410
J. Phys. Chem. 1980, 84, 410-414
were used, cells made of rectangular cross-section quartz tubing were emp10yed.l~(Sizes currently available include 3 X 10, 4 X 10, and 2 X 8 mm cross section). Acknowledgment. The author gratefully acknowledges the gift of the compounds mentioned above, the synthesis of compound I by Mr. M. Kaganowitch, and the technical assistance of Mrs. Nelly Castel. I1
I11
IV
V
mp 61-62 “C). trans-Stilbene was a commercial scintillation-grade product. Solvents. MCH and 2-MP were purified as described.l* Other solvents were commercial “spectral quality” or “fluorescence grade” used directly. Absorption and Emission Spectra. Absorption spectra were measured on a Cary 14 spectrophotometer. Emission spectra were measured with a Perkin-Elmer Hitachi Type MPF-44 instrument. Both instruments were furnished with our usual cooling arrangements,laJ5 based on controlled passage of liquid air through a copper block surrounding the spectrophotometric cell inside a quartz Dewar. All emission spectra are uncorrected. Excitation was at right angles, with care taken to excite the solution layer close to the emitting surface to reduce distortions due to reabsorption of emitted light. To achieve this, 2 X 10 mm or 2 X 8 mm cross-section cells were used, excited a t the 2 mm side. This setup has the additional advantage of smaller volumes and faster temperature equilibration. When solvents which tend to crack at low temperatures
References and Notes (1) (a) Part 1: E. Haas, G. Fischer, and E. Fischer, J. Phys. Chem., 82, 1638 (1978), and earlier papers detailed therein; (b) J. Photochem., 9, 277 (1978). (2) Yu. B. Scheck, N. P. Kovalenko, and M. V. Alfimov, J. Lumin., 15, 157 (1977), and earlier papers cited therein. (3) T. Wismontski-Knittel and E. Fischer, J. Chem. SOC.,Perkin Trans. 2 , 449 (1979). (4) (a) G. Fischer and E. Fischer, Mol. Photochem., 8, 279 (1977); (b) H. Greenspan and E. Fischer, J. Phys. Chem., 69, 2466 (1965). (5) E. Fischer, J. Phys. Chem., 77, 859 (1973). (6) T. Wismontskl-Knittel and E. Fischer, Mol. Photochem., 9, 67 (1978-1979). (7) E. L. Eliel, Isr. J . Chem., 15, 7 (1976/77). (8) T. Wismontski-Knittel and K. A. Muszkat, in preparation. (9) 2. Shakked and D. Rabinowitz, in preparation. (10) We are indebted to Dr. K. Ghiggino for informing us about these unpublished results, which are to be publlshed in J. Photochem. (11) Measured on BRUKER 90-and 270-MHz instruments. (12) Dr. Klaus Schuken, Max-Planck-Instkutfur BiophysikalischeChemie, Gottingen, BRD. (13) F. B. Malkxy and C. W. Ma m l ,J. Am. Chem. Soc.,94, 6047 (1972). (14) E. Spaeth, Monatsh. Chem., 35, 463 (1914). The method described by Spaeth for the synthesis of 2,2‘dimethylstilbene was used. (15) E. E. Fischer, Mol. Phofochem., 2, 99 (1970); 8, 1 (1974). (16) A. M. Schaffer, W. H. Waddell, and R. S.Becker, J. Am. Chem. SOC.,96, 2063 (1974).
Evidence for Reductive Quenching of Singlet Excited Methylene Blue by Iron( 11) Terry L.
O M , +Norman N. Lichtin,” Morton 2. Hoffman, and Sugata Rayt:
Department of Chemistry, Boston University, Boston, Massachusetts 022 15 (Received August 29, 1979) Publication costs assisted by the Department of Energy
Quenching of singlet excited methylene blue by Fe11(Hz0)62+ and Fe11(Hz0)5C1+ has been investigated in 0.01 M solutions of HC1 in water and in 50 v/v % aqueous CH3CN by laser flash photolysis-kinetic spectrometry by use of a Q-switched ruby laser emitting at 694.3 nm and time resolutions to 100 ns. Semimethylene blue quenching was monitored at 370 and 840 nm. It has been shown that in 50 v/v % aqueous CH3CN direct reduction is an important pathway in the quenching of singlet methylene blue. A similar conclusion is consistent with data obtained in aqueous solution.
