7aa
R. M .Thibault, D. R . Hepburn, and T. J. Klingen
Charge Scavenging in y Radiolysis of Cyclohexane Solutions of Carboranes Ronald M. Thibault, David R. Hepburn, Jr., and Theodore J. Klingen* Department ot Chemistry and Center for Radiation Research, The University ot Mississippi, University, Mississippi 38677 (Received November 9, 7973) Publication costs assisted by the Department of Chemistry, The University of Mississippi
The 6oCo y radiolysis of cyclohexane solutions of o-carborane and 1-ethyl-, 1-vinyl-, and 1-allyl-0-carbolane has been studied for solute concentration in the range 0.5-200 mM. The value of G(H2) decreases with increase in solute concentration and values of the ion scavenging coefficient LY of 33 M - 1for o-carborane and 1-ethyl-o-carborane and 10.6 M - l for 1-vinyl- and I-allyl-o-carborane were calculated, suggesting efficient electron scavenging by these solutes. Lower limits for the ratio of the specific rate of hydrogen abstraction from cyclohexane to the specific rate of addition to 1-alkenyl-o-carboranes is estimated for the hydrogen atom to be 0.0031 for 1-vinyl-o-carborane and 0.002 for 1-allyl-o-carborane. Yields of cyclohexene, dicyclohexyl, and cyclohexyl addition products with the 1-alkenyl-o-carboranes are consistent with electron and radical scavenging. The ultraviolet absorption spectra of the o-carboranes is discussed and the possibility of energy transfer from electronically excited cyclohexane to the o-carborane solutes is suggested. The results are related to previous studies of radiation-induced addition polymerization of I-alkenyl-o-carboranes.
Introduction Products of the y radiolysis of cyclohexane solutions of 0.8 to 3.4 M 1-vinyl-o-carborane (Figure 1) are mainly vinyl addition dimers of I-vinyl-o-carborane.1 The cyclohexyl group was not detected by mass spectrometric analysis of the oligomeric products. Its absence was interpreted as evidence of steric inhibition of cyclohexyl radical addition to the vinyl double bond. An alternative explanation is that cyclohexyl radicals are not formed efficiently in the presence of 1-vinyl-o-carborane (VOC) in this concentration range owing to capture of free and geminate electrons and/or quenching of electronically excited cyclohexane by VOC. Further study2 has shown that very small yields of cyclohexene and dicyclohexyl are found in these solutions; both of these products have cyclohexyl precursors which, in turn, have both ionic and electronically excited precursor^.^ Thus, electron capture or excited state quenching is implicated. Electron capture by the o-carborane "superaromatic" cage is supported by calculations of Potenza and Lipwomb4 who predict the lowest unfilled molecular orbital .of o-carborane to be 2.44 eV below the ionization threshold. Stable o-carborane anions are found in alkali metalammonia solutions of o - c a r b ~ r a n e . The ~ o-carborane anion also appears to be the precursor of o-carborane radicals in an esr study ofy-irradiated o-carborane a t 77"K.6 The present work was initiated to verify electron scavenging by o-carborane (OC) in y-irradiated cyclohexane. Similar studies of 1-ethyl-o-carborane (EOC), VOC and 1-allyl-o-carborane (AOC) show the influence of the substituent on the radiation chemistry of o-carborane in cyclohexane. Inferences from these studies are related to the radiolysis of pure VOC and AOC and the factors controlling the addition polymerization of VOC and AOC are discussed. Evidence for electronic energy transfer from excited cyclohexane to these four solutes is also considered. Experimental Section The cyclohexane (Mallinckrodt, Spectroquality) was passed through a 2.4 cm X 1.5-m column of freshly actiThe Journalot Physical Chemistry, Vol. 78, No. 8, 1974
