COMMUNICAT~ONS TO THE EDITOR
2791
nisms will combine to give a rapid decrease in the hydroxyl group splitting constant ( ~ 0 ” ) when the group is twisted out of the plane. Thus czOHM = - A (cos20)
+ B (sin2 e)
where A and B are constants and the ( ) brackets denote a time-averaged value. The methoxyl group, on the other hand, couples via a hyperconjugative mechanism to the unpaired spin on oxygen, which is at a maximum when the methoxyl group is in the plane of the ring and will decrease as the methoxyl group is twisted out of the plane such that) aoCHaH =
c (COPe)
The above discussion provides a basis for expecting a greater temperature variation for the hydroxyl group. Further calculations enable us to estimate the depth of the potential well from the temperature dependencies. This may be done by evaluating (cos2 e) for various temperature and barrier heights, using the procedures of Stone and Maki.ll A comparison of calculated and experimental temperature dependencies leads us to estimate the barrier heights for VI, VII, VIII, and I X as 16 i 4,5 f 1, -0.8 and -1.8 kcal/ mol, respectively. Further work on the hydroxyl compounds should enable us to similarly obtain their potential barriers which may then be compared with the potential barriers already obtained by line-width alternation studies.12
Aclcnotdedgments. The author wishes to thank Dr.
J. R . Bolton for his continued encouragement and helpful comments and also Dr. G. Vincow for a copy of his computer program. (11) E. W. Stone arid A. H. Maki, J. Chem. Phys., 37, 1326 (1962). (12) P. D. Sullivan, J . Amer. Chem. SOC.,89, 4294 (1967).
DEPARTMENT OF CHEMISTRY UNIVERSITY OF MINNESOTA MINNEAPOLIS, MINNESOTA55455
PAUL D. SULLIVAN
RECEIVED APRIL10, 1969
The Electron Attachment Cross Section for Hexafluoroacetone
Sir: Among the processes which may occur when an electron suffers a collision with a molecule is electron capture, a negative ion being formed AB
+ e +AB-
Although such interactions have been studied extensively, in few cases has the molecule-ion been suf-
ficiently stable to be detected, the more usual process being the dissociative capture reaction AB
+ e +A- + B
SF6- is the classic example of a stable moleculeion1a2and the SFe- abundance curve has been used to mirror the electron energy distribution and calibrate the electron energy scale. Recently, we reported that hexafluoroacetone formed a stable negative ion3 as a result of secondary electron capture, but we were unable to observe its formation at very low electron energies. Using an improved experimental technique, we now report the formation of the CFsCOCF3- ion at low electron energies following primary electron capture and have obtained a value for the electron attachment cross section of the ketone, relative to that for sulfur hexafluoride. Results were obtained using a Bendix time-of-flight mass spectrometer, Model 3015. The electron energy was measured using a Solatron digital voltmeter LM 1619 and “negative” voltages were obtained by incorporating a 3-V dry cell into the electron energy circuit. Using two channels of the mass spectrometer analog output scanners enabled the SFs- and CF3COCFs- ions to be measured simultaneously on l-mV Kent potentiometric recorders. The electron current was kept constant automatjcally over the energy range studied. Ion source pressures were usually maintained below 5 X 10-6mm. When hexafluoroacetone (HFA) was studied at. electron energies -0 eV a fairly abundant parent ion was observed; admission of sulfur hexafluoride to the ion source considerably reduced the intensity of the parent ion, suggesting that the attachment cross section for the reaction SFa
+e
1 ----f
SFs-
was much greater than for
CFsCOCF3
+ e 2,CF3COCF3-
I n Figure 1 (full circles) we show the data obtained for SFa- ion formation using a 50-50 mixture of HFA and SFs; “negative” voltages were obtained by introducing a 3-V dry cell into the electron energy circuit, The smooth curve reflects the electron energy distribution. I n Figure 1 (open circles) we report our experimental data for the CF3COCF3- ion, the ordinate being 58.9 times more sensitive than that for SFe-. It is apparent that the two curves have a very similar distribution, both reaching a maximum value at the same electron energy. The ketone is slightly broader in the wings; (1) W. M. Hickam and R. E. Fox, J . Chem. Phys., 25, 642 (1956). (2) G. J. Schulz, J . A p p l . Phys., 31, 1134 (1960). (3) LJ. C. J. Thynne, Chem. Commun., 1075 (1968).
Volume 73, Number 8 August 1969
COMMUNICATIONS TO THE
Accordingly, using a 39.2: 1 mixture of HFA-SFs, we measured the intensities of the HFA- and SFG-ions a t ten electron energy intervals over the range 15-60 eV. Our data indicated a constant value for I(SFo-)/ I(HFA-) of 1.44 f 0.06 over the entire energy range, indicating that a(SFe)/a(HFA) = 56 f 2. This ratio is in good accord with our directly measured value a t low electron energies.
a 00
e. 0
e 0
0
.
o 0
0 e 0
0
0
e
(4) R. N. Compton, L. G. Christophorou, G. S. Hurst, and P. W Reinhardt, J . Chem. Phys., 45, 4634 (1966). (6) A. J. Ahearn and N. B. Hannay, {bid.,21, 119 (1963).
. 0
0
e
DEPARTMENT OF CHEMISTRY EDINBURUH UNIVERSITY EDINBURUH 9, SCOTLAND
e
0
0
e
d
EDITOR
P. HARLAND J. C. J. THYNNE
RECEIVED APRIL28, 1969
0
0
0
P
00
P Comments on “The Electrical Conductivity of +I
0
I
I
-I
-2
-3
-4
Boron Trifluoride in Pure and Mixed Halogen
Uncorrected electron energy, V
Figure 1. Ion current us. electron accelerating energy: full circles, SFe-; open circles, CFaCOCF3-. Ion current scale for I-IFA 58 ‘ 9 times more sensitive than that for SFg.
this may reflect a slightly different energy dependence for electron attachment or the experimental inaccuracies in measuring very small ion currents. We therefore conclude that, because of the similar energy dependence, the relative peak heights may be used to evaluate the relative attachment cross sections for reactions 1 and 2. Denoting the X- ion current by I(X-), the electron attachment cross section of X by b(X) and the ion source pressure of X by [XI, then a(SF6) - I(SFa-) [HFA] -a(HFA) I(HFA-) [SFs] We have assumed that both ions have the same collection efEciency. Our experimental data indicate that a(SFe)/a(HFA) = 58.9. By means of an electron, ~ measured a swarm technique, Compton, et ~ l . have value of 3.6 X 10-l6 cm2 for a(SF6). Using this result in conjunction with the above data yields a(HFA) = 0.61 X cm2. Ahearn and Hannay5 have reported that at electron energies >14 e v , SF.5- ion formation occurs as a result of secondary electron capture, the secondary electrons being produced by such processes as
+ e -+-SFbf + F + 2e
Fluorides,” by M. S. Toy and W. A. Cannon Toy and Cannon1 have reported the preparation and identification of difluorobrominium tetrafluoroborate, BrFz+BF4-. A recent study of bromine fluorides in our laboratory resulted in data quite different from those reported by Toy and Cannon for Sir:
Table I : Specific Conductivity of Liquid and Solid BrFs Sp conductivity, ohm-1 am’-1
Liquid 2.32 x 2.44 x 2.66 x 3.00 x 3.42 X 3.00 x 2.71 x
0.2“ 5.8“ 12.2 21.6 36.8 53.0 19.4b
10-3
IO-s 10-3 10-3 10-3
Solid
