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Endor study of the 2-adamantyl radical
.’ Volume
17, number
CHEhIlCAL
3
ENDOR
STUDY
1 December
PHYSICS LETTERS
OF THE ZADAhlANTYL
1972
RADICAL”
R.V. LLOYD arid M.T. ROGERS Department of Chemistry, Michigan State University, Ecrt Lansing, Michigan 48623, USA Received 21 August 1972
The ENDOR spectrum of the Zadamantyt radical in +rradiated ~d~ant~ne reveals B proton hypcrfine splitting of 53.55 G. This has been assigned to the 6 bridgehead protons on the basis of INDO calculations. A m&ximum value ofa& from the ENDOR spectrum has been used to estimate a masimum value ofDo = 0.8 G for the widely employed rel~tionsh~? at ~60 f Bz cos% relating dihedral angle to proton hyperfine interactions in rr etectron radicals. No evidence for other radicais in -y-irradiated ndamantane has been obtained.
purity. The I S-line spectrum assigned by Filby and
1. Introduction The ESR spectrum of the 2-adamantyl radical in solid adamantane has recently been reported f l] but the resolution, even with some motional narrowing of the lines, was such that onfy the cu-proton splitting of 20.5 G (57.4 MHz) could be seen clearly, although a 1:2: f tripIet with splitting = 5 G was a&o present.
Giinther [4] to I-adamantyl has ESR parameters qGte different from those established for this radical [2] and again must be From another radical. We find no evidence in the present study for any radical other than Zadamantyl in y-irradiated adamantane.
By means of an electron-nuclear double resonance
2. Experimental
(ENDOR) experiment we have now obtained the smaller hyperfine splitting so that the interactions in the radical can be analyzed completeI:!_ The I-adnmantyl radical is apparently not obtained by +rradiaticn of suffkientIy pure adamantane at temperatures of 77°K or higher [I] ; however, it has been prepared by photolysis of the perester and the ESR parameters have been reported 121. There have been several ESR studies of irradiated adamantane in which the radicals produced apparently came from impurities. The spectra attributed by Gee et al. [3] to 1- and 2-adamantyi radicals have been shown not to arise from those radicals [l, 2] _ A rather similar spectrum obtained by Bonazzola and Marx [3], and att~buted to a substituted l-methylcyciahexyl radical, presumably also came from an im-
The ENDOR spectrum of the 2-adamantyl radical in adamantnne, y-irradiated at 77% and observed at 113%, is shown in fig. 1. The spectrum was obtained with a Varian V-4502 X-band ESR Spectrometer equipped with the Varian E-700 ENDOR accessory. The large peak centered at the free proton frequency (14.028 MHz) arises from unresolved splittings by matrix protons and, probably, the &proton. The lowfield line from the a proton, which would be expected to aTpear at a frequency 1S7.4/2- 14.0281 * 14.67 MHz, should partly overlap the free-proton line and is probably responsible for the asymmetric shoulder on the high-field side of the center line. The broad line centered at 19.(flO MHz corresponds to a
hyperfine splitting of 2118.010- 14.0281 = rir 0.02 G). Sweeping the nuciear frequency over “Jle range 14--27 MHz, ccrresponding to the @per&e splittings in the range O-9 G, rezkQ.964 MHz (3.55
*This wdrk was supported Esteigy Commission
through a contract with the Atomic and this is AEC Documdnt CO@ 1385-45.
428. .
Volume 17, number
3
CHEh!ICAL
1 December
PHYSICS LETTERS
4.12H
-20.81
14.028
I
MHz
19.010
MHz
Fig. 1. The ENDOR spectrum of the 2-adamontyl -y-irradiated admantane at 133°K.
radical in
3. Results
,S
,wHs
Fig. 2. Results of the INDO calculation of tie proton hypertine splittings in the 2-adamantyl radical. The values are given in gauss.
actions vealed no other ENDOR
1972
to be expected
in 2-adamantyl
radical. We as-
sumed that the radica! site was pIanar and that the rest of the radical had the same geometry as the parent
lines.
