Xov., 1963 in the optical densit,y a t Amax 420 nip). Severtheless, oiily 0.5% of CO and approximately an equivalent amount of CF3H were found in the products. Similar results were obtained in liquid hydrocarbons, e.g., a 0.06 M solution of HFBA in liquid 2,3-dimethylbutane, after being photolyzed a t 2.5' for 9 min., showed a decrease in the optical density a t Anla, 420 mp from 1.14 to 0.19 (the optical path being 1 cm.). Again, analysis of the products gave only 0.8% of CO and an equivalent amount of CF3H, and neither CF3. CHO nor CF,CO.CF3 were detected. It is obvious that under these conditions CF3CO or CF3 and GO are not the main products of the primary reaction. The infrared spectra of the products obtained in the presence of hydrocarbons showed the absence of the very strong band a t 14.25 p , which is characteristic of HFBA. The 5.58 p carbonyl band was observed, although its intensity was reduced to about one-half of its initial value. A new band which appeared a t 2.75 p demonstrates the presence of an OH group. Significantly, an additional band a t 3.75 p , indicating the presence of an OD group, n;as observed when liquid 2methylpropane-d? was uspd as solvent. The formation of an OH group was confirmed by a positive Zerevitinoff test. Wheil the bleached solution was exposed to air, some HFBA ::as re-formed; the increase in the optical d e x i t y a t 420 mp showed its concentration to be a few per cent only of the initial value. The average molecular weight of typical products was 550, and their total mass exceeded that of the initial ketone. It appears that HFBA undergoes photoreduction in the presence OE a hydrocarbon. The excited molecule abstracts a hydrogen forming a ketyl radical CF3. COH.COCF, and a solvent radical. The interaction of these radicals then produces a complex mixture containing such species as CFsC(R)OHCO-CF3,CF3CHOH.CO.CF, etc. The presence of t,he latter compound is indicated by its facile oxidation to HFBA. The complexity of the products arises from the fact that HFBA may react with the resulting alcohols, forming ketals. It would appear that this is the first reported exaniple of a bimolecular gas phase photoreduction of a ketone. Unimolecular rearrangement processes leading to similar products are known,%and, of course, many examples of photoreduction of ketones in solution are described in the l i t e r a t ~ r e . ~ Our results raise the question of how important photoreduction may be in the gaseous photolysis of other ketones, and, in fact, some peculiar observations mentioned in a few papers might be due t,o this phenomenon.4 All the irradiations u-ere carried out in Pyrex vessels using unfiltered light from a G.E. A-H6 high pressure mercury lamp. All solution mixtures were thoroughly degassed before being irradiated. The referee of this Note wonders whether the formation of the hydroxylic product in the gas phase photolysis of hexafluorobiacetyl could be due to the two reactions (2) P. Ausloos and R. E. Rebbert, J . Am. Chem. Soc., 83,4897(1961). (3) (a) A. Sohdnberg and A. Mustafa, Chem. Rev., 40, 181 (1947); (b) C. R. Masson, V. Boekelheide, W. A. Noyes, Jr., In A. Weissberger, Ed., "Techniques of Organlc Chemistry," Vol. 11, 2nd Ed., Interscience Publishers, Ine., New York, N. Y.,1956, p. 257; ( e ) G. S. Hammond, K. P. Baker, V. R. Moore, J . Am. C h e m . Soc., 83,2795 (1961). (4) (a) W. A. Noyes, Jr., W. A. Mulac, and M. 8. Matheson, J . Chem. Phys., 36,880 (1962):(b) D.R. Weir, %bid.,36, 1113 (1962).
NOTES
2493
+ CF,.CO.CO.CF, --+ (CF3),.(CO.).CO.CF3 (CF,),.(CO.).CO.CFs + HR + (CF~)YCOH.CO*CF~ CF3
The answer is definitely negative. This suggestion requires formation of equivalent quantities of CO and of the hydroxylic product. This obviously is not the case. For example, it was stated a t the beginning of this Note that, 84% of diketoiie was decomposed while oiily 0.5% of CO was formeld. Even this small quantity of CO cannot be att,ributed to the reaction proposed by the referee since, as was stated above, approximately an equivalent amount of CF,H was formed with CO. Acknowledgments.-We wish to thank Dr. Leonard Moore of the Research and Development Depart'ment of Union Carbide Chemicals for a gift of hexafluorobiacetyl, and we a,lso wish to thank Wright-Patterson Air Force Base for financial support of this work through Grant $F-33 (657)-10855.
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ELECTIXON-BEAM INITIATED POLYMERIZATION OF A4N ORGAXIC VAPOR ADSORBED O N A METAL SURFACE BY P. WHITB~ Standard Telecommunication Laboratories Limited, London Road, Harlow, Essex, England Receined April 26, 1965
Interest, has recently developed in the formation of thin polymer layers by radiation treatment of monomer vapors adsorbed or condensed onto solid surfac'es, and several papers have presented details and meohanisms of reactions.2-6 In general, these papers have one thing in common, that the data have been obtained for the formation of polymer on previously deposited polymer, and in no case has any discussion included the first layer of polymer film; ie., when the monomer gas is adsorbed on a surface other than that of polymler. Some experimental results on the formation of polymer layers have been described by Whit'e and n'ortln,6 and interpretation of these results have some bearing on the question of what happens in the formation of an initial polymer layer on a metal surface. White and Korth6 used a 3-kv. electron beam to make a number of very rapid, but precisely timed, vertical scans across the surface of a freshly depositNed lead film in the presence of a monomer gas, butadiene. The polymer layers formed on the surface of the fiJ.m were detected by a simplified electron-beam scanning microscope described by DaSilva and 3Vhite.I These experiments indicahed whether or not a polymer film had been formed and also yielded qualitative data on the thickness of the deposit. It was then possible to correlate the signal from the scaniiing microscope with the writing time defined as the period of time the elect'ron bea,m irrad.iated any one spot on the surface. Undoubtedly, before the electron beam scanned the metal, a t least one monolayer of organic vapor was (1) This urork was performed while the author was employed by International Business Machines Corporation, Thomas J. Watson Research Center, Porktown Heights, X. Y. ( 2 ) R. W.Christy, J . A p p l . Phys., 31, 1680 (1960). (3) I. Haller and P. White, J . P h y s . Chem., 67,1784 (1963). (4) V. A. Kargin and 1 '. A. Kabanov, J . Polymer Sci., 52, 71 (1961).. (5) D.M.Whit,e, J. Am. Chem.Soc., 82,5678 (1960). (6) P. White and D. W.North, I B X J. Res. Develop., in press. ( 7 ) E. 11.DaSilva and P. White, Proceedings of the 8th National Vacuum Symposium, Pergamon Press, London, 1960, p. 830.