Introduction The lowest triplet states of the thiazine dyes thionine and methylene blue have relatively long lifetimes which facilitate their use as photoredox reagents in photogalvanic cells. For example, the intrinsic lifetime of protonated triplet methylene blue, 3MBH2+,pKA = 7.2,’ is 4.35 f 0.2 ps2v3while the intrinsic lifetime of the unprotonated triplet, 3MB+,has been reported t o be 85 ~ 5 Similarly, . ~ the intrinsic lifetime of protonated triplet thionine, 3TH2+, PKA = 6.3,5 is 7.5 f 0.8 p s 2 and that of 3TH+is -60 In contrast, the intrinsic lifetimes of the lowest excited singlet states of thionine and methylene blue in aqueous solutions IIT Center, 3424 S. State St., Chicago, Ill. 60616. *Deceased, Sept. 25, 1979. 0022-3654/80/2064-0410$01.00/0
containing chloride ion a t pH 2.5 are 377 f 257 and 365 f 21 P S , ~respectively. The reductive quenching of 3THz2+by Fe**(H20)62+ is a key reaction in the widely investigated iron-thionine photogalvanic cell. The rates of this reaction and the analogous reductive quenching of 3M13H2+vary widely with mediuma2 Specific rates of quenching of 3THz2+in the range 5.5 X 106-5 x lo8 M-l s-l have been reported2 for solutions in water or 50 v/v% aqueous CH3CN in the presence of CF3S03- or HS04-/S042-. Analogous rate constants for quenching of 3MBH2+are in the range 9 X 105-7 X lo7 M-’ These rates are substantially slower than diffusion controlled. Quenching of 3TH22+by Fe’I(HzO),2+is reported to proceed with essentially quantitative net electron transfer.6 A relatively high efficiency 0 1980 American Chemical
Society
Reductive Quenching of
SIMethylene Blue by
The Journal of Physical Chemistry, Vol. 84, No. 4, 1980 41 1
Fe(I1)
of net electron transfer in quenching of 3MBH2+by a mixture of Fe11(H20)s2+ and Fe(H20)< is suggested by the present work. (However, the efficiency of net electron transfer in quenching of 3MBH2+by stable coordination complexes of Fe(I1) via an electron exchange mechanism is highly sensitive to the nature of ligands and m e d i ~ m . ~ ) Quenching of singlet excited thionine by Fe"(H20):+ is diffusion controlled or nearly so: (2.9 f 0.25) X lo9M-' s-' in water at pH 2.5 in the presence of chloride and (3.5 f 0.65) X lo9 M-'s-l in the presence of sulfate.8 Since the rate of intrinsic decay of TH2+(SJ8is 2.7 X lo9 s-', significant quenching of singlet thionine thus requires [Fe11(H20),2+]> -0.1 M. Information on net electron transfer in the quenching of excited singlet thiazines by Fe(I1) does not appear to have been reported. We here present evidence that significant net electron transfer accompanies quenching of MB+(S1) by Fe"(H20)62+and Fe11(H20)5Cl+.
