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EPR spectra and structure of the chlorotrifluoroethylene and bromotrifluoroethylene radical anions
Volume 61, number 2
CHEMICAL PHYSICS LETTERS
EPR St’EaRA AND !%TRUCl-URE OF THE CHLOROTRIFLUOROETHYLENE BROMOTRiFLUOROETHYLENE RADICAL ANIONS f
15 February 1979
ANI)
Robert I_ hlcNEIL, Ffrancon WILLIAMS and Moon B. YIM Department
of Chemistry,
Universiry of Tennessee. Knoxville.
Tennessee 37916. USA
Received 23 October 1978
EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid soiutions at Iow temperatures. The spin density distrtbution in these rdids strongly suzeststhat the unpaired electron occupies a (I* orbital. a conclusion which is supported by CNDO/Z calculations.
1. Introduction
2. Experimental
Recent work has strongly suggested that the unpaired electron occupies a d orbital rather than the or* orbital in the tetrafluoroethylene radical anion [I] _ Since MO theory predicts that the replacement of one fluorine by a heavier and less electronegative halogen should have a significant effect on the spin distribution in an antibonding sigma orbital, we have carried out an EPR study of the halogenotrifluoroethylene radical anions. The operation of this halogen substitution effect in an unambiguous series of u* radicals is exemplified by the spin density distributions reported for the halogenotriIluoromethane radical anions [2,3] _Thus, for the semi-occupied of orbital of the chlorotrifluoro-
Chlorotrifluoroethylene (b.p. -27O) and bromotritluoroethylene (b.p. -2O) were supplied by PCR Research Chemicals, Inc- and contained d-limonene and tributylamine, respectively, as polymerization inhibitors. The sample of chlorotrifluoroethylene was purified by trap-to-trap distillation from a dry ice-isopropan01 bath on a vacuum line immediately before use_ Samples containing approximately 5 mole % of the halogenotrifluoroethylene in tetramethylsilane (TMS), tetramethylsilane-d12 (TMS-d12), and 2-methyltetrahydrofuran (MTHF) were prepared in Spectrosil and FEP Teflon tubes by vacuum techniques. The radical anions were usually generated by irradiation of the solid solutions with cobalt-60 gamma rays at 77 K, the typical irradiation dose being 0.75 Mrad. EPR measurements were made as described elsewhere [3 3. Confirmation of the spectral assignments to the radical anions was obtained through the generation of matching spectra by the 320 run photoionization of N,N,N’,N’-tetramethyl-p-phenylenediamine (ThIPD) in rigid MTHF solutions of the halogenotrifluoioethylenesat77K [4].
methane radical anion, experimental results agree with theOietic4 calculations in showing that a large spin density is concentrated in +Jle p. orbitsI of the chIorine ligand [3]. Conversely, the effect of haIogen substitution can be used as a diagnostic test for o* radicals, and here we report the characterization of the chloro- and bromo-trifluoroethylene radical anions as o* radicals from their anisotropic EPR spectra.
3_ Results f Research supported by the Division of Basic Ener,T Sciences, U.S. Department of Energy (document no. ORO-2968-113).
The EPR spectra of the radical anions derived from chIoro- and bromo-trifluoroethylene solutions in TMS 293
Volume 61_ number 2
CHE!GICAL PHYSICS LEl-ERS
and MTHF consisted ofanisotropic patterns_ However, the spectra in TMS-k17 and TM!%Zlz showed an orientation dependence whkh is typicaI of partially aligned radicals in a crystalline matrix [S] , and this effect was exploited to obtain the separate spectra of the parallel and perpendicuiar components by opt.imi&g the sample orientations in the magnetic field (vide infra). Orientation-dependent spectra of this kind were previously obtained for CF31- iu TMS solution [S] _ Throughout this paper we differentiate between parallel and perpendicular spectra according to their spectraI extent, the wider set of components being designated as the parallel spectrumFig- 1 shows the welLresolved EPR spectrum obtained by orientating the sample to intensify the parallel features of the cllorotrifluoroethylene radical anion in the ThlSzl,2 matrix at 98 K-This parallel spectrum consisting of 24 observable components outside the central region was reproducible from sample to sample at a particular orientation; moreover. a matching spectrum was similarly obtained from TMShl, soWion, the ordy difference being that the com-
,
*
Fig. L. First-derimrite
7
Z
3
2
EPR spectrum of a +n-adhted
I
=a
I
solid
sohnion of 5 moI % chIorotrifIuoroethyIene in tetmmethyb siIane4r~(TWGcfr~) at 98 Ii -I-hesampie tube was oriented to give the m&mum intensity for the parallel features of the chIorotrifIuoroethylene mdicaI anion_ The stick diagrams gibe the c&oIated line positions according to the parameters for QFC=CF; giver. in table I_ Residual perpetdictdar features in the spectmm are marhed zwxordingIy_
294
15 February 1979
