1008
2.CIECIERSKI-TWOREK, G. B. BIRRELL, A N D 0. H. GRIFFITH
media was calculated by M o z ~ m d e rand , ~ ~hc arrived kinetics has shown that the observcd electrons are only a t an expression which for very long and very short dia fraction of the total escaped clcctronic charge, and electric relaxation times reduces to t.he simple Onsager this fraction steadily decreases as the tcmperature beformula. comes lower (Figure 10). Thc yield of escaped clecSince the yicld of escaped electrons may reasonably trons which is the quantity to bc used in eq 7, therefore, be identificd with their radiation chcmical yield, 1'90decreases less quickly than thc radiation chcmical yicld zumdcr4*calcuated the thermalization length of elecas given by Table I. If we take this into account and trons in water by matching thc computed cscapc probrccalculate T O from the yield of escaped elcctrons, ie., ability with G/G'i, where C; is thc cxpcrimontal radiolytic from G X GH+/G~.-,whcrc &+/Gee- is taken from yicld of solvated electrons (G valuc) and G i the ionizaFigure 10, the thermalization length decreases from 96 tion yicld. By taking Gi = 5 thc thcrmalieatioilolength A a t -10" to 90 A at -50". Taking into considerawas found to increase from 19.5 at 100" to 35 A at 0". tion that the prccision of G H + / G e , - is not so good, the This is to be expected since a t the lower temperature result, dcspitc thc absencc of increase of yo, is reasonable the elcctrons have to lose more energy to become thcrand the kinetics thus provides us with an cxplanation malizcd and the travel distance must bc longer. for the rapid decrease of the G valuc when the temFor ice we gct a quite diffcrcnt rcsult. In this case perature dccreases. thc relaxation time is very long and IbTozumder's equaAcknowledgments. The authors wish to thank thc tion reduces to the Onsager formula. Taking Ei = operator staff of thc accelerator at Riso for skillful e, = 3.1, Gi = 5, and G from Table I jhc thermalization assistance, Mr. T. Dahlgren for valuablc hclp in prcparlength in H 2 0 ice decreases from 94 A a t - 10" to 52 A ing the icc samplcs, and RIr. C . Lissing for programming a t -50". A similar rcsult was found by b . l o z ~ m d e r ~ ~the computer. We also gratefully acknowledge the when using yields reported by Taub and Eibcn.12 financial support of The Swcdish Atomic Rcscarch The reason for t'his uncxpccted behavior is that for ice Council. the radiolytic yield cannot simply bc identified with the yield of cscapcd electrons. The general second-order (48) A . Mozumder, J. Chem. Phys., 50, 3153 (1969).
Electron Spin Resonance of X-Irradiated Heptanal Trapped in a Single Crystal of Perhydrotriphenylenela by Zofia Ciecierska-Tworek,lbG. Bruce Birrel1,lo and 0. Hayes Griffith* Institute of Molecular Biology and Department of Chemistry, University of Oregon, Eugene, Oregon 97.403 (Received Oclober 28, 1971) Publication costs assisted by the National Science Foundation
Free radicals trapped in X-irradiated heptanal oriented in a single crystal of perhydrotriphenylene were investigated using electron spin resonance techniques. An analysis of the data indicates the presence of two free radicals: a short-lived radical, CH3(CH2)rCHCH0, and a stable semidione radical, CH,(CKz)bCOHCO(CHz)sCH3. All spectra are well resolved and agree with computer-simulated spectra. A table of proton coupling constants and g values is given.
I. Introduction radicals in organic Interest in radiation-produccd molcculcs has led to numcrous studics of aliphatic and alcohol^.^^' HOWacids2t3 and ester^,^ cvcr, progress on aliphatic aldchydcs has been limited to frozen glassrs of short-chain aldehydes including f o r m a l d c h y d ~and ~ ~ ~acctaldehyde.lOsll Hoa'cver, beThe Journal of Physical Chemistry, Vol. 76, N o . 7, 1972
cause the aldehyde molcculcs arc randomly oriented in these rigid glasses, csr spectra of the I esulting radicals (1) (a) ThiR work was supported by the National Science Foundation under Grant No. GP-16341; (b) Fulbright-Hays Exchange Scholar; (c) NIH Postdoctoral Fellow (Fellowship No. 5 F03-CA42789-02) from the National Cancer Institute. (2) J. R.Morton, Chem. Rev., 64,453 (1964).