Experimental Section Materials. Reagents were used as received. These included Fluka puriss methylene blue chloride, Fisher Certified ACS KC1, Fisher Reagent ACS hydrochloric acid, Fisher certified FeCl,, Burdick and Jackson UV grade acetonitrile and Belmont Springs Water Co., Inc. distilled water. The molecular weight of the methylene blue chloride was taken as 374,l which is that of MBC1.3H20. Stock solutions of reagents were made up in 0.01 M aqueous hydrochloric acid. The stock solution of FeC12 was protected from atmospheric O2 by continuous purging with N2 which had been deoxygenated by bubbling through an acidified solution of chromous ion which was in contact with zinc amalgam and then through 0.01 M aqueous hydrochloric acid. Flash solutions were purged similarly for a t least 20 min before flashing except that deoxygenated N2 was bubbled through a 0.01 M solution of hydrochloric acid in a solvent of the same composition as that used for the flash solution. Apparatus. Laser Flash Photolysis. Part of the data reported below were obtained by using a Q-switched ruby apparatus with kinetic spectrophotometric monitoring which has been described e1sewhere.l Another part was obtained by using a similar system at Brandeis University. Flash energies (-1.2 J) were sufficient to convert all the methylene blue in the solutions employed to its triplet state in the absence of quencher.' Fluorescence Quenching. Data for Stern-Volmer analysis was obtained by using a Perkin-Elmer MPF 44A spectrofluorimeter and UV grade silica cells. Fluorescence was excited a t 616 nm and monitored at 680 nm. Pulse Radiolysis. Pulse radiolysis-kinetic spectrophotometry experiments were performed a t the U S . Army Natick Research and Development Command. The Febetron-based apparatus has been described.'O Results 1. Fluorescence Quenching. Chloride was found to weakly quench the fluorescence of methylene blue in a 0.01 M acid. The concentration of chloride was therefore kept constant while the concentration of ferrous ion was varied, and the Io of the Stern-Volmer relationship was determined in the presence of the constant concentration of chloride. A Stern-Volmer plot of data with [Cl-] maintained constant at 1.41 M by addition of KC1, [HCl] = 0.01 M and [MB+] = 1pM is presented in Figure 1. From the resulting slope, k,r = 1.20 f 0.15 M-' in aqueous solution s , ~k, = (3.2 and the T of MB+(S1) = (3.77 f 0.25) X f 0.6) X lo9 M-' s-l. The relatively low solubility of FeCl, in 50 v/v % aqueous CH&N (50% AN) prevented a
Stern-Volrner PIC! for Ouenchlnc of 'luorescence 01 M e t t y l e n e Blue by Fe(Q n 001 c? HCI in \niater and in 50 Y / V % oq Cb,Ch(c--.i
01
O
0
K
oi2
oi5
oi4
0'3
Fe(U) ,
,.,
oi7
Oi6
M
Figure 1. Stern-Volmer plot of quenching of fluorescence of methylene blue by Fe(I1) in 0.01 M HCI, [Cl-] = 1.41 M, [MB'] = 1 pM: (0) aqueous solution; ( 0 )solution in 50 v/v % aqueous CH3CN.
1
13 12
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I
I
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1
-
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-
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8-
-
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7
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,
Corrected Absorption Spectrum of Semlmethylene blue in water at p~ 6.9
I
340
360
380
400
420
.I,
440
460
,I 480
nm
Figure 2. Absorption spectrum of semimethylene blue in M aqueous phosphate buffer at pH 6.9 measured by pulse-radiolysis kinetic spectrophotometry in the presence of 0.1 M tert-butyl alcohol.
precise evaluation of kq7 in this solvent. It is apparent from Figure 1 that quenching efficiency in 50% AN is similar to that in water. More than one labile complex of Fe(I1) may have participated in the quenching of MB+(S1) in the work described above. It has been estimated" that the equilibrium constant for association of Fe(I1) with C1- is less than 0.5 M-' in 1 M aqueous acid at 25 "C and may be less than 0.2 M-l. If the value of K,,, was 0.2 M-' under the conditions of this work, then, with 1.41 M C1-. 78% of the Fe(I1) was present as'Fen(H20),2+and the rest was present as Fe11(H90)nC1+. 2. Determhation of the Spectrum of Semimethylene Blue by P u k e Radiolysis. Argon-purged aqueous solutions M phosphate buffer (pH containing 20 pM MB+, 1 X 6.9), and 0.1 M tert-butyl alcohol were irradiated with 30-11s pulses of 2.3-MeV electrons, each of which produced an initial concentration of hydrated electrons of 2.2 p M according to thiocyanate dosimetry.12 The associated radiolytic yield of H atoms produced a 0.44 pM initial concentration of hydrogen atoms. Reaction of MB+ with radiolytically produced radicals was complete in less than 2 ps, as shown by spectral changes. It can be assumed13 that H atoms and e, - reacted essentially quantitatively with MB+. Optical absorptions were measured 2 ps after the pulse. The data were corrected for loss of absorption
412
The Journal of Physical Chemistty, Vol. 84,No. 4, 1980
Osif et al.