ponent linewidths increased from less than 2 G in TMS+ to ca- 5 G in TMS?z,,. As shown in the line diagrams of fig. 1, the spectrum is composed of two patterns due to the presence of two chlorine isotopes, 35CI (Z = 312) and 37CI (Z = 3/2), with an intensity ratio of 3~1. Each pattern consists of 16 lines resulting from hyperfme interactions with one chlorine nucleus and two (vide infra) nonequivalent fluorine nuclei (Z(lg F) = l/2), one of the lgF coupling constants being about twice the other and the 3sCI coupling having a value intermediate between the two (table I). As can be seen in fig_ 1, the 12 outer components of each pattern are clearly resolved while the 4 remaining components are masked by the intense signal of the matrix radical in the centre of the spectrum. When the orientation of the TMS-d12 sample was changed from the parallel position by rotation about the tube axis, the lines belonging to the spectrum of fig. 1 coliapsed and another weII-defmed spectrum was observed after a ca. 90” rotation_ As shown in fig. 2, the width of this perpendicular spectrum is only 202 G as compared to 449 G for the paraIIe1 spectrum- The analysis of the perpendicular spectrum is given by the line diagram showing hyperfime couphngs to three nonequivalent fluorine nuclei in addition to the interaction with one chlorine, the largest fluorine coupling being approximately twice the 35C1 coupling and the other two fluorines having much smaller couplings_ The EPR parameters for the ch.IorotrifluoroethyIene radical anion are colIected in table lOrientation-dependent specrra were also obtained for -y-hrradiatedsolid solutions of bromotrifluoroethylene in TMS. As shown cIearly by a comparison of the outer spectral regions in figs. 3 and 4, almost complete discrimination has been achieved between the paraIIe1 and perpendicular spectra at these orientations, the parallel MZfBr) = S/2 features (fig_ 3) being absent in the perpendicular spectrum of fig_ 4. The paraIIe1 spectrum is readi& analyzed into separate 81 Br (Z = 3/2; natural abundance = 50.6%) and 79Br (Z = 3/2; natural abundance = 49.4%) patterns resulting from hyperfine interaction with a singIe bromine nucleus. each pattern consisting of sixteen lines made u_pof a large 1 :l :I :I bromine quartet further split by coupling to two nonequivalent f9F (f = l/Z) nuclei_ These I9F couplings for the paraIIe1 spectrum (table 1) are remarkably similar to those obtained for the parallel spectrum of the chIorotrifIuoroethyIene radicaI anion,
Volume 61, number 2
CHEMICAL PHYSICS LETTERS
15 February 1979
Table l EPR parameters a) for the tetrafluoroethylene b) and haIogenotrifluoroethylene radical anions Radical
Matrix
T (Q
g-vaIues 81
F2C=CF;
ml=12
120
F&=CFCl-
TWXI,
98
2.011
FzC=CFBr-
-Ixlzxz,*
98
2.036
Nucleus gu
giso
Hypertine couplings (G) AL
All
Aiso
2.0027
19F l3C
94.3 48.7
1997
7-006
ascl WC1 Fa Fb Fc
37.6 31.3 77.2 7.3 4.5
84.0 69.8 132.3 64.8 0
53.1 44.1
2.000
2.024
“‘Br ‘gBr Fa Fb Fc
212.0 197.5 35.4 30.2 10.1
465.3 431.6 130.6 65.3 0
296.4 275.5
-
a) The tgF hvperfme couplings for the perpendicular and parallel spectra of the halogenotrifluoroethylene radical anions are not principal v&es (see text) and are listed in numerical order, the correct pairingsbeing unknown. b) From ref. [I 1.
Fig. 2_ Fiit-derivati~e EPR spectrum of a r-irradiated solid solution of 5 mole % c-blorotrifluoroethylene in TMSdr2 at 98 Ii. The sample tube wzs oriented to intensify the perpendicular features of the chIorotrifluoroethylene radical anion. The stick diagramsgive the calculated line positions according to the parameters for QFC=CF$ given in table 1-
A considerable narrowing of the bromine hyperfiie couplings occurs in going from the parallel to the perpendicular spectrum of the bromotrifluoroethylene radical anion. Fortunately, the bromine splittings are still large enough to prevent overlapping between the substructures (fig- 4)_ The analysis of the perpendicular spectrum is therefore simplified and the MI(J3r) = -312 substructure is shown on an expanded scale in fig. 5. Although only twelve resolved features can be seen, there is an additional strong feature in the centre of the pattern which is masked by the high-field line of atomic hydrogen_ This feature was uncovered by using FEP Teflon sample tubes which do not give signals from hydrogen on y irradiation 163 _ A good fit of the --3/Z perpendicular manifold is given by the line diagram in fig. 5 consisting of two sets (81 Br and 7QBr) of eight lines, each set representing the splitting pattern from three nonequivalent fluorines_ Since three lines from one set are closely overlapped by three lines from the other set, the diagram accounts for the observation of thirteen resolved components in which features 4,7, and 10 appear with enhanced intensities. Although these results appear to exclude the possibility of interaction with only two fluorines, an additional check on the fluorine hyperfiie splitting pattern is afforded by an examination of the 295
Vi?Iume &I, number 2
Rg. 3. F.&t-derimtive EPR spectntmof a+rWkted miidsotution of 5 mole R bromot~~uo~ethy~ene in TXS at 98 K, The urnpfe tube wx o&ttcd to httensify the pamUelfeatures of the bromotritluoroeth~kne md.icalanion. The stick &grams give the calcutawd i*mcpositions WXO&XI~to the gaameters for BrFC=CFG &en in table 3.