1009
ESRSPECTRA OF X-IRRADIATED HEPTANAL
Table I : Hyperfine Coupling Constants and g Values for Heptanal Radicals Trapped in Perhydrotriphenylene Inclusion Crystals" Coupling
Radioal
Short-lived radical
(1) u.E = 28.0 f 0.3 G (2) ulH = 19.0 zk 0.3 G g. = 2.0045 & 0.0002
Stable radical
(4)
usH
= 9 . 5 i 0.4
oonstants and Q valuesb
(1) a z y H = 14.5 f 0.3 G (2) u z y H = 18.0 zk 0 . 3 G g, = 2.0042 f 0.0002 (1) uzVH = 2.9 f 0.1 G (2) uzyH = 9 . 7 f 0.1 G (2) azyH = 9 . 9 f 0 . 1 G gzu = 2.0042 & 0.0002
G
g, = 2.0046 zk 0.0002
" The numbers in parentheses represent the number of protons with a given coupling constant. z and xy indicate the crystal orientation with respect to the magnetic field. Relative to DPPH (g = 2.0036). are necessarily poorly resolved, making an accurate assignment difficult. To date no studies of aldehyde radicals in an oriented matrix have been reported. The purpose of this paper is to report a study of radicals from X-irradiated heptanal, CH3(CH2)&H0, trapped in a single crystal of truns-anti-trans-anti-trans-perhydrotriphenylene (PHTP). It has been shown by Farina, Allegra, and Natta12 that P H T P is capable of forming inclusion compounds with a large variety of guest molecules. Long-chain hydrocarbon molecules, trapped in the tubular cavities of the hexagonal P H T P crystals, are oriented with the long molecular axis of the guest molecule parallel to the crystalline needle axis. P H T P is a suitable host for the otherwise transient free radicals because all trapped molecules are magnetically equi~a1ent.l~Radical lifetimes vary but are generally several minutes to several days a t room temperature.
11. Experimental Section Single crystals of the heptanal-PHTP inclusion compound mere prepared in the following way: 30-35 mg of P H T P (synthesized according to the method of Farina14) was dissolved in 1 ml of freshly distilled heptanal (Aldrich) in a tightly stoppered vial. The resulting solution was then cooled from 313 to 281'K over a period of 2 days in a 600-ml Dewar containing 400 ml of water. The z axis was defined to lie along the needle axis of the hexagonal crystal and the x y plane was chosen to be perpendicular to the z axis. Crystals were irradiated for 2 hr with a GE XRD-5 tungsten target X-ray tube operated at 40 kV and 20 mA. Spectra were recorded on a Va,rian E-3 esr spectrometer. Spectral simulations were performed on a Varian 620/i computer. 111. Results and Discussion A . The Hydrogen Abstraction Radical. When heptanal-PHTP inclusion crystals were irradiated at 77"K, resulting esr spectra indicated the presence of one prominent short-lived radical. Less intense spectral lines belonging to other radicals were also present. The room temperature esr spectrum of the short-lived
radical recorded with the magnetic field perpendicular to the crystalline z axis is shown in Figure l a ; Figure l b is the computer simulation. The esr spectra of the short-lived radical result from two large isotropic proton coupling constants and one anisotropic proton coupling constant. Coupling constants and g values for this radical recorded at the major crystalline orientations are indicated in Table I. An estimate of the isotropic component, ao", of the anisotropic a-proton coupling constant can be obtained from the relation4 aOOL =
+ a,?
