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The acetone cation — A 13C ESR and endor study
Volume
106, number
5
CHEMICAL PHYSICS LETTERS
4 May 1984
IHE ACETONE CATION - A r3C ESR AND ENDOR STUDY Philip J. BOON, Lorraine Lkprtnwnt
HARRJS, Myra T. OLM. Jane L. WYATT and Martyn CR. SYMONS
of Chentirtry. 7be Unir~ersity.Leicester LEI 7RH, UK
Received 6 February
1984
; in tinal form 24 February
1984
Espomre of dilute solutions of acetone in trichlorofluoromethane to 6oCo -r-rays at 77 R gave the radical cation, characterised only by its g-tensor components. An ENDDR study revealed the presence of two types of weakly coupled protons (A r = I.5 G. AZ = 0.3 G) in accord with INDO calculations. The ESR spectrum of acetone labeled with 13C(13CH3) has been studied and the resulting pammeters interpreted in terms of two equivalent carbon atoms. with ~22% spin density on each. This value is significantly less than that on the hydrogen atoms of the formaldehyde cation (==36W). which supports our contention that C-H hypcrconjugation is somewhat more important than C-C hyperconjugation in radical cations.
1. Introduction The recent discovery that radical cations can be prepared and trapped in various solids, especially tri-
chlorofiuoromethane, under conditions-&h that the only welldefied ESR spectrum is that of the cation, has led to a spate of reports describing such spectra [l-6]. However, we know of no published work reporting t3C hyperfine coupling in such cations *. We have suggested that the ESR spectrum for the acetone cation is an unresolved singlet, with three distinct gfeatures in CFCi3 at 77 K [7], and this has also been reported by Shida and his co-workers [8] and by Snow and Williams [9]. The aim of the present work was to establish the form of the 13C interaction, and hence to estimate the degree of hyperconjugative interaction for the C-C bonds. Our results for acetaldehyde cations [7, lo] and those of Knight and Steadman for fcrmatdehyde cations [ 111 show clearly that C-H hyperconjugation is large. It seemed of interest to obtain an estimate of the degree of interaction of the C-C bonds in this cation by measuring the r3C hyperfme coupling. In many cases, the rotation of methyl groups in
radicalcations is strongly restricted, in contrast with most neutral radicals. Our INDO calculations for (Me&O)’ suggest that this should be the case for acetone, even though the degree of involvement of the protons is very small. A second aim was therefore to measure the proton coupling using ENDOR spec-
troscopy . 2. Experimental Acetone enriched in 13C (92.6 atom% MSD isotopes) was used as supplied. Solvents were freon (CFCI,) and tetrachloromethane, both of which give good ESR spectra for the acetone cation after irradiation. Samples were irradiated at 77 K in a Vickrad 6OCo -y-ray source with doses of ~1 Mrad. ESR spectra were measured on a Varian E-109 spectrometer calibrated with a Hewlett-Packard 5246L frequency counter and a Bruker BH12E field probe, which were standardised with a sample of DPPH. ENDOR spectra were measured at 77 K on a Bruker ER2OOD multiaccessory resonance spectrometer.
* Shida and co-workers have also studied (‘%H~)z CO by ESR spectroscopy. their results being similar to ours (personal communication).
408
0 009-2614/84/s 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume
106. number 5
CHEMICAL
PHYSICS LE-lTERS
1
May 198-l
1
3ZlSG
4 a
b
1 32306 52
-H
C
0
-1 I
:;
x
I
J
1
=
1. Firstderivative X-band ESR spectra for (“CH&CO in CFCI3 or CC4 (~0.001 mol fraction) after exposure IO 6oCo yrays at 77 K;(a) in CFClp at 77 I;. showing features assigned to non-librating_(13CH3)zCOf radical cations [the a features are for (‘3CH3)(‘2CH3)CO+Cations in low abundance];(b) a simulation of (a) for (13CHx)2CO*otions only. using the dam in table 1; (19 in CC4 at 77 K. sho~‘ing features assigned to librating (13CH3)2CO+ cations [feature Q is a&ned to (“CH~)(‘*CHJ)CO+ cations].
