206
J. Phys. Chem. 1980, 84, 206-207
the methyl group could be as small as 1/3 that of the ring carbon. This analysis indicates that the methyl groups are undergoing virtually free rotation and the barrier is close to zero. Finally, the T S R values can be used to provide estimates of the barrier to internal rotation which are in agreement with the previous analysis. The TSRvalues found for the methyl carbons in the 3and 4-methylpyridines in the neat phase give V, values near zero from the correlation equation^.^^^ In the methanol-water solvent the Vovalues for the neutral and protonated species of these pyridines are also found to be quite small. For 2-methylpyridine Vois very close to zero in the neat phase but increases to about 1.0 kcal/mol in the neutral and protonated species in the methanol-water solutions. Apparently, hydrogen bonding to the nitrogen lone pair by the solvent molecules in the neutral species and the hydrogen on the nitrogen in the protonated species produces a steric interaction that raises the barriers to internal rotation. However, this effect is approximately the same for both species. It may be slightly larger for the protonated species, but the accuracy of the TsR values and the reliability of the correlation^^,^ are not sufficient to allow us to make such a distinction between two small V,, values. What is clear, however, is that the Vovalues do
not change appreciably upon protonation, for these pyridines, but Vois higher in the neutral and protonated 2methylpyridine species than for 3- and 4-methylpyridine.
References and Notes (1) E. J. King, "AcidBase Equilibria", Permagon Press, New York, 1965. (2) L. G. Hepler and E. M. Woolley, "Heats and Entropies of Ionization" in "Solute-Solvent Interactions", J. F. Coetzee and C. D. Rltchie, Ed., Marcel Dekker, New York, 1969. (3) D. H. Aue, H. M. Webb, M. T. Bower, C. L. Liotta, C. J. Alexander, and H. P. Hopkins, Jr., J . Am. Chem. SOC.,98, 854 (1976). (4) H. P. Hopkins, Jr., and S. Z. Ali, J. Am. Chem. Soc., 99, 2069 (1977). (5) C. H. Rochester and B. Rossal, Trans. Farao'ay Soc., 65, 1004 (1969). (6) A. P. Zens and P. D. Ellis, J. Am. Chem. Soc., 97, 5685 (1975). (7) J. R. Lyeria, Jr., and D. M. Grant, J . Phys. Chem., 76, 3212 (1972). (8) D. E. Woessner, B. S. Snowden, Jr., and G. H. Meyer, J. Chem. phys., 47, 2361 (1967). (9) N. Levy, "Carbon-13 Nuclear Magnetic Resonance for Organic Chemists", Wiley-Interscience, New York, 1972. (10) A. Tancredo, P. S. Pizani, C. Mendonca, H. A. Farach, C. P. Poole, Jr., P. D. Ellis, and R. A. Byrd, J . Mag. Reson., 32, 227 (1978). (11) H. L. Retcofsky and R. A. Friedel, J . Phys. Chem., 72, 290, 2619 (1968). (12) H. L. Retcofsky and R. A. Friedel, J. Phys. Chem., 79, 3592 (1967). (13) R. J. Pugrnire and D. M. Grant, J. Am. Chem. SOC.,90, 4232 (1968). (14) J. H. Noggle and R. S. Schirmer, "The Nuclear Overhauser Effect. Chemical Applications", Academic Press, New York, 1971, (15) F. W. Wehrli and T. Wirthlin, "Interpretation of Carbon-13 NMR Spectra", Heyden, London, 1976. (16) E. M. Arnett, C. Chawla, L. Bell, M. Taagepera, W. J. Hehre, and R. W. Taft, J . Am. Chem. Soc., 99, 5729 (1977).
