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ESR spectrum of the α-styryl radical
Volunie 9, number 5
CHEhliCAL PHYSICS LETTERS
ESR SPECTRUM
OF
THE
a-STYRYL
1 June 1971
RADICAL
3. E. BENNETT and J. A. HOWARD * Sheli Research Limited, Tkom3.m Research Centre, P. 0. Box I, Chesfer, CM 3.9H, UK Received 5 April 19~1 The ESR spectrum showy that the structure of the Q-sty@ radical, CSH&=CHS, is not that of the I-pkenytvinyl radioat bat that of the P-methyleoebenzyl radical in which the unpaired electron is delocaZize&into the r-system of the ammatic ring.
1. INTRODUCMON Several members of the fwo iso-electronic series of free radicals, X-C=0 (I) and X=C=CH2 (ll), have been studied hy electron spin resonance (ESR) and their structures elucidated.
The formyl [ 13aridvinyl [2,3] (X=H) and the dcetyl[4] and I-methylvinyl [2] (X=CH3) fadi~als
adatnan’Lane and its ESEZspectrum ‘WK and higher temperatures.
observed
at
Tbe ESR spectra were recorded on a Varian V-4502 spectrometer with Fieldial regulation of the magnetic field. Tbe magnetic field was calibrated with a proton resonance magnetometer, and the g-factor was determined by comparison with that of (Y, aWIiphenylpicryI.hydrazyI (2.0036).
have been studied in detail and it has been shown that they are all u-type radicals in which the un.. pairedelectron is located mainly in au sp-hybrid
3. RESULTS AND DISCUSSION
benzoyt radical [5] (I; X=CsH5) has also been shown to be a o-type radical, even though it
The ESR spectrum of the CPstyryl radical in adamantane at IlOoK is shown in fig. 1. Apart
stabilized by delocalization
from a slight decrease in resolution, no significant differences in the spectrum were observed at lower temperatures or for the radical trapped in a matrix of C&D6. The spectrum consisted of a major triplet (1: 2 : 1) which is attributed to
orbital on the free valence carbon atom. The
couId have assumed the n-type configuration of an a-substituted benzyl radical, which would be
oI the unpaired elec-
tron in the n-system of the aromatic ring. In this communication.we report the preparation of the cu-styryl radical (II; X=C6H$ and the observation of its ESR spec’truxn at low temperature.
fine coup’ling to the para- and the two o&ho-protons of the ring. The values of the isotropic hyperfine Coupling constants are:
2. EXPERDMENTAL The ahstyrgl radical was prepared
at low using a rotating cryostat [6], by the chemical r&action: _ temperature
hyperfine cotlpling to the two equivalent =CH2 protons, and a smaller quartet (” 1: 3 : 3 : 1) which arises from approximately equal hyper-
ACHY =
4.15 * 0.03 mT ,
(?PK),
CsHsCBr=CH2
+ Na -
.
CsH5C=CH2
+ NaBr _
The radical was trapped in a mat&x of f&Dg or
_ * Vfsitbg Scientist, pemnanent address: Division OF Chemistry; National Research Council of Canada. Ottawa 7. Ontario, Canada.
Apara z Aor*o
=
0.61 f 0.03 mT ,
giso = 2.0023 .
The ~~-&yryl radical may be considered in one of two configurations, ITIor IV.
Volume 9, numbsr 5
I June 1971
CHEMICALPHYSICSLETTERS
lar to those for the unsujxtituted benzyl radical
Y:
[‘7] (Aorao = 0.515 mT; A apa = 0.618 mT). (There is a possibility phat the free valence carbon atom in structure err) is sp-hybridized, rather than sp2-hybridized as assumed, and that the phenyl ring is rotated by 90a from its position in the benzyl radical. This alternative, in which the unpaired electron is still localized, can be eliminated, for although the =CH2 protons would be equivalent, there would be n@Egible coupling to the ring protons. Also, as discussed below, the hyperfine coupling constant for the =CH2 protons would be expected to be much larger than that found experimentally.) The large hyperfine coupling constant for the /3-methylene protons is consistent with the structure of the benzyl radical (IV), in which these protons are constrained to lie in, or cIose to, the antinodal plane of the n-orbital containing the unpaired electron. A large hyperfine coupling (3.48 mT) lxx also been observed [8] for the methylene protons in the substituted ally1 radi-
H
2mT
Fig. 1. First derivative spectrum of a-sty@ in adamantaneat llO°K.
cals (v).
