The Journal of
Physical Chemistry
Registered in U.S. Patent Office
0 Copyright, 2981,by t h e American Chemical Society
VOLUME 85, NUMBER 15
JULY 23,
1981
LETTERS a-Delocalized Radical Cations [H(CH2), H]' of Primary Alkanes: ESR Evidence Kazuml Toriyama, Kelchi Nunome, and Machlo Iwasakl" Government Industrial Research Institute, Nagoya, Hirate, Kna, Nagoya, Japan (Received: March 4, 198 1; In Fhal Form: May 2 1, 198 1)
This paper describes the first clear evidence that n-alkane cations [H(CH2),H]+with extended structures are characterized as delocalized u radicals. The unpaired electron in the in-plane u molecular orbital delocalizes over the entire chain, giving the characteristic 1:2:1three-line ESR spectra due to the strong hyperfine couplings with the two in-plane end protons. The weak couplings with the out-of-plane CH2 protons are unresolvable for most of the cations as is expected from the unpaired electron in the in-plane orbital. The major hyperfine couplings with the in-plane protons decrease with increasing carbon number from 152 G for CzHG+ to 16 G for n-CloH22+depending upon the u delocalization over the extended chain. The coupling values obtained from the INDO calculations using standard geometry are in good agreement with the observation, although a slight tilt of the CH3 axis was required for the lower alkane cations with n 5 4.
In relation to our studies on the role of hydrogen atoms in the radiation chemistry of alkanes,' studies on the structure and reactivity of alkane cations have been undertaken to elucidate the fate of parent cations in the primary processes. Very recently we have found that simple alkane cations such as C2H6+, C3H8+,i-C4H10+,and neo-C5H12+can be stabilized in SF6matrices as the first successful detection of these species in the condensed phases.2 These cations are formed by positive charge transfer from the matrix to the guest molecules with a relatively low ionization potential, as is reported for solute cation formation in some halocarbon matrices such as CC14 and CFC13.3-5 From the extremely large proton hyperfine
coupling constants observed for only two out of six or eight protons, it was concluded that (152 G) and C3H8+ (98 G) are delocalized u cations, in which a large spin density is on the H 1s orbital of the two in-plane end C-H bonds.2 In the present work, we have confirmed this interpretation by examining partially deuterated ethane and propane cations. Moreover, the general pictures for the electronic structures and the spin distributions of n-alkane cations [H(CH2),H]+ with extended structures have been deduced by examining the dependence of the in-plane end proton couplings on the number of carbon ( n = 2-14). From experimental and INDO studies it has been eluci-
(1) K. Toriyama, K. Nunome, and M. Iwasaki, J. Phys. Chem., 84, 2374 (1980),and papers cited therein. (2) M. Iwasaki, K. Toriyama, and K. Nunome, The 43rd Spring Annual Meeting of the Chemical Society of Japan, March 31,1981, Tokyo, abstract Vol. I, pp 456, 528; J. Am. Chem. SOC.,103, 3591 (1981). (3) T. Shida and W. H.Hamill, J. Chem. Phys., 44, 2369 (1966).
(4) (a) T. Shida and T. Kato, Chem. Phys. Lett., 68, 106 (1979); (b) T. Shida, Y. Egawa, J. Kubodera, and T. Kato, J. Chem. Phys., 73,5963 (1980), and papers cited therein. (5) (a) M.C. R. Symons and I. G. Smith, J. Chem. Res., 382 (1979); (b) M.C. R. Symons, Chem. Phys. Lett., 69, 198 (1980); (c) J. T. Wang and F. Williams, J. Phys. Chem., 84, 3156 (1980).
