July, 1959
NON-RESONANT
MICROWAVE ADSORPTION IN H.4LOGEN SUBSTITUTED METHANES
markable that one can work so close to the solubility limit in these surface-chemical systems, and future research along such lines on surface activity in non-aqueous systems is inviting. It should be noted that the fluorochemicals found most surface active 'by measurements of YLV vs. c and calculation of Zl Y L V / ~ C (or of r/c) are among those found most surface active by Jarvis and ZismanZ1through observation of the spreadability. Under the most favorable conditions there may be a surface tensioii lowering AYLVof from 10 to 20 (21) L. Jervis and W. A. Zisman,
THISJOURNAL, 63, 727 (1959).
1127
dynes/cm. with some fluorochemicals, and with such systems monolayer adsorption of solute may occur. This leads to the firm conclusion that fluorochemicals call have high surface activity in organic solvents. The effect is greatest with solvents of highest surface tension and solutes of lowest surface tension. Of course, the arrangement of the fluorine atoms in the molecules is important. A correct oleophobic-oleophilic balance is one of the necessary attributes. It also appears that the atomic proportion of fluorine to hydrogen in the molecule should not be less than 0.5 or more than 2.0 for best results.
NON-RESONANT MICROWAVE ABSORPTION IN CERTAIN HALOGEN SUBSTITUTED METHANES BY JAMES E. BOGGS AND HARMAN C. AGNEW Department of Chemistry and Electrical Engineering Research Laboratory, The University of Texas, Austin, Texas Received November 89,1968
The absorption of microwave radiation of 9400 Mc. frequency by CH3C1,CTaBr, CHFa! CHC12F, CHClF,, CHzF2,CBr2F2 and CCW2 has been studied over a pressure range from 0.2 to 2 atm. a t 26 . Absorption in the first five gases is of the typical non-resonant type associated with molecular inversion transitions. CClzFz and CBr2F2show no appreciable absorption and the absorption by CH2F2 appears to be due to nearby rotational lines.
A number of workers recently have concerned themselves with inversion transitions in non-planar gaseous molecules, studying the effect by measuring non-resonant absorption in the microwave region2+ or by examining the frequency variation of the diFor symmetric electric constant of the top molecules it has been found possible to calculate the observed intensity and frequency variation of the non-resonant microwave absorption from the Van Vleck-Weisskopf equation for the pressurebroadening of a spectral line using the dipole moment matrix element for inversion and assuming the resonant frequency of the inversion to be essentially ze1-0.~ The same model has led to the calculation of the observed difference between the dielectric constant of the gas a t low frequency and that a t microwave frequen~ies.~Both procedures 1 PJK I require the evaluation of the quantity Z ~ J K where ~ J ip K the fractional number of molecules occupying the J K rotational energy level and I PJK is the dipole moment matrix element for inversion. This sum has been obtained in cloeed form only for symmetric top moleculns so that the quantitative treatment has been limited to such substances. Dielectric constant measurements on gases of lower
I
molecular symmetry, however, have shown that for these substances, also, the orientation polarization is lower a t 9400 Mc. than at low frequency, supgesting the same sort of molecular inversion. lo Certain non-planar molecules of C,, symmetry (CHzCL, CHzF,, CH2Br2and possibly CC12F2) have been shown to have the same dielectric constant a t 9400 Mc. as at low frequency within experimental error.1° We have now measured microwave absorption in CHC12F and CHCIFZ, for which dielectric constant data have been reported,' to determine whether or not the relationship between the absorption and the dielectric dispersion is that which would be predicted if both were caused solely by molecular inversion. We have also looked for absorption in CHsFZ, CBr:Fz and CClzFzto see whether the absorption measurements give any indication of inversion. Experimental
Two different methods were used to measure the absorption of microwave energy a t 9400 b4c. over a pressure range from 0.2 to 2 atm. All measurements were made at 26 f 2'. The first method was a fairly direct measurement of at,tenuation in a 100 foot coil of wave guide filled with the gas. A TSl47/I.JP U.S. Navy radar test set fed the signal through a calibrated attenuator and the sample cell to a crystal detector, amplifier and power meter. Tunable (1) This work has been supported by Air Foroe Contract AF transitions were used, and the system was tuned flat looking 33(616)-5581. both ways from the sample container. A slotted section (2) W. D. Hershberger, J. Appl. Phys., 17, 495 (1946). and tuning screw just before the sample cell allowed the (3) J. E. Walter and W. D. Hershberger, ibid., 17, 814 (1946). (4) B . Bleaney and J. H . N. Loubser, Proc. Phus. Soe. (London), voltage standing wave ratio to be monitored and controlled at that point. A directional coupler with power meter perA63, 483 (1950). mitted the incident power to be monitored. In use, the (5) G . Birnbaum, J. Cham. Phys., 27, 360 (1957). sample cell was evacuated, the amplifier adjusted to indicate (6) Krishnaji and G . P. Srivastava, Phys. Rev., 109, 1560 (1958). some given power level, the gas introduced to the desired (7) J. E. Boggs, C . M . Crain and J. E. Whiteford, T H I S JOURNAL, pressure, and attenuation removed at the variable attenua61, 482 (1957). tor until the original power level was restored. This gave (8) J. E. Boggs, C. M. Thompson and C . M. Crain, ibid., 61, 1625 the attenuation coefficient of the gas from which the loss (1957). tangent could be calculated. (9) J. E. Boggs, J. Am. Cham. Sac., 80, 4235 (1958). With this apparatus the average deviation in the loss (10) J. E. Boggs, J. E. Whiteford and C. M. Thompson, THIS JOURNAL,63, 713 (1959). tangent measurements obtained for a given gas was 2.0%,
JAMES E. BOGGSAND HARMAN C. AGNEW
1128