CH2--CR--C=CH2 radical
The unpaired electron is in a localized orbital in (III), which may be considered as the l-phenyl-
vinyl radical. In contrast, the unpaired electron is in a delocalized orbital in (IV), which can be regarded as the cr-methylenebenql radical. A consideration of the ESR spectrum of the a-styryl radical leads us to favour structure (IV) for the
following reasons: (i) the spectrum in CgD6 at W°K does not exhibit the marked asymmetry associated with utype radicals, such as formyl, acetyl, benzoyl, vinyl and I-methylvinyl radicals, in this and other matrices at WOK; (ii) tie g-factor is isotropic and closer to those of n-type radicals (2.0024- 2.0027) than those for u-type radicals (2.0010- 2.0018); (iii) the hyperfine coupling constants for the =CH2 protons are equal, whereas in l-substituted vinyl radicals the =CH2 protons are nonequivalent, withA,,, * 2Acis; (iv) the relatively large hypetiine coupling constants for the ortho- and paxa-protons are typical of a benzyl radical, whereas for the benzag1 radical [5] there is only a very small coupling (0.116 mT) to the meta-protons. The mag-
nitude of these hyperfine coupling constants (0.61 mT) to the ring protons is also very simi-
(R = A or CH3)
(V)
These radicals have a similar configuration to that of the benzyl radical (IV) with the methylene protons lying close to the antinociai plane of the n-orbital. The slight non-equivalence of the
methylene protons, which is observed (81 in the liquid phase, would not be detected for the astyryl radical in our experiments in the solid
phase where the line width is about 0.35 mT. A simple consideration of the gepmetry of the a-radical fragments -C=CH2 and -C-CIi3, suggests that the hyperconjugation, and thus the hyperfine coupling, between the unpaired electron and the &CH2 protons will be very similar to that for a non-rotating B-CH3 group in which one proton lies in the antinodal plane of the p-orbital. Thus, for unit spin density on the a-carbon atom the hyperfine coupling for the methylene protons will be about 5.85 mT (i.e. twice the value, 2.925 mT, for a freely rotating methyl group [Z]). The spin density distribution in the crstyryl radical (IV) is not very different from that of the
unsubstitutedbenzyl radical [7,9], as is evident from the similarity of the ring proton hyperfine
coupling constants. Thus, ihe spin density on the e-carbon atom is close to 0.1, and the predicted hyperfine coupling co&z.t~t.s of the methylene protons is 4.1 mT (=O.? X 5.85 mT), which is in good agreement with the experimental value (4.15 mT).
Similarly the spin density distribution in the
461
Volume 9, number 5
CHEMICAL PHYSICS LETTERS
ally1 radical (V) [8] is close to that in the unsubstituted ally1 radical which has a spin density of 0.6 on each of the terminal carbon atoms [Z, 91. The predicted hyperfine coupling for the methylene protons in V is 3.51 mT (=0.6 X 5.85 mT), which again is close to the observed value (= 3.48 mT). INDO calculations [8] predict a value of 4.2 mT for this coupling constant. On this basis, if the unpaired electron were localized in the pure p-orbital of an sp-hybridized
1 June 1971
substituted
carbon atom, then the predicted hyperfine coupling constants would be about 5.85 mT for both
the benzyl (IV) and ally1 (V) radicals.
culations
[8] predict
(INDO cal-
an even higher value of 6.8
mT.) These values are much higher than those observed and show that, as concluded above, this structure is not acceptable. The good agreement between the predicted and experimental values for the methylene proton hyperfine coupling constants gives some justification for the assumption, based on the geometric configurations, that ‘he hyperconjugation of an unpaired electron in a p- or n-orbital is similar for a proton in either a P=CH2 group or a 13-CH3 group, when it is located in the antinodal plane of the electron orbital. The behaviour of the crstyryl radical in assuming a n-radical configuration is in marked contrast to that of the iso-electronic benzoyl radical, which retains the sp2-configuration Df the free valence carbon atom (i.e. it is a o-type
radical) x=Co.