molecule, with
and discussion
used in the successful refinement of the adamantane The assignment of the observed hyperfine splitting is not immediately obvious. Ferrell et al. [l] assumed that the splitting of ~5 C, which they observed, was from interaction with the two fl protons. However, if the radical is planar at the trivalent carbon, as indicated by the magnitude of the cr-proton splitting (Q; = 20.5 G), the unpaired electron would be in a p orbital on the a: carbon perpendicular to the plane of the 0 protons. It is well-established that fl proton hyperfine splittings for planar radicals are related to the dihedral angle 8 between the Cp-HB bond and the direction of the unpaired electron orbital according to tiH = (BoSB2
co&)
pi )
(1)
3, = 0,6 = 49.4 G for a secondary radical [6], and pi is the spin density in the odd electron orbital. In the present case ak would be predicted to be nearly zero, since 19is necessarily 90°, so it is unlikely that the observed hyperfine splitting is from the P protons. It is known from studies of the I-adamsntyl [I], I-bicyclo[2,2,2] octyl [I] and 2-bicyclo[2,2,2] octyl [7] radicals that -y and 6 proton hyperfine interactions in the range 0.8-5.7 G may be found, depending on the geometry of the radical. To assist us in assigning the proton splitting of 3.55 G we have calculated, by the LNDO approximation [8], the hyperfine interwhere
crystal structure [9] . The results of the calculation are shown in fig. 2. A remarkably large long-range interaction with the 6 bridgehead protons is predicted, while a!1 the other protons have relatively small values. Similar large hyperfine splittings from 6 protons in the 1-adamantyl and I-bicyclo[2,2,2] octyl bridgehead radicals have been observed and were reproduced satisfactorily by INDO calculations [2]. We have therefore assigned the 3.55 G splitting to the two 6 protons; this would also account for the 1: 2: 1 triplet with spacing ~5 G observed in the ESR spectrum [ 11, while the remaining protons would be responsibie for the shoulders on the central line of the ENDOR spectrum (fig. 1). Although we cannot assign specific values to the remaining protons, we can say that the maximum va!ue of a& is the ~0.7 G width of the shoulders on the ENDOR line. This represents the upper limit on the constant B, pi of eq. (1); since pi z a”,/Q X 0.9, from
McConnell’s relation [lo] with Q = 23 G, the upper on B. becomes ~0.8 G and the upper limit on the ratio B,/B2 becomes ~0.016. Previous determinations of B,/f?, have given the values -0.043 for cyclopropyl radical [IO] , +0-l 19 for isopropyl radical [IO] , and +O. 113 for methyl radical [ 1 l] . Our value limit
is considerably smaller than these bgt we simply assumed that the structure is rigid, with 8 = 0’ in eq. (I), 429
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
and did ndt take into account vibrational motions.
1 December 1972
iSI C. Hellerafld H.M. Mdfonnell, J. Chem. Phys. 32 (1960) 1535.
References 111 J.R. Ferrell, G.R. Holdren Jr?, R.V. Lloyd and DE. Wood, Chem. Phys, Letters 8 !1971) 343. ‘121 P.J. Krusic, T.A. Rettig and P.Y.R. Schleyyer, 3. Am. Chem. Sot. 94 (1972) 995. f3] L. Bonazzoia and R. Marx, Chem. Phys. Letters 8 (1971) 413, (41 W.G. Filby and K. Gunther, Chem. Phys, Letters 14 (19721440.
I61 P.J. Krtisic,P. Neakin and J.P. Jesson,3. Phys Chem. 75 (1971) 3438.
171 I_. Bonazzola and R. Marx,Mol. F%YS 19 f1970) 405. IS] D.L. Beveridge,PA. Dobosh and 3-A. Pople, J. &em. Phys. 48 (1968) 4802: 3.k. Pop&, D.L. Bevehdge and P.A. Dobosh, J. Am. Chem. Sot. 90 (1968) 4201, obtained from QCPE, 1~ diana University. 191 f. Donohue and S.H. Goodman, Acta Cryst. 22 (1967) 352. 1101 N. Bauld, C.E. Hudson and 3-S. Hyde, J. Chem. Phys 54 (1971) 1834, aad references therein Ill1 J.R. Morton, J. Chem. Phys. 41 (1964) 2956.
