1900
J. Phys. Chem. B 1999, 103, 1900-1904
Photoionization and Induced Chemical Reactions of Benzotrichloride on Solution Surface Noriko Horimoto,† Fumitaka Mafune´ ,‡ and Tamotsu Kondow*,‡ Cluster Research Laboratory, Toyota Technological Institute, in East Tokyo Laboratory, Genesis Research Institute, Inc., Futamata 717-86, Ichikawa, Chiba 272-0001, Japan, and Department of Chemistry, School of Science, The UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan ReceiVed: June 1, 1998; In Final Form: October 19, 1998
An alcohol solution of benzotrichloride was introduced into vacuum as a continuous liquid flow (liquid beam) and was irradiated with nanosecond (∼5 ns) and femtosecond (∼200 ns) lasers at the wavelength of 274 nm. Ions produced by multiphoton ionization in the liquid beam and ejected from it were analyzed by a reflectron time-of-flight mass spectrometer. The ions produced were assigned to chloroalcoxybenzyl ions, C6H5CCl(OR)+, and dialkoxybenzyl ions C6H5C(OR)2+. A trace amount of C6H5CCl2+ was observed as well. The mass assignment and the structure of C6H5C(OR)2+ were confirmed by a collision-induced dissociation experiment, where the dominant fragment ion was found to be C6H5C(OH)2+. In comparison with the product ions by the nanosecond and femtosecond lasers, it is concluded that C6H5CCl(OR)+ and C6H5C(OR)2+ are produced by the reaction of the ionic precursor, C6H5CCl2+, with alcohol, ROH.
Introduction Photochemical reactions of organic halide compounds in both vapor and liquid phase under irradiation of ultraviolet light have been investigated extensively,1-9 particularly for clarifying the reaction mechanism. The results of early studies are interpreted in terms of a picture that homolytic cleavage of a carbonhalogen bond (bond homolysis) after photoexcitation preludes the photochemical reactions; that is, the radical produced from the bond homolysis behaves as the reaction precursor.1,2 Later, it becomes evident that this interpretation is not wholly satisfactory to explain the scheme of all the reactions of this kind. Photoinduced alcoholysis of alkyl iodides in an alcohol solution to give alkenes falls into the category that the radical does not necessarily play a significant role in the reaction. The reaction of this type is more characteristic of a process involving carbocations rather than radicals. It is argued that a radical pair, R•-X•, generated through the first excited state of an organic halide, RX, undergoes electron transfer to form an ion pair, R+X-, and then the carbocation, R+, thus produced reacts further with other molecules such as solvent molecules (solvolysis).2,3 Several studies also support that the bond homolysis is regarded to be the primary event for the formation of the ion pair, but the bond heterolysis is ruled out because of a marked reduction in the photosolvolysis of RX in the presence of oxygen.2 In summary, the radical and/or the ion acts as the reaction precursor of photosolvolysis involving an organic halide compound, RX:
The photoalcoholysis of benzotrichloride in an alcohol solution is a typical example that the photochemical reaction is believed to proceed through a carbocation produced by electron transfer * To whom correspondence should be addressed. † The University of Tokyo. ‡ Toyota Technological Institute.