Q022-3654/81/2085-2149$01.25/0
0 1981 American Chemical Society
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The Journal of Physical Chemistry, Vol. 85, No. 15, 1981
Letters
TABLE I : Comparison of the Observed Hyperfine Coupling Constants (Gauss) of [H(CH,),H]+ with Those Obtained by INDO Calculations and the Calculated in-Plane 2p Spin Densities on Each Carbon Atoms obsd values .
n
matrices
2 3 4
SF, SF, CFCl,CF,Cl CFC1, CFCl,CF,Cl CFCl,CF,Cl CFC1, CFCl,CF,Cl CFCl,CF,Cl F(cF;),F* CFCl,CF,Cl CFCl,CF,Cl
5 6 7
8 9 10
calcd hyperfine constantsC " $
in- out-01plane plane H in-plane H H 152 98 61 60
57 41 44 30 22 22 17 16
out-of-plane H,
HI
H, -5.2 -4.9 -4.3 -3.4
-5.3 -4.6
"l
(25) (20) (25) (25)
144.9 91.3 68.0
2.8 -3.7 -1.9
(16)
(1,sd
47.2 33.2
-1.2 -0.9
(16) (16)
23.9
-0.7
-2.6
17.6
-0.5
-1.9
-3.9 -3.2
-4.4 -4.0
13.1 9.9
-0.4 -0.3
-1.5 -1.1
-2.6 -2.1
-3.5 -3.0
calcd in-plane C,, spin densitiesC CI c, c, c, c,
H,
0.22 0.23 0.15
0.24 0.24
0.11 0.08
0.20 0.16
0.25 0.22
0.06
0.04
0.12 0.10
0.16
0.03 0.02
0.07 0.06
0.13 0.10
0.19
0.22 0.20
(i5j
(17) (18)
-3.9 -3.6
0.17
0.15
0.19 0.18
a For unresolvable spectra the line widths are given in the parentheses. Methylene proton couplings (see text). Standard geometries are assumed except for n = 2, 3, and 4, in which the CH, axis is tilted by 1 0 , 7, and 7", respectively, toward the neighboring CH, group. The atomic numbering is from the chain end, and the calculated values are given for halves of molecules because of symmetry.
dated that the unpaired electron occupies the in-plane 0 molecular orbital and delocalizes over the extended chain. Since only the two end protons are in the in-plane u molecular frame in the extended structure, the high spin densities appear only on these in-plane protons, giving a characteristic 1:2:1 three-line spectrum, the splittings of which decrease with increasing carbon number. The spin densities on the out-of-plane CH2protons are consequently very small giving unresolvable hyperfine structures. Our ESR results must give direct information on the wave functions of the unpaired electron orbitals of n-alkane cations. Experimental Section SF6 matrices were used for deuterated ethane and propane. However, since it was difficult to obtain good homogeneous mixtures of SF6with higher n-alkanes, we have used CFC12CF2C16with IP = 11.99 eV as matrices throughout this work and some of the results were compared with those obtained from SF6, CFC13, or perfluoroalkane matrices. Frozen mixtures of SFB(Takachiho), CFC12CF2Cl (Daikin Kogyo), CFC1, (Tokyo Kasei), or n-CsF18 (PCR) containing a small amount (nominal concentration of 1-2 mol %) of n-alkanes (CH3CD3,C2D6, and CH3CD2CH3from MCD; n-C7H16from Wako; n-CsHl8and n-Cl&z, from Chemical Samples; n-CJ4, from Phillips; and others from Tokyo Kasei) were prepared in Suprasil ESR tubes by using a vacuum line at 4.2 or 77 K as described previous1y.l Irradiation was carried out at 4.2 or 77 K with 6oCoy rays or X-rays (45 kV, 40 mA) to a dose of about 0.5 Mrd. The experimental setup and procedures for low-temperature irradiation and subsequent computer-assisted ESR measurements were the same as those reported in our previous papers.*v2 Results and Discussion Structures of n-Alkane Cations. Since the ESR signals from the matrix radicals are anisotropically spread over a wide range,7nearly isotropic spectra from solute cations can be observed without serious interferen~e.~ Analogous to and C3H8+giving a three-line spectrum with splittings of 152 and 98 G, respectively,2 all n-alkane cations with n = 4-10 gave an 1:2:1 three-line spectrum with a coupling constant which sharply decreases with (6) B.Hurni and R. E. Buhler, Radiat. Phys. Chem., 15, 231 (1980). (7) M. Iwasaki, K. Toriyama, and B. Eda, J . Chem. Phys., 42, 63 (1965).
