R. D. Small and J. C.
828
% +
'j;
0.0-
c
a
-
0.7-
0
.-+
0"
0.6-
al
$ 0.5-
B
0.4al
0.5 5
P
250
270
290
310
330
W a v e l e n g t h , nm
Transient absorption spectra induced by a 10-ns electron pulse in neat liquid ammonia and ammonia-containing N,O: 0,initial absorption in ammonia at -75 O C ; 0, initial absorption in ammonia at 23 OC;0,initial absorption in ammonia-containing N20at 23 OC;A, absorption after 10 ps of electron pulse to ammonia-containingN20 at 23 'C. Figure 1.
"C as compared to the absorptions at 23 " C for the same wavelengths. These observations suggest that the transient absorption spectra for neat liquid ammonia are composed of absorptions of NH2 and absorptions due to another species. The absorptions due to another species decrease with decreasing wavelength, and their contribution to the transient absorption spectrum is very small at -250 nm. These absorptions are attributed to earn-.A combination of these absorptions of earn-with the previously published absorptions in visible and near-infrared3for em- constitute an absorption spectrum of earn-which has a maximum absorption in the near-infrared and its tail on the highenergy side extends up to UV. For neat liquid ammonia a t various wavelengths, the higher absorptions in the spectrum at -75 "C as compared to the spectrum at 23 "C (Figure 1) are due to a shift to higher energy of the
Scaiano
spectrum of earn-with decreasing temperatures5 For neat liquid ammonia at 23 "C, the transient absorption signals at 250 nm and 1.73 pm decay with the same second-order specific rate (cf. Results). These results confirm the conclusion reached above that in the transient absorption spectrum for neat liquid ammonia, absorption at 250 nm has only small contribution from earn-absorption. The results of decay kinetics of absorption signals a t 250 nm and 1.73 pm also confirm an earlier postulate5that in pulse radiolysis of neat liquid ammonia at 23 "C, em- decays by reacting with NH2. I t has been reported previously3 that NHz may be responsible for the peak at 550 nm in the transient absorption spectrum for ammonia-containing N20. The present results indicate that NH2 also absorbs in the UV. The visible and ultraviolet bands have also been detected in the absorption spectrum of NH2 in aqueous solution.6 Assuming that at 23 "C in neat liquid ammonia the initial absorption at 250 nm is only due to NH2, an extinction coefficient 1.1X lo3 M-l cm-' for NH2 at 250 nm is calculated from 0.022 the ratio of initial optical densities at 250 nm and 1.85 pm and 4.8 X lo4 M-' cm-I for the maximum extinction coefficient3of earn-at 1.85 pm. For the same dose per pulse, a t 250 nm and 23 "C, the ratio of initial optical densities for ammonia-containing NzO and neat ammonia is 2.3. It is reasonable to assume that for ammonia-containingNzO, the initial optical density at 250 nm is composed of optical density of NH2 (which is equal to the initial optical density at 250 nm in neat ammonia) and optical density of 0-(which is produced by the reaction of earn-with N20). At 23 "C for the above assumption, the combination of 1.1 X lo3 M-' cm-' (extinction coefficient of NH2 at 250 nm) with 2.3 (the ratio of initial optical densities at 250 nm for ammonia-containing NzO and neat ammonia) gives 1.4 X lo3 M-l cm-' for the extinction coefficient of 0-at 250 nm.
References and Notes (1) The Radiation Laboratory of the University of Notre Dame is operated under contract with the US. Energy Research and Development Administration. This is Document No. NDRL-1708. (2) J. Belloni, P. Cordier, and J. Delaire, Chem. Phys. Lett., 27, 241 (1974). (3) L. M. Perkey and Farhataziz, Int. J . Radiat. Phys. Chem., 7 , 719 (1975). (4) Farhataziz, L. M. Perkey, and R. R. Hentz, J. Chem. Phys., 60,4383 (1974). (5) Farhataziz and L. M. Perkey, J . Phys. Chem., 79, 1651 (1975). (6) P. B. Pagsberg. Riso Report No. 256, 1972.