vated 20-200 mesh silica gel and fractions were rejected if the light absorbance for a 1-cm pathlength of deaerated cyclohexane us. distilled water exceeded 0.1 a t 190 nm. 0Carborane (Alfa Inorganics) was vacuum sublimed twice with an Ace Glass No. 8022-10 sublimator with the cold surface at 0" and the solid about 50". Continuous pumping maintained a pressure of about 0.01 Torr over the solid. 1-Allyl-o-carborane (Olin, 85% assay) was vacuum-distilled and the fraction distilling at 157" and 20 Torr was collected. 1-Ethyl-o-carborane and 1-n-propyl-o-carborane were obtained by catalytic hydrogenation of 1-vinyl and 1-allylo-carborane in hexane with 30 psi hydrogen at 23" over a palladium-on-carbon catalyst in a Parr hydrogenator. The catalyst was filtered from the hexane solution of 1-alkylo-carborane and the solvent was evaporated. The 1-ethylo-carborane was twice vacuum-sublimed as described above with the solid at 30" and yielded a plastic-crystalline solid melting at 37".? The liquid 1-n-propyl-o-carborane was eluted from a Yz X 4-in. column of activated silica gel with purified cyclohexane and was recovered by vacuum evaporation of cyclohexane. All carboranes used were shown to be better than 99% pure by gas chromatography. For optical study, a trace impurity absorbing near 270 nm was eliminated from VOC, EOC, and AOC by eluting each carborane from a Yz X 18-in. column of activated silica gel with purified cyclohexane followed by solvent evaporation and vacuum sublimation of the residue. Solutions were prepared gravimetrically with volumetric dilution, were deaerated by about five freeze (77°K)pump-thaw cycles, and were sealed in Pyrex tubes. Gas collected above the solid a t 77°K after y-irradiation was transferred to a calibrated volume uia a break-seal with a Toepler pump in two freeze (77"K)-pump-thaw cycles. The quantity of gas was calculated with the ideal gas law. Gas chromatography of the collected gas on a 7' in. x 2.5-m column packed with 60-80 mesh molecular sieve 5A showed only hydrogen. In a few experiments attempts were made to collect other gases by freezing a t -78 and 0". None was observed.
789
y-Radiolysis of Cyclohexane Solutions of Carboranes
.
C12H22
4
G
.
2P
it,
0
'I-
I
0
I
I
I
8 12 16 2 0 LOCI,units of 10-3 p~
4
Figure 2. G Values for products from y-irradiated cyclohexane solutions of o-carborane. The curve is given by eq I and 01 = 33
M-'.
Figure 1. l-Vinyl-l,2-dicarba-closo-dodecaborane (1 1 ) and the icosahedral cage structure of 0-carborane: @, carbon atom: 0 , boron atom; 0 ,hydrogen atom. Solutions were analyzed on a Beckman GC-5 gas chromatograph with a flame-ionization detector. A ?/4 in. x 6-ft column packed with 60-80 mesh firebrick coated with 20 wt % of a solution of 33% AgN03 in glycerol was used to separate cyclohexene from cyclohexane. The carboranes, dicyclohexyl, and cyclohexylhexene were resolved with a y4 in. x 1-m column packed with 60-80 mesh Gaschrom Q coated with 20 wt % Carbowax 20M. Other hightemperature separations were achieved with a Y4 in X 1-m column packed with 60-80 mesh Chromosorb P coated with 20 wt %I SE-30. Absorption spectra were determined with a Cary 17 uv-visible spectrometer. Solutions were irradiated at 31" with ' W o y irradiation eV g- hr- to the Fricke doat a dose rate of 3.61 X simeter solution with G(Fe3+) = 15.6. The electron density of each solution relative to that of the dosimeter was used to calculate absorbed dose. Results Values of G(H2), G(cyclohexene), and G(dicyclohexy1) from y radiolysis of deaerated cyclohexane solutions of ocarborane, EOC, VOC, and AOC are shown in Figures 2-5, respectively, as plots of values of G us. solute concentration. The curves are calculated and are explained in the Discussion. Two high boiling products were found by gas chromatography in solutions of VOC and AOC and were assigned by mass spectrometry to products of the coupling reaction of cyclohexyl radicals with radicals of VOC and AOC. Approximate G values are shown in Table I. The low doses used in these experiments did not produce measurable amounts of oligomeric products. No loss of solute above 570,the cumulative measuring error, was detected. Values of G a t the highest and a t the lowest solute concentration for each solute were obtained from plots of yield US. dose; such plots were always linear through the origin. Doses were in the range (3.6-18) x 1018 eV 8-l for measurement of G(H2) and (5.4-22) x 1019 eV 8-1 for measurement of other products.
