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A reassignments of the EPR spectrum attributed to the radical cation of 2,2,3,3-tetramethylbutane
I ~ore:11ber 1979
A RE_ASSIGNMENr
ATrRIBUI’ED
OF THE EPR SPECTRUM
TO THE RADICAL
CA-i-ION OF X.3.3-TETRABlETHYLBUTANE*
from Aldrich
I_ Introduction
Chcmiczd Co_ vas sublimed
into a SpccIinc- A few samples cont;lining ca_ 5 mole percent of czlrbon tetrxh!oride or other additives known to be effectire electron scabengers \\cre also prepared_ Before irradiation. the scslcd trosii EPR tube on a vacuum
The EPRspectrum of@radiated 2,2.3.3-i,-tramethylbut.mc at 120 K \
Hexamethylcth~e
(2,2,3,3-tc&methylbutane)
* Rcscarch supported by the Division of Chemical Scienccr Office of Basic Energy Sciences, U-S Dep;lrtment of Ener_eu_
202
sample tubewasheated
above themeltrngpoint
of 3,322-tetramcthylbutarlc
and then cooled
(lOl°C) in order
to obtain a well-packed solid (or solid solution) in the Io\ter portion of the tube. The samples w&c irradiated at 77 K \\ith coblt-60 gamma rays for doses up to 2.5 i&ad. EPR measurements were made 3~ 77 K and hi&her temperatures ;IS described elsewhere [ 5]_
3_ Results and discussion The EPR spectrum of y-irradiated 2,2,3,3-tctramethylbutane at IS2 K is shown in the upper portion of fig. I_ and is almost identical to the previously published spectra obtained at I53 K [B] and ~3. I20 K [ I] _ Since our main concern is with the validity of the radical cation assignment [ 11, we defer comment on the first of these reports [6] unti1 the end of this paperTo f&Mare subsequent discussion, we begin by presenting an alternative analysis of the spectrum to that offered previously [I] _ We concur that the components from the radicaIs -CH2CMe2Chle3 and -Chle; are present but instead of ascribing the remaining
\rolumc 67. number i
CtILWC4L
PtfYSICS
LETTERS
stick
1 November 1979
dtagran:s
Thus,
in fig. I_
the fifteen
well-defined features in the experimental spectrum that were attributed to the radical cation [I] are Aso explained by this alternative analysis_ On the other hand. the present interpretation predicts no extra fe.rtures lying between the fifteen-line pattern and the outermost lines of the r-butyl radical,
whereas the IV, = +S components of the nineteen-line spectrum from the radical c.rtion should be observabIe in this region- Although no such Iines are revealed in tlg. 1, it was clamled [ I] that these two additional hnes could
i-i> 1. First-dcrirrtrisr EPR spectra of-y-irrrtdhred 1.X3.3-retmmerhyIbutane recorded under the mme conditions at 181 K before and .!fter anneaIing to 233 I;. The trxzs showing the outermost lines in the upper spectrum (before anneaiing) \kere obtained II increased microx\axe power. modulation and gzin. The stick pIots represent the spectra of the TMe~CIIe~ and TMe3 radicals. and the lines are numbered 1s in the text.
features to the radical cation, \ve propose that they can be accounted for by the seven-line spectrum of the radical Chle2Chle3 which is not an unexpected product in this system. Since the isotropicg factors and ‘H coupling constants of the mdicals -Chle3 (g= 2.0027, A = 21.2-22.6 G 171) and C~Me2C31e3 (g = 2.0092,A =22.0---22.5 G [6,SJ)arevirtualIyidenticai at this level of resolution, the superimposition of the septet spectrum on the ei$rt inner lines of the ten-line spectrum from the r-butyl radical rest&s in a pattern of fifteen almost evenly spaced components, as depicted by the
be detected
at high gain.