Reaction of Type II Biradicals with Paraquat Ions. Measurement of Biradical Lifetimes Rlchard D. Small and Juan C. Scaiano"
'
Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received January 13, 1977) Publication costs assisted by the Division of Physical Research, U.S.Energy Research and Development Administration
A laser photolysis study of the Norrish type I1 reaction of y-methylvalerophenonein the presence of paraquat ions has allowed us to measure for the first time the lifetime of a type I1 biradical and the rate constant for the intermolecular reaction with paraquat ions. In methanol at room temperature TB = (97 f 15) ns and IZT = (4.6 f 1.0) X lo9 M-' s-' . The value of the lifetime is considerably shorter than values estimated previously from competitive studies. A fraction of the paraquat radical ions generated in the trapping step decay via reaction with the radical PhCOCH2CH2CMe2.
Reports providing direct measurements of biradical lifetimes are rather scarce. Platz and Berson2have recently succeeded in the measurement of absolute reactivities for The Journal of Physlcal Chemistry, Vol. 6 1 , No. 9 , 1977
the relatively long-lived trimethylenemethane biradicals. Closs and Buchwalter3 have examined the behavior of matrix isolated 1,3-cyclopentadiyl, which at 5.5 K has a
829
Reaction of Type I1 Biradlcals with Paraquat Ions
half-life of -30 min. Both studies were carried out using electron spin resonance spe~troscopy,~~~ a technique which has also been applied to the study of stable b i r a d i ~ a l s . ~ ' ~ To the best of our knowledge, no reports have been published on the determination of the lifetimes of highly reactive biradicals. In the case of type I1 biradicals generated in reaction 1,where the asterisk denotes a triplet state, a number of k
B 4CH3COPht C,H, B t PQ" authors have been concerned with the estimation of biradical lifetimes from competitive These type of studies can only measure the product of the rate of trapping (whether or intramolecularg) times the lifetime of the biradical ( ~ T T B ) . It is normally assumed that the value of kT can be taken to be identical with that for a free radical with similar substitution at the reactive center." Unfortunately, the free radical reactivities of some of the biradical traps used are not very well established,'a' leading to considerable uncertainties in the values of T B , In a recent preliminary communication'' we have shown that paraquat (l,l'-dimethyl-4,4/-bippidylium, PQ2') ions are very efficient traps of type I1 biradicals, i.e.
k, +
R
+ P Q . t H'
R
+ R +R, (or disproportionation)
R
t
k6
k
PQ+.2 products
R
I
\. 4-
where PQ'. represents the stable paraquat radical ion. The reaction is well known in the case of ketyl radicals.12 As part of the study of intermolecular reactions of type I1 biradicals we have examined the photochemistry of y-methylvalerophenone in the presence of paraquat ions using a time-resolved laser photolysis technique. The [PQ'.] vs. time profile contains information on both kT and T B , since the trapping reaction competes with the unimolecular processes which determine T B . Our approach does not require the direct observation of the biradical but provides accurate information on its kinetic behavior. We report in this paper the first measurement of the lifetime of a type I1 biradical, as well as the first value of a rate constant for an intermolecular reaction of these biradicals.
Results Aralkyl ketones undergo the Norrish type I1 process exclusively from the triplet state.13 In the case of ymethylvalerophenone the intramolecular abstraction takes place with a rate constant of 5.3 X 10' M-' s-' at room temperature.'* The short triplet lifetime (ca. 2 ns)I5makes it a convenient candidate for the study of biradical processes for two reasons: (a) triplet quenchin (whether chemical or physical) tends to be ineffi~ient?'~and (b) excited state processes are fast compared with biradical reactions, making the analysis of the time profiles straight forward (see below). Scheme I shows the proposed reaction mechanism with methanol as solvent. In alcohol solvents the quantum yields of cyclobutanols and fragmentation are known to add to onet6 it is therefore unnecessary to include in the mechanism the back reaction of the biradical to produce the ground state of the parent ketone which is common in nonpolar s01vents.l~
a
4
e
Flgure 1. Oscilloscope traces for the eneration and partial decay in trace a of paraquat radical ions ([PQ"] = 0.0022 M).