Fig.
409
Volume
106, number
Table 1 ESR parameters
5
for acetone
Medium
CHEMICAL
LETTERS
4 May 1984
cations Hyperfiie
coupling x
CFCIJ (77 K)
13C
cc14
‘H
(77 K)
PHYSICS
t3C
constants
g-tensor
(G) a) I
Y
28
9
-
-
12.0
16.4
9
components
iso
gx
g.,
15.3
2.0077
2.0017
2.003
2.006 1
2.0035
2.0023
1.5 0.3 18.4
15.6
a) 1 G = 1O-4 T.
3.
Results and discussion
3.1.
StQtiOtlQrJ~ rudicuk
A typical ESR spectrum for the t3C derivative is shown in fig. 1. This spectrum, in contrast with many “powder” spectra, is not readily interpreted in terms of normal expectation, and it was necessary to resort to computer simulation. The best simulation (fig. 1b) was obtained with the parameters listed in table 1, and any major deviation from these values gave quite unacceptable simulations. Both spectra show the presence of extra features (ol) not reproduced in the simulations, which relate only to (13CH3)2CO+ cations. These are assigned to cations containing only one 13C atom, and can again be satisfactorily accommodated using the parameters in table 1. 3.2.
Libmting
mdiculs
ESR spectra for 12C or 13C radicals in CC], at 77 K differ markedly from those in CFCl, (fig. lc). However, on cooling the CC14 spectra to ~30 K there was
a reversible change to the form of fig. la. For both solvents, the features broadened seriously on cooling to 4 K, even at very low microwave powers. This may be due to incipient chloride interaction of the type clearly detected for acetaldehyde cations [9,10]. These spectral changes were reversible, and spectra similar to that shown for the radicals in Ccl, were obtained from CFC13 solutions on annealing to =1X K. The spectrum shown in fig. lc has greatly simplified, and can now be analysed as indicated in the stick-dia‘gram. In particular, the intermediate features have been 410
lost and a strongM{ = 0 feature has appeared. The resulting parameters (table 1) have Aiso(t3C) almost equal to that deduced from spectrum la, thus strongly supporting our analysis. Furthermore, in both cases, the g values are equal to those derived from the 12C
spectra. Evidently, there is extensive libration, but not complete rotation about any axes since the symmetry is clearly not axial. The result is that the major turning points are now for field along the symmetry axes, so that a more conventional spectrum is obtained. On cooling to ~30 K this spectrum changes reversibly to one similar to that in fig. la. 3.3. ENDOR
spectm
A typical ENDOR spectrum for the normal *2C cat ion in CC14 at 77 K is shown in fig. 3. This shows two proton lines, one with a coupling of 1.3 G and the other of 5-03 C. Similar spectra were obtained from different parts of the ESR spectrum showing that the coupling constants are almost isotropic. However, we were unable to detect any ENDOR features from the cations in CFCI,. We do not place any special significance on this contrast, since our ability or inability to obtain ENDOR signals from powder spectra such as these seems to have no clearly defied scientific basis.
c6@
-------y
Fig. 2. x,y and z axes for the (CH&COt as&.nment.
-
(2.0017)
cation and g-value
Volume
106. number
PHYSICS
CHEhlICAL
5
4 !a)-
LETTERS
1951
3.5. Aspects of smcrure If we accept that the tjC parameters are close to the principal values. we can derive estimates of the 2s and 2p character of the orbit& on carbon from the Aiso and 3_B data in the usual manner [ 12) _IfA, is taken to be negative, we obtain impossibly high values (~80% spin density on the two methyl carbon atoms). We conclude that A, and As are both positive This gives 251 2% 2s and 21% 2p on each methyl carbon atom. Thus the SOMO comprises carbon p orbital rather than s-p hybrids. The extent of delocalisation is signifkmrly less than that ior H,CO+ cations (=X1
on each hydrogen). This semi-quankative result firs in satisfactorily with our conclusions for the cation of
12
13
,,,,
14
15
16
17
( 18
MHZ Fig. 3. First-derivative (“CH&CO+ mtion features
ENDOR spectrum for the in CCb at 77 K showing hyperfie
for two types of protons.
