J . Phys. Chem. 1989, 93, 4005-4009 Appendix We attach the wavenumbers and integrated intensities for three tautomers of cytosine calculated with the 3-21G basis set (Table V). These data are complementary to the spectra presented in Figure 2. Analogical calculations have been recently performed by SzczqSniak et a1.I2 for the oxo-amino (1) and hydroxy-amino
4005
(3) tautomers. We noticed small differences both in wavenumbers and intensities which probably come from small differences between the optimal geometries obtained in the Present Paper and the quoted work. Registry No. Cytosine, 71-30-7.
Photochemistry and Photophysics of Small Heterocyclic Molecules. 2. CW COP Laser Inducement of the Bimolecular Reaction of Thiirane John T. Campbell and Joseph J. BelBruno* Department of Chemistry, Dartmouth College, Hanouer, New Hampshire 03755 (Received: January 20, 1988; In Final Form: December 13, 1988)
The gas-phase IR laser photolysis of ethylene sulfide has been examined in the presence of SF, as a sensitizer molecule. Reaction does not occur in the absence of the sensitizing agent or at laser frequencies not absorbed by both thiirane and SF,. Surprisingly, ethylene, methane, and carbon disulfide are the resultant gaseous products. Pyrolysis and conventional UV photochemistry do not yield the latter two species, and S2, a major product in conventional photochemistry and thermochemistry, is not observed at all. The dependence of the photolysis on laser fluence, gas (total and/or sensitizer) pressure, and laser wavelength is examined. The data are consistent with a mechanism involving the reaction of highly vibrationally excited thiirane molecules (produced by a combination of collisional and radiative processes) via an intermediate S(lD) species. The thiirane system appears to be unique in that CW C 0 2 laser enhancement of bimolecular chemistry is not readily observed in organic molecules.
Introduction The use of pulsed C 0 2 lasers in the vibrational enhancement of unimolecular processes is well-known.' Less well studied is the application of either pulsed or C W infrared lasers to bimolecular chemistry. This is in stark contrast to much of the published work on UV/vis laser enhancement2 in which electronic excitation of one of the reactants in a bimolecular reaction is observed to greatly augment the reaction rate. One exception to this generalization is the use of C 0 2 lasers in the realm of sensitized bimolecular p h o t ~ c h e m i s t r y . ~Both *~ pulsed and C W lasers have been employed in previously reported studies, although our concern in this report is only with the C W studies. Tardieu de Maleissye et aL5 have reported on the SF6-sensitized reaction of ethane, and in a series of related experiments,b8 Pola has employed SF6 sensitization to the decomposition of small fluoro-substituted molecules. While these experiments were successful in driving a chemical reaction, the use of sensitizers in IR laser photochemistry typically results in a loss of molecular specificity in the deposition of the laser energy. The chemistry often becomes indistinguishable from that of (homogeneous) high-temperature p y r o l y ~ i s . ~ ( I ) See, for example: Bagratashvili, V. N.; Letokhov, V. S.; Makarov, A. A.; Ryabov, E. A. Multiple Photon Infrared Laser Photophysics and Photochemistry; Harwood Academic: Amsterdam, 1985. (2) Fontijn, A., Clyne, M. A. A,, Eds. Reactions of Small Transient Species; Academic Press: London, 1983. (3) Danen, W. C.; Jang, J. C. In Laser-Induced Chemical Processes; Steinfeld, J. I., Ed.; Plenum Press: New York, 1984. (4) The use of CW lasers in IR laser induced unimolecular chemistry has been examined in a series of papers. See, for example: Zitter, R. N.; Koster, D. F.; Parvez, M. S. Opr. Commun. 1986, 59, 259. Zitter, R. N.; Koster, D. F.; Ringwelski, A,; Cantoni, A. Appl. Phys. 1983, 830, 19. Zitter, R. N.; Koster, D. F.; Ringwelski, A.; Cantoni, A. Appl. Phys. 1983, 830, 79, and references therein. (5) (a) Tardieu de Maleissye, J.; Lempereur, F.; Marsal, C.; Ben-Aim, R. I. Chem. Phys. Letr. 1976, 42, 46; (b) Tardieu de Maleissye, J.; Lalo, C.; Lempereur, F.; Masanet, J. J . Phys. Chem. 1987, 91, 5899. (6) Pola, J.; Engst, P.; Horak, M. Collect. Czech. Chem. Commun.1981, 46, 1254. (7) Pola, J. Collect. Czech. Chem. Commun. 1981, 46, 2854. ( 8 ) Pola, J. Collect. Czech. Chem. Commun. 1981, 46, 2860.