Paraquat radical ions have a strong wide absorption band in the visible region (Arnm = 603 nm, emax = 1.2 X lo4 M-' This is a rather "clean" and convenient region of the spectrum. We have used as monitor the 632.8-nm line from a He/Ne laser and the pulses from a nitrogen laser (337.1 nm) as an excitation beam. The simplicity and high stability of the system provide excellent signal-to-noiseratios, usually in the range of 200-500. The rise time of the detection system was -25 ns (see Experimental Section). Some typical traces are shown in Figure 1. According to the mechanism in Scheme I, and assuming that the duration of the laser pulse (ca. 8-10 ns) and the lifetime of the triplet ketone are short compared with the biradical lifetime, the following relations should hold -d[B]/dt=
(kg
f kq
+ kS[PQ"])[B]
d[PQ'*] / d t = k,[PQZ'] [B] - k,[R] [PQ'.]
(3)
(4)
if, in the early stages of the reaction
(5) The Journal of Physical Chemistry, Vol. 81, No. 9. 1977
R. D. Small and J. C.
830
/
5c
Time ( n s )
Figure 2. Plot corresponding to eq 9, for the following concentration of paraquat ions: 0,0.0088M; 0,0.00515 M; 0,0.0037 M; H, 0.0022 M; X and A, 0.0015 M, duplicate runs; A,0.00063 M; V,0.00025 M.
Scaiano
Figure 3. Plot corresponding to eq 9, where kWt = k3 + k4 + &[PQ2+] was obtained from the slopes in Figure 2. The point for [PQ2+] = 0.0088 M is a lower limit (see text).
then d[PQ+.]/ d t
k5[PQ2+][B]
(6)
from which
1In
____
A , - At
- ( k 3 + h 4 + k5[PQ2+])t
I
I
I
I
1000
2000
3000
4000
[ PQ++]-'
(9)
Figure 4. Plot corresponding to eq 11.
= aA,, where a is a proportionality constant
and
where A represents optical density, which is calculated from the oscilloscope traces in the appropriate manner. We attribute the decay which can be observed in the upper trace in Figure 1 to the reaction between PQ'. and R. The fact that this decay is undetectable in trace c justifies the assumption of inequality 5. The concentration of PQ'. radical ions does not decay to zero because a considerable fraction of the radicals R are removed by self-combination. Figure 2 shows the plots of In ( A , / A , - A,) vs. time (eq 9). We note that for the highest concentration of PQ" (0.0088 M) the 'reaction is quite fast and falls within the time response of our system; therefore, the slope from the plot is only a lower limit for kexpt. Figure 3 shows a plot of kexpt(i.e., the slope from Figure 2) vs. the concentration of paraquat ions. The intercept corresponds to 7B-l and the slope of k5 (see eq 9). The values obtained are TB = (97 f 15) ns and k5 = (4.6 f 1.0) x 109 M-I S-l. For laser pulses of constant intensity the values of A , provide an independent estimate of k S T B , since
k s [PQ" 1 k 3 + k 4 + k5[PQ2+] where @'(PQ'.) is the quantum yield of radical-ion production as measured before significant decay via reaction with R takes place (see trace c in Figure 1).Since @O(P&'.) The Journal of Physical Chemistry, Vol. 81, No. 9, 1977
Figure 4 shows a plot of Am-' vs. [P&'"]-'. From the ratio of intercept-to-slope we obtain k5TB = 345 M-l. The value of k 5 7 B predicted from the data in Figure 3 is k 5 7 ~= 445 M-'. The difference probably reflects the uncertainty which is common to this type of plots when the intercept is small. The dotted line in Figure 4 is the calculated line using the data from Figure 3 and assuming that the low concentration point is subject to less error than the others in the case of A,." Some quantum yield data for steady irradiation experiments have been published in a preliminary communication." For 0.0034 M paraquat ions in 1:4 water:acetonitrile @(PQ'-) = 0.24 and @ A c p / ~ ' ~ c p= 0.43, where @ACP and @'OACpare the quantum yields of fragmentation in the presence and absence of paraquat ions. Discussion Previous estimates of the lifetimes of type I1 biradicals derived from aralkyl ketones suggested a value close to 1 p s for solvated biradicals and a lifetime two or three times shorter in nonpolar solvent^.^^' Thiols7 and di-tert-butyl selenoketone' were used as biradical traps. Unfortunately, even when the values of kTTB are reliable, the values of TB reflect the uncertainties in the values of kT assumed. have examined the reIn the gas-phase O'Neal et action of the biradicals from 2-pentanone triplets with s. hydrogen bromide. The authors estimate T B =
