1170
J. Phys. Chem. 1983, 87, 1170-1174
Electron Spin Resonance Studies on Radical Cations of Five-Membered Heteroaromatics. Furan, Thiophene, Pyrrole, and Related Compounds M. Shlotanl,’ Y. Nagata, M. Tasakl, J. Sohma, Facutty of Engineering, Hokkaido University, Sapporo 060, Japan
and T. Shida Department of Chemistty, Faculty of Science, Kyoto UniversHy, Kyoto 606, Japan (Received: August 11, 1982; I n Final Form: October 19, 1982)
Radical cations of several five-membered heteroaromatics (furan, thiophene, pyrrole, 2-methylfuran, 2methylthiophene, 3-methylthiophene, and 2,5-dimethylfuran)have been detected and identified by ESR studies on solid solutions of the parent molecules in various fluorinated matrices. The matrices used include CC13F, SFs, c-C6Fl2,c-C6F11CF3, and c-C,F8. The spin density distribution obtained by the ESR analysis suggests that the unpaired electron resides in the x orbital of laz nature for all the radical cations studied. This is consistent with the photoelectron spectra of the parent molecules and with the INDO calculation for the respective cations. Using c-CsFl1CF3as a matrix, we observed well-resolved ESR spectra for 2-methylfuran,2-methylthiophene, 3-methylthiophene, and 2,5-dimethylfuran cation radicals. The effect of CH3substitution on the spin distribution is discussed. Furthermore, bimolecular reactions between a radical cation and a neutral molecule of furan in a matrix were briefly discussed.
Introduction The advantage of the use of halocarbon matrices for the ESR study of radical cations in y-irradiated rigid solutions was first demonstrated for the system of some aliphatic amines.l Since then, various systems have been studied by several authors.2-21 The matrices provide relatively well-resolved ESR spectra of radical cations. Grimison and SimpsonZ2have reported the optical bands for a range of (1)Shida, T.; Nosaka, Y.; Kato, T. J. Phys. Chem. 1978,82, 695. (2)Symons, M. C. R.; Smith, I. G. J. Chem. Res., Synop. 1979,382. (3)Shida, T.; Kato, T. Chem. Phys. Lett. 1979,68, 106. (4)Kato, K.; Shida, T. J. Am. Chem. SOC.1979,101,6869. (5)Murabayashi, S.; Shiotani, M.; Sohma, J. J. Phys. Chem. 1979,83, 844. (6)Symons, M. C. R. Chem. Phys. Lett. 1980,69,198. (7)Takemura, Y.;Shida, T. J . Chem. Phys. 1980,73,4133. (8)Shida, T.; Egawa, Y.; Kubodera, H. J. Chem. Phys. 1981,73,5693. (9)Wang, J. T.; Williams, Ff. J . Phys. Chem. 1980,84,3156. (10)Shida, T.; Kubodera, H.; Egawa, Y. Chem. Phys. Lett. 1981,79, 179. (11)Iwasaki, M.; Toriyama, K.; Nunome, K. J. Am. Chem. SOC.1981, 103,3591. (12)Toriyama, K.;Nunome, K.; Iwasaki, M. J. Phys. Chem. 1981,85, 2149. (13)Kubodera, H.; Shida, T.; Shimokmhi, K. J. Phys. Chem. 1981,85, 2583. (14)Wang, J. T.; Williams, Ff. Chem. Phys. Lett. 1981,82,177. (15)Wang, J. T.; Williams, Ff. J. Chem. SOC.,Chem. Commun. 1981, 666. (16)Wang, J. T.; Williams, Ff. J. Am. Chem. SOC.1981,103,6994. (17)Walther, B. W.; Williams, Ff. J. Chem. SOC.,Chem. Commun. 1982,270. (18)Snow, L.D.;Wang, J. T.; Williams, Ff. J . Am. Chem. SOC.1982, 104,2062. (19)Iwasaki, M.;et al. Radiat. Phys. Chem., submitted. (20)Shida, T.;Takemura, Y. Radiat. Phys. Chem., submitted. (21)Alder, R. W.; Sessions, R. B.; Symons, M. C. R. J. Chem. Res., Synop. 1981,82. (22)(a) Grimison, A.; Simpson, G. A. J. Phys. Chem. 1968,72,1776. (b) Note that Shida and Hamill first discussed the usefulness of some halocarbon matrices fro isolation and spectrophotomeric identification of cationic species generated by ionization radiation. Shida, T.; Hamill, W. H. J.Chem. Phys. 1966,44,2369,4372. QO22-3654l83l2Q87-117Q$Q1.SO10
heteroaromatic radical cations using halogenic matrices whose ESR spectra are described in the present work. In this work we have studied the radical cations of furan, thiophene, pyrrole, and their methyl derivatives to clarify the electronic structure of the radical cations. Photoelectron spectra of these molecules indicate that the HOMO is of a2 nature.23 However, since the next-toHOMO of bl symmetry is within ca. 1 eV of the HOMO, it seems worthwhile to confirm the electronic state of the ground state of the radical cations in solid matrices by ESR studies. Thus, we attempted to investigate the systems using various fluorinated matrices. As a result of the present work the following conclusions were obtained: (1)By choosing a suitable matrix, one can obtain relatively well-resolved ESR spectra. In particular C-CgF12 and c-C6FllCF, are found useful for some systems. (2) For all the radical cations the odd electron orbital is of a2nature. (3) The methyl group gives an isotropic hfc suggesting rapid rotation under the conditions studied. (4) The formation of allylic radicals and dimer cation is found, exceptionally, for furan in c-C6FlICF3.