0022-3654/89/2093-4005$01.50/0
Previous attempts to induce bimolecular reactions via IR excitation have involved both pulsed and C W C 0 2 laser systems. Birely and LymanIo were successful in their utilization of the technique, but the experiments were restricted to di- and triatomic inorganic species and pulsed lasers. The addition of vibrationally excited hydrogen halides to chloro- and fluorocarbonsI1J2 was attempted with high-energy pulsed lasers, but no laser enhancement of the process was observed. The failure of these attempts at enhancement of the bimolecular chemistry was attributed to efficient energy transfer successfully competing with the reaction. The net effect was simple thermal heating of the gas sample. Lenzi et al.I3 have examined the V-T process in SF,/Ar mixtures and have shown that, with a pulsed laser, nonthermal vibrational distributions may be accessed during the laser pulse. This type of nonequilibrium reaction was employed by Mele et al. and Chin et in the pulsed laser sensitized decomposition of cyclohexene and UF,, respectively. Finally, Guckert and C a d s report on the product selectivity in the pulsed photolysis of trans-2-butene. The selectivity in this experiment was attributed to secondary photolysis of products formed from the decomposition of the initial reactant. Laser enhancement of the SN2reaction between NH, and CH,Br to produce an ion-pair product was unsuc~essful,~ but this type of product is not well characterized and the lack of detected product may be related to the chemistry rather than any competitive process. Two successes have been reported in the literature with CW excitation. Continuous-wave irradiation was employed in the addition of C1 radicals to the CH2D2molecule resulting in the formation of methylene chloride.', This process was (9) Shaub, W. M.; Bauer, S. H. Int. J . Chem. Kinet. 1975. 7, 509. (10) Birely, J. H.; Lyman, J. L. J . Photochem. 1975, 4, 269. (11) Herman, I. P.; Marling, J. B. J . Chem. Phys. 1979, 71, 643. (12) Douglas, D. J.; Moore, C . B. In Laser-Induced Processes in Molecules: Physics and Chemistry; Kompa, K. L., Smith, S . D., Eds.; SpringerVerlag: New York, 1979. (13) Lenzi, M.; Molinari, E.; Piciacchia, G.; Sessa, V.; Terranova, M. L. Chem. Phys. 1986, 108, 167. (14) Mele, A,; Salvetti, F.; Molinari, E.; Terranova, M. L. J . Phorochem. 1986, 32, 265. Chin, C.; Hou, H.; Bao. Y.; Li, T. Chem. Phys. Lett. 1983, 101, 69. (15) Guckert, J . R.; Carr, R. W. J . Phys. Chem. 1986, 90, 5679.