F&S 4.. FirsPderiatZve ETR spectrum of o -&radiated solid soWion of 5 mole St,brorno~~u~roeth~~e~e in T&G at 98 R. The sampie tube was oriented to intensify the perpendicular features of the bromotrifhzoroethy~e~emdkal anion, The stick diagrams show the 3 :1:X ; l quartets from hyperfme ~ter;cton with t&e 8t Br and ‘%r nuclei.The substructurecousistiu~of the ‘*F spritting pattern is showx in f& 5. Residual par&et features from the&Q(Brf = +I12 &old are markedaccor&@y.
+312 manifofd. As a ccmsequence of Mgfier-order effects, the 23f2 manifolds are not exact nxhws images because the ewtres ofthe *x Br and 7qBr fluorine spIitt.bg patterns ase more \Gdeiy separated at Iaw 296
field (fig- 4)- Adjacent of the Lile diagram for this difference produced a good fit to the i-312 mar&oId, features 3,4, Ipand 8 of the ten-Ihe substructure reSuMng from ‘3vedapping 8rBr and 79Br components,
Volume 61, number 2
CHEMICAL PHYSICS LETTERS
15 February 1979
F$_ 5. An expanded view of the high-fieId [MfiBr) = -312, r(Br) = 3/Z] features from the pcrpendicuhr spectrum of the bromousing the parameters radical anion in fis_ 4. The line diagrams showing the “F splitting patt er n sere c&x&ted
trfluoroethylene in table 1.
Thus, the high-field analysis of the perpendicular spectrum in terms of coupling to three nonequivalent fluorines is supported. Turning now to the anisotropy of *he bromine hyperfme couplings, a comparison of the 1 r 1: 1: 1 quartet structures in the parallel and perpendicular spectra reveals that higher-order effects are much more pronounced in the perpendicular spectrum, as expected for A ,,(Br) > AI [7] . The assumption of axial symmetry was then examined by caIcuJ.ating the line positions in the parallel and perpendicuiar spectra using a matrix diagonaIization program for axially symmetric @and hyperfime tensors [3] _Excellent fits to the ohserved spectra were obtained for the parameters listed in table 1, this agreement providing firm evidence for cylindrical symmetry of the bromine hyperfme tensor. Further support for the spectral analysis given above is provided by observations linking the parallel and perpendicular spectra in TM.5 to sin@arities in the powder spectrum of the bromotrifluoroethylene radical anion obtained in a rigid MTHF glass, this latter spectrum show&g the lack of orientation dependence which is characteristic of a randomly oriented sample. Despite the poorer resolution of the substruc-
tures, the outer features in the MTHF spectrum can be assigned to the bromotrifluoroethyiene radical anion. Thus, the C3/2 parallel features were identified in the wings of the spectrum, each manifold consisting of five broad humps corresponding to a low-resolution
1:2:2:2: 1 envelope of the eight-line pattern in the spectrum of fig. 3. Similarly, the +1/2 and -3/2 perpendicular bands were recognized by their intensity and pIacement although they showed even less substructure. Accordingly, the spectrum in MTHF is a low-resolution composite of the parallel and perpendicular spectra in TMS. This close identity implies that almost the full anisotropy of the bromine hyperfme coupling is realized in TMS despite the greater molecular motion generally associated with this matrix [3 3 . The spectral features attributed to the chloro- and bromo-trifluoroethylene radical anions ia rigid MTHF solutions were also generated by TMPD photoionization. Although the signal titer&ties were less than for the Tirradiated samples, the strong perpendicular bands of the anions were clearly manifested at high gain. These photoionization experiments are important in showing that the paramagnetic centres of interest are produced specifically by electron attachment cule_
to the solute mole297
CHEMICAL
Volume 61. number 2
Table 2 SCF SDiII densities for the cNorotrifiuoroeth~Iene radical anion uicubtcd
5
P_X
Cl
0.0197 0.2205
0 0 0 0 0 0
G F3
0_0029
F-X FS Cl
0.0001 0.0021 0.0322
Experimental spin densities
pY 0.0016 O-0872 0.0210 0.0003 -0.0030 0.1009
P=
s
p
-0.0023 0.1228 o-0123 -0_0003 0.0361 0.3448
0.032
0.30
a) The following weters were obtained for the minimum-energy moleculargeometry: r(C-F) r(C-C) = 132pm,~ F3C,F4 = 100*.~ F&Cl= 120”.