in a radical pair, which is a decay product of photoexcited benzotrichloride; the carbocation, C6H5CCl2+, is the reaction precursor in this reaction.7,8 However, no direct evidence is provided for the involvement of the carbocation in the photoalcoholysis. In order to clarify the reaction mechanism in more detail, the radical and the carbocation should be prepared and the reaction products from these species should be detected with a less ambiguous manner. In this regard, resonance excitation to an excited state and ionization by one or more photons (multiphoton ionization) by laser is the most convenient way to study this photochemical reaction. Furthermore, the products must be detected with a high sensitivity in vacuum, especially ionic species. In order to perform this task, the liquid beammultiphoton ionization (MPI) mass spectrometry developed by ourselves10-19 was employed in this study. The liquid beam technique enables us to prepare the liquid surface in a vacuum at a pressure of 10-5-10-6 Torr. This pressure is sufficiently low that sensitive spectroscopic techniques employed for identification of chemical species in the gas phase can be utilized for studies of chemical reactions in the liquid phase. We have combined this liquid beam technique with laser multiphoton ionization and mass spectroscopy: Ions produced by multiphoton ionization on the liquid surface are found to be liberated into the vacuum and detected by a mass spectrometer. The ions carry information on how the chemical reactions proceed in the vicinity of the liquid surface. However, simple mass spectroscopy does not allow us to distinguish the reaction products generated from a radical precursor through an electronic excited state or those from an ion precursor. Namely, the radical precursor prepared through the excited state reacts with surrounding molecules and then is ionized within the same laser pulse, while the ion precursor generated by multiphoton excitation directly reacts with the surrounding molecules. In order to solve this problem, we have proposed a method to discriminate one from the other by using an ultrashort pulse laser having a pulse width of ∼200 fs (fs-laser). The ions produced by the fs-laser are free from the reaction in the excited states, because the excited molecules have no chance to be
10.1021/jp9824306 CCC: $18.00 © 1999 American Chemical Society Published on Web 03/02/1999
Reactions of Benzotrichloride on Solution Surface
J. Phys. Chem. B, Vol. 103, No. 11, 1999 1901
Figure 1. Schematic diagram of the liquid beam apparatus.
converted to the radical precursor and to react before the ionization. In contrast, the ions under irradiation of the nanosecond laser (ns-laser) can be produced by the involvement of the radical precursor resulting from decay of the excited state and/or the ionic precursor by resonant MPI without suffering the decay in the excited state. In this report, the ns- and fslasers were used to specify the reaction path by comparing the product ions under the irradiation of the ns- and the fs-lasers. A collision-induced dissociation (CID) method was employed further as an assist of the mass assignment of the product ions. In accordance with the previous studies, the present study shows that C6H5CCl2+ is the ionic precursor for preparation of the products due to the photoalcoholysis. Experimental Section Figure 1 shows a schematic diagram of the experimental apparatus, which consists of a liquid beam source and a reflectron time-of-flight mass spectrometer. Continuous laminar liquid flow (liquid beam) was introduced through an aperture with a diameter of 20 µm into a vacuum chamber downward from the top, being pressurized by a Shimadzu LC-6A pump at typically 20 atm. The flow rate was kept at 0.2 mL/min. The liquid beam was captured by being frozen on a cylindrical liquid N2 trap at 5 cm downstream from the aperture. The chamber was further evacuated by a 1200 L/s diffusion pump down to 10-5-10-6 Torr. In order to prevent the liquid beam from being frozen due to heat leaking to the liquid nitrogen trap, a rotating bar was employed to chop the liquid beam just above the surface of the cold-trap. Traveling a distance of 5 mm from the aperture, the liquid beam was crossed with a UV laser beam at the first acceleration region of the TOF mass spectrometer. A laser beam with a pulse width of ∼5 ns (ns-laser) was obtained by frequency-doubling the output of a Quanta-ray PDL-3 dye laser pumped by the third harmonics of a Quanta-ray GCR-3 Nd:YAG laser. An ultrashort pulse laser with a pulse width of ∼200 fs (fs-laser) was obtained by amplifying the output of a TSUNAMI Ti:sapphire laser. The maximum output of the laser was 25 mJ/pulse at 810 nm. The pulse width of this fundamental output was measured to be ∼150 fs by an autocorrelator. The output was frequency-tripled by use of a BBO crystal. In the present experimental setup, the pulse width of the third harmonics could not be measured. However, it is likely that the pulse width does not expand so much by frequency-tripling. The laser was focused onto the liquid beam by a lens with a focal length of 400 mm. The ions ejected from the liquid beam were accelerated by a pulsed electric field in the first acceleration region in the direction perpendicular to both the liquid and the laser beams. A delay time from the ionization to the ion extraction was varied in the range of 0-5 µs so as to improve the mass resolution.11 The ions were then steered and focused by a set of vertical and horizontal deflectors and an einzel lens. After traveling in a 1 m field free region, the ions were reversed by the reflectron tilted by 2° off the beam axis and were detected by a Murata EMS-6081B Ceratron electron multiplier. Signals from the
Figure 2. Mass spectrum of ions produced from 1.5 M benzotrichloride solutions of ethanol, 1-propanol, and 1-butanol by irradiation of a 274 nm laser. The two intense peaks (marked with + and *) show a shift with an increase in the chain length of R in ROH (R ) C2H5, C3H7, C4H9). The inset shows expansion of the m/z region, where the species containing the two different chlorine isotopes are observed.