a31
Reaction of Type I1 Biradicals with Paraquat Ions
This value is about two orders of magnitude longer than our value fgr y-methylvalerophenone in methanol. S i y e the rate constants for the reactions of free radicals wlth hydrogen bromide are well established,6s21the difference cannot be attributed to oncertainties in k p Two explanations are possible: (a) the assumption that biradicals react with the same rate constant as a free radical of similar substitution is incorrect; or (b) the lifetimes are in fact different, as a result of the differences in substitution and/or phase. Unfortunately, we cannot provide a conclusive answer to the question; however, we believe that (a) is unlikely to account for a difference in lifetimes of this mpgnitude. We favor explanation (b), and the reasons for this preference are discussed later. The combination of the data from the time profiles, A , values, and quantum yield measurements lead to a consistent picture of the photochemistry of the y-methylvalerophenone-paraquat system, providing meaningful explanations for the quantum yields, fate of the radical ions, rates of cyclization and fragmentation, and nature of the reactive intermediate. The values of k g and TB allow us to predict the quantum yields of reaction assuming that the quantum yield of biradical production is one (i-e.,no triplet quenching). The qudntum yield of acetophenone production will be given bY
Scheme I1 fragmentation k
T B L SB
G-
cyclization
a value if k7 from the initial rate and the value of [PQ'.] at the maximum. From this treatment we estimate k7 = (2.1 f 0.6) x 109 M-' 8-1 . We could not find any previous reports of the rate qf reaction of PQ'. with free radicals, but the bleaching effect is well k n o ~ n . ~ ~ ? ~ ~ One aspect that probably deserves to be emphasized is that our results can only be attributed to the reaction of the biradical. Let ub assume that we want to consider the triplet state as responsible for our results. The following contradictions become apparent: (i) Our kinetic analysis (see above) shows that the reactive intermediate has a lifetime of 97 f 15 ns; therefore, the triplet state would be required to have this lifetime. The evidence showing that the triplet lifetime is in the range of 2 ns is conclu~ive.'~,'~ (li) Even if we ignore all our time resolved results, 345 M-', the plot from Figure 4 would require kqTT where k, would be the rate of quenching and TT the triplet 2X lifetime; since the latter is known we estimate k, 10" M-' s-', a value considerable larger than the rate of c p for diffusion. (iii) From the value of @ p ~ c p / @ ~of~ 0.43 0.0034 M PQ" from quantum yield measurements, and using a Stern-Volmer treatment, we obtain kqrT 390 M-l, leading to the same contradiction as in the previous point. (iv) If we assume a reasonable high limit for k,, e.g., 10" M-' s-', we can estimate that the highest possible quantum yield of any triplet state reaction with 0.0034M PQ" is 0.06 which should be compared with the experimental value of 0.24. (v) Finally, the increase in quantum yields with decreasing triplet lifetime previously reported'' is not compatible with a triplet state process. Up to this point we have discussed the kinetic behavior of the biradical B, but have avoided the question of what are the factors which determine the lifetime of B. It is widely accepted that the biradical is produced initially in its triplet state and that intersystem crossing is a requirement for the occurrence of unimolecular reaction^.^^'^ Scheme I1 shows the basic set of possible reactions, where *B and 'B represent triplet and singlet biradicals respectively. In methanol k12