3.4. g values We follow the assignment of Knight and Steadman 1 I] in fig. 2. It is normally argued for structures of is the g. value along the C-O axis this type ~atgrn, (z) (or C-N axis for R&N radicals). This reasoning is based on the absence of u bonds to oxygen other than the C-O bond, so that orbital angular momentum is relatively free for spin on oxygen. However, Knight [
ethyl benzene [ 131. In this case, the two methylene protons have hyperfine couplings of 19 G each. Since for toluene, the average proton coupling is ==I9 G [ 14 this must mean that the methyl group is constrained t( lie in the plane of the benzene ring, which is a positior of masimum steric energy. We esplained this by postulating that, for radical cations, C-H hyperconjugation is energetically more favourable than C-C hyperconjugation. Finally, we note that in our study of a range of aldehyde and ketone cations with Shida and his coworkers [IS], we reported INDO calculations of the isotropic proton coupling constants for the acetone cation. The results (4H at -1.3 C and 3H at -1.6 G) are quite close to those now obtained by ENDOR spectroscopy. In particular, we show that methylgroup rotation is resrrlcred despite the minor extent of spin delocalisation onto the protons, and despite the occurrence of quite extensive libration of the cation in Ccl, at 77 K.
and Steadman show conclusively that this cannot be correct for H,CO+ cations, whereas if gmax is alongx,
Xcknowledgement
normal to the radical plane, andgint is alongz, the data are nicely accommodated. Our simulations also seem to require this assignment. The reason for the unexpectedly small shift for gz can perhaps be understood in terms of coupling both to the filled and the empty n orbit&, giving considerable cancellation of shift, but we are nevertheless surprised by this result.
We thank Professor for helpful discussion_
T. Shida and Dr. J.R. Morton
References and T. Shidzt, J. Am. Chrm. Sot. 101 (1979) 6669. [ 21 h1.C.R. Symons and LG. Smith, J. Chem. Rcs. (S)
II ] T. km
(1979)
38L 411
Volume 106, number 5
CHEMICAL PHYSICS LETTERS
[ 31 K. Toriyama, I;. Nunome and hl. iwsaki, J. Phys. Chem. 85 (1981) 1149; J. Chem. Phys. 77 (1982) 5891. 141 J.T. Wang and F. Wiiams, Chem. Phys. Letters 82 (1981) 177. [ 51 Y. Takemura and T. Shida, 1. Chem. Phys. 73 (1980) 4133. [6] hl.CR. Symons. J. Chem. Sot. Chem. Commun. (1981) 1251. [ 7] M.C.R. Symons and P.J. Boon, Chem. Phys. Letters 89 (1982) 516. [ 81 T. Shida and I;. U&i&. 24th Japanese Symposium on Radiation Chemistry (1981) p. 110. [ 91 L.D. Snow and F. Wtiarns, Chem. Phys. Letters 100 (1983) 198.
412
(lo]
1I I ] [ 121
[ 131 1141 1151
4 hlay 1984
P.J. Boon and M.C.R. Symons, Chem. Phys. Letters 100 (I 983) 201. L. Knight and G. Stcadman, J. Chem. Phys., to bc published. M.C.R. Symons. Chemiul and biochemic;il aspects of electron spin resonance spectroscopy (Van Nostrand, Princeton, 1978). D.N.R. Rao, H. Chandn and hl.C_R. Symons, J. Chem. Sot. Perkin Trans. Ii (1984). to be published. L. Harris and M.C.R. Symons. J. Chem. Res. (S) (1982) 268; 01) (1982) 2746. P.J. Boon, I;. Ushida. T. Shida and hl.C_R. Symons, to be published.