NOTES
2494
adsorbed on the metal surface. The first step in the polymer formation must be polymerization of the gas already adsorbed on the metal surface, subsequent layers being formed by adsorption of further gas prior to polymerization. Experimental measurements with a gas pressure of 5 X torr and a beam current of 5 pa. indicated that at all writing times down to 1 X see. the polymer film thickness decreased continuously with decrease in writing speed. This showed that a t least more than one layer of polymer must have been deposited, since a deviation from the smooth dependence of signal height on writing time would be anticipated when less than a monolayer was present on the surface. Also, the dependence of film thickness, a t a constant writing speed, on both pressure and current density was consistent with the kinetics previously deduced for formation of polymer on p ~ l y m e r . ~ Both these facts indicate that the po!yrr,er film formed at miting times down to 1 x sec. was composed of more than one layer. It appears therefore that under the conditions of the experiment, 1 X sec. must represent an upper limit of the time required for complete polymerization of the gas already adsorbed on the metal surface. Consider nom what may be inferred from this conamp. is equivaclusion. A beam current of 5 x lent to 3 X 1013electrons striking the surface per second. Assuming R cross section area of 5 X cmS2for butadiene, there would be about molecules adsorbed in a close packed monolayer with an area equivalent to that of the electron beam (about 5 X cm.2). During the interwl of time necessary to form the polymer layer, 3 x 10” electrons would have collided with the surface. It seems, therefore, that one electron is sufficient to cause polymerization of about 30 adsorbed molecules. This compares with approximately 100 electrons/molecule required for polymer formation on a polymer surface, with a similar current and electron energy of about 0.3 kv. Preliminary experiments indicated very little or no dependence of the rate of polymer formation on electron energya3 The very large difference in the number of electrons required to form a single layer of polymer in the two cases must therefore be related to the difference in degree of bonding of the adsorbed gas to the underlying material and the consequence of this on the electron distribution in the adsorbed molecuiz. This is not unusual, since in a number of caaes it has heen shown that the absorption spectra of molecules are appreciably altered when they are chemisorbed on surfaces. It seems, therefore, that the monomer molecules adsorbed on a metal surface are in an energy state from which they can very easily enter into a chain reaction and form polymer molecules containing initially up to 30 monomer units. This initial layer must then form the base from which the polymer film qrows by the mechanism already described. SCALING AND THE VIRIAL THEOREM BY G. HUNTER, D. G. RUSH,AND H. 0. PRITCHARD Chemistry Department, Universitg of Manchester, Manchester 13, England Receised May 3, 196.9
I n some recent LCAO calculations on Hzf, de Carlo and Griffingl found that the virial theorem (2T
+
Vol. 67
t 2.06
2.04
s
i 2.02 ,g 6
2.00
1.98
1.96
I
I 1.0
I 1.2
I
I
I
1.4
1.6
2.
Fig. 1.-Variation
of Rminwith 2 for Pia+.
V = 0) was best obeyed for their 3-term wave function a t R = 2 a.u. when the scale parameter 2 was 1.2, whereas a much higher Z of 1.45 gave the best energy (see Tables V and IX of ref. 1). Dr. Griffiiig asked us if we could investigate this apparent anomaly further, and in so doing, we have found that the situation is somewhat complicated by the fact that, for severely truncated basis sets, the calculated equilibrium bond length is quite sensitive to the scale parameter. There is no anomaly, however, because neither ( E T ) nor (E T R dE/dR) ever become zero when Z is around 1.2. Pritchard and Sumner2 investigated the energy of Hz+ a t R = 2 a.u. for nine values of the scale parameter 2. Since all the integrals involved are actually functions of RZ, it is a simple matter to reinterpret these so that we can choose a fixed value of 2 and calculate the energy for nine different values of R. The minimum in the potential curve for each Z was then located by interpolation or extrapolation of the differences between our calculated energies and the exact energy function derived from the results of Bates, Ledsham, and S t e ~ a r t . The ~ values of R m i n for 2, 3, 4, and 10 basis functions are shown in Fig. 1; for 10 functions with Z = 1.415, Rmin = 1.997 a.u., and coincides with the value given by the exact solution. Figure 2 shows the variation of the virial function (E T ) with scale parameter for internuclear separations R = Rmin. In the range accessible to us, these
+ +
+
+
(1) V. de Carlo and V. Gliffing, J . Phys. Chcm., 66, 845 (1962), and personal communication. (2) H. 0. Pritchard and F. H. Sumner, abzd., 67, 641 (1961). (3) D. R. Bates, K. Ledsham, and A. L. Stewart, Phil. Trans Roy. Xoc. London, A146,215 (1953).