multiplier were amplified and processed by a Yokogawa DL 1200E transient digitizer based on an NEC 9801 microcomputer. The mass resolution, m/∆m, was found to be more than 200 at m ) 150 in the present experimental condition. The identification of the product ions by the mass spectroscopic technique was further tested by collision-induced dissociation (CID) of the product ions as follows: A mass gate and a pulsed nozzle were mounted in the field free region of the TOF chamber before the reflectron. The mass gate consists of two sets of horizontal deflectors mounted serially with a gap of 2 cm between them two. A grounded metal grid is placed between the two deflectors. A pulsed electric field was applied to the mass gate, and only an ion of desire was admitted through the mass gate. The ion was allowed to collide with argon gas supplied by a pulse nozzle, and fragment ions produced by the collision were detected after the reflectron. Benzotrichloride and benzotrifluoride (Tokyo Kasei Kogyo Co., Ltd. 99% purity) were used without further purification. Results Figure 2 shows typical TOF mass spectra of ions produced from a 1.5 M solution of benzotrichloride in alcohol by irradiation of the ns-laser at 274 nm. The observed ions must be formed in the liquid beam by multiphoton ionization and ejected from it, because these ions are observed only when the laser beam irradiates the liquid beam.11 In panel a, two intense peaks appear at m/z ) 169 and 179. When the ethanol solution of benzotrichloride is replaced with a 1-propanol and a 1-butanol solution of benzotrichloride, the intense peaks shift; the peak at m/z ) 169 for the ethanol solution (panel a) shifts to m/z ) 183 for the 1-propanol solution (panel b) and to m/z ) 197 for the 1-butanol solution (panel c). The peak shift corresponds to the mass difference between ethanol and 1-propanol or between 1-propanol and 1-butanol. These results indicate that the ion with m/z ) 169 contains the alkyl moiety of the ethanol solvent. Therefore, the ion with m/z ) 169 (marked by + in panel a) is assignable to C6H5CCl(OEt)+. On the other hand, the peak at
1902 J. Phys. Chem. B, Vol. 103, No. 11, 1999
Horimoto et al.
Figure 4. Flight times of the parent and daughter ions plotted as a function of the reciprocal retarding voltage Vr. The daughter ion is assigned to C6H5C(OH)2+. Figure 3. TOF spectra of ions produced from C6H5C(OPr)2+ in collision with Ar gas. Panels a and b show the spectrum of a massselected parent ion, C6H5C(OPr)2+, and that of daughter ion (marked with an arrow), respectively. The collision energy is 2.2 keV in the laboratory frame.