Experimental Section The commercial sources of the reagents used are indicated in parentheses as follows: furan, thiophne, 2methylfuran, 2-methylthiophene, 3-methylthiophene, 2,5dimethylfuran, trichlorofluoromethane, and perfluoromethylcyclohexane (Tokyo Kasei); pyrrole-14N (Nakarai Kagaku); pyrrole-15N of 90 at. % (CEA); perfluorocyclohexane (Pierce); perfluorocyclobutane (Matheson Gas Products); and sulfur hexafluoride (Allied Chemical). The heteroaromatics were dissolved in the fluorinated solvents to a concentration of ca. 1 mol%. After being degassed in a Spectrosil ESR cell, the solutions were yirradiated at 77 K with a dose of less than 1 Mrd. The ESR measurement was carried out with a JEOL spec(23)(a) Baker, A. D.; Betteridge, D.; Kemp, N. R.; Kirby, R. E. Anal. Chem. 1970, 42, 1064. (b) Robinson, J. W., Ed. ‘Handbook of Spectroscopy”; CRC Press: Cleveland, OH, 1974;p 334.
0 1983 American Chemical Society
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Radical Cations of Five-Membered Heteroaromatlcs
la2 (TI)
1171
in Czv
Figure 2. Experimental isotropic hfc constants of furan and pyrrole radical cations are compared with theoretical ones (values in parentheses) calculated by the INDO MO method. The experimental hfc of thiophene radical cation is ghren in the same figure. Calculated spin densities in carbon 2 p r orbitals are also shown.
V -30
-20
-10
0
10
’20
30
H-H, ( gauss) Flgure 1. Experimental ESR spectra of the radical cation of furan generated in a CCl3F matrlx (A) and in a cCBF,, matrlx (B). (C) Simulated theoretical spectrum of B based on the parameters given in Table I and a Lorentzlan line width of 3.5G. Hostands for 3271 0.
trometer (JES-PES)at various temperatures between 77 K and the melting point of each matrix. The hfc was determined with reference to the Mn2+/Mg0sample.
Results and Discussion Furan. Figure 1 shows ESR spectra for the solutions of furan in CC1,F (A), and in c-C6F12(B) at the temperatures which gave the best-resolved spectra. The triple triplet at the bottom (C) was obtained by a simulation with the isotropic hfc of al = 14.4 G and a2 = 3.8 G (see Table I). Poasible paramagnetic species other than the presumed radical cation of furan in the c-C6F12solution are c-C6Fl1. and c-C6F12-,both of which have been studied previously by one of the present However, the spectrum in Figure 1 is different from the spectra of these radicals, and the spectrum is almost solely attributed to the radical cation of furan. The ionization potential of c-CPl2appears not to be available in the literature but it must be much higher than that of furan (8.8 eV)23due to the well-known perfluoro effect26so that the hole transfer from c-C6F12+ to furan should take place efficiently. The difference between the solutions in CC1,F and in c-C6F12in Figure 1indicates that the molecular motion of the radical cation of furan in CC1,F is not rapid enough to average out the A and g tensors. The result in Figure 1shows that the matrix of c-C6F12is superior to CC13F in resolving the ESR spectrum of solute cations. In order to interpret the triple triplet in Figure 1 a standard INDO calculationn was carried out for the radical ~~
~~
(24) Chachaty, C.; Forchioni, A,; Shiotani, M. Can. J.Chem. 1970,48, 435. (25) Hasegawa, A.; Shiotani, M.; Williams, Ff. Faraday Discuss.Chem. SOC.1978, No. 63, 157. (26) Rabelais, J. W. ‘Principles of Ultraviolet Photoelectron Spectroscopy”; Wiley: New York, 1977; p 323.
