J. Pbys. Cbem. 1980, 84, 401-403
(11) B. P. Levitt, J . Cbem. Pbys., 42, 3, 1038 (1965). (12) Lord Rayleigh, Phil. Mag., 34, 94 (1917). (13) F. R. Gilmore, California Institute of Technology Hydrodynamic Lab Report No. 26 (1952). (14) B. E. NoRingk and E. A. Neppiras, Proc. Pbys. SOC.London, Sect. 6 , 63, 674 (1950). (15) T. Oka, A. R. Knight, and R. P. Steer, J . Cbem. Pbys., 63, 2414 (1975). (16) C. Sehgal, R. P. Steer, R. G. Sutherland, and R. E. Verrall, J . Chem. Phys., 70, 2422 (1979). (17) E. L. Mead, R. G. Sutherland, and R. E. Verrall, Can. J . Cbem., 54, 7, 1114 (1976). (18) L. D. Rozenberg, Akust. Zb., 11, 1, 121 (1965). (19) J. H. Todd, Ultrasonics, 8, 234 (1970). (20) F. D. Rossinl and D. D. Wagman, "Selected Values of Chemical Thermodynamic Ropertiis", United States Government Printing m e , Washington, D.C., 1952. (21) C. Sehgal, R. P. Steer, R. G. Sutherland, and R. E. Verrall, J . Pbys. Chem., 81, 2618 (1977). (22) C. Sehgal, R. G. Sutherland, and R. E. Verrall, J . Pbys. Cbem., preceding paper in thls issue. (23) G. Herzberg, "Electronic Spectra of Polyatomic Molecules", Van Nostrand, Princeton, N.J., 1967.
401
(24) H. G. Ftynn, "physical Acowtics", Vol. 18, W. P. Mason,Ed., Academii Press, New York, 1964, p 62. (25) M. G. Sirotyuk, Sov. Pbys. Acoust. (Engl. Trans/.),8, No. 3 (1963). (26) M. A. Margulis, Sov. Phys. Acoust. (Engl. Trans/.),15, No. 2 (1969). (27) R. Hickling, J . Acoust. SOC.Am., 35, 967 (1963). (28) F. R. Young, J . Acoust. SOC.Am., 60, 100 (1976). (29) J. G. Kirkwood and H. A. Bethe, OSRD, ref 588 (1942). (30) D. Srinivasan and L. V. Holyroyd, Pby. Rev., 99, 633 (1955). (31) D. Srinivasan and L. V. Hoiyroyd, J . Appl. Phys., 32, 446 (1961). (32) P. Gunther, W. Zeil, U. Gisar, and E. Heim, Z. Ekctrochem., 61, 188 (1957). (33) P. Gunther, E. Heim, and H. 0. Burgsted, Z . Electrochem., 63, 43 (1959). (34) T. F. Hueter and R. H. Bolt, "Sonics", Wiley, New York, 1955. (35) Assuming the front plate of the insonam cell to be a perfect reflector, we estimated the pressure amplitude PAof a sinusoidal ultrasonic field by using the following equation^:^' I,, = 2€,, =
PA2
2PC
where I,, is the ultrasonic intensity. Taking p = lo3 kg m-3 and c = 1530 ms-', we estimated PAto be 6.2 X lo5 N m-'.
Emission Spectrum of Fluorobromocarbene in Solid Argon at 12 K John C. Miller and Lester Andrews. Department of Chemistry, University of Virginia, Charlottesville, Virginia 2290 1 (Received February 26, 1979; Revised Manuscript Received November 12, 1979)
The CFBr intermediate has been synthesized by vacuum ultraviolet photolysis of CHzFBr and CHF,Br and subsequently trapped in solid argon at 12 K. The fluorescence, following laser excitation at 424 and 428 nm, consisted of a 13-member progression in the ground-state bending mode with an average spacing of 327 cm-' and an electronic origin near 23 300 cm-'.