although the absolute accuracy is felt to be considerably worse. The attainment of greater accuracy probabl waa prevented by the construction of the sample cell whicz consisted of a coil made up from eight semi-circular sections joined together. Each joint presented a small discontinuity to the radiation and produced some reflection of the wave. Although the wave was effectively flat before entering the sample, the reflections within the sample, which could not be tuned out, could interact so as to produce either an additive or subtractive effect on the apparent loss. The second method made use of a bridge circuit constructed of x-band wave guide. The signal from the TS147/UP test set passed through an attenuator to a magic tee which divided the power equally into the two arms of the bridge. One arm consisted of a phase shifter, the sample cell 1.05 meters in length and a variable attenuator. The other arm was similar with a physically identical sample container used to make the two arms of the bridge electrically equivalent. The signals from the two arms were summed by a second magic tee and fed into a third tee where the signal was mixed with the output from a local oscillator operating at a frequency 30 Me. above or below 9400 Mc. The resultant 30 Me. signal was amplified and displayed as power. To make a measurement, the sample cell was evacuated and the bridge was balanced to minimum signal by adjusting the phase shifters and variable attenuators. The balance was sufficiently precise that the ratio of summed to subtracted signals was in excess of 60 db. a t all times. The sample then was introduced and the brid e rebalanced to minimum signal using only the phase shifters, the power level of the minimum being noted. The sample cell was again evacuated, the bridge was rebalanced to maximum power using the phase shifters, and attenuation was introduced with the calibrated attenuator in the TS147/UP signal source until the power output of the bridge was reduced t o the level of the minimum with the sample in the sample cell. This attenuation, read in decibels, is equal to 20 log [( 1 eT*l)/( 1 - e - a l ) ] , where 1 is the length of the sample and a is the attenuation coefficient of the gas. The accuracy obtained by this second method was again not as high as had been anticipated. The over-all accuracy can only be checked by comparison with measurements by other methods that have been reported for certain gases as shown below. While neither method of measurement gave results of as high accuracy as needed to study the fine details of the absorption, the accuracy was sufficient to answer the original questions proposed.
Vol. 63 TABLE I tan 8.tm
x 104 CHBCl 2.5 3.0
Method 1 Method 2 . Bleaney and Loubser4 Birnbaums Krishnaji and SrivastavaG Theoretical6
2.7
CH3Br Method 2 2.2 Bleaney and Loubser4 Birnbaum6 1.6 Krishnaji and Srivastavaa Theoretical5 Method 2 Birnbaum' Theoretical6
CHFa 4.2 3.7
(tan 8 / p h x x 107
4.6 5.2 4.8
AY/P 104
x
I .8 1.9 1.G 1.5 1.6
5.4 3.8 3.8
1.3 1.6 1.9 1.6
4.1
14.8 16.3 18.3
0.8 0.7
curvature at low pressure caused by the effect of nearby rotational lines. The data for CHClzF gave values of (tan 6/p)max = 6.5 X IO-' mm.-l and Av/p = 1.3 X cm.-I/mm. If it is assumed that only a low-frequency inversion transition is responsible for the observed loss and also for the dielectric dispersion observed by Boggs, Crain and Whiteford,' the dielectric constant measurements can be used to calculate a value of 5.7 x IO-' mm.-I for (tan 6/ p)max. This agreement is as good as can be expected considering the uncertainty in the dielectric constant measurements as well as those in the present, investigation. The agreement would suggest that essentially all of the observed electrical interaction is due to the inversion transition and that the low-frequency rotational lines are weak enough to have little effect. Results and Discussion Absorption measurements also were made on I n order to evaluate the accuracy of the experi- CHClF2. For this substance the plot of p2/tan 6 mental methods, measurements were made on ab- against p2was again a straight line as expected, but sorption by CHaC1, CH3Brand CHFI, for which loss the slope of the line was so small that major uncertangents a t 9400 Mc. have been reported by other tainties were introduced into the calculation of (tan investigators. As shown by B i r n b a ~ mif, ~the loss and A v / p from the graph. The published tangent is measured at various gas pressures md dielectric constant data7 would predict a value of if the loss is due entirely to a pressure-broadened (tan G/p)m,x of 6.6 X mni.-l, a value which a t low-frequency inversion transition, a plot of p3/tan least is not inconsistent with our loss measure6 against p2 should give a straight line. The maxi- ments. mum absorption, (tan 6 / ~ ) , , ~ , and line broadening Measurements were also made on CHzF2,CBr2F2 constant, A y / p , are related to the slope m and the and CC12Fato check the dielectric constant data intarcept c of this line by the equations showing that there was no measurable dispersion in A v / p = (rn/c)% these gases. The interpretation of the absorption data is complicated by the fact that low-frequency and rotational transitions can contribute to the loss a t (tan 6/p),,a = (4mc)-'/a 9400 Me. Thus, a measurement showing no loss is Table I shows the observed loss tangent a t one at- significant, while one showing appreciable loss mosphere pressure and the values of (tan 6/p)max in would not necessarily be. mm.-' and Av/p in cm.-l/mm. for the reference Measurements of microwave absorption in CBr2gases. Of the various measurements shown, those F2and CC12F2showed that there is no measurable by Birnbaum are probably most accurate. loss at 9400 Me. Any loss which occurs must The remaining gases were studied by the Fecond correspond to s1 tan 6 of less than 0.2 X at 1 method described above. For CHClzF and CHC1- atmosphere pressure. Thus, for these two comF%the typical non-resonant absorption curve was pounds the absorption measurements support the diobtained in a plot of @/tan 6 against p 2 . The electric constant experiments in showing that low curve was linear at the higher pressures with a frequency radiation cannot produce a n inversion
+