cation of furan assuming the same geometry as that of the neutral molecule.2s The calculation yielded hfc constants of -10.3 and -2.5 G for the protons attached to the carbons Cz and C3, respectively, for the radical cation in the 1A2 state. They are favorably compared with the observed splittings of 14.4 and 3.8 G in absolute magnitude (cf. Figure 2, which also includes the result of thiophene and pyrrole). The observed hfc constants slightly vary when the other matrices CC13F and SF6are used, as listed in Table I. As the sample was warmed, the triple triplet diminished uniformly without yielding any new spectra for the above-mentioned three matrices. However, when cC6FllCF3was used as the matrix, a completely different spectrum, shown in Figure 5, was obtained, which will be discussed later. Thiophene. Thiophene also gave a similar triple triplet. Contrary to the case of furan the solution of thiophene in CC13F at 140 K gave the best-resolved spectrum. A residual anisotropy is noticeable when the observed and simulated spectra are compared closely. The parameters used for the simulation are given in Table I. Although the hfc constant of the major triplet is slightly smaller in thiophene than in furan, the similarity suggests that the electronic structure of thiophene cation is also of laz nature. Nagai and Gillbro observed a triplet of hfc constant equal to ca. 16 G with additional substructures for thiophene adsorbed on silica gel when the ratio of thiophene vs. silica gel was smalLa Referring to the result for the adsorbed benzene and its methyl derivatives where both3Gu monomeric and dimeric radical cations are generated by y irradiation, they tried to assign the spectrum to the radical cation of thiophene. However, they abandoned the assignment because of the disagreement with the result of CND0/2 calculation for the ESR parameters and tentatively attributed the spectrum to the 3-thionyl radical. We should like to point out a possibility that their original assignment was correct because of the close similarity between their spectrum and ours. Pyrrole. The spectra of y-irradiated pyrrole-14Nand -15N in CC13F gave the well-resolved spectrum of each isotope. The major triplet of ca. 18 G was unchanged when (27) INDO calculation was carried out by using the CNDO-INDO MO program presented in: Pople, J. A.; Beveridge, D. L. ’Approximate Molecular Orbital Theory”;McGraw-Hill: New York, 1970; modified by H. Itoh and deposited in the Program Library at Hokkaido University Computing Center. (28) Bak, B.; Christensen, D.; Dixon, W. B.; H-Nygaard, L.; R-Anderson, J.; Schottlander, M. J.Mol. Spectrosc. 1962, 9, 124. (29) Nagai, S.; Gillbro, T. J . Phys. Chem. 1979, 83, 402. (30) Edlund, 0.;Kinell, P.-0.; Lund, A.; Shimizu, A. J. Chem. Phys. 1967,46, 3679; Adu. Chem. Ser. 1968, No. 82, 311. (31) Wong, P. K.; Willard, J. E. J. Phys. Chem. 1968, 72, 2623. (32) Kinell, P.-0.; Lund, A.; Shimuzu, A. J. Phys. Chem. 1969, 73, 4175. (33) Nagai, S.; Ohnishi, S.; Nitta, I. Bull. Chem. SOC.Jpn. 1971, 44, 1230. (34) Komatsu, T.; Lund, A.; Kinell, P.-0. J. Phys. Chem. 1972, 76, 1721.
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The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Shiotani et al.
H
G:
G-1
Il8G
138G
; * 3< CHI H I 8 7 G 12 8G
S
' 2 8C
Flgure 4. Experimental hf spliiings of the radical cations of methylated furans and thiophenes compared with theoretical ones (values in parentheses) calculated by INDO calculation.
