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Observation of a free radical during the photolysis of phenyliodoacetylene
‘Volume ll,~numbex
3
‘. : 15 October
CHEMICAL PHYSICS LETTERS
1971
OBSERVATION OF A FREE RADICAL DURING THE PHOTOLYSIS OF P’HENYLIODOACEI’YLENE J.S. COLEMAN,A.HUDSON,KDJ. ROOT,and D&M. WALTON School o,fMolecu[m Sciences, University of Sussex, Brighton, BNI 9QJ, UK Received 9 August 1971
&-I ESR spectrum observed during the photolysis It
is suggested
t&t
a crossing
of
o-and
~-&et
0-s
of phenyJicdoacetybne indicates the formation h the ptenylethynyl radial
It has been shown [l] that photolysis ofphenyliodoacetylene (I) is a source of phenylethynyl radicals (II) and the reactivity of these radicals towards a range of monosubstituted benzenes has been investigated by Tiecco and his co-workers [2]. We report the results of an ESR study of the following photolysis hv PhC=CI+PhC_=C.+l-. (1) (II) initial exneriments using techniques described elsewhere [3]’and solutions of (I) in carbon tetrachloride prepared by the method of Shostakovskii and his coworkers [4] yielded only weak ill-defined signals. More positive results were obtained’when quartz: tubes containing (I) in the absence of solvent were irradiated with unfiltered light from a 25OW high pressure mercury ‘amp in the cavity of a Varian E3 spectrometer. The same spectrum was observed fr,om samples of (I) prepared according to NePs method [S] . Best. results were obtained when (I) was supercooled below its melting point of 9°C to temperatures in the range 0” to -10°C before irradiation. A well resolved spectrum (fig. I) was.obtained in the supercooled glassy phase, indicating that the radicals are stiil rotating freely although their r&e of recombination tis lower than in the liquid phase. The spectrum’ ti be analysed.in~te@ of triplet splittings of 2;22 .: and.0.81d and a doublet splitting of i.IM;. Theg.I‘factoris2.0030~0.0003. ;-, ,.
ora n-radical
The number of coupling constants is consistent with splittings from pairs of ortho and meta protons and a single para proton. The magnitudes of the hyperfme interactions however are not those expected for a o-radical (II). The pattern of three approximately equivalent protons and a smaller splitting from a pair of equivalent protons is more typical of n-radicals such as benzyl. The coupling constants are in fact very close to those reported [6] for the ring protons in the radical PhC = C’CH2 (Ai =A$ = 2.55,Ag = 0.82G). The g-factor is also typical of a n-radical. The ESR results therefore suggest that, if our assignment of the radical is correct, phenylethynyl has the n-structure (III) rather than the o-structure (II). The possibility of a crossing of &-and n-levels in aryl radicals has been discussed by Kasai et al. [7] but an examination of the ESR spectra of p_henyl, I- and 2naphthyl,.l- and 9-anthracyl and 1-pyrenyl showed that in all cases the unpaired electron occupied the essentially non-bonding odrbital corresponding to the broken o-bond. However it has recently been shown[8] that the ar-s&yl,radical has the *methyleneberuyl structure (rv) rather than the u-structure (V). ‘This system has obvious similarities to the phenylethynyl radical. Preliminary caIc&tions using the INDO method, whilst not conclusive have demonstrated the possibility of a (~4 crossover; moreover ,: @f&i
.&&
g+\#,.
cHELh4Ic4LPHYsIcs
Volume 11, number 3
Fig.
&e proton
LETTERS
1. The ESR spectrum observed during photolyl
hyperfiie
coupling coxtants calculated (III) are in good agreement with
for the sr-structure those determined experimentally
[9].
We thank Dr. H.G. Benson for performing the INDO calculations.
References [I ] N. Kha.rasch, W. Wolf, T. Erpelding, Tokes, Chem. Ind. (1962) 1720.
P.G. Naylor and L.
15 October 1971
of phenyliodoacetyiene.
[2] G. MarteNi, P. SpagnoIo and M. Tie-,
Chem. Commun (1969) 282; J. Chem. Sk. B. (1970) 1413. I31 A. Hudson and HA. Humin, I. Chm. SOC. B. (1969) 793. 141 Ms. Shostakovskii N.V. Komarovand Q.G. Yarosh, Izv.
