6836
J. Phys. Chem. 1986, 90, 6836-6842
flavin. Similar results have been reported for the oxidation of uracil by SO,'-, where electron loss is rapidly followed by deprotonation.l* Some support for the above hypothesis is provided by the results obtained with 3-methyllumiflavin in which deprotonation from N(3) is blocked. Firstly, no pronounced pH dependence in the shape of the spectrum of Fl" can be observed (Figure 5 ) . Secondly, the spectra a t pH 3.8 and 7.7 closely resemble those obtained for lumiflavin at low pH (Figure 2) rather than the proposed deprotonated form observed at pH 7.0 (Figure 1). The rapid decay of the F1" form derived from 3-methyllumiflavin observed at high pH values is consistent with the rapid hydrolysis reported for other cation radicals19 via a reaction with H 2 0or OH- ions. A more rapid decay of the cation derived from lumiflavin is not observed at high pH, presumably since the cation is present in its deprotonated form. (18) Bansal, K. M.;Fessenden, R. W. Radial Res. 1978, 75 497. (19) Von Sonntag, C.; Schuchmann, H. P. Int. J. Radiar. Biol. 1986, 49,
Conclusion Several studies4s5 have reported the observation of the oneelectron reduced form of lumiflavin (FP- or FlH') by flash photolysis in aqueous solution in the absence of added electron donors. Hemmerich and co-workers suggested the formation of F1'- could be accounted for by eq 2. The failure to observe the Fl'+ radical was suggested to be due to either it absorbing at short wavelengths only or its rapid decay via hydrolysis. As we have shown, F1'+ does in fact absorb at longer wavelengths than found in either the F1'- or FlH' spectra. The failure to observe this species is most likely due to its close similarity to the spectrum of 3Fl itself at X > 500 nm and also because its decay rate is comparable to the decay rate of the triplet state. Acknowledgment. This work was supported in part by a grant from the Cancer Research Campaign. Registry No. K2S208, 7727-21-1; SO4'-, 12143-45-2; t-BuOH, 7565-0; t-BuO', 3141-58-0; tetranitromethane, 509-14-8; lumiflavin, 1088-56-8;3-methyllumiflavin, 18636-32-3;isoalloxazine-N(10)-butanoic acid, 105103-62-6.
1.
Structures and Reactions of Radical Cations of Linear Alkanes: ESR Evidence for Selective Deprotonation Kazumi Toriyama,* Keichi Nunome, and Machio Iwasaki* Government Industrial Research Institute, Nagoya, Hirate, Kita, Nagoya 462, Japan (Received: June 2, 1986; In Final Form: July 28, 1986)
It has been shown that the linear alkane radical cations (C4-C,) radiolytically produced in SF6 at 77 K exhibit the planar extended structure with no detectable gauche conformers. The two in-plane C-H bonds at the chain end preferentially participate in the u delocalized molecular orbital resulting in higher unpaired electron density as compared with that of the out-of-plane C-H bonds. Upon warming above 100 K, all the linear alkane radical cations in SF6exclusively undergo selective deprotonation from the chain end to form 1-alkyl radicals, as is expected from the unpaired electron distributions. The results are consistent with our recent observation of the selective formation of 1-alkyl radicals in the 4 K radiolysis of neat crystalline linear alkanes. Formation of 2-alkyl radicals previously observed in CFCl2CF2C1is reasonably ascribable to the reaction via the gauche at C2conformation, in which the unpaired electron is highly populated in the in-plane C-H bond at C2 rather than the chain end. The present paper provides further support for our correct interpretation recently given for the reversible temperature change of the spectrum of n-butane radical cations and clarifies the confusing argument made by Lund and his co-workers on the structure and reactions of the alkane radical cations.
Introduction Radical cations of alkanes produced by a loss of an electron from the u bonding orbital must be of most fundamental importance as reaction intermediates. Since our first ESR observation of ethane and other prototype alkane radical cations in SF,,' we have elucidated structures and reactions of radical cations of alkanes based on the systematic studies on linear, branched, and cycloalkane radical cation^.^-^ The important concepts hitherto established are as follows: Firstly, the unpaired electron is de(1) Iwasaki, M.; Toriyama, K.; Nunome, K. J. Am. Chem. Soc. 1981,103,
3591.