Introduction Recently, dihalocarbenes have been extensively studied, primarily by the technique of low-temperature, matrixisolation spectroscopy. CFZ,ld CCl,,7-13and CBr$4-16have been investigated in this manner, as well as the asymmetric carbenes CFC117-20and CC1Br.'2J4J6 These intermediates were synthesized by reactions of carbon atoms with haloof alkali metals with tetrahalog e n ~ by , ~ reaction ~ ~ or by vacuum UV photolysis of the appropriate dihalomethane~.~~~~J~J~J~ In general, infrared studies have provided antisymmetric stretching frequencies, and optical emission spectra have given the bending modes for the ground state. Excited-state constants have been obtained from either absorption or tunable dye laser excitation spectra, and electronic lifetimes have also been measured. A useful summary of the spectroscopic constants and lifetimes has been given by Bondybey and English.16 This paper describes the laser-induced emission spectrum of fluorobromocarbene, CFBr. No previous results have appeared on this molecule, although an unanalyzed absorption band in the region 390-440 nm following flash photolysis of CHFBr, has been tentatively attributed to CFBr.*l In a related matrix study, the C-Br and C-F stretching fundamentals of CFBr have been identified in the infrared spectrum at 656 and 1157 cm-l, respectively, following argon resonance photolysis of CHzFBr.22 Experimental Section The CFBr intermediate was produced by codepositing CHzFBr/Ar mixtures (1/300 mol ratio) a t about 1-3 0022-3654/80/2084-0401.$0 1.OO/O
mmol/h with simultaneous irradiation from a windowless argon resonance lamp onto a polished copper wedge. As the argon from the lamp was also condensed on the cold surface, the final concentration is about 1/600 mol ratio. The lamp output, described p r e v i o u ~ l yconsists ,~~ mainly of 106.7- and 104.8-nm Ar resonance lines and 121.6-nm light from impurity hydrogen Lyman a emission. The light is energetic enough to photodetach hydrogen atoms, which then can diffuse away as the matrix freezes. In one experiment CHFzBr was used as the precursor and produced identical results. The synthesis of CHzFBr and CHFzBr have been reported e l s e ~ h e r e . ~The ~ , ~ substrate ~ was cooled to about 12 K by a CTI Model 21 closed-cycle refrigerator, and standard vacuum and gas-handling techniques were used. Excitation was provided by a pulsed nitrogen laser (Molectron UV 14) and a pumped dye laser (Molectron DL11) using the dye DPS (396-416 nm). The resulting emission was focused on the slit of a Spex 1401 double monochromator and detected photoelectrically with a RCA C31034 phototube and a Keithley 414s picoammeter.
Results and Discussion The emission spectrum (shown in Figure l), recorded from a sample of CH,FBr diluted in argon and subjected to argon resonance photolysis, consists of a long, 13-member progression of broad bands with an average vibronic spacing of 327 cm-'. The band positions and spacings are listed in Table I. The spacing undoubtedly reflects the ground-state bending mode vi', and the Franck-Condon intensities indicate a substantial change in bond angle between the ground and the excited state. The average 0 1980 American Chemical Society
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Miller and Andrews
The Journal of Physical Chemistry, Vol. 84, No. 4, 1980
I
-C 11
I
1, , LA19000
20000
21000
*'A V E N U W B E R 5 [ c
22033 "7'.
23003
)
Figure 1. Laser-excited fluorescence spectrum of CFBr in solid argon at 12 K.
