278
NOTES
counterJ3the fact that collisions occur in “sets” is then of little relevance. The situation is akin to that in the l/zRT, gas phase and we again expect a value of E* assuming that 2, the collision number, in solution is also proportional to T ‘ / ~ . The situation at low values of E* has been treated4 using an expression for the temperature dependence of the rate constant which is valid over the whole range, from very low values of p , the probability that a collision results in reaction, to p = 1 or reaction on the first collision. This show that even if the reaction probability is energetic in nature, as when p = exp(-E*/ RT)j E, may be less than 3 at low values of E*, by an amount which is quite sensitive to the average number of collisions per encounter. In discussing these cal~ulations,~ it was remarked that the figure then available5for the activation energy of the eaqH+ reaction, 3.2 =k 0.3 kcal mol-1, implied that the activation energy for the diffusion of eaq- was slightly higher than that for the self-diffusion of water. The more recent data,6 giving the figure of 2.6 i 0.3 kcal suggests that the first may be a little high and that the implication is not correct. (It seems a pity that the conductivity work of Schmidt and Buck’ was not repeated at other temperatures with a view to estimating the activation energy for the diffusion of the hydrated electron.) The other result quoted by Cercek and Eberts is, with a little difficulty, compatible with the results of these calculations;4 when the charge product is positive, the minimum in E , as a function of E* is lower and a value of about 2 kcal mo1-I appears acceptable for this reaction. However, the authors of this informatied deny this possibility. When the erroneous quotation of the values of these workers6 is corrected in the table of activation energies derived8 by competition studies, this selection of reactions, with rate constants ranging from 6 X 1O1O to less than lo7M-1sec-1, no longer presents quite so uniform a list of Arrhenius activation energies. A larger variation than this was found in values obtained directlyg by pulse radiolysis. In the case of the very slow reactions, the reasonable conclusion is that although the probability that a collision results in reaction is small chiefly because of a very low temperature-independent factor, there is also a factor of the form exp(-E*/RT), where E* lies in the range of approximately 2-4 kcal moI-I. The earIier implication8 that the value of E* was constant seemed very surprising. The only invariants in the reactions listed were the hydrated electron and the aqueous solvent, but the reaction, eaqHzO, has been shown’o to have a significantly higher activation energy of 6.7 k 0.7 kcal mol-’. Anbar and Hartg attributed the small and scarcely significant difference in their activation energies for the reaction eaq- CICHzCOO- in H20 and DzO to what they claim is a very small difference in the B values for
+
+
+
+
T h e Journal of Physical Chemistry
these two solvents. However, the influence of B on the activation energy is not important unless the reaction is very fast; from the previous calculations4 it may be deduced that a variation of 0.4 kcal mol-’ will not be significant in a reaction whose rate constants are less than about 3 X lo9 M-1 sec-l. (3) E. Rabinowitchand W. C. Wood, Trans. Faraday SOC., 32, 1381 (1936). (4) S. R. Logan, ibid., 63, 1712 (1967). ( 5 ) J. K. Thomas, S. Gordon, and E. J. Hart, J . Phys. Chem., 68, 1524 (1964). (6) E. Cercek and M. Ebert, ibid., 72, 766 (1968). (7) K. H. Schmidt and W. L. Buck, Science, 1 5 1 , 70 (1966). (8) M. Anbar, 2. B. Alfassi, and H. Bregman-Reisler, J . Amer. Chem. SOC.,89, 1263 (1967). (9) M. Anbar and E. J. Hart, J . Phys. Chem., 71, 3700 (1967). (10) E. M. Fielden and E. J. Hart, Trans. Faraday SOC., 63, 2976 (1967).