L
TABLE I : Yields of Semimethylene Blue (MBH'. t MBHZ2+.) Produced from 5.5 pM MB+in the Presence of 0.240-0.546 M FeCI, in 0.01 M Aqueous HC1
0Al=.085
[FeCl,], M 0.240 0.273 0.546
b
quenching fraction probaquenchbility,a % ed,b % 22 25 40
79 83 94
yield,c % 97 105 101 i 2
Figure 3. Individual traces of photomultiplier output vs. time for 5 pM MB+ in 0.01 M aqueous HCI: (a) 0.273 M FeCI,; (b) 0.540 M FeCI,.
a Probability of quenching by Fe(II)/molecule of MB' (SI ). Because of pumping of MB+(S, ) during the laser pulse the indicated probabilities/molecule of MB+(S,)are substantially less than the actual fractions of dye in the solutions which underwent quenching from the S I state. Fraction of 3MBHZ+quenched by Fe(I1). Calculated from concentrations with the aid of rate constants from Yield of MB. t MBH'. calculated from A,,,. ref 3, Taking = 6300 M - ' cm-' for semimethylene blue and 3280 for MB'.
by reacted MB+ to give the absorption spectrum of semimethylene blue shown in Figure 2. This spectrum is similar to that obtained by Keene et al.13 by pulse radiolysis of solutions of M MB+ containing 0.1 M sodium formate at pH 7.8. Their values of ,A, 420 nm, ,E, 10500 M-' cm-', shoulder at 380-400 nm, and t,h 9000 M-' cm-' can be compared with A,, 420 nm, ern= 13 100 M-' cm-', shoulder at 390-400 nm, and t,h 10 200 M-l cm-' found in the present work. The pKA of MBH22+.is reported to be -2.13-15 The absorption spectrum measured at pH 6.9, where semimethylene blue is MBH'., is not identical with the spectrum of the radical a t pH 2, where approximately equal concentrations of MBH'. and MBHZ2+are present. However, comparison of spectral3 of the semimethylene blue at pH 7.8 and -1.75 indicates that there is an isobestic point at 370 nm. The value of 6370 determined in the present work, 6300 M-' cm-l, is therefore used in the following section to estimate yields of semimethylene blue produced by flash photolysis at pH 2. This value can be compared with €370 7000 M-' cm-' reported by Keene et al.13 3. Yields of Semimethylene Blue when Quenching of MB+(Sl)Is Significant. Five micromolar solutions of methylene blue in 0.01 M aqueous hydrochloric acid were subjected to laser flash photolysis a t 694.3 nm in the presence of 0.240, 0.273, or 0.546 M FeC12. On the basis of the data summarized in section 1,under these conditions the probabilities that each MB+(S1) molecule will be quenched by Fe(I1) are, respectively, 0.23,0.25, and 0.40. Since the lifetime of the laser flash was more than ten times the intrinsic lifetime of MBt(S1), many cycles of pumping of the dye from Soto S1 occurred during the flash. The probabilities indicated above are therefor less than the fractions of MB+ molecules which underwent quenching from the S1 state. (The entire population of dye molecules was excited during the flash.') Without quantitative knowledge of the relative proportions of quenching directly to So, to TI, and to semimethylene blue, it is not possible to calculate the actual fraction of MB+ which underwent quenching from the S1 state. In the absence of Fe(I1) all the dye is converted to its T1state (3MBH2+)under the conditions emp1oyed.l The value of €370 for 3MBH2+is 12 500 M-' s-l in 0.01 N aqueous acid,l twice that of semimethylene blue under these conditions. However, time resolution for the generation of 3MBH2+and its quenching by 0.24-0.55 M Fe(I1) could only be partially accomplished since the lifetime of 3MBH2+under these conditions was 0.7-0.2 ks3 and the time constant of the monitoring system used in these experiments was 100 ns. As illustrated in Figure 3, how-