106, number
5
CHEMICAL PHYSICS LETTERS
4 May 1984
IHE ACETONE CATION - A r3C ESR AND ENDOR STUDY Philip J. BOON, Lorraine Lkprtnwnt
HARRJS, Myra T. OLM. Jane L. WYATT and Martyn CR. SYMONS
of Chentirtry. 7be Unir~ersity.Leicester LEI 7RH, UK
Received 6 February
1984
; in tinal form 24 February
1984
Espomre of dilute solutions of acetone in trichlorofluoromethane to 6oCo -r-rays at 77 R gave the radical cation, characterised only by its g-tensor components. An ENDDR study revealed the presence of two types of weakly coupled protons (A r = I.5 G. AZ = 0.3 G) in accord with INDO calculations. The ESR spectrum of acetone labeled with 13C(13CH3) has been studied and the resulting pammeters interpreted in terms of two equivalent carbon atoms. with ~22% spin density on each. This value is significantly less than that on the hydrogen atoms of the formaldehyde cation (==36W). which supports our contention that C-H hypcrconjugation is somewhat more important than C-C hyperconjugation in radical cations.
1. Introduction The recent discovery that radical cations can be prepared and trapped in various solids, especially tri-
chlorofiuoromethane, under conditions-&h that the only welldefied ESR spectrum is that of the cation, has led to a spate of reports describing such spectra [l-6]. However, we know of no published work reporting t3C hyperfine coupling in such cations *. We have suggested that the ESR spectrum for the acetone cation is an unresolved singlet, with three distinct gfeatures in CFCi3 at 77 K [7], and this has also been reported by Shida and his co-workers [8] and by Snow and Williams [9]. The aim of the present work was to establish the form of the 13C interaction, and hence to estimate the degree of hyperconjugative interaction for the C-C bonds. Our results for acetaldehyde cations [7, lo] and those of Knight and Steadman for fcrmatdehyde cations [ 111 show clearly that C-H hyperconjugation is large. It seemed of interest to obtain an estimate of the degree of interaction of the C-C bonds in this cation by measuring the r3C hyperfme coupling. In many cases, the rotation of methyl groups in
radicalcations is strongly restricted, in contrast with most neutral radicals. Our INDO calculations for (Me&O)’ suggest that this should be the case for acetone, even though the degree of involvement of the protons is very small. A second aim was therefore to measure the proton coupling using ENDOR spec-
troscopy . 2. Experimental Acetone enriched in 13C (92.6 atom% MSD isotopes) was used as supplied. Solvents were freon (CFCI,) and tetrachloromethane, both of which give good ESR spectra for the acetone cation after irradiation. Samples were irradiated at 77 K in a Vickrad 6OCo -y-ray source with doses of ~1 Mrad. ESR spectra were measured on a Varian E-109 spectrometer calibrated with a Hewlett-Packard 5246L frequency counter and a Bruker BH12E field probe, which were standardised with a sample of DPPH. ENDOR spectra were measured at 77 K on a Bruker ER2OOD multiaccessory resonance spectrometer.
* Shida and co-workers have also studied (‘%H~)z CO by ESR spectroscopy. their results being similar to ours (personal communication).
408
0 009-2614/84/s 03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
Volume
106. number 5
CHEMICAL
PHYSICS LE-lTERS
1
May 198-l
1
3ZlSG
4 a
b
1 32306 52
-H
C
0
-1 I
:;
x
I
J
1
=
1. Firstderivative X-band ESR spectra for (“CH&CO in CFCI3 or CC4 (~0.001 mol fraction) after exposure IO 6oCo yrays at 77 K;(a) in CFClp at 77 I;. showing features assigned to non-librating_(13CH3)zCOf radical cations [the a features are for (‘3CH3)(‘2CH3)CO+Cations in low abundance];(b) a simulation of (a) for (13CHx)2CO*otions only. using the dam in table 1; (19 in CC4 at 77 K. sho~‘ing features assigned to librating (13CH3)2CO+ cations [feature Q is a&ned to (“CH~)(‘*CHJ)CO+ cations].