(2) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Phys. Chem. 1981, 85, 2149. (3) Toriyama, K.; Nunome, K.; Iwasaki, M. J . Chem. Phys. 1982, 77,
5891.
(4) (a) Nunome, K.; Toriyama, K.; Iwasaki, M. J. Chem. Phys. 1983,79, 2499. (b) Iwasaki, M.; Toriyama, K.; Nunome, K. Radiat. Phys. Chem. 1983, 21,147. (c) Nunome, K.; Toriyama, K.; Iwasaki, M. Chem. Phys. Lett. 1984, 105, 414. ( 5 ) Iwasaki, M.; Toriyama, K.; Nunome, K. Faraday Discuss.Chem. Soc. 1984, No. 78, 19. (6) Nunome, K.; Toriyama, K.; Iwasaki, M. 'Tetrahedral Symposia-inPrint on Structure and Reactivity of Organic Radical Ions", in press. (7) Iwasaki, M.; Toriyama, K.; Nunome, K. Chem. Phys. Lert. 1984,111, 309.
localized over the u molecular frame in the linear and cycloalkane radical cations, in which particular C-H bonds preferentially participate ,in the u molecular orbital, giving extremely large hyperfine coupling constants as compared with those of neutral alkyl 7~ radicals,'" whereas the unpaired electron in the branched alkane radical cations tends to be more confined to a particular C-C bond so as to maximize hyperconjugation to the branched methyl Secondly, linear alkane radical cations normally take the fully extended structure, in which the two in-plane end C-H bonds preferentially participate in the u molecular orbital, giving a particularly large hyperfine coupling constant which decreases with increasing chain length and thus with increasing u delocalization.2 The interactions with matrix, however, sometimes stabilize the gauche conformer at the C2 atom with an appreciable p ~ p u l a t i o nsince , ~ the energy difference of the rotational isomers is very small. The 120' rotation about the C2-C3 bond results in the shortening of the extended chain length and thus in the increase of the coupling constants of the in-plane C-H bond at C2 as well as at the other end.3 Thirdly, deprotonation to form alkyl radicals, which is exclusively observed in SF6 and in CFC12CF2C1,takes place selectively from the C-H bond having the highest unpaired electron The reaction path is matrix dependent, and elimination of H2or CH, to form olefinic K radical cations is observed in CFC13.3 Fourthly, the matrix effect
0022-3654/86/2090-6836$01 .50/0 0 1986 American Chemical Society
Radical Cations of Linear Alkanes
The Journal of Physical Chemistry, Vol. 90, No. 26, 1986 6831
TABLE I: Hyperfine CoupUng Constants of Linear Alkane Radical Cations (from C4 to C,) with the Extended Structure in SF6Obtained at 77 K and the Comparisons with Those Observed in Other Matrices as w d as with the INDO Calculations
cation n-C4Hla+
matrix SF6
CFC12CF2Cl CFCIP n-C4F10
CFPCCl, INDO'
n-CSHl2+
F6
CFC12CF2Cl n-C4F10
CF$Cl, INDO" SF, CFC12CF2C1 CFC13 CF$Cl, INDO"
4CHP)/G in-plane (2 H) out-of-plane (4 H) +61.5 0 61.0 +60.0 +7.8 +63.0 0 +59.0 +7.8 +68.0 -1.9
cr(CH2)/G for out-of-plane CP
c 4
(-)10.5
56.5 57.0 +47.2 +40.8 41.0 44.0 41.0 +33.2 31.0
CFC12CF2C1 CC14 CFPCC1,
30.0 30.5 30.0 +23.9
ref this work 2 2, 11 3, 11 b 2
(-)5.5
(-)8.0 (-)4.5
-4.9
this work
49.5 57.0
SF6
INDO"
c2
2 3
(-)8.0 -4.3
-1.2 0
(-)3.6
10b -5.3 (-)6.8
2
this work 2 2
(-)4.1
(-)4.0 -0.9
-3.4
-0.7
-3.6
-6.4
(-)4.0 -3.9
-4.6
1 oc 2 this work 2 b 10 2
"Calculated for the extended conformer. bunpublished work by K. Toriyama and M. Iwasaki. plays a crucial role in some cases to determine not only the geometrical but also the electronic structure, when the cation has low-lying excited electronic states close to the ground statea3 Fifthly, there is isotopic preference in determining the stable geometry, as is found in partially deuteriated ethane radical
a)
