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three was sodium fluorescein, a dye used also by us. However, they based their conclusion on the fact that fluorescein was (almost) not consumed in the reaction. Note that our mechanism allows for possible complete recovery of the dye: It forms a radical which then becomes the electronically excited molecule, and finally reverts back to ground state. In conclusion, our present interpretation emphasizes the chemical interaction between dye and hydrazide and its products under e-beam excitation. The fact that fluorescein was found to be the most efficient emitter under these conditions must therefore be traced to its radical chemistry. In order to achieve better light yields, more should be learned on the chemistry of dye radicals. The failure of Forster's mechanism may be traced to the extremely short lifetime of the would-be donor. One possible way to utilize energy transfer may consist of finding hydrazide derivatives with long emission lifetimes. Unfortunately, such derivatives have yet to be prepared.
Pemberton et al.
References and Notes (1) E. Wurzherg and Y. Haas, Chem. Phys. Left., 55, 250 (1978), and preceding paper in this issue. (2) R. B. Brundrett, D. F. Roswell, and E. H. White, J . Am. Chem. Soc., 94, 7536 (1972). (3) J. D. Gorsuch and D. M. Hercules, Photochem. Photobloi., 15, 567 (1972). (4) E. H. White and D. F. Roswell, J. Am. Chem. Soc.,89, 3944 (1967). (5) D. F. Roswell, V. Paul, and E. H. White, J . Am. Chem. Soc., 92, 4855 (1970). (6) D. R. Roberts and E. H. White, J. Am. Chem. Soc., 92, 4861 (1970). (7) J. Nikokavours, C. Papadopoulos, A. Perry, and G. Vassilopoulos, Chim. Chron., 5, 223 (1976). (8) B. A. Rusin, V. N. Emokhonov,E. L. Frankevich, and V. L. Tal'rose, High Energy Chem., 10, 77, 81, 85 (1976). (9) Th. Forster, Ann. Phys., 2, 55 (1948). (10) V. I. Vasil'ev and R. F. Vasil'ev, High Energy Chem., 8, 398 (1974). (11) E. Wurzberg, Ph.D. Thesis, 1977. (12) C. A. Parker and W. T. Rees, Analyst, 85, 587 (1960). (13) W. Prutz and E. J. Land, Biophysik, 3, 349 (1967). (14) W. Prutz and E. J. Land, J . Phys. Chem., 78, 1251 (1974). (15) J. H. Baxendale, Trans. Faraday Soc., 69, 1665 (1973). (16) E. H. White and M. M. Bursey, J. Am. Chem. Soc., 86 941 (1964). (17) W. Ware, J. Am. Chem. SOC.,83, 4374 (1961).
Reactions of Cation Radicals of EE Systems. 9. Kinetics and Mechanism of the Reaction of Thianthrene Cation Radical with Nitrate Ion' Jeanne E. Pemberton, Gregory L. McIntire, Henry N. Blount," and John F. Evans* Brown Chemical Laboratory, The University of Delaware, Newark, Delaware 19711, and the Department of Chemistry, The University of Minnesota, Minneapolis, Minnesota 55455 (Received May 22, 1979)
The stoichiometry and dynamics of the reaction of thianthrene cation radical with nitrate ion have been studied in acetonitrile, butyronitrile, nitromethane, nitrobenzene, and methylene chloride. In all cases, the reaction is found to yield thianthrene 5-oxide exclusively, with no detectable thianthrene being regenerated during the reaction. The observed rate law in all solvents studied was found to be second order in thianthrene cation radical and first order in nitrate ion. The mechanism proposed for this reaction is analogous to the halfregeneration pathway but differs in that a bridged intermediate comprised of two cation radicals and a nitrate ion decomposes to form two molecules of the oxide and NO'. The observed rate constants, corrected for association equilibria involving nitrate ion, exhibit a linear inverse dependence on Dimroth-Reichardt solvent polarity (ET)parameters, thereby suggesting formation of the bridged intermediate as the rate-determining step in the reaction.