('/8)(2azYO
(1)
where azy" and a," are the values of the a-proton coupling constant measured with the magnetic field direction in the x y plane and parallel to the e axis, respectively. Using eq 1 and the data of Table I, aoa = 19.0 G. Similarly the isotropic g value, go, can be obtained from the relationship4
+
90 = (1/8)(2gz~
f?z>
(2)
Employing eq 2 and the data of Table I, go = 2.0043. These coupling constant data clearly suggest a radical having a large a-electron spin density on the a-carbon atom. The most logical choices are the radicals RCHOH or ReHCOR'. However, the values of aOa and go are substantially higher than would be expected (3) D. Kivelson and C. Thomson, Ann. Rev. Phys. Chem., 15, 197 (1964). (4) 0. H. Griffith, J . Chem. Phys., 41, 1093 (1964). (5) 0. H. Griffith, ibid., 42, 2644 (1965). (6) P. J. Sullivan and W. 8. Koski, J . Amer. Chem. Soc., 85, 384 (1963). (7) G. B. Birrell and 0. H. Griffith, J . Phys. Chem., 75, 3489 (1971). (8) J. A. Brivati, N. Keen, and M. C. R. Symons, J . Chem. Soc., 237 (1962). (9) F. J. Adrian, E. L. Cochran, and V. A. Bowers, J . Chem. Phys., 36, 1661 (1962). (10) C. Charhaty and R. Marx, J . Chim. Phys. 58,787 (1961). (11) V. I. Smirnova, G. S. Zhuravleva, K. G. Yanova, and D. N. Shigorin, Zh. Fiz. Khim., 38, 742 (1964). (12) M. Farina, G. Allegra, and G. Natta, J. Amer. Chem. Soc., 86, 516 (1964). (13) 0 . H. Griffith, Proc. Nut. Acad. Sci. U.S., 54, 1296 (1965). (14) M. Farina, Tetrahedron Lett., 30, 2097 (1963). The Journal of Physical Chemistry, Vol. 76, N o . 7 , 1.978
Z. CIECIERSKI-TWOREK, G. B. BIRRELL,AND 0. H. GRIFFITH
1010
n
w 30 G
30 G Figure 1. Experimental (a) and computer simulated (b) esr spectra of the short-lived radical from an X-irradiated heptanal-perhydrotriphenyleneinclusion crystal. The experimental spectrum was recorded at room temperature with the magnetic field perpendicular to the crystalline z axis. Coupling constants used in the simulation were uH = 18.0 G (two protons) and aH = 14.5 G (one proton).
Figure 2. Experimental (a) and simulated (b) esr spectra of the stable free radical from an X-irradiated heptanal-perhydrotriphenylene inclusion crystal. The experimental spectrum was recorded a t room temperature with the magnetic field parallel to the crystalline z axis. The simulation was performed on a Varian 620/i computer using the coupling constants of Table I.
from an alcohol radical, RCHOH. For example, Zeldes and Livingston's observed a free radical formed in the solution photolysis of acetaldehyde which they identified as CH&HOH with aoa = 15.22 G, UO@ = 22.11 G, and go = 2.0032. Similarly, Dixon and Normanle examined the free radical produced by the reaction of ethanol with .OH in acidic solutions of T i c k H202 and found that the CH&HOH radical was formed with uoa = 15.0 G and aos = 22.0 G. I n addition, Birrell and Griffith' studied the radicals formed in Xirradiated long-chain alcohols trapped in single crystals of urea and reported the predominant species to be RCHOH, with aoa = 14.4 G, a$ = 20.8 G, and go = 2.0031. The data of Table I, however, are reminiscent of the uoaof 18.5 G and go of 2.0041 obtained for a variety of aliphatic ketone radicals, ReHCOR', in X-irradiated urea inclusion compounds,b which are derived from the parent hydrocarbon by abstraction of a hydrogen atom from the carbon adjacent to the carbonyl group. Based on these similarities in uOa and go, we conclude that the short-lived radical formed by X-irradiating heptanal a t 77°K is
A similar radical, 6H2CH0, was observed by Livingston and Zeldes" during the solution photolysis of
0
/I
CH3(CH2),CH2eCH
I
H The Journal of Phgsical Chemistry, Vol. 76, No. 7 , 1978
ethylene glycol. The coupling constants reported by Livingston and Zeldes were aoH= 19.2 G, uoH = 18.7 G, and a o H = 0.5 G; go = 2.0046. A small splitting of the order of 0.5 G would be effectivcly obscured in the 2-3-G line width of the heptanal spectra. 23. The Semidione Radical. At room temperature, spectral lines belonging to the short-lived radical from heptanal-PHTP rapidly disappeared, leaving the esr spectrum of a second radical,'* as shown in Figures 2 and 3. Figures 2a and 3a are experimental spectra recorded at room temperature with the magnetic field along the crystalline z axis and in the xy plane, respectively; Figures 2b and 3b are the respective simulations. The spectra result from four isotropic and nearly equivalent proton coupling constants (of 9.5-10 G), which are further split by one small (-3 G) proton coupling constant when the magnetic field is in the crystalline xy plane. The second radical was stable for several weeks a t room temperature. Similar spectra (15) H.Zeldes and R. Livingston, J . Chem. Phus., 47,1465 (1967). (16) W.T.Dixon and R. 0. C. Norman, J . Chem. SOC.,3119 (1963). (17) R. Livingston and H. Zeldes, J . Amer. Chem. Soc., 88, 4333 (1966). (18) Although it is possible that the short-lived radical is the precursor of the second radical, it was not possible to verify this in the present study. Both of these radicals, of course, are relatively stable and are not necessarily the initially formed species.
