J . Phys. Chem. 1990, 94, 1032-1037
7032
hydroxy
N1 H-oXO
N~H-oxo
Figure 7. SCF/3-21 G* optimized geometries of three tautomeric forms of S-methylated 2-thiouracil: hydroxy (A), N,H-oxo (B), and N3H-oxo (C).
terize the cytosine, isocytosine, and S-methylated 4-thiouracil vapor. A nearly equimolar hydroxy:oxo mixture should exist in the gas-phase 4-hydroxypyrimidine and S-methylated 2-thiouracil. In the polar environment, however, the hydroxy-oxo tautomeric equilibrium should be strongly shifted toward the oxo form. We do not expect the hydroxy tautomeric form to be detectable in the polar (e.g., aqueous) solutions. Another tautomeric rearrangement corresponding to the proton transfer between the endocylic nitrogen atoms (N( 1)-H to N(3)-H and vice versa) appears to be highly unfavorable in the gas phase. On the other hand, such a process can easily occur in the polar solvents due to an additional strong (mostly electrostatic) solvent stabilization of the energetically unfavored tautomeric form. We may expect that by means of the variation of the environment polarity one may enforce the presence (and the pre-
dominance) of a particular tautomeric form. This, in consequence, may influence the rate of the chemical exchange of the mobile proton on the electrophilic reagent (e.g., the methyl group). Acknowledgment. This study was supported by an institutional grant from the National Cancer Institute and by a Biomedical Research Support grant provided by The University of Arizona. We are greatly indebted to Drs. H. Rostkowska, K. Szczepaniak, M. J. Nowak, J. Leszczynski, K. KuBulat, and W. B. Person for sending us their paper (ref 72) prior to its publication. Appendix
The optimized SCF/3-21G* geometries for isocytosine and 2and 4-(thiomethyl)uracil are found in Figures 5-7 and Tables XII-XIV.
Comparative SO2 Infrared Spectra: Type I and I 1 Clathrate Hydrate Films, Large Gas-Phase Clusters, and Anhydrous Crystalline Films Fouad Fleyfel, Hugh H. Richardson,+ and J. Paul Devlin* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: April 6, 1990)
The mechanism by which SO2is incorporated into microparticles of ice in the vapor phase is receiving special interest because of the unexpectedly high efficiency with which SO2 is scavenged by ice crystals. A possible explanation of this efficiency might be found in the tendency for small polar molecules, such as the small ring ethers, to form clathrate hydrates at low temperatures and low partial pressures. This possibility has been examined by spectroscopic studies at 120 K of large gas-phase clusters formed from anhydrous SO2and H20-S02 mixtures with a ratio appropriate for clathrate hydrate formation. On the basis of a comparison with new thin-film infrared data for the simple type I SO2hydrate, the mixed SO,-ethylene oxide type 1 hydrate and the type I1 double hydrate with tetrahydrofuran, it is apparent that SO2is not enclathrated under the conditions that are known to cause formation of clathrate hydrate crystalline clusters of ethylene oxide, trimethylene oxide, and tetrahydrofuran. This indication, that the SO2clathrate hydrate, although stable once formed, grows relatively slowly, reduces the likelihood that SO2enclathration is the basis for its large uptake in ice crystallites. The nature of the anhydrous-SO, cluster spectra, together with existing data for anhydrous-C02clusters, prompted an examination of large cluster spectra from a macroscopic dielectric approach. A remarkable similarity of cluster spectra with spectra of thin films, at off-normal incidence, has revealed a close relationship between the cluster and thin film spectra.
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I. Introduction Sulfur dioxide is an important atmospheric species of natural and anthropogenic origin that is generally believed to be implicated in the formation of acid rain. As such, the form and mechanism by which SO2is incorporated into icy crystals and water droplets is of particular interest. A recent study has revealed that a surprisingly high concentration of SO2is captured and deposited 'Present address: Department of Chemistry, Ohio University, Athens, OH.
in ice particles formed within cold chambers containing a few parts per million of SO2.' Since the scavenging of SO2exceeded that anticipated, in view of the tendency of solutes to be excluded from ice grown in solutions, a model was offered on the basis of the enrichment of SO2in an aqueous surface film of the growing ice particles. The existence of such a liquid film a t the surface of ( 1 ) Valdez, M. P.; Dawson, G . A.; Bales, R. C. J . Geophys. Res. 1987.92,
9789.