When a so!id solution of TMS containing C,F3Br w-as-yirradiated at 4 K and observed at 20 K, the EPR spectrum was identical to that obtained by irradiation at 77 K. This result is consistent with the radical anion assignment and shows that the pammagnetic centre produced at 77 K is not the product of a thermal dissociation which occurs only at the higher temperatureCalcuiations on the chiorotrifluoroethylene radical anion, by the CMD0/2 method [S] resulted in the energy-optimized geometry and the SCF spin densities given in table 2_ These resuks are in remarkably good agreement with the experimental observations on two crucial points_ First, the calculations predict that one of the fluorines (F4) possesses extremely small spin densities in its vaIence orbitals, the 2s spin density corresponding to an isotropic coupling constant of only ca_ 2 G_ Since this is on the order of the narrowest linewidth in the TMS spectra, it is not surprising to fmd that the spectrum obtained at one of the two canonical orientations shows hyperfiie interaction with only two fluorines. Second, the cakulated spin densities in the chlorine orbitals compare closely with the experimental vaiues (tabIe Z), the most significant point being the large spin density (ca. 0.3) in the u orbital of the chlorine ligand.
298
1979
by the CNDO/2 method a)
Spin densities in atomic orbit& AtOm
15 Febnwy
PHYSICS LETTERS
= 136 pm, r(C-Cl)
= 193 pm,
4. Discussion The EPR spectra of the TMS solid solutions re-
ported here are remarkable for their fuiIydeveloped orientation dependence and exceCent line resolution. In both of these attributes, the spectra are more typicaI of single crystals than of powder samples. Nevertheless, close e xamination of the lineshapes reveals that they have the characteristic appearance of parallel and perpendicular features as would be found in a powder spectrum resuking from a radical with axially symmetric hyperfme and g tensors. In view of the apparent cylindrical symmetry of the bromine hyperfme tensor in the radical anion of bromotrifluoroethylene, the most reasonabIe choice of principal axis is along the C-Br bond. However, tk direction is unlikely to be a principal axis for the fluorine hyperfiie tensors and therefore the fluorine couplings observed at the canonical orientations are not principal valuesThis series of radicals derived from the halogenotrifluoroethylenes is characterized by a large spin density in the p orbital of the heavy halogen atom, the values deduced from the hyperfme tensors being 0.30,0.34, and 0.47 [9] for chlorine, bromine, and iodine, respectively. Whereas such results are difficult to reconcile with z* radicals, they are comparable to those obtained for o* radicals [2,3,10] with carbon-halogen and nitrogen-halogen bonds- Moreover, the increase in spin density with decreasing halogen electronegativ-
Volume 61, number 2 ity is diagnostic
of an aatibonding
CHEMICAL semioccupied
orbit-
al. Therefore, we conclude that the unpaired electron occupies a u* orbital in the halogenotrifluoroethyIene radical a-tions.
References [l] [21 [3] [4] [5]
PHYSICS LETTERS
R-1. McNeil, hl. Shiotani, F. Wi!liams and h1.B. Yim. Chem. Phys. Letters 51 (1977) 433. A. Hasegawz and F. Wiims, Cbem. Phys. Letters 46 (1977) 66. A. Hasegawa. hi. Shiotani and F. Williams, Faraday Discussions Chem. Sot. 63 (1977) 157. J. Lin, K. Tsuji and F. Williams, Trans. Faraday Sot. 64 (1968) 2896. P.H. Kasai, W_ Weltner Jr. and E-B. Whipple, J. Chem. Phyr 42 (1965) 1120; C.ILL Kerr and F_ Williams, J. Am. Chem. SOC. 94 (1972) S2I 2; RL Hudson and F. Williams. J. Am. Chem. Sot. 99 (1977) 7714.
1.5 February
1979
161 R-L McNed, PhD. Thesis, University of Tennessee (1978) p- 75. 171 B_ Elesney, Phil_ Ma~_42 (1951) 441; A. Horsfield, J.R. Morton snd D-H. Whiffen. MoL Phys. 4 (1961) 475; C-hI.L Kerr, K. Webster and F- Williams, J. Phys. Chem. 79 (1975) 2650. [S] J-A. Pople and G-k Segal, J. Chem- Phys. 44 (1966) 3289; J.A. GopIe, D-L. Beveridge and P-A. Dobosh, J. Chem. Phys. 47 (1967) 2026; A.R. Gregory, J. Chem. Phys. 60 (1974) 3713. [9] R-1. McNeil, J-T. Wang and F. Williams, unpublished results. [lOI G-W_ Neilson and M.C.R. Symons, J. Chem. Sot. Faraday II 68 (1972) 1582; G-W_ Neilson and &l.C_R. Symons. Mol. Phys. 27 (1974) 1613; D-J_ Nelson-and U.C.R. Symons, Chem. Phys. Letters 47 (1977) 436; M.C.R. Symons, J. Chem. Sot. Chem. Commun. (1977)
408; S. Nagai and T. Gillbro, J. Phys. Chem. 81 (1977)
1793.
