KOTES
a t very lorn frequencyQincluding the strong v7 vibration a t 429 cm.-l and the very strong v12 vibration a t 750 cm.-I. Thus ail atomic polarization of 2.82 cc. does not appear unreasonable for Y 2 0 4 . From internal evidence, the work of Zahn5 appears to be less precise than that of S ~ h u l z . Under ~ corresponding conditions of temperature and pressure, the results of Zahn are a little higher. On reanalysis of Zahn's data, it is found that dipole moments of 0.316 D. for NOz and 0 D. for S20a and distortion polarizations of 8.2 cc. for 9 0 2 and 22 cc. for N204will reproduce the data with an average error of only about 1.5%. Again, a small atomic polarization for KO2 and a large atomic polarization for S 2 0 4 are indicated. The data of Williams, Schwingcl, and WinningK also appear to have less precision than those of Schulz and, while they are roughly the same, they exhibit a pressure variation that is not compatible with the analysis suggested here nor with the data of Schulz or of Zahn. It appears that the best dielectric constant data in the literature are compatible with the microwave dipole moment measurement for NOz and with a zero moment for X204,and that previous difficulties in interpretation arose from ignoring the possibility that N201 might have a large atomic polarization. The atomic polarization of S201is on the order of 3 cc., but a more precise determination must await measurements of infrared absorption intensities.
Generation o f Acidity in Silica Gel
by Ionizing Radiation
by C. Barter and C. D. Wagner Shell Deve1opmen.t C o m p a n y , Emeryeille, Californid (Reeeized M a r c h 20, 1.964)
Irradiation of silica, gel has been reported to cause the formation of hydrogen atonis1V2and color centers. These entities, however, do not appear to be connected with the enhanced catalytic activity for hydrogendeut,erium exchange and ethylene hydrogenation shown by the irradiated gel4 since the hydrogen atoms are not stable above -120°1 and irradiated silica gel retains catalytic activity after color centers have been removed by chemisorption of hydrogen or bleaching by irradiation with ultraviolet. Recently, it has been reported that carbonium, ions are formed when silica gel and certain adsorbed hydrocarbons are ir-
2381
radiated sim~ltaiieously.~On this basis it has been suggested that irradiation increases the acidity of silica gel and that this may account for its catalytic activity for hydrogen-deuterium exchange. However, t,he fact that carbonium ions were found when hydrocarbon and gel were irradiated together leaves open the question that acidity can be manifested in the gel in the absence of hydrocarbon. The investigation reported here is a study of the effects of radiation on silica gel by chemical investigation after irradiation. Detection and ineasurement of acid sites is by indicators and by polymerization of isobutylene.
Experimental Gel Preparation. Three grams of. Davison 950 sXca gel (surfa,ce area 625 ni.z/g.) was pretreated in air a t 520' for several days and then was degassed a t 520° under high vacuum for 20 hr. After the system was sealed off, the gel was irradiated a t -196' vYith bremsstrahlung from the gold target of a 3-Nev. Van de Graaff to a total dose of 3 X lo7 rads, in 1 hr. Irradiated samples were maintained a t - 196' until used. Measurement of Acidity. One-tenth iiiilliliter of a 5.4 X M solution of p-dimethylaniinoaeobenzene in carbon tetrachloride was degassed by successive freeze-thaw and pumping operations; it was added to the gel through a break-seal. Gel suspended in reagent was transferred by pipet to a spectrophotometer cell for recording of the absorption spectrum. Addition of pure n-butylamine, for titration purposes, was made by a micro hypodermic syringe directly to the optical cell. I n experiments with triphenylmethane and anthracene, the 50 mg. of hydrocarbon was distilled under vacuuni onto the gel. Measurement of Polymerization Activity. For this experiment, Phillips research grade isobutylene (8 nil.) was distilled in vacuo through a break-seal into the gel a t -196'. The mixture was then warmed to - 78" and stirred magnetically. After a specified time of polymerization, 10' nil. of n-hexane was added by distillation. The mixture was then brought to rooin temperature. The supernatant solution of (1) V. R. Kazansky, G. B. Pariisky, and V. V. Voevodsky, Discussions Faradnu Soc., 31, 203 (1961).
(2) P. H. Emmett, K. Livingston. H. Zeldes, and R . J. Kokes, J . P h y s . Chem., 6 6 , 921 (1962). (3) H. W. Kohn, .l'ature, 184, G30 (1959). (4) H. W. Kohn and E. H. Taylor, Proceedings of Second International Congress on Catalysis, Editions Technip, Paris, Vol. 2, 1962, p. 1481.
( 5 ) H. TV. Kohn, J . P h y s . Chem., 6 6 , 1185 (1962).