-40
-20
0
2c
H-k(
gauss )
40
Flgure 3. Experimental ESR spectra of the radical cation of 2methylthlophenegenerated in CC13F matrii (A) and in CCgF,,CF3matrii (B). (C) Simulated theoretical spectrum of B based on the paramters given in Table I and a Lorentzian line width of 4.0 G. H , is 3285 G.
the 15N isomer was used instead of the 14N. Thus, the triplet is attributed not to the N atom but to the protons at C2 by analogy with the radical cations of furan and thiophene. The minor structures were not analyzed precisely, but in reference to the INDO c a l ~ u l a t i o nshown ~~ in Figure 2 they are attributed to the nitrogen and the protons attached to C3 and C4. Methylated Furans and Thiophenes. In order to see the effect of the electron-repelling methyl substitution upon the spin density distribution, we also studied several methylated furans and thiophenes. The observed spectrum of 2-methylfuran in c-C6F11CF3 was successfully analyzed by the simulation method using the parameters listed in Table I. The major quartet of 18.7 G is obviously due to the methyl protons. The calculated hfc constant of the methyl protons is an average of the values obtained for various configurations of the methyl group assumed. The INDO calculation strongly suggests that three different doublets of 16.0, 5.0, and 1.0 G are due to the protons attached to C5, C3, and C4, respectively. The spectrum of Figure 3B was observed for 2methylthiophene in c-C6F11CF3at 110 K. The general spectral pattern is similar to that of the observed spectrum of 2-methylfuran in the same matrix. The parameters used to obtain the simulated spectrum are given in Table I. By analogy with the radical cation of 2-methylfuran, the assignment of the four hfc constants was made as shown in Figure 4 and Table I. It is noted that the observed hfc constant of the methyl protons is exactly the same between 2-methylfuran and thiophene. Furthermore, the hfc constants of the other protons are of the same order of mag(35) INDO calculation was carried out for the radical cation of pyrr0le-l" assuming the same geometry 88 that of the neutral molecule reported by: Bak, B.; Christensen, D.; Hansen, L.; R-Andersen, J. J. Chem. Phys. 1956, 24, 720.
nitude between these two radical cations. Therfore, it may be concluded that the replacement of oxygen with sulfur does not affect the spin density distribution appreciably. The solutions of 2-methylthiophenein CC1,F yielded the spectrum of Figure 3A which was less resolved than in the solution of c-C6F11CF3.Similarly, better spectrum were observed also for 2-methylfuran and 2,5-dimethylfuran in the perfluoromethylcyclohexanethan in the CC1,F matrix. A well-resolved spectrum is indispensable to determine precise hfc constants and identify a paramagnetic species formed. In this view point the c-C6FlICF3is a better matrix than the CC13F. The spectrum of 2,ti-dimethylfuran radical cation in c-C6F1,cF, was easily analyzed as the triple septet with the hfc constants given in Table I. In reference to the other methylated systems thus far discussed the assignment of the larger hfc constant to the methyl protons is obvious. The experimental hfc constants are favorably compared with the theoretical ones calculated by the INDO method as shown in Figure 4. The results for 3-methylthiophene are also summarized in Table I and Figure 4. Allylic Radicals Produced by an Ion-Molecule Reaction in the C - C $ ~ ~ CMatrix. F~ Spectra in Figure 5A-D show a continuous change of spectra with increasing initial concentration of furan in c-C6Fl1CF3.Figure 5A gives a triplet with a hfc constant of 14.6 G and is essentially the same as Figure 1B (i.e., monomer cation of furan: species I) although the smaller triplet in Figure 1B is unresolved. The triplet persisted up to 165 K for this sample of 0.05 mol%. With increases in the concentration to ca. 0.1 mol%, a spectrum with a smaller hfc constant (species 11) appeared to superpose upon the major triplet as shown in Figure 5B. As the sample was warmed above 130 K, spectra of both species I and I1 irreversibly changed into a new spectrum which was