299
CHEMICAL PHYSICS LETTERS
EPR St’EaRA AND !%TRUCl-URE OF THE CHLOROTRIFLUOROETHYLENE BROMOTRiFLUOROETHYLENE RADICAL ANIONS f
15 February 1979
ANI)
Robert I_ hlcNEIL, Ffrancon WILLIAMS and Moon B. YIM Department
of Chemistry,
Universiry of Tennessee. Knoxville.
Tennessee 37916. USA
Received 23 October 1978
EPR spectra attributable to the halogenotrifluoroethylene radical anions have been generated by electron attachment in solid soiutions at Iow temperatures. The spin density distrtbution in these rdids strongly suzeststhat the unpaired electron occupies a (I* orbital. a conclusion which is supported by CNDO/Z calculations.
1. Introduction
2. Experimental
Recent work has strongly suggested that the unpaired electron occupies a d orbital rather than the or* orbital in the tetrafluoroethylene radical anion [I] _ Since MO theory predicts that the replacement of one fluorine by a heavier and less electronegative halogen should have a significant effect on the spin distribution in an antibonding sigma orbital, we have carried out an EPR study of the halogenotrifluoroethylene radical anions. The operation of this halogen substitution effect in an unambiguous series of u* radicals is exemplified by the spin density distributions reported for the halogenotriIluoromethane radical anions [2,3] _Thus, for the semi-occupied of orbital of the chlorotrifluoro-
Chlorotrifluoroethylene (b.p. -27O) and bromotritluoroethylene (b.p. -2O) were supplied by PCR Research Chemicals, Inc- and contained d-limonene and tributylamine, respectively, as polymerization inhibitors. The sample of chlorotrifluoroethylene was purified by trap-to-trap distillation from a dry ice-isopropan01 bath on a vacuum line immediately before use_ Samples containing approximately 5 mole % of the halogenotrifluoroethylene in tetramethylsilane (TMS), tetramethylsilane-d12 (TMS-d12), and 2-methyltetrahydrofuran (MTHF) were prepared in Spectrosil and FEP Teflon tubes by vacuum techniques. The radical anions were usually generated by irradiation of the solid solutions with cobalt-60 gamma rays at 77 K, the typical irradiation dose being 0.75 Mrad. EPR measurements were made as described elsewhere [3 3. Confirmation of the spectral assignments to the radical anions was obtained through the generation of matching spectra by the 320 run photoionization of N,N,N’,N’-tetramethyl-p-phenylenediamine (ThIPD) in rigid MTHF solutions of the halogenotrifluoioethylenesat77K [4].
methane radical anion, experimental results agree with theOietic4 calculations in showing that a large spin density is concentrated in +Jle p. orbitsI of the chIorine ligand [3]. Conversely, the effect of haIogen substitution can be used as a diagnostic test for o* radicals, and here we report the characterization of the chloro- and bromo-trifluoroethylene radical anions as o* radicals from their anisotropic EPR spectra.
3_ Results f Research supported by the Division of Basic Ener,T Sciences, U.S. Department of Energy (document no. ORO-2968-113).
The EPR spectra of the radical anions derived from chIoro- and bromo-trifluoroethylene solutions in TMS 293
Volume 61_ number 2
CHE!GICAL PHYSICS LEl-ERS
and MTHF consisted ofanisotropic patterns_ However, the spectra in TMS-k17 and TM!%Zlz showed an orientation dependence whkh is typicaI of partially aligned radicals in a crystalline matrix [S] , and this effect was exploited to obtain the separate spectra of the parallel and perpendicuiar components by opt.imi&g the sample orientations in the magnetic field (vide infra). Orientation-dependent spectra of this kind were previously obtained for CF31- iu TMS solution [S] _ Throughout this paper we differentiate between parallel and perpendicular spectra according to their spectraI extent, the wider set of components being designated as the parallel spectrumFig- 1 shows the welLresolved EPR spectrum obtained by orientating the sample to intensify the parallel features of the cllorotrifluoroethylene radical anion in the ThlSzl,2 matrix at 98 K-This parallel spectrum consisting of 24 observable components outside the central region was reproducible from sample to sample at a particular orientation; moreover. a matching spectrum was similarly obtained from TMShl, soWion, the ordy difference being that the com-
,
*
Fig. L. First-derimrite
7
Z
3
2
EPR spectrum of a +n-adhted
I
=a
I
solid
sohnion of 5 moI % chIorotrifIuoroethyIene in tetmmethyb siIane4r~(TWGcfr~) at 98 Ii -I-hesampie tube was oriented to give the m&mum intensity for the parallel features of the chIorotrifIuoroethylene mdicaI anion_ The stick diagrams gibe the c&oIated line positions according to the parameters for QFC=CF; giver. in table I_ Residual perpetdictdar features in the spectmm are marhed zwxordingIy_
294
15 February 1979
