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and by reaction with stannous chloride. Both techniques gave the same e.s.r. spectrum but resulted in vastly different visual appearance. The solution obtained from the stannous chloride reaction remained a yellow of approximately the same hue as the solution of the parent bianthrone while the portion treated with potassium t-butoxide gave a blood-red solution. The spectrum obtained consisted of a major heptet of lines (intensities 1, 6, 15, 20, 15, 6, l ) , each of which showed further splitting into a quintet. The splitting of the major heptet of lines was 2.83 gauss and the minor splitting was 0.79 gauss. The pattern was centered a t g 2.0028. The major heptet of lines can result from a set of six protons giving nearly equivalent hyperfine interaction and the remaining five-line minor interaction results from four equivalent protons. The observation that Acknowledginents. We wish to thank A h . J. Menes hyperconjugation in the ethyl radical’ gives nearly for her experinierital contribution, Nrs. P. Charpin the same splitting for the P-protons as for the a-protons for X-ray studies, ;\Iiss S. Repellin for establishing the lends credulity to the interpretation that the major solubility isotherms, and N r . G. Dirian for fruitful heptet arises from three ring protons and the three discussions. 3Iost of the isotopic analyses have been methyl protons with nearly equal interactions while carried out ir the Section of Mass Spectrometry a t the minor interaction is a result of four ring protons. Saclay. A tentative assignment of the major heptet to interaction with positions 4, 5, and 7 ring protons, together (7) F. A. Lewis, Platinum Metal Rev., 5 , 21 (1961). with the three 2-methyl protons and the minor splitting (8) H. London, Ed., “Separation of Isotopes,” George Newnes Ltd., London, 1961. arising through interaction with protons on carbons 1, F. BOTTER 3, 6, and 8, is indicated. SERVICE DES ISOrOPES STABLES CENTRED’ETUDBS N U C L ~ A I RDE E SSACLAY The spectrum observed is comparable to that obFRANCE tained by Wasserman2 for the A1nsln’-bianthronetherRECEIVED DECEMBER 18. 1964 mochromic form if one assumes that, instead of the assignment suggested, the major splittirigs of the A1Oslo’-bianthrone resulted from the protons in the 2, 4, The Electron Spin Resonance Associated with 5, and 7 positions, rather than the 1, 3, 6, and 8 positions. Photochromism in 2,2’-Dimethylbianthrone The spectrum obtained from photolysis of I a t - 100” arid observed a t - 195” shows the same major features Sir: The electron spin resonance (e.s.r.) spectrum of as the negative ion spectrum, but it is less fully resolved solutions of the 2,2’-dimethyl-A108 ‘O’-bianthrone negadue to dipolar broadening in the glassy solid. The obtive ion in 2-methyltetrahydrofuran have been observed major splitting of the photolytically produced tained and compared with the e.s.r. spectrum observed species is approximately 3 gauss, but due to the greatly during low temperature photolysis of solutions of the reduced resolution and larger niodulations involved i n parent, 2,2’-dimethyl-A’0~1n’-bianthrone (I). this measurement this is probably equivalent to the 0 measured 2.83 gauss in the room temperature liquid. The signal intensity observed a t - 195’ is not directly cH3* \4 10 related to the visually estimated production of the green form but seems to maximize before maximum (visual) conversion. The above observations indicate that the observed CH3 resonance signal does not arise from a biradical of the 0 I
will provide a better understanding of the nature of the alloys “Pd-Pt”7 and “Pt, Pd-H, D” and a possible explanation for the considerable discrepancy between cy1 (20°, 1 atni.) = 1.95 obtained here and the solubility ratio of each isotope considered individually under the same conditioiis (pressure, temperature, 10% Pt alloy). The same eff evt is observed with pure palladium, where the isotopic solubility ratio at 20” under atmospheric pressure is 1.08,’ while the observed separation factor is 2.0.2 On the other hand, in the electrochemical experiments mentioned above,’ the over-all separation factor might be the product of a solubility separation factor and of a high kinetic factor arising from the cathodic process; we have examples of this,8 mainly in acid solutions like those used in these experiments.
The negative ion Of I was produced by two different methods, by reaction with potassium t-butoxide
(1) R. Fessenden and R. Schuler. J . Chem. Phys., 39, 2147 (1963). (2) E. Wasserman, J . Am. Chem. SOC..si, 5006 (1959).