0022-3654 l9012094-7032%02.50/0 0 1990 American Chemical Societv
Comparative SO2 Infrared Spectra ice, particularly for the temperatures of the SOz uptake experiments (258 K), is supported by significant, if not irrefutable, evidence.2 However, since SO2 is known to form a structure I clathrate hydrate (CH),3 it was also speculated that the high SO2 content might reflect enclathration of the sulfur dioxide, despite the moderately high temperatures and low partial pressures used in sampling. Through a series of FT-IR spectroscopic measurements of icy gas-phase clusters, it has been established that certain molecules do readily form CH microparticles at temperatures not uncommon in the upper a t m ~ s p h e r e . ~For example, it has been shown that trimethylene oxide, present in one part per thousand in a moist mixture with nitrogen gas at -200 K, exists largely as the guest molecule of the type I and/or type I1 clathrate hydrate. These observations, combined with evidence that icy thin films react with ambient air molecules at 1 atm and 200 K to produce stable C H S , ~ has prompted this investigation of the relative ease with which sulfur dioxide forms large gas-phase C H clusters. The recognition of the structural phase(s) assumed by molecular species under conditions of cluster formation, from the cluster FT-IR spectra, requires both extensive information about the infrared spectra of the particular species in various phases and some insight to the unique influence the cluster nature of a sample has on the spectrum of a particular phase. Sufficient information has accumulated for a variety of large gas-phase clusters to allow generalizations about comparative features of the spectra of clusters and of the corresponding "bulk" samples. For examples, it has been established that the infrared bands of molecular vibrations having large oscillator strengths are markedly different for pure crystalline gas-phase clusters than for the corresponding bulk crystal sample^.^*^-* Intermolecular coupling through the vibrational transition dipoles produce polar modes within large clusters that absorb strongly at frequencies significantly blueshifted from the usual transverse mode band positions. This effect has been reported for C02,8 HCI, ethylene oxide (EO), trimethylene oxide, and tetrahydrofuran (THF)! By contrast, modes of weak oscillator strength give rise to bands that are nearly superimposable on the features measured for crystalline films. Further, the decoupling of vibrations of large oscillator strength, as occurs for dilute isotopomeric species or for guest molecules separated (uniformly) within CHs, results in cluster spectra that match the "bulk" thin-film specta. The recognition of the phase(s) assumed by SO2clusters, from the FT-IR gas-phase cluster spectra reported here, requires a comparison with known spectra. Although the SO2 type I C H was characterized as early as 1949? and a vibrational spectrum was subsequently reported,I0 that spectrum was later reasigned to an amorphous mixture.)' Therefore, particular attention will be directed to new thin-film infrared spectra of the SO2 type I hydrate, the mixed hydrate with EO, and the structure I1 double C H with tetrahydrofuran. Spectra of crystalline films of anhydrous SO2,which have previously been reported,'&12 have been redetermined as a function of film thickness and angle of incidence. An investigation of this angle dependence has been prompted by the recognition that a relationship should exist between the Berreman effect for off-normal-incident sampling of ultrathin filmsI4 and the spectra observed for gas-phase cluster^.^ In this (2) Nenow, D. In Progress in Crystal Growth and Character, Pamplin, B. R.. Elwell, D., Eds.; Pergamon: New York, 1984; Vol. 9, p 185. (3) Davidson, D. W. In Wafer,a Comprehensive Treatise; Franks, F., Ed.; Plenum: New York. 1973; Vol. 2, Chapter 2. (4) Fleyfel, F.; Devlin, J. P. J . Cfiem. Pfiys. 1990, 92, 36. (5) Mayer, E.; Hallbrucker, A. J . Cfiem. SOC.1989, 12, 749. (6) Cardini, G.; Schettino, V.; Klein, M. L. J . Cfiem.Pfiys. 1989, 90,4441. (7) Barnes, J. A,; Gough, T. E. J. Cfiem. Pfiys. 1987,86, 6012. (8) Fleyfel, F.; Devlin, J. P. J. Pfiys. Cfiem. 1989, 93, 7292. (9) Stackelberg, M. V.; Muller, H. R. Z . Elekrrocfiem. 1954, 58, 25. (IO) Harvey, K. B.; McCourt, F. R.; Shurvell, H. F. Can. J. Cfiem.1964, 42. 960. ( 1 1 ) Hardin, A. H.; Harvey, K. B. Can. J . Cfiem. 1971, 49, 4114. (12) Giguere. P. A,; Falk, M. Con. J . Cfiem. 1956, 34, 1833. (13) Wiener, R. N.; Nixon. E. R. J . Cfiem. Pfiys. 1956, 25, 175. (14) Berreman, D. W. Pfiys. Rev. 1963, 6, 2193.
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7033 context, thin-film spectra for C02(s) will also be reexamined in an effort to establish a qualitative relationship between the published C02cluster spectra and expectations from macroscopic dielectric theory. 11. Experimental Section
Ultrathin films (