However,
a care-
ful examination of the published spectrum [l] indicartes th.rt the line marked D near the start of the amplified trace (on the high-fieId side of the spectrum) does not have the ant,icipsted position for the 111~= -8 component. We suggest that this feature is part of the anisorropicaliy broadened MI= -7P_ component of the spectturn from the r-butyl radical; this r H hyperfine anisotropy has been described [9] and is plainly evident in the upper spectrum of fig. 1. To clarify the problem further. we turned to a detarled study of spectral changes on progressive annealing at temperatures exceeding 200 K. A comparison of the spectra taken under the same conditions before and after annealing (fig- 1) reveals that a diminution in signal strength is accompanied by marked changes in the intensity ratios of the specrral components, as expected for a composite spectrum derived from two or more radicals which decay at different rates- What is remarkable, however, is that all of the odd-numbered lines in the fifteen-line pattern h,rve decIined uniformly to about 30% of their initial intensities. There is also a correlated decrease in the intensities of lines 2,4, 12, and 14, but for this group the reduction is only to about 55-60% of the initial values- Several other annealing esperiments confirmed these correlations in the decay characteristics of the spectrum. The above results are clearly in accord with the analysis we have given, whereby the odd-numbered lines ah belong to the f-butyl radical spectrum while the other group of lines (2_ 4, 12, 14) consists of the four outer components of the -CMe3Chle3 radical- On the other hand, it would be difiicult to account for the correlated decay of the odd-numbered lines if these were composed of contributions from both the radical cation and the r-butyl radical. In this case it can easily be shown from the binomial coefficients that the r-butyl radical 203
would contribute a progressiwly Isrger share to the tot31 line inrcnsity ab [he IilZ~Ividuc of the lint incrcas-
4_ Conclusion
es_ A corrclatr’d dscay wouid thcreforc require thrtt the two rztditzds decay at the same iarc \thich is clearly no1 the Cxie. Another argument against the r.rdicrtl ation msignment L’oIIlcS front the soorntilous Ime intcnsitics of the fifteen-lint pattern in the lower spectrum of fig_ I - Even dkdforving any contribution from the r-butyl mdical to the strcngtbs of the odd-numbcrcd lines. the intcnsity mtios for both the 5 I 4 (or I 1 : 12) zmd 7 I 4 (or 9 : I3 pairs are much Iowx than those predicted (2. I7 and 5-I 1, respecthAy) by the binomial coefticicnts for
presented in this paper argues against assignment of a spectrum in y-irradiated ~.~,~,~-t~1rJIIlcth~~butJIle to the rJdicd CJtion of thC
the radica1 cation spectrum. When a solid solution of carbon tctrxhioride in 2,13d-tctrarnet~lylbutane NLISirrqdiated. the resulting spectrum was complicated by several broad fe.mmzs,
especially on the !ow-ticId side. rendering rtnalysis more difficuk Also, strong signals from the CCIj radical were detected in the centre of ttls spectrum on annealing the s;tmpIc to IS0 IL CoPtray to the previous report
[II.
wz found
no significant
improvement
JikJIW
c\idecc~
It is hOWI
hbctorrl~
r.idica[s.
interpreted
that the EPK
in terms
-C[[2C\fe2C~&[e3.
spectrum
of the three
-C~kzC%k3
\t’e tIl.ulh .\Ir_ Steve E_ Dorris carq ing out these studies_
is more
sxt-
neutral
Jnd -C.\fc~*_
for Ilis .asatnnce
in
* A rcfcrec f1.c. bindI\ dmun our ;Ittwtion
to .I stud) of X2.3.3-tctr.tmeth) lbutanr under clccrron irr~dltion (ref. [ lO[) ahirh aIs0 prestmts .m an&Js of the tPR spectrum in rc‘rnls of these three mdicaIs_
References
in the
resolution of the fifteen-line pattern and. in paticular, we doubt the clsim thA t-butyl radicak are not produced in this system Incidentally, the suggestion that E-butyl radicals originate from dissociative eIectron enpture by the alkane seems surprising in view of the absence of any reports of alkane ridicixl anions. FinaIIy, we turn to the earIicr interpretation of the fifteen-line pattern by GeroIa et al. [6] _ These authors attributed the spectrum to the neutral -CMe2CH2CMe3 which they suggested was formed from -CH1Cble,CBle3 by trasposition of the Cii2 and Ckfe2 groups under irradiation. Although the suggestion is ingenious in that the predicted spectrum contains fifteen fines with the observed spacing, we submit that such a reaction is most improbable-