m/z ) 179 (panel a) shifts by m/z ) 28 with replacement of ethanol to 1-propanol and 1-propanol to 1-butanol. It is concluded similarly that the ion with m/z ) 179 contains two alkyl moieties of the ethanol solvent. The peaks marked with * in panel a are assigned to C6H5C(OEt)2+, and those marked with + and * in panels b and c to C6H5CCl(OR)+ and C6H5C(OR)2+, respectively, where R represents propyl and butyl groups, respectively. The mass assignment is consistent with the natural isotope abundance ratio of 35Cl to 37Cl (see the inset of panel a); the peaks at m/z ) 169 and 171 are assigned to C6H5C35Cl(OEt)+ and C6H5C37Cl(OEt)+, respectively. Furthermore, a CID experiment was performed to attest the mass assignment mentioned above. The ion assignable to C6H5C(OPr)2+ was admitted to the TOF tube through the mass gate and was allowed to collide with Ar gas. Figure 3 shows typical mass spectra after the mass gate when Ar gas was introduced (panel a) and was not (panel b). As shown in panel a, a peak at the flight time of about 53 µs arises from the collisional dissociation of the incoming parent ion. The flight times, t, of the parent and daughter ions were measured as a function of a retarding voltage, Vr, applied at the reflectron. The parent and daughter ions have the same velocity before entering the reflectron and after leaving it, so that t is given as
m t ) t0 + C Vr
(2)
where t0 is the total flight time outside the reflectron, m is the mass of the ion of interest, and C is the coefficient: Equation 2 shows that the flight time increases linearly with the reciprocal of the retarding voltage, 1/Vr, and the slope is proportional to the mass of the ion. Figure 4 shows that the flight times of the parent ion, C6H5C(OPr)2+, and the daughter ion increase linearly with 1/Vr. The daughter ion was estimated to have m/z ) 123 by the comparison of the slope of the solid line for the daughter ion (open circles) with that for the parent ion (solid circles) on the premise that the parent ion has m/z ) 207: The m/z value thus obtained agrees with that obtained by the TOF mass
Figure 5. The intensities of C6H5CCl(OEt)+ and C6H5C(OEt)2+ plotted as a function of the incident laser power in panels a and b, respectively. An eye guide is given by the solid lines.
spectroscopy. The daughter ion should be assigned to C6H5C(OH)2+. Formation of C6H5C(OH)2+ supports that the parent ion is C6H5C(OR)2+, as is the case of C6H5C(OH)2+ formation by electron impact on C6H5C(OR)2+.21 Figure 5 shows the intensities of C6H5CCl(OPr)+ and C6H5C(OPr)2+ as a function of the incident laser power (0-200 µJ/ pulse) when a solution of benzotrichloride in 1-propanol is irradiated by the ns-laser at 274 nm, where the solid curves show the calculations based on the Coulomb ejection model14 together with two-photon excitation. As the laser power increases, the intensity of C6H5CCl(OPr)+ increases similarly to those of C6H5C(OPr)2+. Figure 6 shows a typical mass spectrum of ions ejected from a 1.5 M benzotrichloride solution in benzene by ns-laser irradiation at 274 nm. The most intense peak at m/z ) 159 is assignable to C6H5CCl2+. In addition, the peaks assignable to C6H6+, C6H5CC6H5+, and C6H5CClC6H5+(C6H6) are observed. The mass assignment of C6H5CCl2+ is supported by the intensity ratio of the ions containing 35Cl to 37Cl (see inset). Figure 7 shows a mass spectrum of ions ejected from a 1.5 M benzotrifluoride solution in 1-butanol under irradiation of the ns-laser at 266 nm. The peaks at m/z ) 181 and 235 are assignable to C6H5CF(OBu)+ and C6H5C(OBu)2+, respectively. The peak assignable to C6H5CF2+ is also observed in the spectrum, along with a series of peaks (m/z ) 75, 149, 223, 297, ...) assignable to protonated 1-butanol cluster ions, H+(BuOH)n.