TABLE I: Band Energies and Spacings of the Progression of CFBr in Solid Argon at 1 2 K
cm-'
v2
v,
2 3 4 5 6 7 8 9 10 11 12 13 14
22 620 22 280 21 940 21 600 21 260 20 930 20 610 20 270 1 9 955 1 9 630 19 305 1 8 990 1 8 715
A
vZ"
cm-' 340 340 340 340 330 320 340 315 325 335 315 285 -
av 327
frequency of 327 cm-' probably extrapolates to an w2 value of around 340 cm-', which compares well with the trend established in CF2 and CFCl of 668 and 442 cm-', respectively. The large bandwidth (- 150 cm-', fwhm) is indicative of a high degree of guest-host interaction, and the observed bands must reflect multiphonon transitions. No trace of zero-phonon lines was observed. Unfortunately, the breadth of the lines and their great intensity obscure any weaker progressions in either of the stretching modes. In principle, because of the lowered symmetry of the molecule, all three modes could be active, and indeed for CFCl and CClBr, other vibrations have been observed. Determination of the O,O,O O,O,O band is complicated by the first apparent band being overlapped by another emission of unknown origin, making its measurement uncertain. However, as the lowest excitation frequency resulting in the emission system occurs at 23 355 cm-', about one v2 spacing from the first emission band, the vibronic numbering of Table I, is suggested. As we are observing multiphonon transitions, the highest energy emission band and the lowest energy excitation band are not expected to coincide exactly, The observed band system thus corresponds to the O,O,O 0,2,0 to O,O,O 0,14,0 bands. In previous experiments on dihalocarbenes, the use of tunable lasers allowed determination of several excitedstate frequencies. Unfortunately, in this case, the broad multiphonon emission precludes observation of sharp lines in the excitation spectra. Only two very broad and poorly resolved excitation thresholds were observed spaced about 240 f 40 cm-', which is appropriate for the excited-state bending mode. The assignment of the observed emission to the CFBr radical is based on several factors. First, it is the most reasonable product to be formed under these conditions. In each previous case vacuum-UV photolysis of the appropriate CH2X2compound has resulted in only a single emission system due to the CX2 species. Second, the compound CHFBrz was used as the precursor in one experiment, and an identical emission spectrum was re-
-
-
-
corded. Third, the energy of the band system and the observed frequencies are in agreement with extrapolations from the other, very well characterized dihalocarbenes. In addition, the overall Franck-Condon envelope and dominance of the bending fundamental are also indicative of all of the CX2 species previously studied. Finally, the observation of a 390-440-nm absorption after flash photolysis2' of CHFBr2 is supportive of the present CFBr assignment which required 424-428-nm excitation. In similar infrared experiments performed to study the positive and negative molecular ions in the CH2FC1, CH2FBr,and CH2FI systems,22argon resonance photolysis of CH2FBrrevealed a very strong band at 1157 cm-l and a strong site split band a t 656 cm-'. By comparison to the CH2FClexperiments,"~~ which give a strong 1147-cm-' band with a C-13 counterpart at 1120 cm-' for the C-F stretching mode of CFC1, the strong 1157-cm-' absorption is due to the C-F stretching mode of CFBr. The 656-cm-' absorption shows an appropriate displacement from the chlorine isotopic doublet at 742 and 738 cm-' due to the C-C1 stretching mode in CFC1, and accordingly the 656cm-l absorption is assigned to the C-Br stretching mode of the carbene CFBr. The observation of the infrared spectrum of CFBr in similar experiments supports the present identification of the emission spectrum of CFBr. Similar experiments using CHF21and CHF12with argon laser excitation in hopes of observing CFI emission yielded only a strong fluorescence due to IF.25 Apparently, the very strong iodine monofluoride emission masked any possible signal from CFI in these experiments. Finally, additional attempts to observe C12 utilizing the Na/C14 reaction described previ~usly'~ were also negative. It was hoped that, if the C12transition were greatly red-shifted from CBr2,use of the previously unavailable Kr+ infrared lines (676.4, 687.0, and 799.3 nm) might excite C12 luminescence. Unfortunately, it thus remains unknown whether C12is in fact unstable, even at low temperatures, or whether it is unobservable because of experimental difficulties. In summary, a banded emission system with its origin near 23 300 cm-' has been assigned to the bending mode progression of the CFBr intermediate. Determination of the bending frequency of about 340 cm-', coupled with infrared observation22of the C-F stretch at 1157 cm-' and the C-Br stretch at 656 cm-', completes the ground-state description of this molecule. The upper state bending mode is reduced to about 240 cm-'.