204
The
the previous
[6[ 171 [S[ [S[ [ 101
WC-R_ S\ mans. J_ Char Sot_ Chem. Cornmun. f 1975) 6S6. K.\V_~essmdert .md RIL Schuler. .\dun_ Raiiitmn Chem :! 11970) l_ I_ NilIiams. Quarr. Rr\. 17 (1963) 101; \V_R Buskr. D-EL Vsrfin snd I< \\iiIi.ms. Discussions I-;lradzp Sot_ 36 (1963) 103 RI__ Iludson and F_ Williams. J. Am_ Chrm SOG 99 ( 1977) 7714. X- IIzseg~~i.t, Y. Shiotani and 1:. Wilhrtms, I‘arada? DisrussionsChcm Sot. 63 (1977) 157. J_ CeroIa. B. Lsmotte ad R Marx. J.. Chimie Ph>r 66 (1969) 316s D-L Wood and RI_ Sprechcr. MoL Phys 7-6 (1973) I31JG-B_ Watts and KIJ. IngoId. J. Am Chem Sot_ 9-t (1977) J91_ M.C_R S>mons. Mot Phys 21 (1972)461. IL Shimishi. IL Kadoi. S_ II~~~~zwzI. Y_ rabara and K. Oshirnz. BuIL Chcm Sot Japan 47 (1971) IWO_
A RE_ASSIGNMENr
ATrRIBUI’ED
OF THE EPR SPECTRUM
TO THE RADICAL
CA-i-ION OF X.3.3-TETRABlETHYLBUTANE*
from Aldrich
I_ Introduction
Chcmiczd Co_ vas sublimed
into a SpccIinc- A few samples cont;lining ca_ 5 mole percent of czlrbon tetrxh!oride or other additives known to be effectire electron scabengers \\cre also prepared_ Before irradiation. the scslcd trosii EPR tube on a vacuum
The EPRspectrum of@radiated 2,2.3.3-i,-tramethylbut.mc at 120 K \
Hexamethylcth~e
(2,2,3,3-tc&methylbutane)
* Rcscarch supported by the Division of Chemical Scienccr Office of Basic Energy Sciences, U-S Dep;lrtment of Ener_eu_
202
sample tubewasheated
above themeltrngpoint
of 3,322-tetramcthylbutarlc
and then cooled
(lOl°C) in order
to obtain a well-packed solid (or solid solution) in the Io\ter portion of the tube. The samples w&c irradiated at 77 K \\ith coblt-60 gamma rays for doses up to 2.5 i&ad. EPR measurements were made 3~ 77 K and hi&her temperatures ;IS described elsewhere [ 5]_
3_ Results and discussion The EPR spectrum of y-irradiated 2,2,3,3-tctramethylbutane at IS2 K is shown in the upper portion of fig. I_ and is almost identical to the previously published spectra obtained at I53 K [B] and ~3. I20 K [ I] _ Since our main concern is with the validity of the radical cation assignment [ 11, we defer comment on the first of these reports [6] unti1 the end of this paperTo f&Mare subsequent discussion, we begin by presenting an alternative analysis of the spectrum to that offered previously [I] _ We concur that the components from the radicaIs -CH2CMe2Chle3 and -Chle; are present but instead of ascribing the remaining
\rolumc 67. number i
CtILWC4L
PtfYSICS
LETTERS
stick
1 November 1979
dtagran:s
Thus,
in fig. I_
the fifteen
well-defined features in the experimental spectrum that were attributed to the radical cation [I] are Aso explained by this alternative analysis_ On the other hand. the present interpretation predicts no extra fe.rtures lying between the fifteen-line pattern and the outermost lines of the r-butyl radical,
whereas the IV, = +S components of the nineteen-line spectrum from the radical c.rtion should be observabIe in this region- Although no such Iines are revealed in tlg. 1, it was clamled [ I] that these two additional hnes could
i-i> 1. First-dcrirrtrisr EPR spectra of-y-irrrtdhred 1.X3.3-retmmerhyIbutane recorded under the mme conditions at 181 K before and .!fter anneaIing to 233 I;. The trxzs showing the outermost lines in the upper spectrum (before anneaiing) \kere obtained II increased microx\axe power. modulation and gzin. The stick pIots represent the spectra of the TMe~CIIe~ and TMe3 radicals. and the lines are numbered 1s in the text.
features to the radical cation, \ve propose that they can be accounted for by the seven-line spectrum of the radical Chle2Chle3 which is not an unexpected product in this system. Since the isotropicg factors and ‘H coupling constants of the mdicals -Chle3 (g= 2.0027, A = 21.2-22.6 G 171) and C~Me2C31e3 (g = 2.0092,A =22.0---22.5 G [6,SJ)arevirtualIyidenticai at this level of resolution, the superimposition of the septet spectrum on the ei$rt inner lines of the ten-line spectrum from the r-butyl radical rest&s in a pattern of fifteen almost evenly spaced components, as depicted by the
be detected
at high gain.
