 The Journal of Physical Chemistry, Vol. 95, No. 13, 1991
and molecule-rare gas complexes.* Unfortunately, such spectroscopic results have been considerably more difficult to obtain for other classes of binary complexes, including ones involving two aromatic hydrocarbons.'* In the case of these latter species their large size has effectively precluded measurements of spectra at rotational resolution! Thus,although spectroscopic experiments on dimers have provided insight into aromatic-aromatic interactions, one still does not know much,about aromatic dimer equilibrium structures. A consequence is that one has little to work with in assessing the kinds of interactions that dominate forces between aromatics at close range. Further, one lacks a data base to develop the kinds of qualitative and quantitative descriptions of intermolecular forces that have been developed for hydrogen-bonded c o m p l e x e ~for , ~ ~example. ~~ One relatively recent approach to high-resolution rotational spectroscopy involves the use of picosecond optical techniques to monitor the evolution of rotational superposition states.l0*" This so-called rotational coherence spectroscopy (RCS) is particularly advantageous in the study of large species in molecular beams. Indeed, numerous applications of RCS to structural studies of such species, including c o m p l e ~ e sand ~ ~clusters,I4 ~ ~ ' ~ ~ have ~ ~ been reported. Given this, one expects that the application of RCS may prove fruitful in structural studies of aromaticaromatic complexes as well. In this Letter we report the first measurement of the rotational constants of a dimer formed from two aromatic hydrocarbons. RCS results have been obtained for four isotopomers of the fluorene-benzene van der Waals complex, a species first observed by Even and Jortner' in fluorescence excitation spectroscopy. The results show that the species are near-oblate symmetric tops. Fits to the observed rotational constants using structural degrees of freedom as fitting parameters yield two possible gross structures. The structures (between which we cannot distinguish because of the symmetry of the moment of inertia tensors of the species) are both approximate displaced parallel-plate structures with the benzene moiety tilted into the plane of the fluorene moiety. These structures are discussed in terms of the results of other theoretical and experimental studies of aromatic dimer structures.
Experimental Section RCS was implemented by the time-resolved fluorescence de(2) For example, see: Novick, S.E. In ref la, p 201. (3) (a) Janda, K. C.; Hemminger, J. C.; Winn, J. S.;Novick, S.E.; Harris, S.J.; Klemperer, W. J . Chem. Phys. 1975,63,1419. (b) Steed, J. M.; Dixon, T. A.; Klemperer, W. J . Chem. Phys. 1979, 70,4940. (4) There are such measurements for dimers involving single-ring heteroaromatics, however. See: (a) Levy, D. H.; Haynam, C. A.; Brumbaugh, D. V. Faraday Discuss. Chem. Soc. 1982.73, 137. (b) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J . Chem. Phys. 1983,79, 1581. (c) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J . Chem. Phys. 1984,81. 2282. ( 5 ) (a) Vernon, M. F.; Lisy, J. M.; Kwok H. S.;Krajnovich, D. J.; Tramer, A.; Shen, Y.R.;Lee, Y . T. 1.Phys. Chem. 1981,85,3327. (b) Hopkins, J. B.; Powers, D. E.; Smalley, R. E. J . Phys. Chem. 1981,85, 3739. (c) Langridge-Smith, P. R.R.;Brumbaugh, D. V.; Haynam. C. A.; Levy, D. H. J . Phys. Chem. 1981,85, 3742. (d) Fung, K. H.; Selzle, H. L.; Schlag, E. W. J . Phys. Chem. 1983,87, 51 13. (e) BBrnsen, K. 0.; Selzle, H. L.; Schlag, E. W. J . Chem. Phys. 1986,85, 1726. (9Law, K.; Schauer, M.;Bernstein, E. R. J . Chem. Phys. 1984,81,4871. (g) Kolenbrander, K. D.; Lisy, J. M.J . Chem. Phys. 1986,85,6227. (h) Henson, B. F.; Hartland, G. V.; Venturo, V. A.; Hertz, R. A.; Felker, P. M.Chem. Phys. Left. 1991, 176. 91. (6) Syage, J. A.; Wessel, J. E. J . Chem. Phys. 1988, 89, 5962. (7) Even, U.; Jortner, J. J . Chem. Phys. 1983, 78, 3445. (8) Doxtader, M. M.;Mangle, E. A.; Bhattacharya, A. K.; Cohen, S.M.; Topp, M. R. Chem. Phys. 1986,101, 413. (9) For a review, see: Buckingham, A. D.; Fowler, P. W.; Hutson, J. M. Chem. Rev. 1988, 88, 963. (10) (a) Felker, P. M.; Baskin, J. S.;Zewail, A. H. J . Phys. Chem. 1986, 90,724. (b) Baskin, J. S.;Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1986, 84, 4700. (1 I ) (a) Felker, P. M.; Baskin, J. S.;Zewail, A. H.J . Chem. Phys. 1987, 86.2460. (b) Baskin, J. S.;Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1987, 86. 2403. (12) Baskin, J. S.;Zewail, A. H. J. Phys. Chem. 1989, 93, 5701. (13) Connell, L. L.; Corcoran, T. C.; Joireman, P. W.; Felker, P. M.J . Phvs. Chem. 1990. -. 94.. 1229. ~- (14) Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M.J. Chem. Phys. 1991, 94, 4668. -
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Delay (psec) Figure 1. TRFD traces for fluorene-benzene isotopomers: (a) experimental (top) and best-fit calculated (bottom) traces for the hi& species; (b) experimental (top) and best-fit calculated (bottom) traces for the hI0-d6 species. The best-fit traces were calculated by using the 2C - A - B and A + B constants listed in the second column of Table I and by taking A - B = 5 MHz. In addition, a temperature of 5 K and a Gaussian temporal response of 32 ps fwhm were assumed.
pletion (TRFD)IS scheme of picosecond spectroscopy. The experimental apparatus has been described in previous publications.I3J4J6 Briefly, the picosecond laser source is a home-built, cavity-dumped dye laser (rhodamine 610 as dye, I-kHz repetition rate) pumped by the doubled output of a Qswitched, mode-locked C W Nd:YAG laser (Spectron). After frequency-doubling the dye laser output in @-bariumborate, the UV excitation pulses had several microjoules of energy in a bandwidth of about 2 cm-' and had pulsewidths that allowed for a temporal resolution of about 30 ps. The doubled output of the dye laser was directed through a Michelson interferometer, one of the reflectors of which was mounted on a stepper motor-driven translation stage. The output of the interferometer, consisting of linearly polarized pump and probe pulse trains (with parallel polarizations), was softly focused several millimeters downstream of a continuous free-jet expansion (about 50-pm expansion orifice) of fluorene (0.01%) and benzene (