Reactions of Benzotrichloride on Solution Surface
J. Phys. Chem. B, Vol. 103, No. 11, 1999 1903 SCHEME 1
Figure 6. Mass spectrum of ions produced from a 1.5 M benzotrichloride-benzene solution under irradiation of a 274 nm laser. The peak at m/z ) 159 is assigned as C6H5CCl2+. The inset shows expansion of the m/z region, where the species with the different chlorine isotopes are given.
benzotrichloride in the liquid beam. Note that the IP of benzotrichloride in the gas phase is 9.5 eV.20 Three photons are needed for photoionization of a benzotrichloride molecule in the gas phase. No detection of ions under irradiation of the ns-laser implies that three-photon ionization scarcely takes place in the fluence range of the ns-laser used. It is well-known that IP of a molecule on a liquid surface is lower by 0.6-1.4 eV than that in the gas phase,22 due to stabilization of the molecule on the liquid surface by a polarization field built by its surrounding molecules. The energy of the polarization, P, is given by the Born equation as
P)
Figure 7. Mass spectrum of ions produced from a 1.5 M benzotrifluoride solution of 1-butanol under irradiation of a 266 nm laser. The precursor ion, C6H5CF2+, is observed in a moderate amount. A series of cluster ions, H+(BuOH)n, appear in the spectrum.
Figure 8. Mass spectrum of ions produced from a 1.5 M benzotrichloride-1-propanol solution under irradiation of a 270 nm laser having a pulse width of 200 fs (FWHM). The ions appearing in a mass spectrum for the ns-laser at 274 nm are also observed in this spectrum.
Figure 8 shows a mass spectrum of ions ejected from a 1.5 M benzotrichloride solution in 1-propanol by irradiation of the fs-laser at 270 nm. The ions, C6H5CCl(OPr)+ and C6H5C(OPr)2+, appearing in a mass spectrum by irradiation of the nslaser at 274 nm are also observed in the mass spectrum for the fs-laser. Discussion Two-Photon Ionization of Benzotrichloride in Liquid Beam. The dependence of the ion intensities on the laser power shows that benzotrichloride molecules are ionized by two-photon absorption in the liquid beam. This result shows that the energy of two photons at 274 nm (9.0 eV) exceeds the ionization potential (IP) of benzotrichloride in the liquid beam, and the ionic species observed are produced by the ionization of
[ ]
-e2 1 18π0R r
(3)
where R is the radius of the positive ion, e is the electronic charge, 0 is the permittivity of the vacuum, and r is the relative dielectric constant of the liquid. The effective radius of a positive ion on a liquid surface is reported to be reduced by ∼60% of that of its neutral.23 The polarization energy of a 1-propanol molecule on the surface of liquid 1-propanol is determined to be 0.73 eV by Faubel et al.,24 giving rise to the effective dielectric constant of 1.2. Therefore, a trichlorobenzene molecule on the surface of liquid 1-propanol should have the polarization energy of 0.6 eV. This estimated polarization energy is consistent with the finding that the IP of a benzotrichloride molecule is lowered by more than 0.5 eV when it is placed on the surface of a 1-propanol liquid. Formation of C6H5CCl2+. Under irradiation of the ns-laser on the liquid beam, a trace amount of C6H5CCl2+ is produced instead of C6H5CCl3+, while the fs-laser excitation gives C6H5CCl2+, but not C6H5CCl3+. The same phenomenon is observed under irradiation of the fs-laser on molecule in the gas phase, where no solvent molecule exists. These findings lead us to conclude that dissociative photoionization occurs in the solution as