assigned to a new species I11 as shown in Figure 5B'. The total spin concentrations were of the same order of magnitude between the two spectra of Figure 5, B and B'. The spectrum shown in Figure 5B' (species 111) is readily reproduced by simulation as shown in the broken curve of the same figure in terms of a double triplet with hfc constants of 33.7 G for the doublet and 13.2 G for the triplet. The triplet decreases with further increases in the concentration to ca. 0.3 mol%, and the double triplet denoted with asterisks in Figure 5 appeared concomitantly with the spectrum due to species I1 as shown in Figure 5C. The radical conversion from species I1 to species I11 was also observed upon warming the sample above ca. 130 K. A proper subtraction of the double triplet from the spectrum in Figure 5C yields a quintet of a binominal intensity distribution as shown in the broken curve of the same figure. The hfc constant of the quintet is 7.2 G, which happens to be nearly equal to half the hfc constant of the observed value for the dilute solution of ca. 0.05 mol % . With further increase of the concentration to ca. 8.8 mol
The Journal of Physical Chemistry, Vol. 87, No. 7, 1983
Radical Cations of Five-Membered Heteroaromatics
1173
TABLE I: ESR Parameters of Radical Cations of Five-Membered Heteroaromatics
solute furan
matrix CC1,F SF,
thiophene
I2
a
A, G 1 4 . 3 * 0.5 ( 2 H ) 15.1 * 0.5 ( 2 H ) 15.0 * 0.5 ( 2 H ) 3.4 i- 0.5 ( 2 H) 14.4 ( 2 H ) 3.8 ( 2 H ) 14.6 ( 2 H) 14.2 t 0.5 ( 2 H ) 11.8( 2 H)']! 3.2 ( 2 H ) 1 2 . 8 ( 2 H) 2.9 ( 2 H ) 14.0 i 0.5 ( 2 H ) 13.8 t 0.5 ( 2 H) -- 18.0 3.5 -2.0
180 110 77
CC1,F
140 12
SF, pyrrole
g 2.003
100
c-C6Fl1CF, CC1,F
c-C6F
radical
77 77
SF, c-C6F
temp, K
c-C,F, CC1,F
2.003
175 77 140 150
2.00 3
2 -meth ylf w a n
c-C,FllCF,
100
2.003
2-methylthiophene
c-C,F ,,CF,
100
2.003
-
2 and 2 and 2 and 3 and 2 and 3 and 2 and 2 and 2 and 3 and 2 and 3 and 2 and 2 and 2 and
5 5 541 4 5 4 5 5 5 4 5 4 5 5 5
3 and 4
18.7 ( 3 H) 16.0 (1H)b 5.0 f 0.5 (1H) 1.0 i- 0.5 (1H) 18.7 ( 3 H ) 13.7 (1H ) 4.0 -f. 0.5 (1H) 1.0 f 0.5 (1H) 12.4 ( 5 H ) 12.8 ( 5 H)
4 3 CH, at 3;422 and 5 CH, at 3;422 and 5 CH, a t 2 and 5 3 and 4
3-methylthiophene
c-C6FllCF, CC1,F
167 145
2.003
2,5 -dimethylfuran
c-C6F1,CF,
100
2.003
16.9 ( 6 H ) 4.3 ( 2 H )
furan
c-C6Fl1CF3
110
2.003
7.2 (4 H )
Average value of Ail = 13.5 G and A1 = 11.0 G.
assigned position of carbon atom and nitrogen a t o m s
CH, at 2 5 4 3 CH, at 2
5
2 and 5
Average value of Ail = 21.0 G and A1 = 13.5 G.
cation giving the triplet spectrum appears only in dilute solutions. (ii) At the concentration of ca. 0.3 mol 70 most furan molecules are in dimeric forms at 77 K, and irradiation produces dimeric cations. There seem to be two dimeric radicals. One is a sandwich T dimer cation which gives the quintet with half of the hfc constant of the monomer cation. This is analogous to the monomer-dimer cations of benzene and its methyl The other, which gives the persisting double triplet, is tenetatively attributed to the neutral radical, 111, produced by the ion-molecule reaction in eq 1. This assignment is based
+
H'
Figure 5. Concentration dependence of the ESR spectra of irradiated solution of furan in c-COF1,CFB(A-D). As the initial concentration is increased, the spectrum of the monomer radical cation (species I) is replaced with new spectra. B' shows that both the triplet (I) and the quintet (species 11) are changed into the double triplet (species 111) upon warming. The broken curves in 6' and C are the simulated spectra of 111 and I1 based on the parameters given in Tables I1 and I, respectively.
% the quintet was almost completely replaced by the
double triplet as shown in Figure 5D. These results indicate the following features peculiar to the system of furan in c-C6F11CF3. (i) Furan in this matrix is not as soluble as in the other matrices, such as CC13F, and SF6,and the monomer
on a comparison with a number of similar allylic radicals of furan and its analogues which have been studied previously by many author^.^^^^^^ (36) Schuler, R.H.; Laroff, G . P.;Fessenden,R.J.Phys. Chem. 1973, 77, 456.