462
exhibited by the other acyl radicals,
: :
-
.
We thank Messrs. C. T. Rimmer and R. Summers for assistance with the experimental work. RRFERENCES [l] -F. J. Adrian, E. L. Cochran and V. A. Bowers, J. Chem. Phys. 36 (1962) 1661; J.A.Briv&.i, N.keen-and M. C. R. Symons, J. Chem. Sot. (1961) 237: E. L. Co&ran, F. J. Adrian and V. A. Bowers, J. Chem. Phys. 44 (1966) 4626; R.W. Holmberg, J. Chem. Phys. 51 (1969) 3255. [2] R.W. Fessenden and R.H. Schuler. J. Chem. Phys. 39 (1963) 2147. [3] E.L. Co&ran. F. J. Adrian and V. A. Bowers, J. Chem. Phys. 40 (1964) 213: P. H.Kasai and E. B.Whipple, J. Am. Chem. SGC. 89 (1967) 1034. 141 B. hlile and B. Ward. Chem. Commun. . _ J. E. BeMdt. (1969) 13. . [5j P. J.Krusic and T.A.Rettig, J. Am. Chem. See. 92 (1970) 722. 162J.E.Bennett and A.Thomas, Proc. Roy. Sot. A280 (i964) 123; J. E. Bennett. B. Mile. A, Thomas and B. Ward. in: Advances in physical brganic chemistry, Vol. 8. ed. V. Gold (Academic Press, New York, 1970) p. 1. [7] A. Carrington and I. C. P. Smith, Mol. Phys. 9 (1965) 137. [8] J. K. Kocht and P. J. Krusic, J. Am. Chem. Sot. 92 (1970) 4110. [9] A.D.McLachlaa. Mol. Phys. 3 (1960) 233.
CHEhliCAL PHYSICS LETTERS
ESR SPECTRUM
OF
THE
a-STYRYL
1 June 1971
RADICAL
3. E. BENNETT and J. A. HOWARD * Sheli Research Limited, Tkom3.m Research Centre, P. 0. Box I, Chesfer, CM 3.9H, UK Received 5 April 19~1 The ESR spectrum showy that the structure of the Q-sty@ radical, CSH&=CHS, is not that of the I-pkenytvinyl radioat bat that of the P-methyleoebenzyl radical in which the unpaired electron is delocaZize&into the r-system of the ammatic ring.
1. INTRODUCMON Several members of the fwo iso-electronic series of free radicals, X-C=0 (I) and X=C=CH2 (ll), have been studied hy electron spin resonance (ESR) and their structures elucidated.
The formyl [ 13aridvinyl [2,3] (X=H) and the dcetyl[4] and I-methylvinyl [2] (X=CH3) fadi~als
adatnan’Lane and its ESEZspectrum ‘WK and higher temperatures.
observed
at
Tbe ESR spectra were recorded on a Varian V-4502 spectrometer with Fieldial regulation of the magnetic field. Tbe magnetic field was calibrated with a proton resonance magnetometer, and the g-factor was determined by comparison with that of (Y, aWIiphenylpicryI.hydrazyI (2.0036).
have been studied in detail and it has been shown that they are all u-type radicals in which the un.. pairedelectron is located mainly in au sp-hybrid
3. RESULTS AND DISCUSSION
benzoyt radical [5] (I; X=CsH5) has also been shown to be a o-type radical, even though it
The ESR spectrum of the CPstyryl radical in adamantane at IlOoK is shown in fig. 1. Apart
stabilized by delocalization
from a slight decrease in resolution, no significant differences in the spectrum were observed at lower temperatures or for the radical trapped in a matrix of C&D6. The spectrum consisted of a major triplet (1: 2 : 1) which is attributed to
orbital on the free valence carbon atom. The
couId have assumed the n-type configuration of an a-substituted benzyl radical, which would be
oI the unpaired elec-
tron in the n-system of the aromatic ring. In this communication.we report the preparation of the cu-styryl radical (II; X=C6H$ and the observation of its ESR spec’truxn at low temperature.