\
O'
2
4 6 8 10 Number of Carbon Members
"
"
"
"
Flgure 1. (a) Observed and calculated coupling constants for the in-plane end protons in [H(CH,),H]+ vs. the carbon number: (0)in CFCI,CF,CI at 77 K (A) in SF, at 4.2 K; (0)in CFCI, at 77 K; (X) INDO values. Typical examples of the ESR spectra of n-alkane cations are also given for (b) [H(CH2),H]+ and (c) [H(CH,),H]+ In CFC12CF2Cl matrices at 77 K.
increasing carbon numbers as shown in Figure 1and Table I. Thus, cations with n = 11-14 gave unresolvable single-line spectra. The g values of these species were found to be 2.0030 f 0.0005. It should be mentioned that the same three-line spectrum was observed for [H(CH,),H]+ both in SF6 at 4.2 K and in CFC12CF2C1at 77 K, although the cation yield was lower in the former matrix because of poor homogenity. The coupling values for C2H6+and C3H8+in SF6given in Table I were obtained in our previous work2 from the samples irradiated and measured at 4.2 K while the others were at 77 K in the present work. As is described in our previous paper,2 the spectrum of CzH6+exhibits a reversible temperature change at 4.2 and 77 K. However, the coupling values of other cations at 77 K are essentially the same as those at 4.2 K. As is previously shown: the unpaired electron of C2H6+ in SF6 occupies a Jahn-Teller split le, orbital: for which a large spin density appears on the two in-plane end (8) See, for example, W. C. Herndon, M. L. Ellzey, Jr., and K. S. Ranghnveer, J . Am. Chem. SOC.,100, 2645 (1978).
Letters
protons. In the present work, we have confirmed this by examining CH3CD3+as well as C&+ in SF, at 4.2 K. CH3CD3f gave the two-line spectrum with a splitting of 152 G with substructures arising from deuteron couplings which are equivalent to the 152-G proton coupling. The spectrum of C&6+ was just what is expected from our interpretation. The unpaired electron of C3H8+in SF6 occupies a 4bl orbital,218in which a large spin density appears on the two in-plane end protons as is the legxorbital for C2H6+. In the present work, we have further confirmed that CH3CD2CH3+in SF6gives essentially the same three-line spectrum as that of CH3CH2CH3+in SFs as is expected from the above interpretation. The results also indicate that the hyperfine couplings with the central CH2protons are very small. A similar three-line spectrum with 61-G splittings obtained from n-C4Hlo+in CFC12CF2C1indicates that the unpaired electron of [H(CH,),H]+ with C2h symmetry occupies the following 5% orbital similar to the 4b1 orbital of C3H8+with czv symmetry:
Thus, the characteristic three-line hyperfine structures of other odd and even n-alkane cations [H(CH,),H]+ can be interpreted in terms of similar unpaired electron orbitals. The decrease of the three-line splittings with increasing carbon number clearly indicates that the spin density on the in-plane end protons decreases with increasing u delocalization over the entire chain. This is further confirmed by the INDO MO calculation assuming an extended structure with standard geometry except for a slight tilt of the CH3 axis for lower alkane cations (n I 4). As shown in Table I and Figure 1, the INDO calculations gave large equivalent hyperfine coupling constants for the two in-plane end protons and small coupling constants of less than 6 G for the out-of-plane protons. In addition, the conformers other than the fully extended ones did not give coupling values consistent with the observed values. The observed trend of the decrease of the in-plane end proton couplings with increasing carbon number was also well revealed by our calculation for the extended structures. Similarly, chain folding makes the in-plane proton coupling larger as a result of the shortening of the extended chain. As a measure of spin delocalization over the extended chain the calculated spin densities in the in-plane 2p orbitals are also given for each carbon atom in Table I. The rest of the spin densities on the carbon atoms is only a minor part as is understood from the magnitudes of the in-plane 2p spin densities given in Table I. As is well-known, a simple LCBO treatment of n-alkane cationsg shows that the unpaired electron density a t the sth C-C bond is proportional to sin2 ( m l n ) . The INDO results for the extended structures can be qualitatively predicted by this relation. Although it was not possible to resolve small couplings with the out-of-plane protons in the CH3 and CH2 groups, the line widths of the three-line spectra are consistent with the calculated small couplings. It should be mentioned here that better resolution of the spectra can be obtained (9)(a) J. Lennard-Jones and G. G. Hall, Trans. Faraday SOC.,48,581 (1952); (b) N. D. Coggeshall, J. Chern. Phys., 30,595 (1959).
