NOTES
10-9 einstein/sec.). If it is assumed that the cyclopropyl radicals observed in this study are formed, at least in part, by the quenchingreaction and that reaction 3 occurs, then it follows that at greater absorbed light intensities a larger proportion of the cyclopropyl radicals should be scavenged. Further studies along these lines will be necessary to prove if this is indeed the mechanism of allyl radical formation. The results with added I4CzH4also provide insight into the mechanism of formation of high molecular weight products.1° It is suggested that the observed labeled 3-methylhexane and 5-methyl-1-heptene (Table I) are formed by secondary reactions in which hydrogen atoms add to the major olefinic products of the sensitization (l-pentene-I4C and 1,5-hexadiene) forming radicals1' which combine with I4C2H5 radicals. These results suggest that the polymerization could occur by a series of reactions in which intermediate radicals, formed by hydrogen atom addition to olefin, combine with allyl radicals regenerating olefin which subsequently is attaoked by a hydrogen atom, and the process is repeated.
Acknowledgment. The authors wish to thank Dr. Hadley Ford for lending us his Ph.D. thesis and for his criticisms and suggestions during the preparation of this note. (10) No analysis for polymer was made in this work; visual examination of the cell afterward revealed no deposit. (11) The radical formed by H-atom addition to 1,bhexadiene could conceivably rearrange by adding intramolecularly to itself forming a Smethylcyclopentyl radical. This reaction may be the origin of the cyclopentane rings observed in the polymer.*
Electron Spin Resonance of Aliphatic Semiquinones
by E. Thomas Strom, Socony Mob2 Field Research Laboratory, DaUas, Texas, and pioneer in^ Research Division, U.S. A m y Natick Laboratories,
Natick, Massach~setts,~
Glen A. Russell, and Robert D. Stephens Departof C h i a t r y , Iowa State Univeraity, Amea, Iowa (Received January 28, 1966)
Characteristic colors have been observed when compounds of the formula ArCHOHCOAr react with base and oxygen.2 Michaelis and Fetcher8 suggested that these species were semiquinones of type I.
2131
0-
0-
I
I Ar-C=C-h
R-C=C-R
-0
.O
I
I
I
(CHZ)X
I 0I _
-0
II
m
The paramagnetic nature of these intermediates has since been dem~nstrated.~ One would not expect the corresponding aliphatic semiquinones (I1 and 111) to be readily observed for two reasons: the absence of aromatic rings in which the electron can be delocalized, and the possibility of condensation reactions occurring in the basic media in which the radicals are usually generated. For example, it was found that the reduction of biacetyl with zinc in basic solution resulted in the formation of the corresponding p-benz~semiquinone.~ Recent work has shown that the radical anions of cyclic a-diketones (111),6pivalil,6-s isobutyril,s and butyril,* can be formed. Thus, delocalization in the .O-C=C-Osystem in I1 and I11 is sufficient to impart stability to the radical, and cyclization to the benzoquinone is not necessarily an unsurmountable problem. We have found that ketones of the type (CHB(CH2),)zC=0 readily oxidize in dimethyl sulfoxide (80%)-&butyl alcohol (20%) containing potassium & butoxide to give yellow-green solutions containing semiquinones of type 11. Smaller amounts of other radicals are also formed. For semiquinones with n = 2-10, the spectra consist of a main quintet, aH = 4.68 =k 0.05 gauss, due to interaction with the four @-protons. Each peak of the quintet is split further from interaction with the four y-protons, aH = 0.22 gauss. A typical example is shown in Figure 1. In general, the 7-splitting was a quintet splitting except for n = 2 where the first, third, and fifth peaks of the main quintet were split into triplets and the second and fourth peaks into quartets or possibly sextets of which the wing peaks are not observed.
(1) At second address, on military leave of absence. (2) E. Fischer, Ann., 211, 214 (1882); A. Hantzsch and W. H. Glower, Ber.,40, 1520 (1907). (3) L. Michaelis and E. S. Fetcher, Jr., J. Am. Chem. SOC.,59, 1246 (1937). (4) B. Venkataraman and G. E. Fraenkel, ibid., 77, 2707 (1955); J. L. Ihrig and R. G. Caldwell, dbid., 78,2097 (1956); G. A. Russell, E. G. Janzen and E. T. Strom, i b a . , 84,4155 (1962). (5) M. Adbms, M. S. Blois, Jr., and R. H. Sands, J . Chem. Phys., 28, 774 (1958). (6) G. A. Russell and E. T. Strom, J. Am. Chem. Soc., 86, 744 (1964). (7) G. R. Luckhurst and L. E. Orgel, Mol. Phys., 7, 297 (1963). (8) H. C. Heller, J. Am. Chem. SOC.,86, 5346 (1964).