Akad. Nauk SSSR (1968) 908. [5] J.V. Nef, LiebigsAnn. 308 (1899) 293;J. Chem. SOC. abstracts (1900) 20. [6j J.K. Kochi and PJ. Krusic, 3. Am. Chem. Sot. 92 (1970) 4110. [71 P.H. Kwi, PA. Clarke and E.B. Whippie, I. AI-II.Chcm. Sot. 92 (1970) 2640. 181 J.E. Bennett ati JA. Howard, Chem. Fhvs: Letters 9 (1971) 460. [9] H.G. Benson. Ph.D. Thesis. University of Sussex (1971)
3
‘. : 15 October
CHEMICAL PHYSICS LETTERS
1971
OBSERVATION OF A FREE RADICAL DURING THE PHOTOLYSIS OF P’HENYLIODOACEI’YLENE J.S. COLEMAN,A.HUDSON,KDJ. ROOT,and D&M. WALTON School o,fMolecu[m Sciences, University of Sussex, Brighton, BNI 9QJ, UK Received 9 August 1971
&-I ESR spectrum observed during the photolysis It
is suggested
t&t
a crossing
of
o-and
~-&et
0-s
of phenyJicdoacetybne indicates the formation h the ptenylethynyl radial
It has been shown [l] that photolysis ofphenyliodoacetylene (I) is a source of phenylethynyl radicals (II) and the reactivity of these radicals towards a range of monosubstituted benzenes has been investigated by Tiecco and his co-workers [2]. We report the results of an ESR study of the following photolysis hv PhC=CI+PhC_=C.+l-. (1) (II) initial exneriments using techniques described elsewhere [3]’and solutions of (I) in carbon tetrachloride prepared by the method of Shostakovskii and his coworkers [4] yielded only weak ill-defined signals. More positive results were obtained’when quartz: tubes containing (I) in the absence of solvent were irradiated with unfiltered light from a 25OW high pressure mercury ‘amp in the cavity of a Varian E3 spectrometer. The same spectrum was observed fr,om samples of (I) prepared according to NePs method [S] . Best. results were obtained when (I) was supercooled below its melting point of 9°C to temperatures in the range 0” to -10°C before irradiation. A well resolved spectrum (fig. I) was.obtained in the supercooled glassy phase, indicating that the radicals are stiil rotating freely although their r&e of recombination tis lower than in the liquid phase. The spectrum’ ti be analysed.in~te@ of triplet splittings of 2;22 .: and.0.81d and a doublet splitting of i.IM;. Theg.I‘factoris2.0030~0.0003. ;-, ,.
ora n-radical
The number of coupling constants is consistent with splittings from pairs of ortho and meta protons and a single para proton. The magnitudes of the hyperfme interactions however are not those expected for a o-radical (II). The pattern of three approximately equivalent protons and a smaller splitting from a pair of equivalent protons is more typical of n-radicals such as benzyl. The coupling constants are in fact very close to those reported [6] for the ring protons in the radical PhC = C’CH2 (Ai =A$ = 2.55,Ag = 0.82G). The g-factor is also typical of a n-radical. The ESR results therefore suggest that, if our assignment of the radical is correct, phenylethynyl has the n-structure (III) rather than the o-structure (II). The possibility of a crossing of &-and n-levels in aryl radicals has been discussed by Kasai et al. [7] but an examination of the ESR spectra of p_henyl, I- and 2naphthyl,.l- and 9-anthracyl and 1-pyrenyl showed that in all cases the unpaired electron occupied the essentially non-bonding odrbital corresponding to the broken o-bond. However it has recently been shown[8] that the ar-s&yl,radical has the *methyleneberuyl structure (rv) rather than the u-structure (V). ‘This system has obvious similarities to the phenylethynyl radical. Preliminary caIc&tions using the INDO method, whilst not conclusive have demonstrated the possibility of a (~4 crossover; moreover ,: @f&i
.&&
g+\#,.
cHELh4Ic4LPHYsIcs
Volume 11, number 3
Fig.
&e proton
LETTERS
1. The ESR spectrum observed during photolyl
hyperfiie
coupling coxtants calculated (III) are in good agreement with
for the sr-structure those determined experimentally
[9].
We thank Dr. H.G. Benson for performing the INDO calculations.
References [I ] N. Kha.rasch, W. Wolf, T. Erpelding, Tokes, Chem. Ind. (1962) 1720.
P.G. Naylor and L.
15 October 1971
of phenyliodoacetyiene.
[2] G. MarteNi, P. SpagnoIo and M. Tie-,
Chem. Commun (1969) 282; J. Chem. Sk. B. (1970) 1413. I31 A. Hudson and HA. Humin, I. Chm. SOC. B. (1969) 793. 141 Ms. Shostakovskii N.V. Komarovand Q.G. Yarosh, Izv.
Akad. Nauk SSSR (1968) 908. [5] J.V. Nef, LiebigsAnn. 308 (1899) 293;J. Chem. SOC. abstracts (1900) 20. [6j J.K. Kochi and PJ. Krusic, 3. Am. Chem. Sot. 92 (1970) 4110. [71 P.H. Kwi, PA. Clarke and E.B. Whippie, I. AI-II.Chcm. Sot. 92 (1970) 2640. 181 J.E. Bennett ati JA. Howard, Chem. Fhvs: Letters 9 (1971) 460. [9] H.G. Benson. Ph.D. Thesis. University of Sussex (1971)