However,
a care-
ful examination of the published spectrum [l] indicartes th.rt the line marked D near the start of the amplified trace (on the high-fieId side of the spectrum) does not have the ant,icipsted position for the 111~= -8 component. We suggest that this feature is part of the anisorropicaliy broadened MI= -7P_ component of the spectturn from the r-butyl radical; this r H hyperfine anisotropy has been described [9] and is plainly evident in the upper spectrum of fig. 1. To clarify the problem further. we turned to a detarled study of spectral changes on progressive annealing at temperatures exceeding 200 K. A comparison of the spectra taken under the same conditions before and after annealing (fig- 1) reveals that a diminution in signal strength is accompanied by marked changes in the intensity ratios of the specrral components, as expected for a composite spectrum derived from two or more radicals which decay at different rates- What is remarkable, however, is that all of the odd-numbered lines in the fifteen-line pattern h,rve decIined uniformly to about 30% of their initial intensities. There is also a correlated decrease in the intensities of lines 2,4, 12, and 14, but for this group the reduction is only to about 55-60% of the initial values- Several other annealing esperiments confirmed these correlations in the decay characteristics of the spectrum. The above results are clearly in accord with the analysis we have given, whereby the odd-numbered lines ah belong to the f-butyl radical spectrum while the other group of lines (2_ 4, 12, 14) consists of the four outer components of the -CMe3Chle3 radical- On the other hand, it would be difiicult to account for the correlated decay of the odd-numbered lines if these were composed of contributions from both the radical cation and the r-butyl radical. In this case it can easily be shown from the binomial coefficients that the r-butyl radical 203
would contribute a progressiwly Isrger share to the tot31 line inrcnsity ab [he IilZ~Ividuc of the lint incrcas-
4_ Conclusion
es_ A corrclatr’d dscay wouid thcreforc require thrtt the two rztditzds decay at the same iarc \thich is clearly no1 the Cxie. Another argument against the r.rdicrtl ation msignment L’oIIlcS front the soorntilous Ime intcnsitics of the fifteen-lint pattern in the lower spectrum of fig_ I - Even dkdforving any contribution from the r-butyl mdical to the strcngtbs of the odd-numbcrcd lines. the intcnsity mtios for both the 5 I 4 (or I 1 : 12) zmd 7 I 4 (or 9 : I3 pairs are much Iowx than those predicted (2. I7 and 5-I 1, respecthAy) by the binomial coefticicnts for
presented in this paper argues against assignment of a spectrum in y-irradiated ~.~,~,~-t~1rJIIlcth~~butJIle to the rJdicd CJtion of thC
the radica1 cation spectrum. When a solid solution of carbon tctrxhioride in 2,13d-tctrarnet~lylbutane NLISirrqdiated. the resulting spectrum was complicated by several broad fe.mmzs,
especially on the !ow-ticId side. rendering rtnalysis more difficuk Also, strong signals from the CCIj radical were detected in the centre of ttls spectrum on annealing the s;tmpIc to IS0 IL CoPtray to the previous report
[II.
wz found
no significant
improvement
JikJIW
c\idecc~
It is hOWI
hbctorrl~
r.idica[s.
interpreted
that the EPK
in terms
-C[[2C\fe2C~&[e3.
spectrum
of the three
-C~kzC%k3
\t’e tIl.ulh .\Ir_ Steve E_ Dorris carq ing out these studies_
is more
sxt-
neutral
Jnd -C.\fc~*_
for Ilis .asatnnce
in
* A rcfcrec f1.c. bindI\ dmun our ;Ittwtion
to .I stud) of X2.3.3-tctr.tmeth) lbutanr under clccrron irr~dltion (ref. [ lO[) ahirh aIs0 prestmts .m an&Js of the tPR spectrum in rc‘rnls of these three mdicaIs_
References
in the
resolution of the fifteen-line pattern and. in paticular, we doubt the clsim thA t-butyl radicak are not produced in this system Incidentally, the suggestion that E-butyl radicals originate from dissociative eIectron enpture by the alkane seems surprising in view of the absence of any reports of alkane ridicixl anions. FinaIIy, we turn to the earIicr interpretation of the fifteen-line pattern by GeroIa et al. [6] _ These authors attributed the spectrum to the neutral -CMe2CH2CMe3 which they suggested was formed from -CH1Cble,CBle3 by trasposition of the Cii2 and Ckfe2 groups under irradiation. Although the suggestion is ingenious in that the predicted spectrum contains fifteen fines with the observed spacing, we submit that such a reaction is most improbable-
204
The
the previous
[6[ 171 [S[ [S[ [ 101
WC-R_ S\ mans. J_ Char Sot_ Chem. Cornmun. f 1975) 6S6. K.\V_~essmdert .md RIL Schuler. .\dun_ Raiiitmn Chem :! 11970) l_ I_ NilIiams. Quarr. Rr\. 17 (1963) 101; \V_R Buskr. D-EL Vsrfin snd I< \\iiIi.ms. Discussions I-;lradzp Sot_ 36 (1963) 103 RI__ Iludson and F_ Williams. J. Am_ Chrm SOG 99 ( 1977) 7714. X- IIzseg~~i.t, Y. Shiotani and 1:. Wilhrtms, I‘arada? DisrussionsChcm Sot. 63 (1977) 157. J_ CeroIa. B. Lsmotte ad R Marx. J.. Chimie Ph>r 66 (1969) 316s D-L Wood and RI_ Sprechcr. MoL Phys 7-6 (1973) I31JG-B_ Watts and KIJ. IngoId. J. Am Chem Sot_ 9-t (1977) J91_ M.C_R S>mons. Mot Phys 21 (1972)461. IL Shimishi. IL Kadoi. S_ II~~~~zwzI. Y_ rabara and K. Oshirnz. BuIL Chcm Sot Japan 47 (1971) IWO_