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1774
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The Multiplicity of CF2l
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by Julian Heicklen, Norman Cohen, and Dennis Saunders I
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Aerospace Corporation, El Segundo, California (Received January 27, 1966)
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Figure 3. Absorption spectra of 2,1,3-beneothiadiaeole (), 2,1,3-benzoselenadiazole(-), and [1,2-c]naphthoselenadiazole(- - - -) radicals in THF.
nance absorption.s The reactions were followed spectrophotometrjcally by contacting only portions of the solutions with the alkali metal in order to establish equilibrium between the parent compound and the reduced species. Representative absorption spectra, a t various stages of reaction, are illustrated in Figures 1 and 2. The data obtained for the radicals of the other compoundsginvestigated are shown in Figure 3. In every case studied, the new spectra develop through well-defined isosbestjc points as the reduction proceeds. The number and distribution of the points over the entire spectral range suggest the formation of only a single new species. Isosbestic points will exist only if no consecutive reactions occur.1o The reaction of two radicals to form a dimer, as in the reduction of pyridine,ll can therefore be excluded. These conclusions are supported by the fact that oxidation of the radicals by exposure to dry air regenerates SO-SO% of the original spectra. Solutions of' the benzene and the two naphthalene derivatives were analyzed for sodium content after reaction. The molar ratio of alkali metal to piaselenol is 1.0 (k0.1). These results establish the mononegative radicals as the predominant products obtained in the reaction of sodium metal with piaselenol and related derivatives. (8) A detailed report of the e.8.r. spectra will be published elsewhere. (9) The sulfur and oxygen analogs of piaselenol were included for comparison. The oxygen derivative, 2,1,3-benzoxadiaeole, reacts irreversibly with sodium without radical formation. (IO) M. D. Cohen. and E. Fischer, J. Cham. SOC.,3044 (1962). (11) R. L. Ward, J . Am. Chem. Sac., 83, 3623 (1961).
The Journal of Physical Chemistry
A transient species identified as CF2 has been produced by several methods. The question that arises is whether the CF2 diradicals produced by the various methods are indeed identical. The hydrocarbon analog CH2 exists as both a singlet and a triplet, with the triplet believed to be the ground state. The evidence, summarized in Table I, suggests that both states of CF2 can be produced also, with the singlet probably being the ground state. The evidence is based on the observed reactivity of CF2 with C2H4, C2F4, and 0 2 . The various rate constants are tabulated in Table I; for two of the cases, the reactivities are very much greater than those from the other methods of preparation. Direct Photolysis of C2F4 The flash photolysis of C2F4 has been recently reported by Dalby.2 The CF2radical concentration was monitored by its ultraviolet absorption spectrum, and the recombination rate constant was measured. Using this value and the apparent nonreactivity of CF2 to C2F4, C2H4,or 02,one can deduce upper limits for the respective rate constants. This nonreactive state is presumably the singlet because triplet molecules should be scavenged. Spin conservation rules permit the formation of either two singlets or two triplets. The fact that two singlets are formed suggests that this is the ground state. Hg-Sensitized Decomposition of C2F4 CF2 has been produced in th,e Hg-sensitized decomposition of C2F4 using 2537-A. r a d i a t i ~ n . ~If, ~one assumes that the radical concentration was uniform throughout the cell and uses Dalby's recombination rate constant, then a value of about 6 X lo2 l./molesee. is obtained for ICC2s4. When C2H4 or 02 was added, no reaction with CF2 was observed; thus, upper limits for I C C 2 ~ 4and kol could be established. The CF2 in this system behaves in a manner similar to that in (1) This work was supported by the U. S. Air Force under Contract No. AF 04(695)-469' (2) F. W. Dalby, J. Chem. Phys., 41, 2297 (1964). (3).(a) B.Atkinson, J . Chem. SOC.,2684 (1952); (b) J. Heicklen, V. Knight, and S . Greene, J . Chem. Phys., 42, 221 (1965). (4) Unpublished results of this laboratory.
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Table I : Rate Constants (l./mole-sec.) for CF2 a t Room Temperature Source of CFz
Photolysis of CzF4 Hg sensitization of C Z F ~ Photolysis of CFZHBr, CFzBrs Glow discharge of CF4 O( *P) GzF4 Photolysis or pyrolysis of CFzNa
+
io4
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Dalby’s system and is presumably the singlet. Spin conservation rules predict that of the two CF2 species formed from each C2F4-Hg(3P)collision, one should be singlet and one triplet. Apparently, the spin conservation rule does not apply in this system. Professor George Hammond of the California Institute of Technology has suggested to us the possibility that the triplet C2F4 molecule passes through excited vibrational levels of a singlet state before dissociating, and thus only singlet CF2 is formed.
Photolysis of CF2HBrand CFzBr2 CF2Br2and CF2HBr were photolyzed,5 and the absorption spectrum of CF2was observed. The addition of a large excess of oxygen did not reduce the intensity of the bands; presumably singlet CF2was present.
Glow Discharge of CF, CF2 has been produced from an electrodeless discharge of CF4 vapor, and the emission bands,6 as well as the absorption bands,’ were measured. From these optical observations, it was concluded that the species involved was nonlinear singlet CF2. In other experiments with discharged CF4,*ethylene was added downstream, and the products of the reaction were collected and analyzed. With as little as 5 or 10 p of c2H4, the C3 adducts of CF2 to C2H4were found, and their concentrations did not increase with additional C2H4. Under these conditions, the CF2 radical concentration was about 0.1 p in the absence of C2H4. Consequently, could be computed to be at least lo7l./mole-see., or lo4 times faster than that expected for singlet CF2. In addition, the reaction with C2H4 produced chemiluminescence which reached a maximum value with as little as 5 p of C2H4. The luminescent state had a long lifetime and presumably corresponded to a triplet molecule, as would be expected if the CF2reactant were a triplet. Additions of small amounts of air between the discharge and the C2H4 inlet caused the chemiluminescence t o vanish, thus indicating scavenging of the precursor or the luminescing species, or both. It is difficult for us to understand these results unless triplet CF2 is present.
ko2
kC2%
k W d