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The Journal of Physical Chemistry, Vol. 87,No. 7, 1983
Shiotani et al.
TABLE 11: ESR Parameters of Neutral Radicals Originating from Five-Membered Heteroaromatics
matrix
temp, K
g
A, G
assigned position of carbon and nitrogen atoms
acidic soln
room temp
2.00314
basic s o h
room temp
2.00277
basic soln
room t e m p
c-C,F ,,CH,
132
35.55 ( 2 H ) 13.41 ( 2 H) 2 . 2 0 (1H) 21.12 (1 H ) 1 3 . 5 9 (1 H ) 1 . 9 7 (1 H) 14.28 (1 H ) 21.0 (1H) 14.0 (1H ) 2 . 0 (1 H ) 1 4 . 8 (1 H ) 33.7 (1H ) 13.2 ( 2 H)
2 3 and 5 4 2 3 4 5 2 3 4 5 2 3 and 5
33.5 ( 2 H ) 14.0 ( 2 H ) 3.0 (1 H ) 31 (1H ) 16 (2 H)
2 3 and 5 4 2 3 and 5
solute furan
thiophene
pyrrole
thiophene
2.003
ref 36 36
37
present work 29
193-203
2.003
203
2.003
c-C,F,,CF,
150
2.003
2 9 . 1 (1H) 13.1 ( 2 H )
2 3 and 5
present work
adamantane
260
2.0030
4
1 1 2 3 4 5 2 3 and 5
38
Ar
3.64 (1N ) 4.56 (1H ) 38.40 ( 2 H ) 1 1 . 3 0 (1H ) 1 . 7 0 (1 H ) 11.10 (1 H) 45 ( 2 H ) 11 ( 2 H ) 29.1 (1N ) 13.26 ( 2 H) 3.55 ( 2 H)
1 2 and 5 3 and 4
40
basic soln
aR=AorB
radical
room t e m p
X = 0 o r S.
2.00232
d
29
39
B
The data in the literature relevant to the assumed radical I11 are listed in Table 11. The hfc constant of 33.1 G for the double triplet is very close to that of the methylene protons of allylic radicals produced by the addition of a H atom to the C2 carbon of furan. The triplet of 13.2 G is attributed to the protons of I11 attached to C3 and C6. The survey of Table I1 reveals that hfc constants of the protons at C3 and C5are accidentally equal for various ~~
(37) Shiga, T.;Isomoto, A. J . Phys. Chem. 1969, 73, 1139. (38) Lloyd, P. V.;DiGregorio, S.; Wood, D. E. J. Chem. Phys. 1978, 68,1813. (39) Kasai, P.H.;McLeod, D., Jr. J. Am. Chem. SOC.1973, 95,27. (40) Samuni, A,; Neta, P. J. Phys. Chem. 1973, 77, 1629. (41) At this temperature a radical of an F-atom adduct was clearly observed together with the radical cation. The details will be published elsewhere (Shiotani, M.; Nagata, Y.; Sohma, J.). (42) Although the hydrogens at C2and Caare not equivalent, the ESR spectrum shows that the two hydrogens are magnetically equivalent within the line width of 4.0 G. Furthermore, the hfc constant of the methyl protons happens to be equal to that of the two protons.
allylic radicals. The hfc constant of the proton at C4 in I11 may be too small to be resolved in reference to the data in Table 11. Similar behavior of monomer and dimer cations in the c-C6F11CF3matrix was also observed for thiophene. It is apparent that an analogous argument is applicable to this system also. Table I1 contains relevant data for thiophene and pyrrole.
Acknowledgment. We thank Dr. H. Kubodera for preliminary experiments with regard to this study. The present research was partially supported by the Subsidy for Scientific Research of the Ministry of Education in Japan, Grant No. 56380024. Registry No. Furan radical cation, 66429-00-3; thiophene radical cation, 34480-70-1; pyrrole radical cation, 34468-30-9; 2-methylfuran radical cation, 84752-89-6; 2-methylthiophene radical cation, 84752-90-9; 3-methylthiophene radical cation, 84752-91-0; 2,fi-dimethylfuran radical cation, 84752-92-1.
