Nonplanarity of Tetrafluorocyclobutadiene E. James Petersson, †,‡ Jason C. Fanuele, † Mark R. Nimlos, § David M. Lemal, † G. Barney Ellison, ‡ and J. George Radziszewski* ,§ National Renewable Energy Laboratory (NREL) 1617 Cole BlVd., Golden, Colorado 80401 Department of Chemistry, Dartmouth College HanoVer, New Hampshire 03755 Department of Chemistry & Biochemistry UniVersity of Colorado, Boulder, Colorado 80309 ReceiVed June 10, 1997 The examination of tetrafluorocyclobutadiene, C 4 F 4 (1), illuminates striking differences between fluorocarbons and their hydrocarbon analogues. 1,2 While cyclobutadiene, C 4 H 4 , has been studied extensively, 3-6 due to interest in its antiaromaticity, so far only indirect evidence for the formation of 1 has been provided. 7 Analysis of the IR spectrum of C 4 F 4 leads us to believe that this unstable molecule is nonplanar. This result, though not anticipated by any treatment of C 4 F 4 as a mere analog of cyclo- butadiene, is reminiscent of the nonplanarity of the calculated structure for the perfluoroallyl radical. 8 We observe the IR spectrum of 1 (Figure 1, center) when we irradiate tetrafluorocyclobutene-3,4-dicarboxylic anhydride (2) 7a isolated in an Ar matrix at 12 K with a 248 nm light from KrF excimer laser. In addition to C 4 F 4 , CO 2 and CO are formed. The C 4 F 4 was also generated, with lower yield, by irradiation of cis- or trans-tetrafluoro-3,4-diiodocyclobutene. 9 As for an IR spectrum of C 4 F 4 , we identify a set of peaks which (a) grows in at the same rate on irradiation of 2, (b) is produced at the same rate that the precursor is destroyed, and (c) disappears at the same rate when 1 is destroyed by prolonged irradiation. The photochemical transformation of 2 to 1 is very clean, with only a trace amount of the perfluorocyclopenta- dienone 7c detected. The equimolar concentrations of CO 2 , CO, and C 4 F 4 in the matrix allow us to estimate the absolute intensities of the C 4 F 4 absorptions. 11 Partial bleaching of the C 4 F 4 by linearly polarized 248 nm light leads to partial photoorientation of 1. Polarization measurements indicate that electronic absorption 12 at 248 nm proceeds along the C 2 axis, and the three a u vibrations show negative dichroism in the IR; for each of them absorption along the photoselection axis Z (E Z ) is smaller than along direction Y (E Y ) perpendicular to it (E Z < E Y ). At the same time, the two observed b u modes show positive dichroism (Figure 1, bottom). The fundamental vibrations for tetrafluorocyclobutadiene (Table 1) were identified by comparison of observed IR bands † Dartmouth College. ‡ University of Colorado. § NREL. (1) Chemistry of Organic Fluorine Compounds II: a Critical ReView; Hudlicky, M., Pavlath, A. E., Eds.; American Chemical Society: Wash- ington, DC, 1995. (2) Organofluorine Chemistry, Principles and Commercial Applications; Banks, R. E., Smart, B. E., Tatlow, J. C., Eds.; Plenum: New York, 1994. (3) Bally, T.; Masamune, S. Tetrahedron 1980, 36, 343. (4) Hess, B. A.; C ˇ arsky, P.; Schaad, L. J. J. Am. Chem. Soc. 1983, 105, 695. (5) Maier, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 309. (6) Arnold, B. R.; Michl, J. in Kinetics and Spectroscopy of Carbenes and Biradicals; Platz, M. S., Ed.; Plenum Publishing Corporation: New York, 1990; pp 1-35. Bonac ˇic ´-Koutecky ´ , V.; Scho ¨ffel, K.; Michl, J. J. Am. Chem. Soc. 1989, 111, 6140. (7) (a) Gerace, M. J.; Lemal, D. M.; Ertl, H. J. Am. Chem. Soc. 1975, 97, 5584. (b) Fanuele, J. C., Senior Honors Thesis, Dartmouth College, 1996. Earlier workers had transformed 3,4-diiodotetrafluorocyclobutene into a polymer of tetrafluorocyclobutadiene (Anderson, R. W.; Frick, H. R., US Pat. 3 682 876, 1972), a trimer of cyclobutadiene (Hertler, W. R. J. Fluor. Chem. 1975, 6, 171), and octafluorocyclooctatetraene (Barlow, M. G.; Crawley, M. W.; Haszeldine, R. N. J. Chem. Soc., Perkin Trans. 