The Journal of Physical Chemistry, Vol. 85, No. 15, 1981 2151
1
Figure 2. Second derivative ESR spectrum of [H(CH&H]+ in CFCIB matrices irradiated at 4.2 K and measwed at 77 K. The arrow indicates the position of the DPPH field marker.
from CFCl, matrices. As is seen in Figure 2, a fairly well-resolved 9-line substructure with splittings of 4.0 G was observed at 77 K for [H(CHz)6H]+.10As is suggested from the INDO results given in Table I, the substructure is ascribable to the eight out-of-plane methylene protons with -3.4 (C2,C5) and -4.6 G (C3, C4). The weakest coupling (0.9 G) with the end CH2 protons in the CH3 groups was not resolved. Since the unpaired electron is delocalized in the in-plane u orbital in the extended chain, the spin density in the out-of-plane pseudo T CH2 orbitals must be very small, as is observed. The INDO calculations for lower alkane cations with n I 4 show that not only the spin distributions but also the energy level of the unpaired electron orbital are quite sensitive to the tilt angle of the end CH3groups. The effect of the tilt angle on the spin distribution in C&6+ has already been described in our previous paper.2 For C3H8+ the standard geometry without tilt leads to a 2b2 unpaired electron orbital,8 which is essentially the pseudo T orbital giving strong couplings with the out-of-plane protons, whereas weak couplings with the in-plane protons, that is, 131.5, 48.7, and -1.8 G for the central CH,, the end CH2, and the in-plane CH protons, respectively. However, the slight tilt ( 7 O ) of the CH3 axis from the C1-C2 bond toward the neighboring CH2 group alters the unpaired electron orbital from 2b2 to 4bl, giving coupling values which are consistent with those observed in SF6 (see Table I).'' A similar level crossing by the slight tilt (7') leads to a 5a, unpaired electron orbital for [H(CH,),H]+, giving coupling values consistent with the observations. Since it was doubtful that a long chain molecule such as n-C8H18+posesses an extended form in the CFClzCFzCl matrices, we have further examined frozen mixtures of n-C8& and n-C8F18in which a higher probability of taking the extended form can be expected for n-C&Ils+. As shown in Table I, essentially the same three-line spectrum as that in CFC12CF2Clwas obtained giving the experimental confirmation for the extended structure in these matrices. Hexamethylethylethane Cations (HME+). It may be worthwhile to give a brief comment on HME+ in this paper, since our observation may terminate the controversy in the spectral identifi~ation.~ Using CFC1, matrices, we have observed a small substructure with a splitting of 3.8 G on the 28.8-G seven-line spectrum, whereas the substructure was not well resolved in CFClzCFzCl or per(10) A similar substructure was also observed for n-C4HIoCin CFC13 at 77-120 K. (11)I t should be mentioned that C3Hsf in CFC1, (at 77 K) gave the hyperfine structure (- 105 G for the central CH2 and -48 G for the end CH2 protons), which can be interpreted in terms of the 2bz unpaired electron orbital. The difference from those observed in SF, is ascribable to the level crossing caused by the matrix effect. The detaih will be given in a separate paper.
2152
J. Phys. Chem. 1981, 85, 2152-2155
fluoroalkane matrices. A similar matrix effect on the spectral resolution was observed for n-C6HI4+as is mentioned before. The matrix effect may solve the controversy about the presence and absence of the substructure as well as a small difference in the seven-line splittings reported by two groups.6 As is expected from our previous results for branched alkane cations,2the unpaired electron of HME+ must be mainly confined to the central C-C bond so that large couplings (29 G) can be expected from the six protons in the trans C-H bonds while the small ones (4 G) arise from the remaining 12 protons with the 60’ conformations. Similar 6-proton couplings with rigid CH3 protons in C-C cr radicals have already been determined by ENDOR studies in our laboratory for the deprotonated cations of a-aminoisobutyric acid.12 The averaged 6-proton couplings for the two CH3 groups are 22.1 and 3.3 G for the (12) H.Muto, M. Iwasaki, and Y. Takahashi, J.Chem. Phys., 66,1943 (1977).
