COMMUNICATIONS TO THE EDITOR
2038 tions for hindered alkyl nitroxides are I, 11, and 111.
perfluoroethyl and the ala-difluorobenzyl-t-butylnitroxide is found to dewease with increase in temperature for both radicals (Table I). This observation thus supports the conclusion that I1 is the preferred limiting conformation for DIP-difluoroalkylnitroxide derivatives and that it is for this reason that the @-fluorinecoupling increases with substitution as first observed by Strom and Bluhm. The ,&fluorine coupling situation when 8 = 90" is by I I1 no means clear. From INDO calculations Morokuma6 concluded that the coupling should be near zero when 0 = 90" whereas Kosman and Stock14as well as Underwood, et aLJ6show that a residual coupling should be present a t this angle. Experimental data differ for each system studied. Perfluoroisopropyl-t-butylnitroxide shows a relatively small @-fluorinecoupling which decreases in magnitude to a very small value when the temperature is lowered (Table I). This result is conThe @-hydrogen coupling in numerous examples indisistent with @-fluorinecoupling decreasing to zero as e cates that I is the most probable conformation. In+ 90". I n triptycene ~erniquinone,'~ however, the spection of space-filling models of the analogous @,@-di- bridgehead fluorine coupling is appreciable (approxifluoro derivatives indicates that I is not obviously better mately one-third of the value found for the freely rotatthan I1 or 111. This result is due mainly to the ining trifluoromethyl group) and a very large value for creased size of the fluorine atom which causes significant the @-fluorine coupling is found in 2-perfluoroazoprosteric interaction with the oxygen atom and the R' pane radical aniong ( a p F= 62.45 G). group in I.12 Conformations I1 and I11 appear at least (12) I n P,P-dichloro nitroxide derivatives this interaction very clearly equally favorable since the fluorine atoms interact less makes I the least probable conformation shown: E . G. Janzen, B. with R' or oxygen, respectively, in spite of the fact that R . Knaner, L. T. Williams, and W. B. Harrison, submitted for publication. R eclipses oxygen or R', respectively. (13) The same conclusion has been reached by Underwood, et C L L . , ~ Further consideration of the probable dipole-dipole based on a comparison of literature data on the preferred conformainteraction existing in such compounds leads to the contions of alkyl aldehydes and ketones with nitroxides of similar structure. clusion that I1 should be the most probable conforma(14) D. Kosman and L. M. Stock, J . Amer. Chem. Soc., 92, 409 tion for the difluoro nitroxide derivatives. Thus when (1970). R = phenyl
-
.-
UBPARTMENT OF CHEMISTRY UNIVERSITY OF GEORGIA ATHENS,GEORQIA30601
EDWARD G. JANZEN BRUCER. KNXUER JOHN L. GERLOCK
DEPARTMENT OF CHEMISTRY UNIVERSITY OF Iowa IOWA CITY, IOWA52240
KENNETH J. KLABUNDE
R4-o F
I
R I
11
I11
On the basis of the combined effect of steric and dipoledipole interactions it is our prediction that the limiting conformation preferred will be I1 in @,@-difluoroalkyl nitroxides.I3 This prediction should be testable. Thus if one assumes that the freedom of rotation of the substituent will increase with increase in temperature so that eventually the apparent average NCF dihedral angle will be 45' as in the freely rotating trifluoromethyl group, the average dihedral angle mill become smaller in I and larger in I1 with increase in temperature. Furthermore, if the angular dependence of @-fluorinecoupling approximately follows a cos2e relationship, as for @-hydrogen coupling, the fluorine coupling should increase in I and decrease in I1 with increase in temperature. The @-fluorinetemperature dependence for the The Journal of Physical Chemistry
RECEIVED FEBRUARY 16, 1970
Electrical Conductivity of Bromine Trifluoride'"
Xir: Karl ChristeIb has recently commented on the report by Toy and Cannon2 of the preparation and identification of difluorobromium tetrafluoroborate BrF2+ BrF4-. We were indeed skeptical of Toy and Cannon's report for the reasons Christe has outlined and are inclined to accept his conclusion, but his paper includes comments on the conductivity of bromine trifluoride that appear to be incorrect and may prove mis(1) (a) Work performed under the auspices of the U. s. Atomic Energy Commission; (b) K. 0. Christe, J . Phys. Chem., 73, 2792 (1969).
(2) M.S. Toy and W.A. Cannon, ibid., 70, 2241 (1966).