C6H5CCl3 + hν f C6H5CCl3+ f C6H5CCl2+ + Cl (4) Ion-Molecule Reaction in Alcohol Solution. As discussed previously, C6H5CCl2+ produced by two-photon ionization acts as the precursor of the reaction, where the positively charged carbon atom of the precursor reacts with a neighboring alcohol molecule. This conjecture is supported by the finding that only a trace amount of C6H5CCl2+ is present in the alcohol solutions but is abundant in its benzene solution; the precursor reacts much more readily with alcohol than benzene. As shown in Scheme 1, the positively charged carbon atom of the precursor ion reacts with one solvent alcohol molecule for the C6H5CCl(OR)+ formation and with two solvent molecules for the C6H5C(OR)2+ formation. These ions are produced from the same precursor, C6H5CCl2+, because their intensities depend almost identically on the laser power (see Figure 5). The reaction could proceed through the excited state of the parent molecule, as observed in the photochemical reaction in a haloaniline solution in alcohol.19 In this scheme, C6H5CCl3 is raised to the first excited state by one-photon absorption and then dissociates into C6H5CCl2 and a Cl atom. The radical,
1904 J. Phys. Chem. B, Vol. 103, No. 11, 1999 C6H5CCl2, reacts with one and two alcohol molecules to form C6H5CCl(OR) and C6H5C(OR)2, respectively. These products further absorb one more photon in the duration of the same ns-laser pulse and are ionized into C6H5CCl(OR)+ and C6H5C(OR)2+. However, this scheme should be ruled out because the ions produced by irradiation of the ns-laser are almost identical with those by the fs-laser. Under the irradiation of the fs-laser, even if such a reaction takes place, the product radicals, C6H5CCl(OR) and C6H5C(OR)2, should not have chance to absorb another photon for the ionization. The duration of the fs-laser is regarded to be too short. Actually, simple intracluster exchange reactions proceed in a time regime of several picoseconds, which is much longer than the pulse duration (∼200 fs).26,27 This fact indicates that the reaction proceeds through the ionic precursor, C6H5CCl2+, but not through molecular rearrangements before ionization. The dialkoxybenzyl cation, C6H5C(OR)2+, is stable in a solution in the presence of a counter anion such as BF4-,28 since the positively charged carbon of C6H5C(OR)2+ is stabilized by the electron-donating functional groups, C6H6 and OR. The ion is attacked by the nucleophilic reagent, Nu, and C6H5C(OR)2+‚ Nu or C6H5COOR, are produced.29 As mentioned above, C6H5CCl(OR)+ and C6H5C(OR)2+ are produced selectively in the present experiment. Therefore, photoionization of benzotrichloride in an alcohol solution can be a novel method to prepare C6H5C(OR)2+. Reaction in Benzene Solution. A benzene molecule is considered to be less reactive with C6H5CCl2+ than an alcohol molecule due to the less nucleophilicity of benzene, so that the reaction intermediate is likely to survive in the solution. Actually, C6H5CCl2+ is the dominant ionic species, as shown in the mass spectrum (see Figure 6). In addition, a small amount of C6H5CClC6H5+ is observed. This finding implies that the precursor, C6H5CCl2+, reacts with a benzene molecule and C6H5CClC6H5+ is formed through the attacking of a π-electron of the benzene molecule at the positively charged carbon atom of C6H5CCl2+. Effect of Halogen Atom in the Reaction. In an alcohol solution of benzotrifluoride, almost identical reactions occur and corresponding product ions, such as C6H5CF(OR)+ and C6H5C(OR)2+ are produced. As C6H5CF2+ is observed in the spectrum, benzotrifluoride seems to be less reactive than benzotrichloride in an alcohol solution. In fact, it has been reported that benzotrifluoride hardly reacts under irradiation of a UV light.7,25 In addition, protonated alcohol cluster ions, H+(ROH)n, are observed in the spectrum. The formation mechanism of these ions has not been elucidated, but benzotrifluoride must be associated with the ion formation, because the ion intensities of H+(ROH)n increase with the concentration of benzotrifluoride in the alcohol solution. Conclusion Ion-molecule reactions are found to proceed on the surface of alcohol solutions of benzotrichloride and benzotrifluoride by