Volume 69, Number 7
July 1965
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structure proposed by Wasserman., Furthermore, it appears that the e.s.r. signal may not arise from the major colored species but may be associated with parallel reactions of the parent material or subsequent reactions of the colored modifications. The spectra of the negative ion and of the low-temperature photolytic species both indicate structures in which the unpaired spin is effectively localized in one half of the molecule and are probably skewed conformations.
fourth vibrational level and above. These molecules are energetic enough to dissociate fluorine via reaction 3, although de-excitation via reaction 4 seems more likely. In the reaction between hydrogen atoms and chlorine, 1.3% of the HC1 is excited to the fourth level or above. This would seem a conservative estimate for reaction 2, since the heat of reaction is larger relative to the size of the vibrational quantum. Thus we might expect CY 0.013-0.02. By applying the steady-state approximation to the AIR FORCE MATERIALS LABORATORY LARRYA. HARRAH hydrogen and fluorine atom concentrations and also the WRIQHT-PATTERSON AIR FORCE BASE,OHIO concentration of vibrationally excited HF*, the followDEPARTMENT OF CHEMISTRY RALPHBECKER ing rate expression for the kinetics is easily obtained UNIVERSITY OF HOUSTON
-
HOUSTON, TEXAS
d[HFI 2~~12k3[H212[F21 dt k5[~~wl(ks[F:,I k4[&1(4)1)
+
RECEIVED MAY14, 1965
A Suggested Mechanism for the Hydrogen-Fluorine Reaction S i r : Two recent papers by Levy anL Copeland1s2 report experimental studies of the rate of reaction of hydrogen and fluorine. These authors were not able to devise a mechanism to explain their data quantitatively, although they did discuss their results in a qualitative manner. I t is the purpose of this communication to suggest a mechanism which quantitatively accounts for the results of Levy and Copeland; it is hoped that this will provide a useful working hypothesis for planning future work. In their first paper,' Levy and Copeland investigated the reaction of hydrogen-fluorine mixtures diluted with nitrogen in a flow system a t 110". They found the reaction rate to be proportional to fluorine concentration and independent of hydrogen concentration. This result can be explained by the reactions
+ Hz -% HF + H H + Fz --%aHF* + (1 - a ) H F + F HF* + Fz -% H F + 2F HF* + -% H F + M(4) 2F + &I --% ,,) Fz + (or perhaps F
(1)
(2)
(3)
M(4)
(4)
&)
(5)
2HF for
= H,)
The first two reactions-both exothermic-are analogous to steps occurring in the hydrogen-bromine and hydrogen-chlorine reactions. In the case of the reaction H Clz --t HC1 C1 nearly 25% of the product HCl is vibrationally e ~ c i t e d . ~It seems reasonable to expect the same phenomenon in reaction 2, and HF* represents hydrogen fluoride molecules excited to the
+
+
The Journal of Physical Chemistry
This corresponds to the observed kinetics provided k4[M(,)] >> kI([Fz]and provided the inert collision partners M(4)and are both preferentially hydrogen. The condition k4[M(4)]>> k3[Fz]is almost certainly satisfied in Levy and Copeland's experiments, since their fluorine concentrations were always low (mole fraction less than 0.05). Furthermore, reaction 4,a vibrational relaxation, is expected to be very fast, whereas reaction 3 may have a very low steric factor. The assumption that M(4) is preferentially hydrogen is also reasonable. Reaction 4 is a vibrational de-excitation, and light molecules seem to be especially effective in robbing diatomic molecules of excess vibrational energy. Thus Millikan and White's4 correlation predicts that hydrogen should be thirty times as effective as nitrogen and over forty times as effective as fluorine in relaxing H F from the first vibrational level. I n addition, the fundamental vibrational frequencies of H F and HZ differ by only about 6%, so that there is the possibility of de-excitation by transfer of vibrational quanta from H F to Hz. On the other hand, there is no a priori reason to expect that is preferentially hydrogen; hydrogen does not seem to be especially effective in recombining other halogens.5 It is possible to provide an ad hoc explanation by postulating an intermediate with a few kilocalories of stability, so that recombination occurs stepwise HzF 11 F Hz 31 HzF F --+ products
+ + +
+
J. B. Levy and B. K. W. Copeland, J . Phys. Chem., 67, 2156 (1963). (2) J. B. Levy and B. K. W. Copeland, ibid., 69,408 (1965). (3) J. T.Airey, R. R. Getty, J. C. Polanyi, and D. R. Snelling, J. Chem. Phys., 41, 3255 (1964). (4) R.C.Millikan and D. R. White, ibid.,39,3209 (1963). (6) D. L. Bunker and N. Davidson, J . Am. Chem. SOC.,80, 5086 (1958). (1)