ponent linewidths increased from less than 2 G in TMS+ to ca- 5 G in TMS?z,,. As shown in the line diagrams of fig. 1, the spectrum is composed of two patterns due to the presence of two chlorine isotopes, 35CI (Z = 312) and 37CI (Z = 3/2), with an intensity ratio of 3~1. Each pattern consists of 16 lines resulting from hyperfme interactions with one chlorine nucleus and two (vide infra) nonequivalent fluorine nuclei (Z(lg F) = l/2), one of the lgF coupling constants being about twice the other and the 3sCI coupling having a value intermediate between the two (table I). As can be seen in fig_ 1, the 12 outer components of each pattern are clearly resolved while the 4 remaining components are masked by the intense signal of the matrix radical in the centre of the spectrum. When the orientation of the TMS-d12 sample was changed from the parallel position by rotation about the tube axis, the lines belonging to the spectrum of fig. 1 coliapsed and another weII-defmed spectrum was observed after a ca. 90” rotation_ As shown in fig. 2, the width of this perpendicular spectrum is only 202 G as compared to 449 G for the paraIIe1 spectrum- The analysis of the perpendicular spectrum is given by the line diagram showing hyperfime couphngs to three nonequivalent fluorine nuclei in addition to the interaction with one chlorine, the largest fluorine coupling being approximately twice the 35C1 coupling and the other two fluorines having much smaller couplings_ The EPR parameters for the ch.IorotrifluoroethyIene radical anion are colIected in table lOrientation-dependent specrra were also obtained for -y-hrradiatedsolid solutions of bromotrifluoroethylene in TMS. As shown cIearly by a comparison of the outer spectral regions in figs. 3 and 4, almost complete discrimination has been achieved between the paraIIe1 and perpendicular spectra at these orientations, the parallel MZfBr) = S/2 features (fig_ 3) being absent in the perpendicular spectrum of fig_ 4. The paraIIe1 spectrum is readi& analyzed into separate 81 Br (Z = 3/2; natural abundance = 50.6%) and 79Br (Z = 3/2; natural abundance = 49.4%) patterns resulting from hyperfine interaction with a singIe bromine nucleus. each pattern consisting of sixteen lines made u_pof a large 1 :l :I :I bromine quartet further split by coupling to two nonequivalent f9F (f = l/Z) nuclei_ These I9F couplings for the paraIIe1 spectrum (table 1) are remarkably similar to those obtained for the parallel spectrum of the chIorotrifIuoroethyIene radicaI anion,
Volume 61, number 2
CHEMICAL PHYSICS LETTERS
15 February 1979
Table l EPR parameters a) for the tetrafluoroethylene b) and haIogenotrifluoroethylene radical anions Radical
Matrix
T (Q
g-vaIues 81
F2C=CF;
ml=12
120
F&=CFCl-
TWXI,
98
2.011
FzC=CFBr-
-Ixlzxz,*
98
2.036
Nucleus gu
giso
Hypertine couplings (G) AL
All
Aiso
2.0027
19F l3C
94.3 48.7
1997
7-006
ascl WC1 Fa Fb Fc
37.6 31.3 77.2 7.3 4.5
84.0 69.8 132.3 64.8 0
53.1 44.1
2.000
2.024
“‘Br ‘gBr Fa Fb Fc
212.0 197.5 35.4 30.2 10.1
465.3 431.6 130.6 65.3 0
296.4 275.5
-
a) The tgF hvperfme couplings for the perpendicular and parallel spectra of the halogenotrifluoroethylene radical anions are not principal v&es (see text) and are listed in numerical order, the correct pairingsbeing unknown. b) From ref. [I 1.
Fig. 2_ Fiit-derivati~e EPR spectrum of a r-irradiated solid solution of 5 mole % c-blorotrifluoroethylene in TMSdr2 at 98 Ii. The sample tube wzs oriented to intensify the perpendicular features of the chIorotrifluoroethylene radical anion. The stick diagramsgive the calculated line positions according to the parameters for QFC=CF$ given in table 1-
A considerable narrowing of the bromine hyperfiie couplings occurs in going from the parallel to the perpendicular spectrum of the bromotrifluoroethylene radical anion. Fortunately, the bromine splittings are still large enough to prevent overlapping between the substructures (fig- 4)_ The analysis of the perpendicular spectrum is therefore simplified and the MI(J3r) = -312 substructure is shown on an expanded scale in fig. 5. Although only twelve resolved features can be seen, there is an additional strong feature in the centre of the pattern which is masked by the high-field line of atomic hydrogen_ This feature was uncovered by using FEP Teflon sample tubes which do not give signals from hydrogen on y irradiation 163 _ A good fit of the --3/Z perpendicular manifold is given by the line diagram in fig. 5 consisting of two sets (81 Br and 7QBr) of eight lines, each set representing the splitting pattern from three nonequivalent fluorines_ Since three lines from one set are closely overlapped by three lines from the other set, the diagram accounts for the observation of thirteen resolved components in which features 4,7, and 10 appear with enhanced intensities. Although these results appear to exclude the possibility of interaction with only two fluorines, an additional check on the fluorine hyperfiie splitting pattern is afforded by an examination of the 295
Vi?Iume &I, number 2
Rg. 3. F.&t-derimtive EPR spectntmof a+rWkted miidsotution of 5 mole R bromot~~uo~ethy~ene in TXS at 98 K, The urnpfe tube wx o&ttcd to httensify the pamUelfeatures of the bromotritluoroeth~kne md.icalanion. The stick &grams give the calcutawd i*mcpositions WXO&XI~to the gaameters for BrFC=CFG &en in table 3.