Reactions of the cation radical of thianthrene, a sul(Tho). That the thianthrene cation radical/nitrate fur-centered EE system,2 with a broad spectrum of nucleophiles and reducing agents have been ~ t u d i e d , l as ~~~-~~ have similar reactions involving phenoxathiin,12J4J5J7 p h e n o t h i a ~ i n e , ~and J ~ substituted phenothiazine~.~"J~J~ The cation radical of thianthrene is also known to initiate cationic polymerizations3 as well as to play a significant Th'. Tho mechanistic role in models for oxidative phosphorylation.20 system behaves in this unprecedented manner may well Although both disproportionation and half-regeneration represent an exception to our findings that a general have been argued to be the dominant mechanism for half-regeneration mechanism suitably describes addition reactions of these EE systems with nucleophiles,2a conreactions of cation radicals of EE systems with nucleosistent observation is the re-formation of cation radical philes.2 Both as a consequence of this and in order to precursor in the amount of 50% of the cation radical probe the effect of solvent on the rate of the reaction of consumed en route to formation of the addition (or Th+. with nitrate ion, the work reported here was unsubstitution) product. dertaken. Kinetic parameters and reaction stoichiometries A notable exception to this pattern of precursor rewere determined in acetonitrile, butyronitrile, methylene generation is the significant finding of Shine et al.5 that chloride, nitromethane, and nitrobenzene. the reaction of thianthrene cation radical (TH+-)with both nitrate and nitrite gives rise to 100% conversion of the Experimental Section cation radical to the corresponding thianthrene 5-oxide Materials. Sources and purification procedures for thianthrene, thianthrene 5-oxide, and thianthrene cation *Address correspondence t o H.N.B. at The University of Delaware radical perchlorate have already been reported,16as have and to J.F.E. at The University of Minnesota. n
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0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83,No. 21, 1979 2697
Reaction of Thianthrene Cation Radical with Nitrate Ion
TABLE I: Stopped-Flow Kinetic Characterization of the Reaction of Thianthrene Cation Radical with Nitrate Ion in Acetonitrile at 25.0 ( i 0 . 2 ) O c a series 104CN03-,bM 1 0 3 [ T h ] ,M 1 0 3 [ T h 0 ] ,M replicates 10-2kot,~,CA-' S-' 104[N0,-],d M 1 2 3 4 5 6 7
2.00 5.00 10.0 25.0 25.0 25.0 50.0
1.02 1.02 1.02 1.02 0.00 1.00 1.02
0.00 0.00 0.00 0.00 0.00 0.51 0.00
17 15 15 15 9 8 14
1.12 (k0.08)e
1.34 3.35 6.70 16.7 16.7 16.7 33.2
1.88( i 0 . 0 2 ) 3.83 8.99 9.05 9.16 17.85
(k0.22) (kO.11)
(k0.14) (k0.08) (k0.21)
a Both Th'. (as the C10,- salt) and NO,? (as the tetra-n-butylammonium salt) solutions contained 0.10 M tetra-n-butylt o 5.0 X M. ammonium perchlorate t o control ionic strength, Initial concentrations of Th'. ranged from 1.0 X hobd defined as 1/A5# vs. time. All data treated for minimum of two half-lives. CoeffiAnalytical concentration. Equilibrium concentration; see text. e Parentheses contain one standard cients of correlation exceeded 0,999 in all cases. deviation.
those for acetonitrileS2l Tetra-n-butylammonium perchlorate (TBAP, Fluka/Tridom) was purified as reported elsewhere.16 Tetra-n-butylammonium nitrate (TBAN, Fluka/Tridom) was twice recrystallized from benzene, crushed, and dried under vacuum (52 "C, 20 h), mp 117.5-118.5 "C (lit.22mp 118 "C). Butyronitrile (Aldrich) was purified after the manner of Mel10r~~ as follows: After reflux for 3 h with benzoyl chloride (15 mL/L), this solvent was distilled through an 18-in. Vigreaux column, the first and last 15% being discarded. The retained fraction was refluxed over and then distilled first from anhydrous sodium carbonate (45 g/L, 16-h reflux) and then from potassium permanganate (25 g/L, 18-h reflux). That fraction boiling above 115 "C, exclusive of the final 15%, was retained. Phosphorus pentoxide (5 g/L) was then added and the mixture refluxed for 2 h, after which the middle 70% was distilled onto calcium hydride (5 g/L). Following 18-h reflux and subsequent distillation, the middle 80% of this final fraction was retained [bp 118 "C (760 mm)]. Gas chromatographic determination of the water content (Poropak QS) showed less than 1mM water in this purified solvent. Nitrobenzene (Fisher) was twice passed through an 18 X 1 in. column of activated (500 "C, 48 h) alumina and then twice vacuum distilled. The middle 80% of the final distillation [bp 58-60 "C (5 mm)M]was retained. Following distillation after the manner of Silber,8 nitromethane (Eastman, Spectrograde) was twice passed through an 18-in. column of activated (500 "C, 48 h) alumina immediately prior to use. Methylene chloride (Burdick & Jackson, UV grade) was twice distilled from calcium hydride (40 mesh)25 [bp 41 "C (760 mm)]. All other chemicals used were reagent grade or equivalent. All solutions used in stopped-flow, electrochemical, spectroelectrochemical, and conductance studies were prepared in a nitrogen-filled drybox. Apparatus. The instrumentation and data acquisition system employed in stopped-flow, electrochemical, and spectroelectrochemical studies have been described elsewhere.lbv2 Conductivity determinations were made at 25.0 (f0.2) "C in a nitrogen-filled drybox by the sequential additions,of preweighed portions of TBAN or TBAP to the desired solvent. After thermal reequilibration, conductance measurements were made by using a dip cell (1-cm2 platinized platinum electrodes, K = 0.0982 cm-l) in conjunction with a Surfass conductance bridge. Numerical analyses of these conductance data (cf. eq 8) and subsequent calculation of equilibrium salt concentrations were performed on a Data General Corp. NOVA 1200 computer system employed for data acquisition and kinetic analysis.26 Product Characterization. Reaction product distributions in all five solvents employed in this study were characterized by thin-layer chromatography (silica gel,
Eastman). These characterizations spanned the range of [NO