ever, absorption at 370 nm reached constancy in less than 2 ps and could be used to estimate yields of semimethylene blue resulting from the sum of quenching of MB+(S1)and 3MBH2+.Table I summarizes the yields of semimethylene blue calculated in this way by using €370 = 6300 M-l cm-'. These data do not have a unique interpretation. However, they do not rule out the possibility that quenching of MB+(S1)by a mixture of Fe"(H20)t+and Fe"(H,O),Cl+ produces semimethylene blue directly and/or via 3MBH2+ resulting from intersystem crossing induced by the quencher 4. Kinetics of Formation of Semimethylene Blue by Reaction with 0.05 M FeC12 in 0.01 M Solutions of HCl in 50 v/v % Aqueous CH3CN. In 0.01 M solutions of HC1 in 50 v/v % aqueous CH3CN (50% AN), neither Ml3+ (So) nor 3MBH2+absorbs detectablyl at A,, 1 800 nm. Semimethylene blue has been reported14 to absorb in this re= 880 gion a t 3 I pH 5 8 in aqueous solution, with A,, nm. Similar absorption was observed in the present work consequent upon flashing solutions of MB+ and FeC12 in 0.01 M solutions of HC1 in 50% AN. Under these con= 865 nm with e,, 1 8200 ditions, we have observed ,A, M-' s-'. In the measurements described below, absorbance was monitored at 840 nm, = 0.75 emax, because of better response of the photomultiplier. Data reported below were obtained by using a monitoring system with a time constant I10 ns. It is readily shown that, under the circumstances indic;ea%g40Prodlq.It and Ifare the cated above, log [It/IflslO.= intensity of light transmitted through the sample at time t and when the reaction is complete, respectively, c y t is the concentration of reactive but unreacted dye species at time t , tg40prod is the molar absorbancy index of semimethylene blue, 1 is the optical path length, and q is the fraction of reactant (dye) converted into product (semimethylene blue). If the reactive species is quenched by excess quencher in a pseudo-first-order process, In log [Z,/Zf]~o = -(ki + k,[Q])t + In c y t + In (tslOprdlq), where ki is the intrinsic specific rate of decay of the reactive species, iz, is the specific rate of quenching, and [Q] is the concentration of quencher. Under these circumstances, a plot of In log [Zt/Zf]g40 vs. t will be linear with a slope equal to -k,[Q]. If semimethylene blue were formed solely via quenching of 3MBH2+and both the lifetime of the flash and the time required for intersystem crossing and protonation of the triplet were negligible, extrapolation to the time of the flash of such a plot, recorded over a time period appropriate to the complete quenching of the triplet, would give a value of (Io/If)840 identical with (ZOo/If)840, where Zoo is the intensity of transmitted light
time, 2/ls/div
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- --
-
-
-
-
.
The Journal of Physical Chemistty, Vol. 84, No. 4, 1980
Reductive Quenching of SI Methylene Blue by Fe(I1)
413
TABLE 11: Prompt Yields of Semimethylene Blue from Quenching of 5 . 5 pM Methylene Blue by 0.05 M FeCl, in 0.01 M HC1 in 50 v/v % Aqueous CH,CN Monitored a t 840 nm ~
expt no.
10~5k,,,t,
L34D L34E L34F
8.2 8.0 8.6
L33A L33C L33D
16.2 14.5 15.2
s-l
ra
A t , , b PS
fs,,
At,,C PS
,d
f,,,," 7%
70
[Cl-] = 0.11 M f 0.9960 0.19 0.9967 0.14 0.9950 0.18
0.28 0.21 0.26
16 11 14
21 15 19
1Cl-I = 0.36 Mg 0.14 0.16 0.19
0.24 0.26 0.29
20 20 24
32 31 34
0.9967 0.9994 0.9984
Delay between t o oand beginning of flash. See text for dea Correlation coefficient of linear plot of In log ( I t / I f )vs t . Fraction of yield of semimethylene blue calculated from I , extrafinition of t o o . Delay between t o , and end o€ flash. polated t o beginning of flash. e Fraction of yield of semimethylene blue calculated from I t extrapolated to end of flash.
-36-38-
-40-42i
1
I
I
I
l
I
I
I
0 05 I O 15 20 25 30 35 40 45 t IIS
Figure 4. Decay of 3MBH2+monitored via absorption of semimethylene blue at 840 nm: 5.5 KM MB', 0.05 M FeCI2, 0.01 M HCI in 50 vlv % aqueous CH3CN. Experiment L33C.