F&S 4.. FirsPderiatZve ETR spectrum of o -&radiated solid soWion of 5 mole St,brorno~~u~roeth~~e~e in T&G at 98 R. The sampie tube was oriented to intensify the perpendicular features of the bromotrifhzoroethy~e~emdkal anion, The stick diagrams show the 3 :1:X ; l quartets from hyperfme ~ter;cton with t&e 8t Br and ‘%r nuclei.The substructurecousistiu~of the ‘*F spritting pattern is showx in f& 5. Residual par&et features from the&Q(Brf = +I12 &old are markedaccor&@y.
+312 manifofd. As a ccmsequence of Mgfier-order effects, the 23f2 manifolds are not exact nxhws images because the ewtres ofthe *x Br and 7qBr fluorine spIitt.bg patterns ase more \Gdeiy separated at Iaw 296
field (fig- 4)- Adjacent of the Lile diagram for this difference produced a good fit to the i-312 mar&oId, features 3,4, Ipand 8 of the ten-Ihe substructure reSuMng from ‘3vedapping 8rBr and 79Br components,
Volume 61, number 2
CHEMICAL PHYSICS LETTERS
15 February 1979
F$_ 5. An expanded view of the high-fieId [MfiBr) = -312, r(Br) = 3/Z] features from the pcrpendicuhr spectrum of the bromousing the parameters radical anion in fis_ 4. The line diagrams showing the “F splitting patt er n sere c&x&ted
trfluoroethylene in table 1.
Thus, the high-field analysis of the perpendicular spectrum in terms of coupling to three nonequivalent fluorines is supported. Turning now to the anisotropy of *he bromine hyperfme couplings, a comparison of the 1 r 1: 1: 1 quartet structures in the parallel and perpendicular spectra reveals that higher-order effects are much more pronounced in the perpendicular spectrum, as expected for A ,,(Br) > AI [7] . The assumption of axial symmetry was then examined by caIcuJ.ating the line positions in the parallel and perpendicuiar spectra using a matrix diagonaIization program for axially symmetric @and hyperfime tensors [3] _Excellent fits to the ohserved spectra were obtained for the parameters listed in table 1, this agreement providing firm evidence for cylindrical symmetry of the bromine hyperfme tensor. Further support for the spectral analysis given above is provided by observations linking the parallel and perpendicular spectra in TM.5 to sin@arities in the powder spectrum of the bromotrifluoroethylene radical anion obtained in a rigid MTHF glass, this latter spectrum show&g the lack of orientation dependence which is characteristic of a randomly oriented sample. Despite the poorer resolution of the substruc-
tures, the outer features in the MTHF spectrum can be assigned to the bromotrifluoroethyiene radical anion. Thus, the C3/2 parallel features were identified in the wings of the spectrum, each manifold consisting of five broad humps corresponding to a low-resolution
1:2:2:2: 1 envelope of the eight-line pattern in the spectrum of fig. 3. Similarly, the +1/2 and -3/2 perpendicular bands were recognized by their intensity and pIacement although they showed even less substructure. Accordingly, the spectrum in MTHF is a low-resolution composite of the parallel and perpendicular spectra in TMS. This close identity implies that almost the full anisotropy of the bromine hyperfme coupling is realized in TMS despite the greater molecular motion generally associated with this matrix [3 3 . The spectral features attributed to the chloro- and bromo-trifluoroethylene radical anions ia rigid MTHF solutions were also generated by TMPD photoionization. Although the signal titer&ties were less than for the Tirradiated samples, the strong perpendicular bands of the anions were clearly manifested at high gain. These photoionization experiments are important in showing that the paramagnetic centres of interest are produced specifically by electron attachment cule_
to the solute mole297
CHEMICAL
Volume 61. number 2
Table 2 SCF SDiII densities for the cNorotrifiuoroeth~Iene radical anion uicubtcd
5
P_X
Cl
0.0197 0.2205
0 0 0 0 0 0
G F3
0_0029
F-X FS Cl
0.0001 0.0021 0.0322
Experimental spin densities
pY 0.0016 O-0872 0.0210 0.0003 -0.0030 0.1009
P=
s
p
-0.0023 0.1228 o-0123 -0_0003 0.0361 0.3448
0.032
0.30
a) The following weters were obtained for the minimum-energy moleculargeometry: r(C-F) r(C-C) = 132pm,~ F3C,F4 = 100*.~ F&Cl= 120”.