17, number
CHEhIlCAL
3
ENDOR
STUDY
1 December
PHYSICS LETTERS
OF THE ZADAhlANTYL
1972
RADICAL”
R.V. LLOYD arid M.T. ROGERS Department of Chemistry, Michigan State University, Ecrt Lansing, Michigan 48623, USA Received 21 August 1972
The ENDOR spectrum of the Zadamantyt radical in +rradiated ~d~ant~ne reveals B proton hypcrfine splitting of 53.55 G. This has been assigned to the 6 bridgehead protons on the basis of INDO calculations. A m&ximum value ofa& from the ENDOR spectrum has been used to estimate a masimum value ofDo = 0.8 G for the widely employed rel~tionsh~? at ~60 f Bz cos% relating dihedral angle to proton hyperfine interactions in rr etectron radicals. No evidence for other radicais in -y-irradiated ndamantane has been obtained.
purity. The I S-line spectrum assigned by Filby and
1. Introduction The ESR spectrum of the 2-adamantyl radical in solid adamantane has recently been reported f l] but the resolution, even with some motional narrowing of the lines, was such that onfy the cu-proton splitting of 20.5 G (57.4 MHz) could be seen clearly, although a 1:2: f tripIet with splitting = 5 G was a&o present.
Giinther [4] to I-adamantyl has ESR parameters qGte different from those established for this radical [2] and again must be From another radical. We find no evidence in the present study for any radical other than Zadamantyl in y-irradiated adamantane.
By means of an electron-nuclear double resonance
2. Experimental
(ENDOR) experiment we have now obtained the smaller hyperfine splitting so that the interactions in the radical can be analyzed completeI:!_ The I-adnmantyl radical is apparently not obtained by +rradiaticn of suffkientIy pure adamantane at temperatures of 77°K or higher [I] ; however, it has been prepared by photolysis of the perester and the ESR parameters have been reported 121. There have been several ESR studies of irradiated adamantane in which the radicals produced apparently came from impurities. The spectra attributed by Gee et al. [3] to 1- and 2-adamantyi radicals have been shown not to arise from those radicals [l, 2] _ A rather similar spectrum obtained by Bonazzola and Marx [3], and att~buted to a substituted l-methylcyciahexyl radical, presumably also came from an im-
The ENDOR spectrum of the 2-adamantyl radical in adamantnne, y-irradiated at 77% and observed at 113%, is shown in fig. 1. The spectrum was obtained with a Varian V-4502 X-band ESR Spectrometer equipped with the Varian E-700 ENDOR accessory. The large peak centered at the free proton frequency (14.028 MHz) arises from unresolved splittings by matrix protons and, probably, the &proton. The lowfield line from the a proton, which would be expected to aTpear at a frequency 1S7.4/2- 14.0281 * 14.67 MHz, should partly overlap the free-proton line and is probably responsible for the asymmetric shoulder on the high-field side of the center line. The broad line centered at 19.(flO MHz corresponds to a
hyperfine splitting of 2118.010- 14.0281 = rir 0.02 G). Sweeping the nuciear frequency over “Jle range 14--27 MHz, ccrresponding to the @per&e splittings in the range O-9 G, rezkQ.964 MHz (3.55
*This wdrk was supported Esteigy Commission
through a contract with the Atomic and this is AEC Documdnt CO@ 1385-45.
428. .
Volume 17, number
3
CHEh!ICAL
1 December
PHYSICS LETTERS
4.12H
-20.81
14.028
I
MHz
19.010
MHz
Fig. 1. The ENDOR spectrum of the 2-adamontyl -y-irradiated admantane at 133°K.
radical in
3. Results
,S
,wHs
Fig. 2. Results of the INDO calculation of tie proton hypertine splittings in the 2-adamantyl radical. The values are given in gauss.
actions vealed no other ENDOR
1972
to be expected
in 2-adamantyl
radical. We as-
sumed that the radica! site was pIanar and that the rest of the radical had the same geometry as the parent
lines.