Horimoto et al. multiphoton excitation in nano- and femtosecond time regimes. The dialkoxybenzyl ion is produced by substitution of dichlorobenzyl ion with alcohol. It was revealed that the dichlorobenzyl ion is the reaction precursor in the formation of the dialkoxybenzyl ion. Acknowledgment. The authors are grateful to Mr. Y. Hashimoto for the helpful discussion about the reaction mechanism. This work is undertaken under the Cluster Project of the Genesis Research Institute, Inc. and the International Joint Research Program of NEDO. References and Notes (1) Majer, J. R.; Simons, J. P. AdV. Photochem. 1964, 2, 137. (2) Gilbert, A.; Baggot, J. Essentials of Molecular Photochemistry; Blackwell Scientific Publications: London, 1991. (3) Kropp, P. J. Acc. Chem. Res. 1984, 17, 131. (4) Bunnett, J. F. Acc. Chem. Res. 1978, 11, 413. (5) Havinga, E.; Cornelisse, J. Pure Appl. Chem. 1976, 47, 1. (6) Appleton, D. C.; Brocklehurst, B.; McKenna, J.; Mckenna, J. M.; Thackeray, S.; Walley, A. R. J. C. S. Perkin Trans. 2 1980, 87. (7) Ishigami, T.; Kinoshita, Y.; Sugimori, A. Chem. Lett. 1974, 149. (8) Izawa, Y.; Tomioka, H.; Natsume, M.; Beppu, S.; Tsuji, H. J. Org. Chem. 1980, 45, 4835. (9) Izawa, Y.; Watoh, Y.; Tomioka, H. Chem. Lett. 1984, 33. (10) Mafune´, F.;Takeda, Y.; Nagata, T; Kondow, T. Chem. Phys. Lett. 1992, 199, 615. (11) Mafune´, F.; Kohno, J.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 7. (12) Mafune´, F.; Takeda, Y.; Nagata, T.; Kondow, T. Chem. Phys. Lett. 1994, 218, 234. (13) Kohno, J.; Mafune´, F.; Kondow, T. J. Am. Chem. Soc. 1994, 116, 9801. (14) Mafune´, F.; Kohno, J.; Kondow, T. J. Chin. Chem. Soc. 1995, 42, 449. (15) Matsumura, H.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1995, 99, 5861. (16) Mafune´, F.; Hashimoto, Y.; Hashimoto, M.; Kondow, T. J. Phys. Chem. 1995, 99, 13814. (17) Kohno, J.; Horimoto, N.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1995, 99, 15627. (18) Horimoto, N.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1996, 100, 10046. (19) Hashimoto, Y.; Mafune´, F.; Kondow, T. J. Phys. Chem. 1998, 102, 4295. (20) Peel, J. B.; Nagy-Felsobuki, E. Aust. J. Chem. 1987, 40, 751. (21) (a) Budzikiewicz, H.; Djerassi, C.; Williams, D. H. Mass Spectroscopy of Organic Compounds; Holden-Day: San Francisco, 1967. (b) Schroll, G.; Jakobsen, H. J.; Lawesson, S.; Brown, P.; Djerassi, C. Ark. Kemi 1967, 26, 279. (22) Nemec, L.; Gaehrs, H. J.; Chia, L.; Delahay, P. J. Chem. Phys. 1977, 66, 4450. (23) Lacmann, K.; Koizumi, H.; Schmidt, W. F. In Linking the Gaseous and Condensed Phases of Matter; Christophorou, L. G., Illenberger, E., Schmidt, F., Eds.; Plenum Press: New York, 1994; p 525. (24) Faubel, M.; Steiner, B.; Toennies, J. P. J. Chem. Phys. 1997, 106, 9013. (25) Grinter, R.; Heilbronner, E.; Petrzilka, T.; Seiler, P. Tetrahedron Lett. 1968, 35, 3845. (26) Gruebele, M.; Sims, I. R.; Potter, E. D.; Zewail, A. H. J. Chem. Phys. 1991, 95, 7763. (27) Zhong, D.; Ahmad, S.; Cheng, P. Y.; Zewail, A. H. J. Am. Chem. Soc. 1997, 119, 2305. (28) Pindur, U.; Flo, C. Synth. Commun. 1989, 19, 2307. (29) Pindur, U.; Mu¨ller, J.; Flo, C.; Witzel, H. Chem. Soc. ReV. 1987, 16, 75.