COMMUNICATIONS TO THE EDITOR
2039
leading to workers in the field. The conductivity of bromine trifluoride has been studied by several groups on separate batches of material purified by their individual techniques and in apparatus constructed of diverse materials. The observations of Banks, Emeleus, and Woolf3 and Quarterman, Hyman, and Kate4 are in excellent agreement both qualitatively and quantitatively and involve authentic bromine trifluoride samples of reasonably high purity. We have recently been studying the bromine trifluoride-hydrogen fluoride system which will be discussed elsewhere. Like bromine and bromine pentafluoride, whose effects were studied by Quarterman, et al., hydrogen fluoride acts to reduce the electrical conductivity of bromine trifluoride (Table I). For distilled material these three
Impurity
Conon, mol %
Impurity
None Brz
3.99
8.03 7.21
HF
BrFS
Specific Conductivity of BrFs
Temp, OC
Cond 10-3, ohm-1 om-'
10.5 18.0 25.0 30.0
8.34 8.20 8.03 7.97
Cond, ref 4
Temp, "C
Cond 10-3, ohm-' om-]
8.01 7.92
35.0 40.0 45.0 50.0
7.82 7.71 7.49 7.31
x
x
Cond, ref 4
7.80 7.65 7.47 7.27
Electrical conductivities were measured using polychlorotrifluoroethylene cells with bright platinum electrodes essentially as described in ref 4 and a Wayne Kerr autobalance precision bridge Model B331. (3) A. A. Banks, H . J. Emeleus, and A . A. Woolf, J . Chem. Soc., 2861 (1949).
Table I : Conductivity of Dilute Solutions of Volatile Impurities in Bromine Trifluoride Cond X lo-*, ohm-' om-1 (at 2 5 O )
Table I1 : Temperature Dependence of the
Cond X l o - * , ohm-1 Conon, om-' mol 7% (at 2 5 " )
3.0 8.1
7.30 7.52
volatile materials are the most likely impurities in bromine trifluoride. We believe that the high conductivity of this reagent is associated with a high mobility of the parent ions, perhaps via a chain conducting mechanism. For this reason we suspect that the effect of any impurity, whether an inert diluent or any solute other than a strong acid or base, will lead to a net reduction in the specific conductivity. After Christe's communication appeared, we reexamined a new batch of carefully distilled bromine trifluoride. We found as expected that removing volatile impurities led to a small increase in conductivity. Various batches showed values between 7.94 and 8.04 X ohm-' cm-l with a mean of 8.01 X loe3 ohm-' em-' at 25". I n a single case where a distilled bromine trifluoride sample was refractionated by a simple trap-to-trap distillation the distillate showed a conductivity of 7.89, the residue of 8.03 X ohm-' cm-l. In exploring the specific conductivity as a function of temperature the negative temperature coefficient was confirmed over the range 10.5-50.0" (Table 11) in quite good agreement with the 1957 measurements. At this time we are not prepared to speculate on the nature of the impurities that either Toy or Cannon and/or Christie were dealing with. We simply suggest that anyone working with bromine trifluoride with an electrical conductivity substantially below 8 X at 25' must be presumed to be working with a mixture and bears the burden of a more complete identification of his material.
(4) L. A. Quarterman, H . H . Hyman, and J. J. Kats, 61, 912 (1957).
CHEMISTRY DEPARTMENT ARGONNE NATIONAL LABOR.4TORY ARGONNE, ILLINOIS 60439
J. Phys. Chem.,
HERBERT H. HYMAN TERRY SURLES LLOYDA. QUARTERMAN
DEPARTMENT O F CHEMISTRY MICHIGAN STATEUNIVERSITY EASTLANSING, MICHIGAN48823
ALEXANDER POPOV
RECEIVED DECEMBER 16, 1969
Electrical Conductivity of Bromine Trifluoride
Sir: The discrepancy between our previously reported' conductivity values of BrFa and those of Hymari and coworkers2 has successfully been resolved. A reinvestigation of the BrF3 system showed that the BrF3 samples used in our study were of high purity and that the difference in conductivity could not be attributed to impurities as suggested by Hyman.2 Using the same conductivity cell as in our original experiments, our previously reported data' could be reproduced. However, it was found that the measured resistance was dependent on the applied ac voltage indicating interfering reactances. When a new conductivity cell (having larger electrode surface areas and a much higher cell constant) was used, no detectable interferences were encountered and values were obtained which differed by less than one per cent from those obtained by Hyman.2 Consequently, the values given in the preceding paper2 are confirmed. (1) K. 0. Christe, J . Phys. Chem., 73, 2792 (1969).
(2) H. H. Hyrnan, T. Surles, L. A . Quarterman, and A. Popov, ibid., 74, 2038 (1970).
ROCICETDYNE, A DIVISIONOF NORTH AMERICAN ROCKWELL CORPORATION CANOGA PARK, CALIFORX~A 91304
KARL0. CHRISTE
RECEIVED FEBRUARY 24, 1970 Volume 74, Number 0 April 80, 1070