Mass Spectra of Hydrolyzed Bromine Fluorides’
by Eric N. Sloth, Lawrence Stein, and Clayton W. Williams Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60@0 (Received August 1.4, 1968)
A bromine(VI1) oxyfluoride, BrOzFa, has been observed in samples of partially hydrolyzed bromine pentafluoride examined with a modified2 Bendix Timeof-Flight mass spectrometer. The oxyfluoride is an analog of the recently discovered perbromate compounds.384 A bromine(V) oxyfluoride, Br02F, previously reported by Schmeisser and Pammer,s has also been observed in samples of partially hydrolyzed bromine trifluoride. The higher-valent compounds are believed to arise from disproportionation of BrF3 and BrF6 during hydrolysis. An unexpected impurity, BrC1, was found both in the original and product fluorides. In hydrolyzed bromine trifluoride, the observed masses correspond to the ions Br+ HBr +
BrO HBrO + BrF + +
BrOz+ BrCl+ BrF2+
BrOzF + BrFs+
Brz +
In hydrolyzed bromine pentafluoride, the observed masses correspond to the ions Br+ HBr+
BrO+
BrOz+
BrOa+
BrOzFZ+
Br02F3+
HBrO+ BrF+
BrCZ+ BrFz+
Br02F+ BrFa+
BrF4+ Brz+
BrF&+ BrlO +
The italicized ions arise from parent species of the (1) Based on work performed under the auspices of the U. S. Atomic Energy Commission. (2) XI, H. Studier, Reo. Sci. Instr., 34, 1367 (1963). (3) E. H. Appleman, J . Amer. Chem. SOC.,90, 1900 (1968). (4) M. H. Studier, ibid., 90, 1901 (1968). (5) M. Schmeisser and E. Pammer, Angew. Chem., 67, 156 (1955).
same m s a formula whereas the others are primarily fragmentation products. Bromine trifluoride and bromine pentduoride were obtained from the Matheson Chemical Co. and purified by distillation in the Chemical Engineering Division at Argonne. Infrared spectra revealed only small amounts of CF. and CzFa in the pentduoride; the spectra otherwise agreed with those in the literature.BJ Approximately 5-g samples of the fluorides in Kel-F test tubes or nickel containers were frozen with liquid nitrogen. Water was added by vacuum distillations until 0.1 to 0.5 g of ice was present as a separate layer. The samples were allowed to thaw, whereupon vigorous and sometime8 violent reactions occurred. Oxygen was liberated both by the hydrolysis and by the partial decomposition of bromine oxide and oxyfluorides. This was pumped off after refreezing Vapors were then analyzed as the products at -196'. the products warmed slowly to room temperature and distilled into cold traps on a manifold made of '/,-in. Kel-E' tubing, which comprised the inlet to the mass
I Cli
I
E;
I BrF'
I BrCi Br F:
Y. BF ;;
HE:
I Er:
Figure 1. Mam spectrum of bromine trifluoride containing several volatile impurities.
spectrometer. Ions produced by electron impact on the vapors were identilied by their masea and characteristic isotope abundance patterns. In some instances, hydrolyzed samples were distilled at -20 to 0 ' in a copper and Monel still fitted with cooling coils. The distillate fractions were then stored for periods up to 1 week in Kel-F test tubes woled with a carbon dioxide-trichloroethylene mixture. Since carbon dioxide and trichloroethylene M u s e through Kel-F plastic, mass spectra of both compounds were observed. The trichloroethylene was eliminated in later experimenta to avoid mass spectral interference. I n the hydrolyzed samples, ions arisiig from water, nitrogen, oxygen, hydrogen fluoride, hydrogen peroxide, carbon monoxide, and carbon dioxide were observed as well as CFa+, COF+, COFZ+, COBr+, COFBr+, and other ions attributable to Kel-F. The most abundant ion was OZ+. Figure 1 shows the maw spectrum of bromine trifluoride which still contained chlorine, bromine mono-
I
I Br0; BrCI' BrF2+
Br02F' BrF3+
Figure 2. Mass spectra of producta from partially hydrolyzed bromine tdluoride (Br*+pealu, not shorn).
chloride, and bromine impUrities after distillation. Due to the higher volatility of the impuritiea, their maw peaks appeared sooner and sometimes at higher intensities than the m s a peaks of the trifluoride. The doublet of the parent ion BrF8+ was prominent, whereas in spectra of bromine pentafluoride, the doublet of the parent ion BrFs+ appeared with very low intensity. I n partially hydrolyzed bromine trifluoride, BrF and BrChF were present as independent species (Figure 2). Both were ascribed to the reaction ZBrF,
. , I
I BrO' HBrO' BrF'
I
E:
+ 2H,O
---t
BrF
+ BtOzF + 4HF
Diatomic bromine was also present and contributed to the formation of BrF through reaction with bromine tri8uoride. Several bromine oxides and oxyfluorides were observed in partially hydrolyzed bromine pentduoride (Figures 3-6). BrOzF was present in most samples and was ascribed to the reaction BrFs
'+ 2Hz0 +Br02F + 4HF
1
1
I
I
Br+
Brb' Br&+ BrOZF+ B r F 2 BrFc HBrO' BrCI' BrF; Brz BrF' BrFZ'
I Hg+
Figure 3. Mam spectra of producta from partially hydrolysed bromine pentrduoride (Hg+ peaks due to mercury from diHuaion pump). (6)
H.H.C-,
B. Weinnto&, and J. 0. Mdm. 3. C M . Phw..
28. 285 (1868). (7) R.8. M c D o d and L. B. bPmY, M.,37,166 (1882).