cation^.^ In order to confirm the third concept for linear alkane radical cations starting from n-C4Hlo+,we have encountered some difficulties, because of the coexistence of the gauche conformer and its increasing equilibrium population at elevated temperature. It was, however, difficult to find a suitable matrix, in which only the extended conformer is stabilized and the contribution from the gauche conformer is negligible even at elevated temperature, at which the cations undergo deprotonation. In the present paper, we wish to report that linear alkane radical cations (C4-C,) with the extended conformation can be stabilized in the sF6matrix by an improved technique and that these cations indeed undergo selective deprotonation to give I-alkyl radicals at elevated temperature, as is expected from the unpaired electron distribution. These results are further discussed in relation to the selective formation of the terminal alkyl radicals in the 4 K radiolysis of crystalline neat linear alkanes previously reported by us.8
Our conclusion may also shed light on the confusing argument by Lund and his co-workers, who proposed a slight nonplanar structure for n-butane radical cations as well as reaction mechanisms to give 2-alkyl radicals in CFC12CF2C1?~10In the present paper, conclusive evidence for not only the planar extended structure of n-C4HIo+but also its conversion into the 1-butyl radicals is mentioned together with that for other higher homologues.
Experimental Section SF6and n-butane were obtained from Takachiho Kogyo, and other linear alkanes from Tokyo Kasei. Frozen solutions of linear alkanes in SF, were prepared at 77 K by quenching liquid solutions containing 0.5-1.0 mol % of solutes rather than from gas-phase (8) (a) Toriyama, K.; Iwasaki, M.; Fukaya, M. J. Chem. SOC.,Chem. Commun. 1982,1293. (b) Iwasaki, M.; Toriyama, K.; Fukaya, M.; Muto, H.; Nunome, K.J. Phys. Chem. 1985,89,5278. (9) Tabata, M.; Lund, A. Radiat. Phys. Chem. 1984,23, 545. (10). (a) Lindgren, M.; Lund, A.; Dolivo, G. Chem. Phys. 1985,99, 103. (b) Dolivo, G.; Lund, A. J. Phys. Chem. 1985,89,3977. (c) Dolivo, G.; Lund, A.2.Naturforsh., A: Phys., Phys. Chem., Kosmophys. 1985,40A,52. (d) Lund, A.; Lindgren, M.; Dolivo, G.; Tabata, M . Radiar. Phys. Chem. 1985, 26, 491.
Figure 1. ESR spectra of the radical cation of n-butane radiolytically produced at 77 K in SF6and of the 1-butyl radical: (a) n-butane radical cation observed at 77 K; (b) thermally induced irreversible change into the spectrum of the 1-butyl radical observed at 110 K; (I:) simulated spectrum of (b) using the parameters listed in Table 11. The dotted line in (a) is due to color center of the sample tube.
mixtures. Dispersion of the solutes was greatly improved by this technique. Cation radicals were generated by '%20 y-irradiation at 77 K to a dose of about 0.5 Mrad. The experimental setup for computer-assisted ESR measurements is the same as that described in the previous paper.3 Results
n-Butane Radical Cations in SF,. The three-line spectrum with a splitting of 61.5 G (2 H) observed in SF6 (Figure l a ) is essentially the same as those in halocarbon matricesZexcept for the substructure, as shown in Table I. The main three-line splitting is due to the two in-plane end protons in the extended trans conformation, as is established in our previous work, in which the details of the unpaired electron distribution in the SOMO are Although it is trivial, Lund and his co-workers
6838 The Journal of Physical Chemistry, Vol. 90, No. 26, 1986
Toriyama et al.
TABLE Ik 1-Alkyl R d i h Formed from Linear A b n e Radical Cations in SFs by Selective Deprotollrtionc dihedral alkyl A(H,)/G A(H6)/G angles/deg (2 H ) (1 H) (1 H) &/G 0, O2 radical C,H