prior to the flash, since neither M13+(So)or W 3 H 2 +absorb at 840 nm. Examples of such plots in which If was determined 4.8 and 6.8 half-lives after the flash, respectively, are shown in Figures 4 and 5. Linearity is excellent but values of log (I,/If)s40 at the time of the flash are significantly smaller than log (Im/If)f)840. Extrapolated values of log (It/If)840 equal to log (Ioo/If)&Io are achieved only at times (designated in Table I1 as too)significantly before the flash. Because flash lifetimes are significant on the time scale of the data, both the beginning and end of the flash are indicated on these figures. Lengths of intervals prior to the beginning and end of the flash required for to equal log &/If) are extrapolated values of log (It/If) designated in Table I1 as At, and At,, respectively. These intervals are due to formation of semimethylene blue during the flash by a process more rapid than the reductive quenching of relaxed 3MBH2+. It is readily shown that, for the case of sharply defined Io, log [Zm/I,,]Mo/log [Im/If]840 f, is equal to the fraction of the total yield of semimethylene blue which is generated by a process more rapid than quenching of 3MBH2+. In the present case, values off, calculated by using values of I extrapolated to the beginning (f,,J and end (f,,,) of the flash are available and are presented in Table 11. Since the rate constant for quenching of MB+(S,) in 50% AN is -3 X lo9 M-l s-l, the probability that each molecule of MB+(S1)will be quenched by Fe(I1) is about 5% in the presence of 0.05 M FeC12. (See section 1.) In the absence of quantitative information on the quantum yield of intersystem crossing, however, the values o f f , presented in Table I1 (all >> 5%) cannot be interpreted quantitatively. It can, however, be concluded from the data that quenching of MB+(S1)by Fe(I1) which is present
Figure 5. Decay of %BH2+ monitored via absorption of semimethylene blue at 840 nm: 5.5 KM MB', 0.05 M FeCI,, 0.01 M HCI, 0.25 M KCI in 50 vlv % aqueous CH3CN. Experiment L34D.
largely as Fen(H,O)t+ and to some degree as Fen(H20)6C1+ proceeds a t least in part by electron transfer. The data also suggest that the quantum yield for intersystem crossing from MB+(S1) to 3MB+ is significantly less than unity because several cycles of excitation of MB+(So)and decay (intrinsic + quenching) would be required to account for prompt yields of semimethylene blue in the ranges 11-15 and 24-3470 which have been observed.
Discussion It should be emphasized that the observed prompt yield of semimethylene blue cannot result from production of relaxed triplet dye in the quenching process. Such "extra" triplet would react with quencher at the same rate as does triplet produced by intrinsic intersystem crossing. There appears to be no alternative to the conclusion that reduction is an important pathway in the quenching of MB+(SJ by Fe(I1) present in the state studied in this research, i.e., Fen(H20),2+and possibly some Fen(H20)J!1+. The data do not distinguish between direct reduction, Le., electron transfer from Fe(I1) to MB+(S1),and more complicated reductive processes occurring in the encounter complex. For example, a reviewer has suggested that electron transfer may conceivably involve a vibrationally excited dye triplet resulting from induced intersystem crossing. The pseudo-first-order rate constants for reduction of 3MBH2+to semimethylene blue, kexpt,show a dependence on [Cl-] which has been studied in some detaiL3 Values of f,,l and f,,, presented in Table I1 are significantly larger with [Cl-] = 0.36 M than with [Cl-] = 0.11 M. Efficiency of net reduction in quenching of MB(S1) by Fe(I1) also appears to be enhanced by chloride but the data do not
414
J. Phys. Chem. 1900, 84, 414-419
support a unique explanation for this observation.
(5) J. Faure, R. Bonneau, and J. Joussot-Dubien, Photochem. Photobbi., 6, 331 (1967). (6) C. G. Hatchard and C. A. Parker, Trans. Faraday SOC.,57, 1093 (1961). (7) A Harriman, private communication. (8) M. D. Archer, M. I. C. Ferreira, G. Porter, and C. J. Tredwell, Now. J. Chim., 1, 9 (1977). (9) . . T. Ohno and N. N. Lichtin. J. Am. Chem. SOC.. submitted for publication. (10) M. Simic, P. Neta, and E. Hayon, J. Phys. Chem., 73, 3794 (1969). (11) H. N. Po and N. Sutin, Inorg. Chem., 7, 621 (1968). (12) J. H. Baxendale, P. L. T. Bevan, and D. A. Stott, Trans. Faraday Sm., 64, 2398 (1968). (13) J. P. Keene, E. J. Land, and A. J. Swallow in "Pulse Radiolysis", M. Ebert, J. P. Keene, A. J. Swallow, and J. M. Baxendale, Ed., Academic Press, London, 1965, pp 227-245. (14) J. Faure, R. Bonneau, and J. Joussot-Dubien, J. Chim. Phys., 65, 369 (1968). (15) J. P. Keene, E. J. Land, and A. J. Swallow, J. Chim. Phys., 65, 371 (1968).