Volume 69, Number 6 June 1966
NOTES
2132
6.0 and ~ c =H 4.7 ~ gauss ~ yields cos B ~ : H , - H / = 1.13. Since 84:HpH would be expected to be >45", Cos 8-cH-a: is 251"; ie., R'in R'CH2 = R [formula 111 has a preferred trans position relative to the carbon-oxygen bond. Using the value of 51" for 8, the spin densities of the carbonyl carbon atoms in I1 with R = ethyl to n-undecyl are calculated to be pc = 0.205. =
COS 0,CH-H
Experimental
Figure 1. ( a j First derivative e.s.r. spectra of n-tridecane-6,7-dione radical anion; (b) second multiplet of (a) with expanded scale.
Methyl ketones (CH~COCHZR) do not form radical anions under these conditions. A superior method for formation of radical anions of the type I1 even when R or R' is methyl involves the reaction of the a-bromoketone with a basic dimethyl sulfoxide solution. By a modification of this procedure, the semiquinones can be formed directly -from ketones. Bromination Of the ketones (e'g', 2-butanone) in containing potassium t-butoxide followed by dilution with dimethyl sulfoxide containing potassium & butoxide forms I' spontaneously* A seven-1ine spectrum, & H = ~ 6.06 ~ gauss, is observed for 11, R = R' = CH3, while for 11, R = CHa, R' = C2H5, the spectrum = 5.96 and @H,H = 4.97 gauss. yields The spin densities a t the carbonyl carbon atoms in the radical anions derived from ketones with n = 2-10, as evaluated from the equation upH = 29.25pcl6t9are 0.16. This is rather close to the spin density previously calculated [ p = 0.121 for the carbonyl carbon atoms in cyclopentadecane-1,2-dione radical anion, wherein the conformation is frozen relative to spectrometer frequency.6s10 That there is not free rotation in the acyclic semiquinones is shown by the nonequivalence ~ H of @HaH and &HaH noted above. Since Q QCCHaJH= 58.5 cos2 e, where e is the time average dihedral angle between the carbonyl carbon pz orbital and the P-carbon-hydrogen bond, it is possible to calculate relative values of e for the 6-carbon-hydrogen bonds via the equation ag = Q cos2 Using The Journal of Physical Chemistry
E.8.r. spectra were obtained with Varian V-4502 and V-4500 spectrometers with 23- and 15-cm. magnets, respectively, and with 100-kc.p.s. field modulation. Dimethyl sulfoxide (Crown Zellerbach Corp.) was distilled from calcium hydride before use. Potassium t-butoxide was purchased from Alfa Inorganics, Inc., and from Mine Safety Appliance Corp. Commercially pure ketones were used without further treatment. Solutions of the ketone or a-bromoketone (-0.10 M ) in dimethyl sulfoxide (80%)-&butyl alcohol (20%) and in pure dimethyl sulfoxide were deoxygenated in the U-type mixing cell described previously'' and mixed with a solution of potassium t-butoxide in the appropriate solvent. In the case of oxygenation experiments, the cell was then opened to air for 10-20 sec., and the oxidate was shaken down into a flat fused silica e.s.r. cell ("aqueous sample cell").lZ (9) R. W.Fessenden and R. H. Schuler, J . Chem. Phys., 39, 2147 (1963). . . (10) Even though in some systems & c - c H ~may ~ be less than 29.25 [C. deWaard and J. C. M. Henning, Phys. Letters, 4, 31 (1963); C. A. MoDowell and K. F. G. Paulus, Mol. Phys., 1, 541 (1963); B. L. Barton and G. K. Fraenkel, J . Chem. Phys., 41, 1455 (1964); E. T. Strom, G. A. Russell, and R. Konaka, ibid., in press], the indicated similarity would remain unchanged. (11) G. A. Russell, E. G. Janzen, and E. T. Strom, J. Am. Chem. SOC.,86, 1807 (1964). (12) This work was supported in part by a grant from the National Scieiice Foundation.
The Electrostatic Forces within the Carbon Monoxide Molecule'
by Peter Politzer2 Department of c . h i s t r y , Western &serve University, Cleveland, Ohw (Received January 28,1966) ~
~
This study was undertaken with the purpose of obtaining more precise knowledge of the roles played by the various molecular orbitals of carbon monoxide. It was hoped that this would lead to a better understand-