V o l u m e 68, N u m b e r 8
August, 196.4
XOTES
2382
polyisobutylene was decanted from the gel, and the solvent and unreacted isobutylene were removed by distillation. The weight of residue was taken after pumping on the residue until it reached constant weight. Results and Discussion
Spectrophotometry of ilcid Reactions. Absorption spectra of organic molecules adsorbed on silica gel are easily obtained with suspensions of the gel in a solvent of suitable refractive index. Spectra so obtained on gel with adsorbed p-dimethylaminoazobenzene in carbon tetrachloride are shown in Fig. 1. Spectra of the acid and base forms were obtained by adding small amounts of acetic acid or butylamine to the suspension of unirradiated gel in the indicator solution. Addition of the indicat,or alone to baked-out but unirradiated gel gives the spectruni shown with a small acid component (visually the color is ~ d l o whowever). , -4ddition of the indicat,or to irradiated gel gives almost the typical acid spectrum. Sddition of excess butylamine then gives a basic form. This basic form, though it is visually yellow, has a different absorption spectrum from the basic form on unirradiated gel, showing that the chemical properties of the surface are changed in ways other than just the production of strong acid. These changes are reversible, for heating irradiated gel a t 500' for 10 hr. reniove,d both effects of radiation. The amount of strong acid generated in the gel was determined by titration with butylamine. Addition in small increments reduced the optical density a t 5500 A. approximately linearly until it reached the density characteristic of unirradiated gel. This amount was taken as the amount of acid generated by radiation. A dose of 3 X 107 rads produces about 3 pmoles of acid sites/g. Distillation of triphenylmethane onto the colorless preirradiated silica gel resulted in an immediate yellon- coloration. A similar effect was observed by KohnJ5but as a result of the simultaneous irradiation of silica gel and triphenylmethane. In the present case an absorption spectrum of a slurry of the colored gel, in isooctane solvent, showed that the yellow color was due to triphenylnlethane cation,E demonstrating that hydride abstraction from triphenylniethane had taken place.'S8 hnthracene distilled onto preirradiated silica gel resulted in a green coloration, but in this case the absorption spectrum of the colored gel was not determined. Kohnj observed a similar coloration following the simultaneous irradiation of silica gel and anthracene and concluded that the carbonium ion of anthracene had been formed since anthracene adsorbed on silicaThe Journal of Physical Chemisfry
alumina results in a similar, visually green coloration. However, it has been pointed outg that anthracene adsorbed on silica-alumina may be present as a positive molecule ion as well as in carboiiium ion forms since the absorption spectrum of adsorbed anthracene shows a band a t 7500 -%.lo which has been attributed to the positive molecule ion of anthraceiie.l1
~
A B AI BI N
~~~~
~
- Acid
-
-
Form, U n i r r a d i a i e d Basic Form Unirradiated Acid F o r m . I r r a d i a t e d Basic F o r m . I r r a d i a t e d Indicator Added 10 m i i r r a d i a t e d Gel
k
c m
A 4
,g 0" 4
2000
3000
ti000
6000
10
Figure 1. Absorption spectra of p-dimethylaminoambenzene on irradiated and unirradiated silica gel.
Polymerization Studies. When polymerization was effected using 5 g. of isobutylene with 3 g. of irradiated gel, approximately 0.5 g. of polymer was obtained. Its molecular weight by ebullioscopic and osmometric methods mas 2400. Correlation with the number of acid sites gives about 10-20 polymer molecules formed per acid site. Blank tests were niade to determine whether polyinerization could be effected by (1) unirradiated silica gel and (2) the irradiated flask with no silica gel. Less than 0.002 g. of polymer was formed in each blank experiment. L. C. bnderson, J . Am. Chem. Soc.. 57, 1673 (1936). (7) H. P. Leftin and R. K. Hall, ref. 4, Vol. 1, 1960, p. 1353. (S) H. 1'. Leftin. J . P h y s . Chem., 64, 1714 (1960). (9) D. lf. Brouwer, Chem. I n d . (London), 177 (1960). (10) R. M. Roherts, C. Barter, and H. Stone, J . PhUs. Chem., 63, 2077 (1959). (11) W. P.iialbersberg, G. J. Hoijt,ink. E. I,. Mackor, and W. P. Weijland. J. Chem. Soc., 3049 (1959). (6)
2383
NOTES