Vol-
75.
Nu&
1 JMWY lseS
Now
280
I
I
I
B r o i l BrFz
I
BrOZF'BrF:
BrCI+
Figure 4. Mass spectra of bromine pentduoride produotn on expnded scale; BrOtF+prominent, BrOF,+ a k n t .
Br02F;
Br;
Figure 5. Partial maPS Spectrum of bromine pentduoride products showing BrO,F.+ and Br,+.
, I
1
I
Ill
Figure 6. Partial mass spectrum of bromine pentduoride products showing Bn+ and BrrO+.
BrOZFa was found in two end fractions of distilled hydrolysate which had been stored for 5 days at -78'. The first fragment, BrOzFz+,was seen at moderately high intensity, and the parent ion, BrOzF*+, a t low intensity. This product was ascribed to the reaction 2BrF,
+ 2HZ0
--P
BrFa
+ BrOzF8+ 4HF
although bromine trifluoride could not be identified as an independent species due to the interference of bromine pentafluoride.' I n earlier mass spectral studies of halogen fluorides, Irsa and Friedman9 observed bromine oxyfiuoride ions in BrFa, BrFs, and BrFs-Oz mixtures, which were ascribed to the parent BrOFa. I n the present study The J w d of Ph&d
Chemidw
there was no evidence of this species, as BrOF*+ and BrOF+, the expected fragments from BrOFr, were not observed. Sin= the two strongest m w peake of. B e l + coincide with the peaks of BrOF+, bromine monochloride impurity, if present in Irsa and Friedman's samples, may.have been ascribed to BrOF+. In almost all of our spectra, bromine monochloride produced peaks at 114, 116, and 118 mass units with intensity ratios of approximately 3:4: 1. This pattern can be noted clearly in Figure 4. In the hydrolyzed bromine pentnfluoride samples, BrO+, BIG+, BrOa+,and BhO+ were also observed. The first two appeared to be fragments from B a F , whereas the last two appeared to be independent species. (BrOa+might be a fragment from the parent BrOzF, but no ion corresponding to this formula was observed.) Twin peaks a t mass 206 and 208 were seen at low intensity several times and ascribed to the ion BrO%Fs+, possibly derived from a complex or a peroxy compound. A compound of this composition, 02BrF6,has been reported by Streng.'o Iodine monobromide was considered to be an unlikely snurce of the m a ? 206 and 208 peaks, since I+ion a t mass 127 was not detected. The ability of bromine monochloride to coexht with bromine trifluoride and bromine pentduoride was confirmed in the following manner. Equal parts of bromine and chlorine were mixed to provide an authentic sample of BrCl (Figure 7). One half of the mixture
Figure 7. Mass spectrum of bromine monochloride in a bromin-hlorine mixture.
was frozen at - 196" with a fivefold excess of bromine trifluoride and the other half with a fivefold excess of bromine pentafluoride. When the mixtures were warmed to room temperature and examined, BrCI+ peaks were found to be still prominent in the spectra.
Ackm-. We are indebted to Roger L. Jsny of the Chemical Engineering Division for the samples of bromine trifluoride and bromine pentafluoride.
-.
One oi the referem has augsestat reaetions involving hydrogen ~d alternatm to disproportionstion: HIO. BrF, 2HF and HIO. BrFi -+ FIrOzFi + 2HF. Several alternate reactions to disproportionation may be postulated using oxygeo-mntaining compounds or o x g w t m intermediates. (9) A. P. Irss and L. Friedman, J. I n m g . Nurl. Chnn.. 6,77 (195.8). (IO) A. 0. Strew, J . A m . G h m . Snc., 85. 1380 (1883). (8)
peroxide
BlO.F
+
+
+