6L
t
e
.
4 0
0
i 0
8
4 8 12 IEOCI, units of
16
M
20
Figure 3. G Values for products from y-irradiated cyclohexane solutions of 1-ethyl-o-carborane.T h e curve is given by eq I and a = 33 M-'. TABLE I: G Values for Cyclohexyl Adducts to 1-Alkenyl-o-carboranes in Cyclohexane Solutesa
voc
AOC
GI
0.9 f 0 . 2 1 . 2 & 0.1
G2
1.5 It 0 . 2 2 . 0 =k 0.3
Gtotal
2.4 0.4 3 . 2 =k 0 . 4
Concentration range is 40-200 mM. Tentatively assigned to 1-(2cyclohexylethy1)-o-carborane for VOC and to 1-(3-cyclohexylpropyl)-ocarborane for AOC by mass spectrometry. Tentatively assigned to 142cyclohexylviny1)-o-carborane for VOC and to 1-(3-cycl~hexylaLlyl)acarborane for AOC by mass spectrometry.
The ultraviolet absorption spectra of the carboranes shown in Table I1 were measured in cyclohexane a t 23". Previously observed absorption near 270 nm8 was removed by careful purification. Maxima of the absorption peaks were observed only for the o-carboranes with unsaturated substituents. Beer's law was obeyed a t several wavelengths for all absorptions. Discussion The initial yield of G(H2) = 5.8 found in the present study for deaerated cyclohexane exposed to e°Co y -irradiation is in fair agreement with other reported The Journalof Physical Chemistry, Val. 78, No. 8, 1974
790
R. M. Thibault, D. R. Hepburn, and T. J . Klingen
I
t
’
I
I
I
I
-
3
--
“!
I
I \
A
Q-carborane
o l-cthyla-carborane
Figure 4. G Values for products from y-irradiated cyclohexane solutions of 1-vinyl-o-carborane. The dashed curve is eq I with a = 10.6 M - l . The solid curve is eq I l l with a = 10.6 M-’ and k 2 / k l = 0.0031.
I
I
0
8
I
I
I
I
32 40 6 ,u n i t s of 10-2~-’’* 16
24
Figure 6. Plot of eq II with hydrogen yields from cyclohexane solutions of o-carborane and 1-ethyl-o-carborane. T h e slope gives a = 33 M - I .
6l
A
t
1
.. ,.,
yo I
H2 C6H10
0
0
0
I
I
0
Equation I, introduced by Schuler and coworkers,3JoJ1 describes the dependence of hydrogen yields from y -irradiated cyclohexane on concentration, s, of solutes which scavenge free and geminate electrons. The term CY,analogous to a ratio of specific rate constants, is an empirical parameter which describes the efficiency of the inhomogeneous electron scavenging relative to electron-cation recombination. Bansal and Schuler3 estimate a value of G(H2) = 1.8 which is formed independent of electron-cation recombination and a value of G(H2) = 3.9, dependent on electron-cation recombination. The latter value is multiplied in eq I by the fraction of electrons escaping capture at solute concentration, S. Rearrangement of eq I leads to eq I1 which predicts a straight line of unity inter-
4 8 12 16 2 0 I ~ O C Iu ,n i t s of 10-3 M
Figure 5. G Values for products from y-irradiated cyclohexane solutions of 1-allyl-o-carborane. The dashed curve is eq I with a = 10.6 M - l . The solid curve is eq I l l with a = 10.6 M - l and k Z / k l = 0.002.
TABLE 11: Ultraviolet Absorption of Carboranes Somf(4* a x ,
e201,
“ub
M-1cm-1
M’lcm-1
du
Compound
Xmax,nm
o-Carborane (OC) 1-Ethyl-OC 1-Isopropyl-OC 1-Vinyl-OC 1-Allyl-OC 1-Isopropenyl-OC