molecule, with
and discussion
used in the successful refinement of the adamantane The assignment of the observed hyperfine splitting is not immediately obvious. Ferrell et al. [l] assumed that the splitting of ~5 C, which they observed, was from interaction with the two fl protons. However, if the radical is planar at the trivalent carbon, as indicated by the magnitude of the cr-proton splitting (Q; = 20.5 G), the unpaired electron would be in a p orbital on the a: carbon perpendicular to the plane of the 0 protons. It is well-established that fl proton hyperfine splittings for planar radicals are related to the dihedral angle 8 between the Cp-HB bond and the direction of the unpaired electron orbital according to tiH = (BoSB2
co&)
pi )
(1)
3, = 0,6 = 49.4 G for a secondary radical [6], and pi is the spin density in the odd electron orbital. In the present case ak would be predicted to be nearly zero, since 19is necessarily 90°, so it is unlikely that the observed hyperfine splitting is from the P protons. It is known from studies of the I-adamsntyl [I], I-bicyclo[2,2,2] octyl [I] and 2-bicyclo[2,2,2] octyl [7] radicals that -y and 6 proton hyperfine interactions in the range 0.8-5.7 G may be found, depending on the geometry of the radical. To assist us in assigning the proton splitting of 3.55 G we have calculated, by the LNDO approximation [8], the hyperfine interwhere
crystal structure [9] . The results of the calculation are shown in fig. 2. A remarkably large long-range interaction with the 6 bridgehead protons is predicted, while a!1 the other protons have relatively small values. Similar large hyperfine splittings from 6 protons in the 1-adamantyl and I-bicyclo[2,2,2] octyl bridgehead radicals have been observed and were reproduced satisfactorily by INDO calculations [2]. We have therefore assigned the 3.55 G splitting to the two 6 protons; this would also account for the 1: 2: 1 triplet with spacing ~5 G observed in the ESR spectrum [ 11, while the remaining protons would be responsibie for the shoulders on the central line of the ENDOR spectrum (fig. 1). Although we cannot assign specific values to the remaining protons, we can say that the maximum va!ue of a& is the ~0.7 G width of the shoulders on the ENDOR line. This represents the upper limit on the constant B, pi of eq. (1); since pi z a”,/Q X 0.9, from
McConnell’s relation [lo] with Q = 23 G, the upper on B. becomes ~0.8 G and the upper limit on the ratio B,/B2 becomes ~0.016. Previous determinations of B,/f?, have given the values -0.043 for cyclopropyl radical [IO] , +0-l 19 for isopropyl radical [IO] , and +O. 113 for methyl radical [ 1 l] . Our value limit
is considerably smaller than these bgt we simply assumed that the structure is rigid, with 8 = 0’ in eq. (I), 429
Volume 17, number 3
CHEMICAL PHYSICS LETTERS
and did ndt take into account vibrational motions.
1 December 1972
iSI C. Hellerafld H.M. Mdfonnell, J. Chem. Phys. 32 (1960) 1535.
References 111 J.R. Ferrell, G.R. Holdren Jr?, R.V. Lloyd and DE. Wood, Chem. Phys, Letters 8 !1971) 343. ‘121 P.J. Krusic, T.A. Rettig and P.Y.R. Schleyyer, 3. Am. Chem. Sot. 94 (1972) 995. f3] L. Bonazzoia and R. Marx, Chem. Phys. Letters 8 (1971) 413, (41 W.G. Filby and K. Gunther, Chem. Phys, Letters 14 (19721440.
I61 P.J. Krtisic,P. Neakin and J.P. Jesson,3. Phys Chem. 75 (1971) 3438.
171 I_. Bonazzola and R. Marx,Mol. F%YS 19 f1970) 405. IS] D.L. Beveridge,PA. Dobosh and 3-A. Pople, J. &em. Phys. 48 (1968) 4802: 3.k. Pop&, D.L. Bevehdge and P.A. Dobosh, J. Am. Chem. Sot. 90 (1968) 4201, obtained from QCPE, 1~ diana University. 191 f. Donohue and S.H. Goodman, Acta Cryst. 22 (1967) 352. 1101 N. Bauld, C.E. Hudson and 3-S. Hyde, J. Chem. Phys 54 (1971) 1834, aad references therein Ill1 J.R. Morton, J. Chem. Phys. 41 (1964) 2956.