trans and the 60’ conformations, respectively (pc = 0.45). This must give a firm basis for the spectral identification of HME+ with a rigid staggered structure. Conversion of Radical Cations to Neutral Radicals. Preliminary experiments indicate that [H(CH2),H] in CFClzCF2Clconverts predominantly into CH3CHCH2CH3 (I) by proton loss upon warming to 11G120 K. Formation of I rather than CH3CH2CH2CH2(11) is somewhat surprising, but one cannot exclude the possibility of deprotonation via the form having a large spin density on the proton attached to the C2 atom. Although they are less probable, one cannot exclude the possibilities of isomerization from I1 to I after deprotonation from the C1 atom and of H abstraction by matrix radicals from undamaged n-C4Hloto form I. Further studies on the reactions of alkane cations are now in progress. +
Acknowledgment. The authors thank Dr. H. Muto for his cooperation in the experiments at liquid helium temperature.
Chemical Waves In the Acidic Iodate Oxidation of Arsenite Thomas A. Gribschaw, Kenneth Showalter,” Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506
Debra L. Banville, and Irving R. Epstein* Department of Chemistry, Brandeis Universiyy, Waltham, Massachusetts 02254 (Received: March 20, 198 1; In Final Form: May 20, 198 1)
After a brief induction period, an unstirred, initially homogeneous solution containing arsenite and iodate at pH -2 may give rise to a single wave of chemical reactivity. The waves have been studied in thin layers and in narrow tubes of solution. The wave is apparently initiated at a region of high I2 concentration, where autocatalytic production of I- begins and spreads into the rest of the solution by diffusion. Waves were also electrochemicallyinitiated in thin layers of solution at a negatively biased Pt electrode. A simple reaction-diffusion model is given to illustrate wave propagation in such a system.
Introduction The development and propagation of chemical waves in an initially homogeneous solution is a subject of considerable interest.l Studies of chemical wave behavior have thus far been confined almost entirely to bromate systems.2 The development of a general theory of such waves will require the existence of several systems, preferably rather different chemically from one another, in which waves may be observed. Epik and Shub3reported, some 25 years ago, that the oxidation of arsenite by iodate is capable under appropriate conditions of generating a chemical wave. No quantitative study was done, however, and no convincing explanation of the reaction-diffusion behavior was developed. It does not appear that this phenomenon has been investigated further since 1957. In this paper, we describe our initial experiments in a detailed investigation of the reaction-diffusion behavior (1) (a) Kopell, N.;Howard, L. Stud. Appl. Math. 1973,52, 291-328. (b) Winfree, A. T. In “Theoretical Chemistry”; Eyring, H.; Henderson, D.; Academic Press: New York, 1978, Vol. 4, pp 1-51. (2) (a) Field, R.J.; Noyes, R. M. J. Am. Chem. SOC.1974,96,2011-6. (b) Orbin, M.Ibid. 1980,102,4311-4. (c) Showalter, K. J.Phys. Chem. 1981,85,440-7. (3) Epik, P.A.; Shub, N. S. Dokl. Akad. Nauk SSSR 1955,100,50343. Shub, N.S. Ukr. Khim. Zhur. 1957, 23,22-6.
in the iodate oxidation of arsenite. Experiments similar to those conducted by Epik and Shub using an open glass cylinder have been carried out. We also report on the electrochemical initiation of waves in a thin f i i of reaction mixture. In addition, a numerical simulation based on a simple reaction-diffusion model is presented. Experimental Section Solutions were prepared with doubly distilled water and reagent-grade chemicals. In the tube experiments iodate and arsenite solutions were prepared in a sulfate-bisulfate buffer and were thoroughly mixed before being poured into tubes of 11cm length and 1.0 cm diameter. The induction time 7 is defined as the time between mixing and the first appearance of the brown ring at the top of the tube. Wave velocities were measured by recording the times at which the wavefront passed predetermined points marked on the tubes. Experiments were also carried out in which waves were initiated at a negatively biased Pt wire electrode. Reaction mixtures were prepared by pipetting appropriate volumes of stock solutions. Iodate reagent was added last by rapid delivery pipet and complete delivery was defined as time zero. The reaction mixture was throughly mixed and then spread over the bottom of a thermostatted petri dish. A
0022-3654/81/2085-2152$01.25/00 1981 American Chemical Society
