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
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2384
In searching for an explanation for the low value of the dipole moment of CF&O, a comparison with the nitrosyl halides is fruitful. For SOF, SOC1, and NOBr, the nitrogen-halogen bond lengths are longer by 0.18, 0.29, and 0.30 respectively, than would be expected for normal covalent single bonds.71~ This observation has been explained by assuming appreciable contributions from an ionic structure, ( X s O ) +X-. Further evidence comes from an analysiss of the vibrational force constant of nitrosyl chloride which yields an S-0 bond order of 2.4. Furthermore, a microwave studys of the C1 quadrupole coupling indicates 40% ionic character in the S-C1 bond. Nagnuson7 was able to measure the dipole moment components of S O F along the inertial axes and to resolve these into components in the bond directions. Using this procedure, he obtained a moment in the N-F direction of 1.75 D. and a moment in the K=O direction of -0.17 D. These measured values are strikingly different from the usual bond moments2 of 0.17 D. for Y-F and 2.0 D. for K=O, and are a clear indication of the shift of electron density in the direction of the halogen atom that is symbolized by the ionic structure. We propose, on the basis of the unexpectedly low value for the dipole moment of CF3x0, that this compound also has a structure which must be represented by an appreciable contribution from an ionic form, (S=O) +CF3-, indicating a withdrawal of electron density by the “pseudo-halogen” CF3 group. I n addition to the effect on the dipole moment, there should be two other observable effects of such a structure: an exceptionally long C-?; bond length and a large positive chemical shift in the fluorine nuclear magnetic resonance. I n view of the low dipole moment and the complications introduced by the expected very low barrier to internal rotation, the bond lengths will presumably have to await an investigation by electron diffraction techniques. If the C-?\‘ bond is very long, however, one would expect intense, low-frequency vibrations which would make unusually large contributions to the atomic p ~ l a r i z a t i o n . ~For this reason, we suspect that the dipole moment of CF3S0 may be somewhat lower than the 0.31 D. calculated above. We have measured the fluorine n.ni.r. spectrum of CF&O a t - 100’ and find a chemical shift of 89.83 + 0.02 p.p.m. with respect to CFCl,, used as an internal reference. Table I compares this value with the chemi-
w.,
T h e Journal of” Physical Chemistry
cal shift observed for other compounds containing the CF3 group. The authors know of no compound containing the CF3 group for which a more positive chemical shift has been reported. Table I : Fluorine 3T.m.r. Chemical Shifts” Chemical shift, Compound
p.p.m.
13.2 8.5 7.5 6.3 0.00 -2.06 -7.0 -7.6 -11.9 -17.3 -23.1 -76.6 a All values except that for CF&O taken from E. Fluck, “Die Kernmagnetische Resonanz und ihre Anwendung in der Anorganischen C hemie,’’ Springer-Verlag, Berlin, Germany, 1963. Bold face type indicates the fluorine atom to which the listed chemical shift applies.
Further evidence for an abnormal structure for CF3XO comes from the fact that CF3n‘0 dimerizes in an entirely different manner than does CH3S0. We have prepared the dimer by allowing the monomer to stand exposed to light for a long period of time and have obtained its mass spectrum. The appearance of large peaks a t m/e = 168, 149, 133, and 114 corresponding to the fragments (CF3)2S0,(CF3)(CF2)NO, (CF3)(CF2)N, and C2F4Xshows that both CF3groups are attached to the same nitrogen atom. This provides additional confirmation of the structure (CF3)2;C-O--N=0 which was suggested as a possibility by Jander and Haszeldineg and later confirmed by the work of Masonlo and others. (7) D. W. Magnuson, J . Chern. Phys., 19, 1071 (1951); D. F. Eagle, T. L. Weatherly, and Q.Williams, ibid., 30, 603 (1959); L. Pauling, “The Nature of the Chemical Bond,” Cornel1 University Press, Ithaca, N. Y . , 1960. (8) D. J. Millen and J. Pannell. J . Chern. SOC.,1322 (1961). (9) J. Jander and R. N. Hasaeldine, ibid.,696 (1954).
(10) J. Mason, ibid., 4531 (1963).