fine coup’ling to the para- and the two o&ho-protons of the ring. The values of the isotropic hyperfine Coupling constants are:
2. EXPERDMENTAL The ahstyrgl radical was prepared
at low using a rotating cryostat [6], by the chemical r&action: _ temperature
hyperfine cotlpling to the two equivalent =CH2 protons, and a smaller quartet (” 1: 3 : 3 : 1) which arises from approximately equal hyper-
ACHY =
4.15 * 0.03 mT ,
(?PK),
CsHsCBr=CH2
+ Na -
.
CsH5C=CH2
+ NaBr _
The radical was trapped in a mat&x of f&Dg or
_ * Vfsitbg Scientist, pemnanent address: Division OF Chemistry; National Research Council of Canada. Ottawa 7. Ontario, Canada.
Apara z Aor*o
=
0.61 f 0.03 mT ,
giso = 2.0023 .
The ~~-&yryl radical may be considered in one of two configurations, ITIor IV.
Volume 9, numbsr 5
I June 1971
CHEMICALPHYSICSLETTERS
lar to those for the unsujxtituted benzyl radical
Y:
[‘7] (Aorao = 0.515 mT; A apa = 0.618 mT). (There is a possibility phat the free valence carbon atom in structure err) is sp-hybridized, rather than sp2-hybridized as assumed, and that the phenyl ring is rotated by 90a from its position in the benzyl radical. This alternative, in which the unpaired electron is still localized, can be eliminated, for although the =CH2 protons would be equivalent, there would be n@Egible coupling to the ring protons. Also, as discussed below, the hyperfine coupling constant for the =CH2 protons would be expected to be much larger than that found experimentally.) The large hyperfine coupling constant for the /3-methylene protons is consistent with the structure of the benzyl radical (IV), in which these protons are constrained to lie in, or cIose to, the antinodal plane of the n-orbital containing the unpaired electron. A large hyperfine coupling (3.48 mT) lxx also been observed [8] for the methylene protons in the substituted ally1 radi-
H
2mT
Fig. 1. First derivative spectrum of a-sty@ in adamantaneat llO°K.
cals (v).
CH2--CR--C=CH2 radical
The unpaired electron is in a localized orbital in (III), which may be considered as the l-phenyl-
vinyl radical. In contrast, the unpaired electron is in a delocalized orbital in (IV), which can be regarded as the cr-methylenebenql radical. A consideration of the ESR spectrum of the a-styryl radical leads us to favour structure (IV) for the
following reasons: (i) the spectrum in CgD6 at W°K does not exhibit the marked asymmetry associated with utype radicals, such as formyl, acetyl, benzoyl, vinyl and I-methylvinyl radicals, in this and other matrices at WOK; (ii) tie g-factor is isotropic and closer to those of n-type radicals (2.0024- 2.0027) than those for u-type radicals (2.0010- 2.0018); (iii) the hyperfine coupling constants for the =CH2 protons are equal, whereas in l-substituted vinyl radicals the =CH2 protons are nonequivalent, withA,,, * 2Acis; (iv) the relatively large hypetiine coupling constants for the ortho- and paxa-protons are typical of a benzyl radical, whereas for the benzag1 radical [5] there is only a very small coupling (0.116 mT) to the meta-protons. The mag-
nitude of these hyperfine coupling constants (0.61 mT) to the ring protons is also very simi-
(R = A or CH3)
(V)
These radicals have a similar configuration to that of the benzyl radical (IV) with the methylene protons lying close to the antinociai plane of the n-orbital. The slight non-equivalence of the
methylene protons, which is observed (81 in the liquid phase, would not be detected for the astyryl radical in our experiments in the solid
phase where the line width is about 0.35 mT. A simple consideration of the gepmetry of the a-radical fragments -C=CH2 and -C-CIi3, suggests that the hyperconjugation, and thus the hyperfine coupling, between the unpaired electron and the &CH2 protons will be very similar to that for a non-rotating B-CH3 group in which one proton lies in the antinodal plane of the p-orbital. Thus, for unit spin density on the a-carbon atom the hyperfine coupling for the methylene protons will be about 5.85 mT (i.e. twice the value, 2.925 mT, for a freely rotating methyl group [Z]). The spin density distribution in the crstyryl radical (IV) is not very different from that of the
unsubstitutedbenzyl radical [7,9], as is evident from the similarity of the ring proton hyperfine
coupling constants. Thus, ihe spin density on the e-carbon atom is close to 0.1, and the predicted hyperfine coupling co&z.t~t.s of the methylene protons is 4.1 mT (=O.? X 5.85 mT), which is in good agreement with the experimental value (4.15 mT).