1 1980, 122), but there was no clear evidence for the intermediacy of the cyclobutadiene in these reports. (c) Grayston, M. W.; Saunders, W. D.; Lemal, D. M. J. Am. Chem. Soc. 1980, 102, 413. (8) Hammons, J. H.; Coolidge, M. N.; Borden, W. T. J. Phys. Chem. 1990, 94, 5468. (9) 3,4-Dichlorotetrafluorocyclobutene (Haszeldine, R. N.; Osborne, J. E. J. Chem. Soc. 1955, 3880) was transformed into the 3,4-diiodides as described by Dreyfuss, M. P., Ph.D. Dissertation, Cornell University, NY, 1957. (10) Radziszewski, J. G.; Hess, B. A.; Zahradnı ´k, R. J. J. Am. Chem. Soc. 1992, 114, 52. (11) Radziszewski, J. G.; Nimlos, M. R.; Winter, P. R.; Ellison, G. B. J. Am. Chem. Soc. 1996, 118, 7400. (12) The UV absorption spectrum was obtained in the instrument where electronic and IR spectral observations are performed simultaneously. This allows for precise correlation between IR and UV absorption growth or decay rates during the course of photolysis at various wavelengths, and thus the extraction of the weak UV absorption bands. (13) The experimental transition moment directions were determined according to the following formula: 14 tan 2  ) (Kx - Ki)/(Ki - Kz), where Kx and Kz are the principal orientation factors, and Ki values are the orientation factors of individual observed vibrations νi. Since the orientation is uniaxial, as dictated by the photoselected direction Z, we can adapt for Kz ) 0.318 (an average of Kz values for ν10, ν11, and ν12). From the condition that Σi ) 1 we determine Kx ) Ky ) 0.341. Although the error bars for our experimental transition moment direction angles are quite large (at least ( 12°), this result supports our claim for a nonplanar structure for 1. The cal- culated transition moment directions are notoriously unreliable, and advanced ab initio treatment is required to achieve some acceptable precision. 15 (14) Thulstrup, E. W.; Michl, J. Elementary Polarization Spectroscopy; VCH: New York, 1989. Michl, J.; Thulstrup, E. W. Spectroscopy with Polarized Light. Solute Alignment by Photoselection, in Liquid Crystals, Polymers, and Membranes; VCH: New York, 1986. (15) Radziszewski, J. G.; Downing, J. W.; Gudipati, M. S.; Balaji, V.; Thulstrup, E. W.; Michl, J. J. Am. Chem. Soc. 1996, 118, 10275. Figure 1. Calculated (B3LYP/cc-pVDZ), unscaled spectrum of C4F4 (top). Calculated transition moment directions are referenced to the carbon skeleton plane and are determined in a counterclockwise sense about the C 2 axis. The experimental infrared absorption spectrum of tetrafluorocylobutadiene (C 4F4) isolated in Ar matrix at 12 K (center). For abbreviations see, Table 1. Linear dichroism spectrum obtained after photoorientation with linearly polarized light at 248 nm (bottom). C 4F4 bands are marked with appropriate polarizations. Experimental transition moment directions were determined according to ref 13. 11122 J. Am. Chem. Soc. 1997, 119, 11122-11123 S0002-7863(97)01930-6 CCC: $14.00 © 1997 American Chemical Society