A test of yield vs. time of polymerization with irradiated gel ( 3 X 107-rad dose) showed the maxinium yield to be approached asymptotically, with 90% of the maximum reached in 1 hr. A study wax made of the effect of dose; a t a dose rate of 3 X l o 7 rads/hr., a maximum yield was again approached asymptotically, with 9Oy0 of thie maxinium value achieved at a dose of about 1.5 X 107 rads. It was found that pretreatment of the gel is important in developing activity for polymerization by irradiation. Treatment of the gel in vacuo a t 25' for 20 hr. prior to irradiation gave a gel completely inactive for polymerization. Baking out in vacuo at 520' for 20 hr. before irradiation gave only about half the polymerization activity attained by a bake-out time of 40 hr. Decay of Acid Sites. Experiments were conducted in which irradiated gel was stored a t 25 and 100' for specified periods before addition of p-dimethylaminobenzene or addition of isobutylene. The weight of polyisobutylene and the titration with butylamine to determine the amount of strong acid were used to determine relative activities. There was a parallelism between the two properties of the gel. Half-time for decay appeared to be about 2-3 hr. a t 25' and 1-1.5 hr. a t 100". It is of interest to note that although the half-time for decay a t - 196' must be many hours, a saturation activity was reached upon irradiation for only 1hr. This must mean that either there is a limited number of sites that can be made acidic, or there is a, radiation-induced decay process which causes a steady-state activity to be reached in only short irradiation time. The parallelism in decay rate of acid centers and those responsible for isobutylene potynierization probably means that the two types of centers are identical. Radiation-induced acidity in silica gel may well play an important role in its enhancement of G-value for certain radiation-induced polymerizations. l 2 The storage of chemically active centers generated by radiation in silica gel may be an effect similar to that noted by Hentz13 in silica-alumina; he found that irradiated silica-alumina could convert isopropylbenzene t o benzene under conditions where it is normally inactive. Yan~amoto'~observed evidence for acidity developed in Kaolinite by irradiation.
The Dipole Moment of Trifluoronitrosomethane'
by James E. Boggs, DeWitt Coffey, Jr.; and Jeff C. Davis, Jr. Department of Chemistry, the C n i w r s i t y of Texas, A u s t i n 12, Texas (Receioed April 9, 196.4)
From tabulated bond moment values2and any reasonable assumption about the C--N=O bond angle, the dipole moment of CE'aSO may be predicted to be in the range of 1.5-1.9 D., with the oxygen end of the molecule negative. An unsuccessful attempt to observe lines in the niicron ave spectrum of the compound in this laboratory suggested that the actual dipole nionient is much less than this predicted value. We have, therefore, measured the molar polarization of CE'3SO and obtained an estimate of the dipole moment using the apparatus and methods described earlier. The sample used v-as prepared by the method of Mason and Dunderdale4and purified by fractional distillation on a vacuum line. It was carefully shielded froiii light after purification to prevent dimerization. The molar polarization of CF&O, extrapolated to high p r e ~ s u r e was , ~ found to be 14.0 f 0.2 cc. at 296O, where the indicated uncertainty includes the effect of deviation from ideal gas b'ehavior. The electronic polarization is estimated to be 10.9 f 0.2 cc. This value is obtained using 1.83 cc. as the group refraction of C--F2 and 5.4 f 0.2 cc. as the group refraction of C-N=O, the latter based on the measured refractive indices of several halogenated aliphatic nitroso coiiipounds.6 Using the standard assumption that the atoniic polarization is approximately 10% of the electronic polarization, one may obtain a value of 0.31 =t 0.03 D. for the dipole moment of CI:3B0. The uncertainty does not include the uncertainty in estimating the atomic polarization, and, for reasons described below, 0.31 D. is probably an upper limit for the dipole moment. (1) This work was supported in part by 11 grant from the National Science Foundation and in part by a grant from the Welch Foundation. (2) C. P. Smyth, "Dielectric Behavior and Structure," McGraw-Hill Book Co., New Tork, N. P.. 1955. (3) A. B. Tipton, A . P. Deam, and J. E. Boggs, J . Chem. Phys., 40, 1144 (1964).
(12) R. Worrall and A. Charlesby, Intern. J . A p p l . Radiation Isotopes, 4 , 84 (1058); It. Worrall and S. H. Pinner, J . Polymer Sei., 34, 229 (1959). 113) R . R . Hentz, J . Pltus. Chem., 66, 2714 (1962).
(14) D. Yamamoto, N i p p o n K u g n k u Zasshi, 83, 115 (1962).
(4) J. Mason and J. Dunderdale, J . Chem. SOC.,749 (1956).
( 5 ) J. E. Boggs and A.
P.Deam, J . Chem. Phus.,
32, 315 ( 1 9 0 ) .
(6) J. D. Park. A. P.Stefani, G. H . Cmwford, and J. R. Lacher, J . Org. Chem., 26, 3316 (1961); J. D . Park, A. P.Stefani, and ,T. R. Lacher, ibid., 26, 4017 (1961).
Volume 68, .Vumber 8
A u g u s t , 1.964