Acknowledgment. This work was sponsored by the U.S. Department of Energy under Contract EY-76-S-02-2889, We express our appreciation to Professor Henry Linschitz for permission to use his laser flash-photolysis apparatus for part of this work. We also thank Dr. Takeshi Ohno for helpful discussions.
References and Notes (1) T. Ohno, T. L. Osif, and N. N. Lichtin, Photochem. Photobiol., 30, 541 (1979). (2) P. D. Wildes, N. N. Uchtin, M. Z. Hoffman, L. Andews, and H. Linschi, Photochem. Photobiol., 25, 21 (1977). (3) T. L. Osif and N. N. Lichtin, Photochem. Photobioi., in press. (4) R. Bonneau, P. Fornier de Violet, and J. Joussot-Dubien, Photochem. Photobioi., 19, 129 (1974).
Varlable Temperature NMR. A Method of Applying Proton Fourier Transform or Continuous Wave Spectroscopy to Nonisothermal Kinetics D. Le Botlan, M. Berry, B. Mechin, and G. J. Martin" Chimie Organique Physique ERA, CNRS 3 15, Universit6 de Nantes, Facult6 des Sciences, Nantes 44072, France (Received March 5, 1979; Revised Manuscript Received August 10, 1979) Publication costs assisted by the Universit6 de Nantes, CNRS
The authors describe a new way of obtaining activation functions for equilibrating systems in NMR studies of nonisothermal kinetics. The variable temperature NMR procedure is applied to the study of first-order and second-order reactions in the liquid state. The activation energy, E,, and the preexponential term, A , are obtained from curve smoothing of experimental sets of concentration and time values to theoretical expressions derived for first- and second-orderkinetics. A program computes the change of concentration, C, against time, t, and temperature, T , and iterates until convergence of theoretical and experimental values is reached. Relations between E, and A are discussed in terms of enthalpy-entropy compensation. The applicability, precision, and accuracy of the method are illustrated by using the results of experimental studies carried out principally on the syn-anti isomerization of a bridged ferrocenyl ion pair and the Diels-Alder electrocyclization of an ethoxydiene.
Introduction Except for line shape analysis of chemical systems in thermal equilibrium, the study of reaction kinetics by use of NMR as a tool for the detection of concentration changes against time is mainly restricted to special cases in which conventional kinetics techniques are not applicable. Probably because NMR is somewhat inaccurate compared to other methods of detection, and a kinetics experiment makes great demands with regard to spectrometer time, few kinetics data based on NMR determinations appear in the compilation of rate constants and activation functions. In conventional CW NMR, measurements of concentration change are subject to certain important limitations. The method is time consuming and has low sensitivity, and the temperature determinations are relatively inaccurate. FT spectroscopy has improved the sensitivity of the method and made automatic recording of spectra possible. Interesting developments in NMR kinetics have recently taken place with the application of continuous' and stopped2 flow techniques to the NMR detection of transient species. On the other hand, nonisothermal kinetics methods offer attractive possibilities since maximum information on activation parameters can be obtained with a single experiment and involve considerable saving in time and in reactant material. Nonisothermal kinetics has received a certain amount of attention from chemists in the field of 0022-3654/80/2084-0414$0 1.OO/O
solid state thermal analysis and related techiques. The feasibility of performing nonisothermal optical extinction measurements has been discussed recently,3bbut other applications which use the spectroscopic method are restricted, to our knowledge, to the polarimetric determination of activation functions in a first-order r e a ~ t i o n . ~ The proposed NMR method which involves simultaneous time and temperature variations is applicable in both CW and FT spectroscopies and greatly improves the rapidity and quality of kinetic measurements. The variable temperature NMR procedure implies continuous monitoring and measuring of a linear temperature increase by means of an electromechanical programmer. It gives good results as far as rapidity and accuracy in line shape studies of systems at equilibrium are concernedsa and we present its use in the study of chemical kinetics where the reagent concentration changes with time.
Experimental Section Temperature Determinations. The variable temperature device and the temperature programmer have been described p r e v i ~ u s l y . In ~ ~order to achieve a very slow programming rate, the ten-turn 100-Qpotentiometer may be replaced by a precision-grade50- or 25-3 potentiometer. The calibration of the temperature scale of the linear programs is achieved as follows. A platinum sensor (Air Liquide P 500) is inserted into the NMR probe and fixed 0 1980 American Chemical Society