ESRSPECTRA OF X-IRRADIATED HEPTANAL
1011 An INDOlg calculation was performed on the freeradical geometry
H
/ 0 I I1
0
CHsC---CCHs
-+I
30 G
Flgure 3. Experimental (a) and simulated (b) esr spectra of the stable free radical from an X-irradiated heptanal-perhydrotriphenylene inclusion crystal. The experimental spectrum was recorded a t room temperature with the magnetic field in the crystalline zy plane. The simulation was performed on a Varian 620/i computer using the data of Table I.
were obtained from X-irradiated butyraldehyde-PHTP inclusion crystals and also by subjecting a fresh (nonirradiated) butyraldehyde-PHTP inclusion crystal to uv light from a PER 110-W mercury lamp for 12 hr. Coupling constants and g values for the second radical observed in heptanal-PHTP recorded at the principal crystalline orientations are indicated in Table I. The essential features of the spectra are the relatively small coupling constants, suggesting a delocalized spin distribution. This can best be explained by the formation of a bimolecular reaction product of the type RCH&0HCOCH2R where R is (CH2)&H3. For example, a hybrid of the structures
H ,/ 0 0
I
I/
RCH&-CCH2R
H
0
\
II
0
I
*RCH2C--CCHzR
accounts for the esr data. Another explanation would be rapid proton exchange between two radicals of the type
HO 0
I II
RCH&-CCH2R
0 OH
I1 I
RCH2C-CCH2R
The available data, including a limited low temperature study, do not distinguish between these two possibilities.
using as bond lengths Rc-H = 1.08 8, Rc-o = 1.32 8, Rc-c = 1.46 8, and RO-H = 1.82 8 and bond angles L H C H = 109.5", LCCO = 120°, and LCOH = 100". For this calculation we assumed a dihedral angle of 60°, between the axis of the 2p orbital with unpaired spin density and the projection of the C-H, bond onto a plane perpendicular to the C-C bond. (A dihedral angle of 60" is not unusual for molecules trapped in P H T P inclusion As calculated by the INDO method, the two out-of-plane methyl proton coupling constants are 7.3 G, in reasonably good agreement with the experimental results. I n a related study Norman and PritchettZ1obtained U C H ~= 8.3 G (six protons) and yo = 2.0044 for the radical formed in the one electron reduction of biacetyl, CHaCOCOCHa, at pH = 0.5 using the TiC13-H202 system. Norman and PritchettZ1 concluded that their spectra were due to the monoprotonated semidione radical. This free radical evidently exists in the tautomeric forms Me
OH
Me
// c-c / \
0
\,
/ / \
0
Me
HO
Me
with the rate of interconversion of the tautomers varying with pH. Rates of interconversion were postulated to be rapid at pH 0.5 (resulting in the two sets of methyl proton coupling constants being equivalent) and diminishing as the pH was raised (with a resulting nonequivalence of the two sets of methyl proton coupling constants). Differences in coupling constants were also noted for the cis-trans isomers of these tautomers.21 It is interesting to note that in the photolysis of acetaldehyde in lert-butyl alcohol,l6 the acetoin radical, CH3cOHCOCH3, with U C H , ~of 13.57 G (three protons), ucHaHof 2.44 G (three protons), and uH of 1.97 G (one proton) was the only species identified. I n this case, therefore, rapid proton exchange did not occur. (19) J. A. Pople, D. L. Beveridge, and P. A. Dobash, J. Amer. Chem. Soe., 90, 4201 (1968) ; a copy of this program (program 141) was obtained from the Quantum Chemistry Program Exchange, Indiana University, 1969. (20) G. E. Birrell, A. A. Lai, and 0. H . Griffith, J . Chem. Phgs., 54, 1630 (1971). (21) R. 0. C. Norman and R. J. Pritchett, J . Chem. SOC.B, 3119 (1967). The Journal of Physical Chemistry, Vol. 76, h'o. 7 , lQ78
1012
ROBERT D. ALLENDOERFER AND RICHARD J. PAPEZ
These solution studies, then, are consistent with the single crystal results of this paper and serve to point out
the remarkable variety of conditions under which the semidione radical can be observed.