Fig.
409
Volume
106, number
Table 1 ESR parameters
5
for acetone
Medium
CHEMICAL
LETTERS
4 May 1984
cations Hyperfiie
coupling x
CFCIJ (77 K)
13C
cc14
‘H
(77 K)
PHYSICS
t3C
constants
g-tensor
(G) a) I
Y
28
9
-
-
12.0
16.4
9
components
iso
gx
g.,
15.3
2.0077
2.0017
2.003
2.006 1
2.0035
2.0023
1.5 0.3 18.4
15.6
a) 1 G = 1O-4 T.
3.
Results and discussion
3.1.
StQtiOtlQrJ~ rudicuk
A typical ESR spectrum for the t3C derivative is shown in fig. 1. This spectrum, in contrast with many “powder” spectra, is not readily interpreted in terms of normal expectation, and it was necessary to resort to computer simulation. The best simulation (fig. 1b) was obtained with the parameters listed in table 1, and any major deviation from these values gave quite unacceptable simulations. Both spectra show the presence of extra features (ol) not reproduced in the simulations, which relate only to (13CH3)2CO+ cations. These are assigned to cations containing only one 13C atom, and can again be satisfactorily accommodated using the parameters in table 1. 3.2.
Libmting
mdiculs
ESR spectra for 12C or 13C radicals in CC], at 77 K differ markedly from those in CFCl, (fig. lc). However, on cooling the CC14 spectra to ~30 K there was
a reversible change to the form of fig. la. For both solvents, the features broadened seriously on cooling to 4 K, even at very low microwave powers. This may be due to incipient chloride interaction of the type clearly detected for acetaldehyde cations [9,10]. These spectral changes were reversible, and spectra similar to that shown for the radicals in Ccl, were obtained from CFC13 solutions on annealing to =1X K. The spectrum shown in fig. lc has greatly simplified, and can now be analysed as indicated in the stick-dia‘gram. In particular, the intermediate features have been 410
lost and a strongM{ = 0 feature has appeared. The resulting parameters (table 1) have Aiso(t3C) almost equal to that deduced from spectrum la, thus strongly supporting our analysis. Furthermore, in both cases, the g values are equal to those derived from the 12C
spectra. Evidently, there is extensive libration, but not complete rotation about any axes since the symmetry is clearly not axial. The result is that the major turning points are now for field along the symmetry axes, so that a more conventional spectrum is obtained. On cooling to ~30 K this spectrum changes reversibly to one similar to that in fig. la. 3.3. ENDOR
spectm
A typical ENDOR spectrum for the normal *2C cat ion in CC14 at 77 K is shown in fig. 3. This shows two proton lines, one with a coupling of 1.3 G and the other of 5-03 C. Similar spectra were obtained from different parts of the ESR spectrum showing that the coupling constants are almost isotropic. However, we were unable to detect any ENDOR features from the cations in CFCI,. We do not place any special significance on this contrast, since our ability or inability to obtain ENDOR signals from powder spectra such as these seems to have no clearly defied scientific basis.
c6@
-------y
Fig. 2. x,y and z axes for the (CH&COt as&.nment.
-
(2.0017)
cation and g-value
Volume
106. number
PHYSICS
CHEhlICAL
5
4 !a)-
LETTERS
1951
3.5. Aspects of smcrure If we accept that the tjC parameters are close to the principal values. we can derive estimates of the 2s and 2p character of the orbit& on carbon from the Aiso and 3_B data in the usual manner [ 12) _IfA, is taken to be negative, we obtain impossibly high values (~80% spin density on the two methyl carbon atoms). We conclude that A, and As are both positive This gives 251 2% 2s and 21% 2p on each methyl carbon atom. Thus the SOMO comprises carbon p orbital rather than s-p hybrids. The extent of delocalisation is signifkmrly less than that ior H,CO+ cations (=X1
on each hydrogen). This semi-quankative result firs in satisfactorily with our conclusions for the cation of
12
13
,,,,
14
15
16
17
( 18
MHZ Fig. 3. First-derivative (“CH&CO+ mtion features
ENDOR spectrum for the in CCb at 77 K showing hyperfie
for two types of protons.