Similarly the spin density distribution in the
461
Volume 9, number 5
CHEMICAL PHYSICS LETTERS
ally1 radical (V) [8] is close to that in the unsubstituted ally1 radical which has a spin density of 0.6 on each of the terminal carbon atoms [Z, 91. The predicted hyperfine coupling for the methylene protons in V is 3.51 mT (=0.6 X 5.85 mT), which again is close to the observed value (= 3.48 mT). INDO calculations [8] predict a value of 4.2 mT for this coupling constant. On this basis, if the unpaired electron were localized in the pure p-orbital of an sp-hybridized
1 June 1971
substituted
carbon atom, then the predicted hyperfine coupling constants would be about 5.85 mT for both
the benzyl (IV) and ally1 (V) radicals.
culations
[8] predict
(INDO cal-
an even higher value of 6.8
mT.) These values are much higher than those observed and show that, as concluded above, this structure is not acceptable. The good agreement between the predicted and experimental values for the methylene proton hyperfine coupling constants gives some justification for the assumption, based on the geometric configurations, that ‘he hyperconjugation of an unpaired electron in a p- or n-orbital is similar for a proton in either a P=CH2 group or a 13-CH3 group, when it is located in the antinodal plane of the electron orbital. The behaviour of the crstyryl radical in assuming a n-radical configuration is in marked contrast to that of the iso-electronic benzoyl radical, which retains the sp2-configuration Df the free valence carbon atom (i.e. it is a o-type
radical) x=Co.
462
exhibited by the other acyl radicals,
: :
-
.
We thank Messrs. C. T. Rimmer and R. Summers for assistance with the experimental work. RRFERENCES [l] -F. J. Adrian, E. L. Cochran and V. A. Bowers, J. Chem. Phys. 36 (1962) 1661; J.A.Briv&.i, N.keen-and M. C. R. Symons, J. Chem. Sot. (1961) 237: E. L. Co&ran, F. J. Adrian and V. A. Bowers, J. Chem. Phys. 44 (1966) 4626; R.W. Holmberg, J. Chem. Phys. 51 (1969) 3255. [2] R.W. Fessenden and R.H. Schuler. J. Chem. Phys. 39 (1963) 2147. [3] E.L. Co&ran. F. J. Adrian and V. A. Bowers, J. Chem. Phys. 40 (1964) 213: P. H.Kasai and E. B.Whipple, J. Am. Chem. SGC. 89 (1967) 1034. 141 B. hlile and B. Ward. Chem. Commun. . _ J. E. BeMdt. (1969) 13. . [5j P. J.Krusic and T.A.Rettig, J. Am. Chem. See. 92 (1970) 722. 162J.E.Bennett and A.Thomas, Proc. Roy. Sot. A280 (i964) 123; J. E. Bennett. B. Mile. A, Thomas and B. Ward. in: Advances in physical brganic chemistry, Vol. 8. ed. V. Gold (Academic Press, New York, 1970) p. 1. [7] A. Carrington and I. C. P. Smith, Mol. Phys. 9 (1965) 137. [8] J. K. Kocht and P. J. Krusic, J. Am. Chem. Sot. 92 (1970) 4110. [9] A.D.McLachlaa. Mol. Phys. 3 (1960) 233.