Acknowledgment. The authors gratefully acknowledge support from the National Science Foundation under Grants CHE 76-11640 and CHE 77-09296. References and Notes (1) Bass, A. M.; Mann, D. E. J . Chem. Phys. 1962, 36, 3501. (2) Milligan, D. E.; Mann, D. E.; Jacox, M. E.; Mitsch, R. A. J . Chem. Phys. 1964, 41, 1199. (3) Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1968, 48, 2265. (4) Smith, C. E.; Jacox, M. E.; Milligan, D. E. J . Mol. Spectrosc. 1976, 60, 381. (5) Bondybey, V. E. J. Mol. Specfrosc. 1976, 63, 164. (6) Andrews, L.; Prochaska, F. T. J. Chem. Phys. 1969, 70, 4714. (7) Mliligan, D. E.; Jacox, M. E. J . Chem. Phys. 1967, 47, 703. (8) Andrews, L. J . Chem. Phys. 1968, 48, 979. (9) Jacox, M. E.; Milllgan. D. E. J . Chem. Phys. 1970, 53, 2688. (10) Shirk, J. S. J . Chem. Phys. 1971, 55, 3608. (11) Tevault, D. E.; Andrews, L. J . Mol. Specfrosc. 1975, 54, 110. (12) Maltsev, A. K.; Neofedov, 0. M.; Hauge, R. H.; Margrave, J. L.; Seyferth, D. J . Phys. Chem. 1971, 75, 3984. (13) Bondybey, V. E. J . Mol. Specfrosc. 1977, 64, 180. (14) Andrews, L.; Carver, T. G. J . Chem. Phys. 1968, 49, 896. (15) TevauR, D. E.; Andrews, L. J . Am. Chem. SOC. 1975, 97, 1707. (16) Bondybey, V . E.; English, J. H. J . Mol. Specfrosc. 1980, 79, 416. (17) Smith, C. E.; Miillgan, D. E.; Jacox, M. E. J. Chem. Phys. 1971, 54, 2780.
J. Phys. Chem. 1980, 84,403-410 (18) (19) (20) (21) (22)
Tevault, D. E.; Andrews, L. J . Mol. Spectrosc. 1975, 5 4 , 54. Bondybey, V. E. J . Chem. Phys., 1977, 66,4237. Bondybey, V. E.; English, J. H. J. Mol. Spectrosc., 1977, 68,89. Merer, A. J.; Travis, D. N. Can. J. Phys. 1966, 44, 1541. Prochaska, F. T.; Hndrews, L. J. Chem. Phys., in press.
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(23) Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1970, 32, 157. (24) Carver, T. G.; Andrews. L. J . Chem. Phys. 1969, 50, 5100. (25) Miller, J. C.; Andrews, L., submitted for publication in J . Mol. Spectrosc .
Conformational Equilibria in trans-1 ,2-Diarylethylenes Manifested in Their Emission Spectra. 2.‘’ Phenanthryl and Benzophenanthryl Derivatives Ernst Fischer Department of Structural Chemistry, The Weizmann Institute of Science, Rehovof, Israel (Received May 2 7, 1979)
The emission spectra of 10-5-104 M solutions of trans-1,2-diarylethylenes,where at least one of the aryls is 3-phenanthryl or 2-benzo[c]phenanthryl, vary with the excitation wavelength, indicating the existence of two or even three species of each compound with slightly shifted absorption and emission peaks. Measurements in a variety of solvents and at temperatures down to -180 “C show little dependence on the solvent but much better spectral definition at lower temperatures. As suggested earlierls for 2-naphthyl homologues of stilbene, the above two or three species are assigned to almost isoenergetic conformers existing in a dynamic equilibrium in solutions of each compound. In at least one compound some sort of aggregation process takes place in poor solvents at low temperatures and causes almost complete conversion intoone conformer. In 3,3’-dimethylstilbene a slight effect of the excitation wavelength was found, but only below -170 “C. Stilbene proper, in which no conformers are feasible, shows no such phenomena, but in M solutions in aliphatic hydrocarbons below -150 “C, excited at above 334 nm, the ratio between the emission peaks changes drastically in a time-dependent and thermally reversible process also ascribed to aggregation.