3.4. g values We follow the assignment of Knight and Steadman 1 I] in fig. 2. It is normally argued for structures of is the g. value along the C-O axis this type ~atgrn, (z) (or C-N axis for R&N radicals). This reasoning is based on the absence of u bonds to oxygen other than the C-O bond, so that orbital angular momentum is relatively free for spin on oxygen. However, Knight [
ethyl benzene [ 131. In this case, the two methylene protons have hyperfine couplings of 19 G each. Since for toluene, the average proton coupling is ==I9 G [ 14 this must mean that the methyl group is constrained t( lie in the plane of the benzene ring, which is a positior of masimum steric energy. We esplained this by postulating that, for radical cations, C-H hyperconjugation is energetically more favourable than C-C hyperconjugation. Finally, we note that in our study of a range of aldehyde and ketone cations with Shida and his coworkers [IS], we reported INDO calculations of the isotropic proton coupling constants for the acetone cation. The results (4H at -1.3 C and 3H at -1.6 G) are quite close to those now obtained by ENDOR spectroscopy. In particular, we show that methylgroup rotation is resrrlcred despite the minor extent of spin delocalisation onto the protons, and despite the occurrence of quite extensive libration of the cation in Ccl, at 77 K.
and Steadman show conclusively that this cannot be correct for H,CO+ cations, whereas if gmax is alongx,
Xcknowledgement
normal to the radical plane, andgint is alongz, the data are nicely accommodated. Our simulations also seem to require this assignment. The reason for the unexpectedly small shift for gz can perhaps be understood in terms of coupling both to the filled and the empty n orbit&, giving considerable cancellation of shift, but we are nevertheless surprised by this result.
We thank Professor for helpful discussion_
T. Shida and Dr. J.R. Morton
References and T. Shidzt, J. Am. Chrm. Sot. 101 (1979) 6669. [ 21 h1.C.R. Symons and LG. Smith, J. Chem. Rcs. (S)
II ] T. km
(1979)
38L 411
Volume 106, number 5
CHEMICAL PHYSICS LETTERS
[ 31 K. Toriyama, I;. Nunome and hl. iwsaki, J. Phys. Chem. 85 (1981) 1149; J. Chem. Phys. 77 (1982) 5891. 141 J.T. Wang and F. Wiiams, Chem. Phys. Letters 82 (1981) 177. [ 51 Y. Takemura and T. Shida, 1. Chem. Phys. 73 (1980) 4133. [6] hl.CR. Symons. J. Chem. Sot. Chem. Commun. (1981) 1251. [ 7] M.C.R. Symons and P.J. Boon, Chem. Phys. Letters 89 (1982) 516. [ 81 T. Shida and I;. U&i&. 24th Japanese Symposium on Radiation Chemistry (1981) p. 110. [ 91 L.D. Snow and F. Wtiarns, Chem. Phys. Letters 100 (1983) 198.
412
(lo]
1I I ] [ 121
[ 131 1141 1151
4 hlay 1984
P.J. Boon and M.C.R. Symons, Chem. Phys. Letters 100 (I 983) 201. L. Knight and G. Stcadman, J. Chem. Phys., to bc published. M.C.R. Symons. Chemiul and biochemic;il aspects of electron spin resonance spectroscopy (Van Nostrand, Princeton, 1978). D.N.R. Rao, H. Chandn and hl.C_R. Symons, J. Chem. Sot. Perkin Trans. Ii (1984). to be published. L. Harris and M.C.R. Symons. J. Chem. Res. (S) (1982) 268; 01) (1982) 2746. P.J. Boon, I;. Ushida. T. Shida and hl.C_R. Symons, to be published.