Ion Pairing in Alkali Metal Durosemiquinone Solutions by Robert D. Allendoerfer* and Richard J. Papez Department of Chemistry, State University of New York at Buffalo, Buffalo,New York
[email protected]. (Received September $9, 1971) Publication costs assisted by the Petroleum Research Fund
The esr spectra of Li+, Na+, K+, Rb+, Cs+, and TNBAf durosemiquinone in DME solution are reported and precise values of the g values and hyperfine splitting constants given. An equilibrium between a contact ion pair and the free ion is observed for Na, K, Rb, and Cs, and the thermodynamic parameters characterizing this equilibrium are measured. These data are combined with the known rates of intramolecular ion exchange for the tight ion pairs to give a more complete description of the mechanism of ion exchange.
Introduction Since the first electron spin resonance experiment with a p-benzosemiquinone radical, dozens of papers have appeared describing the details of the esr spectra obtained. From these compounds, two effects have been of particular interest: first, the tendency of these ions to show an alternation in line width in their spectra, caused by rapid exchange of the counterion between positions adjacent to the one of the two carbonyl groups; and second, the formation of various types of ion pairs, which give rise to hyperfine couplings with the counterion and distortions of the symmetry of the semiquinone ion. These effects are particularly pronounced in ethereal solutions. Our study is restricted to 1,2-dimethoxyethane (DME) solution in an effort to reduce the problem to feasible proportions. The semiquinone ions have been studied in this solvent by several groups, principally those of Symons,2 Warhurst, G ~ u g h and , ~ Das6 and their coworkers. These references and those contained therein, while not intended to be complete, should provide an adequate introduction to this complex field. While numerous papers have been written about durosemiquinone and related compounds, they have not received the detailed analysis accorded the alkali metal naphthalenide system with respect to the nature of the ion pairs formed in solution, probably because the semiquinone esr spectra are much more complex. The simultaneous observation of contact ion pairs, solvent-separated ion pairs, and free ions was first reported by Hirota6 for the sodium naphthalenide system and has since been studied by several workers. Recently, precise determinations of the kinetic and therThe Journal of Physical Chemistry, Vol. 7 6 , No. 7, 1972
modynamic parameters involved have been made by Szwarc, et al., for the naphthalene ion pair equilibria,’ and the temperature and solvent dependences of the g and various hyperfine coupling values have been detcrmined by Dodson and Reddocha and by Fraenkel, et ~ 1 . ~ I n this paper, we have applied the precise techniques used above to the durosemiquinone ion (DSQe-) in D M E solution to determine the thermodynamic parameters for the various equilibria involved and also tried to correlate the trends in g and hyperfine coupling values with current concepts of the nature of the ion pairs in these solutions.
Experimental Section The alkali metal durosemiquinone ion pairs were prepared using the standard in vacuo reduction techniquelo (1) B. Venkataramen and G. K. Fraenkel, J . Amcr. Chem. SOC.,77, 2707 (1955). (2) J. Oakes and M. C. R. Symons, Trans. Faraday SOC.,66, 10 (1970). (3) D. H. Chen, E. Warhurst, and A. M. Wilde, ibid., 63, 2561 (1967). (4) (a) P. S. Gill and T. E. Gough, ibid., 64, 1997 (1968); (b) T.E. Gough and P. R. Hindle, Can. J . Chem., 49,2412 (1971). (5) M. P. Khakhar, B. 5. Prabhananda, and M. R. Das, J . Amer. Chem. Soc., 89, 3100 (1967). (6) N. Hirota, J.Phys. Chem., 71, 139 (1967). (7) K. Hofelmann, J. Jagur-Grodzinski, and M. Szwarc, J . Amer. Chem. SOC., 91,4645 (1969). (8) C. L. Dodson and A. H. Reddoch, J . Chem. Phys., 48, 3226 (1968). (9) W. G. Williams, R. J. Pritchett, and G. K. Fraenkel, ibid., 52, 5584 (1970). (IO) D. E. P a d , D. Lipkin, and S. I. Weissman, J . Amer. Chem. Sot., 78, 116 (1956).