0 1989 American Chemical Society
4006
The Journal of Physical Chemistry, Vol. 93, No. 10, 1989
successful in the presence of large quantities of Ar buffer gas, and although the authors were able to enhance other analogous reactions, the process was successful only for the addition of atomic reactants to hydrocarbons. Bomse and Beauchampl' have reported a 1000-fold enhancement of an ion-molecule reaction using relatively low-power C W C 0 2 radiation and speculated on the practicality of using changes in reaction rates as spectroscopic probes. Two suggestions have been put forth for improving the prospects of IR laser-induced bimolecular ~ h e m i s t r y : (~1 ) simultaneous irradiation of both of the reactants and (2) the excitation of translational, as well as internal, modes to permit passage over the reaction barrier. The former has been successful, as was recently reported by Keehn and c o - ~ o r k e r s l * for * ' ~ hexafluorobenzene/silane mixtures using pulsed irradiation. The second suggestion has not been implemented3 for organic molecules. We report the SF,-sensitized reaction of ethylene sulfide to yield CS2, CH4, and C2H4 Both the sensitizer3 and the reactant moleculeZo have significant absorption coefficients at the P(20) line of the C 0 2 laser (944 cm-I). Therefore, irradiation of a cell containing both SF6 and thiirane causes direct excitation of both molecules. The rapid equilibration of the sample mixture via the many collisions occurring during the course of irradiation results in the efficient production of vibrationally excited thiirane via resonant V - v (SF6/thiirane) energy-transfer collisions. One now has a system involving a vibrationally hot organic molecule that may then absorb additional laser photons, leading to the formation of products not readily accessible by thermal activation alone. This report describes the effect of added buffer gas, sensitizer pressure, laser fluence, and the laser frequency on the chemistry of thiirane. A mechanism consistent with the observed kinetics and known sulfur chemistry is proposed.
Experimental Section An Apollo Lasers, Inc., Section Model 122 C 0 2 laser was employed in these studies. The beam, with an approximately 8-mm diameter, was passed unfocused through the photolysis cell. For most of the reported experiments, the laser was tuned to the P(20) line, although photolysis at other frequencies was attempted and noted in the Results. The laser output was measured with a Laser Precision Corp. Model RJP-735 pyroelectric detector and ranged from 3 to 45 W. The laser may be operated in an electronically chopped mode such that a single cycle of the output is obtained. In this mode, the irradiation time is adjustable and was determined with an Analogic Data 6000 transient recorder. The exposure was one of the experimental variables and was adjusted to values between 0.1 and 1 s, as noted. The photolysis cells were constructed of 3 16-type stainless steel and fitted with NaCl windows for both photolysis and analysis. The data reported in the table and figures were collected with a cell IO cm in length and 5 cm in diameter. However, a cell of similar diameter and 2.5 cm in length was used to check on the length of the photochemically active region. Details are presented in the Results section of this report. The cells were evacuated by rotary pumps prior to sample introduction and were filled from stainless steel vacuum lines. Thiirane was purchased from Aldrich Chemical Co. and distilled prior to use. The sensitizer (SF,) and background gas (N2) were purchased from Matheson Gas Products and Airco, respectively, and used as received, without further purification. Sample pressures were measured via a McLeod gauge (thiirane) or a Bourdon tube gauge (background gases). All reactions were run with a single exposure to the laser output. In order to minimize the effects of secondary processes, the length of the photolysis time was restricted (the standard exposure was (16) Hsu. D. S. Y.; Manuccia, T.J. A p p l . Phys. Lett. 1978, 33, 915. ( 1 7 ) Bomse, D. S.; Beauchamp, J . L. J . Am. Chem. SOC.1980,102, 3967. (18) Koga, Y.; Serino, R . M.; Chen, R.: Keehn, P. M. J . Phys. Chem. 1987, 91, 298. (19) Koga, Y . ;Serino, R. M ; Chen, R.; Keehn, P. M. J . Phys. Chem. 1987, 91, 306. (20) Thompson, H. W . ; Cave, W . T. Trans. Faraday Soc. 1951, 47, 950.
Campbell and BelBruno I
C M-'
Figure 1. FTIR spectra of the mixture of SF, and thiirane: (a) after and (b) prior to irradiation at 944 cm-'. The reactants and products are identified in both spectra. The fraction of SF, is quite large, and its absorption is saturated. In spectrum b, open arrows indicate SF, bands, while thiirane absorptions are noted by filled arrows.
0.73 s) so that only small yields (