Singlet and Triplet Emission from Difluoromethylene in the Reaction of Ozone with Tetrafluoroethene Sidney Toby* and Frina S. Toby DepaHment of Chemistly, Rutgers University, The State University of New Jersey, New Brunswick, New Jersey 08903 (Received July 16, 1999)
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In addition to t,he previously identified singlet emission (CF#B,) CF2('A,)), the reaction of ozone with tetrafluoroethene also gives triplet emission due to CF2(3B1) CF&Al). The intensities of the emissions are in accord with formation of the excited singlet via 2CF2(3B1) CF2(lB1)+ CF2('A1). The reaction between ozone and tetrafluoroethene at room temperature gives a strong emission in the ultraviolet which was identified by Sheinson, Toby, and Toby1 as due to excited singlet CFJIB1). In addition an unidentified luminescence was seen consisting of peaks in the region 490-625 nm. Recent work by Koda2i3has shown that the reaction of oxygen atoms (3P)with tetrafluoroethene gives both CF2(IB1)and a visible luminescence which he identified as the hitherto unseen triplet CFz(3B1)emission. Reported here is the identification of the visible luminescence of the C2F4 O3 reaction as due to CF2(3B1) and a brief investigation of the kinetics of the ultraviolet and visible luminescence. The apparatus was similar to that previously described.l Ozone was prepared by passing oxygen (Matheson Ultrapure grade) through an ozonizer operated at 7.5 kV and trapping the O3 in silica gel at -78 "C. The silica gel trap was then evacuated to remove adsorbed O2 until appreciable quantities of O3 desorbed, and the trap was then allowed to warm to room temperature, allowing the O3 to expand into a 3-L bulb where it was diluted with helium. In some cases the O3 was eluted directly from the warmed silica gel trap with a stream of helium or oxygen. Total pressures were generally 1torr for kinetic measurements, and -50 torr for spectra.
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0022-3654/80/2084-0206$01 .OO/O
The reaction vessel was approximately 65 cm long and C2F4 (from Columbia Organic Chemicals, distilled before use to remove polymerization inhibitor) was mixed with O3 60 cm from the monochromator window. O3 pressures were measured by absorbance at 254 nm and CzF, pressures were measured via a ball flowmeter which had been calibrated for C2F4 flow under controlled conditions by using a McLeod gauge. A Jarrell-Ash 0.25-m monochromator with a 2360 grooves/mm grating and a 3-nm spectral slit width viewed the reaction vessel axially through a quartz window with a Kodak Wrattan 2A filter to remove the second-order UV spectrum. Integrated singlet and triplet intensities were found by removing the monochromator and Wratten filter to measure total emission and interposing the filter to measure triplet emission. A cooled EM1 9683QKB photomultiplier was used. Results and Discussion The visible emission from the reaction of O3with CzF4 is shown in Figure 1. The assignments are those of Koda2 for CF2(3B, lAl) and are due to the progression for the bending v 2 mode, particularly the transitions from v i = 0. A few lines from vi = 1 and v2' = 2 are partially resolved. The effect of an oxygen rather than a helium carrier is most interesting. The triplet emission is reduced
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@ 1980 American Chemical Society
The Journal of Physical Chemistry, Vol. 84, No- 2, 1980
Reaction of Ozone with Tetrafluoroethene
:!OS
U
E
I
i W
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-
L
I
_
500
l
_
l
l
550
_
I
_
I
_
L
_
I
I
650
600
-
-
700
750
+
rim
Figure 1. Visible emission from the reaction O3 C2F4 with He and O2 carriers: total pressure = 45 torr, spectral slit width = 3 nm, uncorrected for plhotomultiplier response. Assignments are by Koda2 for u,'v2' v,''v~'. +
Scheme 1
A H " , kcalmol-I
3
2C1i',(3B,)l -+ CF,('B,)
+
CF,('A,)
4
CF,('B,)
+
CF,('A,) t h ~ ,
5
CF,(3EI,) -$ CF,('A,) t
UT
-72,a
-107' -572