When a so!id solution of TMS containing C,F3Br w-as-yirradiated at 4 K and observed at 20 K, the EPR spectrum was identical to that obtained by irradiation at 77 K. This result is consistent with the radical anion assignment and shows that the pammagnetic centre produced at 77 K is not the product of a thermal dissociation which occurs only at the higher temperatureCalcuiations on the chiorotrifluoroethylene radical anion, by the CMD0/2 method [S] resulted in the energy-optimized geometry and the SCF spin densities given in table 2_ These resuks are in remarkably good agreement with the experimental observations on two crucial points_ First, the calculations predict that one of the fluorines (F4) possesses extremely small spin densities in its vaIence orbitals, the 2s spin density corresponding to an isotropic coupling constant of only ca_ 2 G_ Since this is on the order of the narrowest linewidth in the TMS spectra, it is not surprising to fmd that the spectrum obtained at one of the two canonical orientations shows hyperfiie interaction with only two fluorines. Second, the cakulated spin densities in the chlorine orbitals compare closely with the experimental vaiues (tabIe Z), the most significant point being the large spin density (ca. 0.3) in the u orbital of the chlorine ligand.
298
1979
by the CNDO/2 method a)
Spin densities in atomic orbit& AtOm
15 Febnwy
PHYSICS LETTERS
= 136 pm, r(C-Cl)
= 193 pm,
4. Discussion The EPR spectra of the TMS solid solutions re-
ported here are remarkable for their fuiIydeveloped orientation dependence and exceCent line resolution. In both of these attributes, the spectra are more typicaI of single crystals than of powder samples. Nevertheless, close e xamination of the lineshapes reveals that they have the characteristic appearance of parallel and perpendicular features as would be found in a powder spectrum resuking from a radical with axially symmetric hyperfme and g tensors. In view of the apparent cylindrical symmetry of the bromine hyperfme tensor in the radical anion of bromotrifluoroethylene, the most reasonabIe choice of principal axis is along the C-Br bond. However, tk direction is unlikely to be a principal axis for the fluorine hyperfiie tensors and therefore the fluorine couplings observed at the canonical orientations are not principal valuesThis series of radicals derived from the halogenotrifluoroethylenes is characterized by a large spin density in the p orbital of the heavy halogen atom, the values deduced from the hyperfme tensors being 0.30,0.34, and 0.47 [9] for chlorine, bromine, and iodine, respectively. Whereas such results are difficult to reconcile with z* radicals, they are comparable to those obtained for o* radicals [2,3,10] with carbon-halogen and nitrogen-halogen bonds- Moreover, the increase in spin density with decreasing halogen electronegativ-
Volume 61, number 2 ity is diagnostic
of an aatibonding
CHEMICAL semioccupied
orbit-
al. Therefore, we conclude that the unpaired electron occupies a u* orbital in the halogenotrifluoroethyIene radical a-tions.
References [l] [21 [3] [4] [5]
PHYSICS LETTERS
R-1. McNeil, hl. Shiotani, F. Wi!liams and h1.B. Yim. Chem. Phys. Letters 51 (1977) 433. A. Hasegawz and F. Wiims, Cbem. Phys. Letters 46 (1977) 66. A. Hasegawa. hi. Shiotani and F. Williams, Faraday Discussions Chem. Sot. 63 (1977) 157. J. Lin, K. Tsuji and F. Williams, Trans. Faraday Sot. 64 (1968) 2896. P.H. Kasai, W_ Weltner Jr. and E-B. Whipple, J. Chem. Phyr 42 (1965) 1120; C.ILL Kerr and F_ Williams, J. Am. Chem. SOC. 94 (1972) S2I 2; RL Hudson and F. Williams. J. Am. Chem. Sot. 99 (1977) 7714.
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161 R-L McNed, PhD. Thesis, University of Tennessee (1978) p- 75. 171 B_ Elesney, Phil_ Ma~_42 (1951) 441; A. Horsfield, J.R. Morton snd D-H. Whiffen. MoL Phys. 4 (1961) 475; C-hI.L Kerr, K. Webster and F- Williams, J. Phys. Chem. 79 (1975) 2650. [S] J-A. Pople and G-k Segal, J. Chem- Phys. 44 (1966) 3289; J.A. GopIe, D-L. Beveridge and P-A. Dobosh, J. Chem. Phys. 47 (1967) 2026; A.R. Gregory, J. Chem. Phys. 60 (1974) 3713. [9] R-1. McNeil, J-T. Wang and F. Williams, unpublished results. [lOI G-W_ Neilson and M.C.R. Symons, J. Chem. Sot. Faraday II 68 (1972) 1582; G-W_ Neilson and &l.C_R. Symons. Mol. Phys. 27 (1974) 1613; D-J_ Nelson-and U.C.R. Symons, Chem. Phys. Letters 47 (1977) 436; M.C.R. Symons, J. Chem. Sot. Chem. Commun. (1977)
408; S. Nagai and T. Gillbro, J. Phys. Chem. 81 (1977)
1793.
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