In a previous paperla we showed that solutions of the trans isomers of l-phenyl-2-(2-naphthyl)ethylene (Ph-2N), 1,2-di(2-naphthyl)ethylene (2N-2N), and 1-(1naphthyl)-2-(2-naphthyl)ethylene(1N-2N), exhibited anomalies in their emission spectra, lifetimes, and quantum yields. These anomalies were ascribed to the existence, in solution, of a dynamic equilibrium among two or three conformers, as illustrated for 2N-2N in Scheme I. The basic idea was that the sharply structured absorption spectra of the various conformers are slightly shifted against each other, so that at any but the longest excitation wavelength light is absorbed by all conformers, but to different extents. If the emission spectra of the assumed pure conformers differ sufficiently from each other, the observed emission spectra should be superpositions of the spectra of the two or three conformers, with the contribution of each conformer varying with the excitation wavelength. Indeed the most prominent and easily observable effect was t,he variation of the emission spectra with ,,A, in a way which can be expressed as a superposition of two sets of emission peaks shifted 10-15 nm against each ether.'* Partly related results were reported by Sheck and co-workers.2 As already pointed out, similar effects may be expected in any related system in which two or more well-defined and almost isoenergetic conformers can exist in solution.1b In the present paper we report rather spectacular emission-spectroscopic observations with 1,2-di(3-phenanthryl)ethylene,(3 + 3), and 1-(2naphthyl)-2-(3-phenanthryl)ethylene, (2 3). Similar but less spectacular observations were made with (1 + 4), (2 + 41, ( 3 + 4), and (4 + 4). (Unless indicated otherwise, all compounds were measured in their trans form.) The reversible trans-cis photointerconversion and the fluorescence of these compounds, as well as the conformational control of the photocyclization products of the
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respective cis isomers, have been reported recentlya3
Results 1. Emission Spectra of (2 3) and ( 3 + 3 ) . The emission spectra of solutions in a variety of solvents were measured as a function of ,,A, and temperature. Some of the results are summarized in Figures 1-5 and 7. Qualitatively similar results were obtained in a variety of solvents. Two sets of emission peaks, denoted t and 1 in the figures, characterize these spectra at all temperatures. We shall refer to the two sets as S (short) and L (long), respectively, where L is the species in which the emission peaks are at longer wavelengths. The position of these peaks shifts slightly with the solvent and the temperature, and the spectra undergo pronounced sharpening on cooling. A comparison of the observations in solutions in decalin and in propylene with those in other solvents at low temperatures indicated that the sharpening is predominantly due to the low temperatures and not to the very high viscosities. Some relevant viscosity data4 are given in Table I. Solvents used but not shown in the figures were 2-propanol, cumene (= isopropylbenzene), and 2-methylpentane (2-MP). 2-MP served as a “poor” solvent (cf. later), whereas toluene (T) was added to methylcyclohexane (MCH) (Figure 3) in order to improve the solubility of the solute and thereby avoid complications due to aggregation or even precipitation on cooling. (An “undisturbed’ 1:l mixture of MCH and T‘ can be cooled4b to -160 “C.) Table I1 summarizes the positions of the emission peaks of the S and L modifications observed with solutions of (2 + 3) and (3 + 3) in MCH/B-MP (3:l) at -150 “C. In toluene and in MCH/T the peaks are all shifted to longer wavelengths by 6-10 nm. 2. Variation of the Emission Spectra with Temperature. Several effects were observed: (a) Both absorption
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0 1980 American Chemical Society