considerably, 81s expected, but a weak luminescence at 762 nm is increased approximately fivefold. This emission is the well-known 02(1zg++32Jtransition4 which was observed by Koda in his oxygen atom work. It presumably arises from the reaction CF2(3B,)+ 02(3Z:,-) CF2('A1) 02('2,+)which is 20 kcal exoergic. The kinetics of the O3+ CzF4system were studied by Toby and Tob,y5who postulated a complex energy pooling mechanism to account for the formation of excited singlet CF2 with up to 117 kcal mol-l of energy above ground. If instead we assume that CF2('B1) is formed via triplettriplet annihilation, then the previously postulated5 mechanism mlay be simplified and the source of the CF2(3Bl)is then likely to be the spin-allowed decomposition of a difliiioromethylene peroxide intermediate. We postulate the rnechanism in Scheme I for the initial steps. Here, hus arid huT refer to singlet and triplet emission and subsequent fates for CF2(1Al)are omitted since they do not affect the luminescence kinetics. In a flow system the steady-stale approximation is of dubious validity for long-lived intermediates such as CF2(3B1),for which the lifetime has been estimated as -1 sa3 On the other hand, since most of the rate constants for the reactions in Scheme I are unknown, computer simulation techniques are of little value. We car1 obtain some quantitative information by assuming a steady state only in the short-lived CF2('R1) and we obtain
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+
I T / I S 1 l 2= k6k3-'/a
.3Ci
.003
.Ol
1
.03
,I
.3
I
R E L A T I V E SINGLET INTENSITY, na
Figure 2. Plot of log triplet emission vs. log singlet emission with heliium carrier: total pressure = 1.0 torr, O3pressure = 0,012-0.024 torr, C2F, pressure = 0.020-0.24 torr: (circles) no added 02, (squares) O2 pressure = 0.23 torr, (triangles) O2 pressure = 0.39 torr.
where IT and Is are the triplet and singlet emission intensities and any radiationless internal conversion steps have been ignored. A log-log plot of relative triplet emission against relative singlet emission is shown in Figure 2 for experiments with a helium carrier to which in some cases O2 had been added. A line drawn with a slope of 112 gives good agreement with the data (the least-mean-squares line has a slope of 0.52 and a correlation coefficient of 0.98). It is interesting to note that the addition of O2 has no effect on the linearity since the expected reaction 02(32:g-) + CF2(3B1) CF20z products
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will not affect IT/Isllz.Addition of sufficient oxygen, however, reduced both singlet and triplet emission. Figure 2 is strong evidence for the formation of CF2(lB1)by triplet-triplet annihilation (step 3) as postulated by Koda2 in the reaction of O(3P)with C2F4. It has been suggestedg that the visible bands reported by Koda2 were due to a N-atom impurity in the 0 atoms. The fact that the same bands were seen in an ozone system argues against this interpretation. We conclude that the reaction of ozone with tetrafluoroethene is most unusual in that two relatively stable gases react at room temperature to form an intermediate which gives emission from its excited singlet and triplet states in comparable intensities. The system is thus well suited for the investigation of comparisons of quenching and of chemical reactivity for singlet and triplet difluoromethylene. Such experiments are planned.
Acknowledgment. We thank the Rutgers University Research Council for support of this work.
References and Notes (1) R. S. Sheinson, F. S. Toby, and S. Toby, J . Am. Chem. Soc., 97, 6593 (1975). (2) S. Koda, Chem. Phys. Lett., 55, 353 (1978). (3) S. Koda, J. Phys. Chem., 83, 2065 (1979). (4) A. U. Khan and M. Kasha, J . Am. Chem. Soc., 92, 3293 (1970). (5) F. S. Toby and S. Toby, J. Phys. Chem., 80, 2313 (1976). (6) S. W. Benson, "Thermochemical Kinetics", 2nd ed,Wiley, New York, 1976. (7) A. S. Rodgers, ACS Symp. Ser., No. 66, 296 (1978). (8) C. W. Mathews, Can. J. Phys., 45, 2355 (1967). (9) D. W. Peters and C. W. Mathews, Symposium on Molecular Spectroscopy, Columbus, Ohio, 1979, Abstract TB2.
