J . Phys. Chem. 1991,95, 8671-8676
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Measurement of the Enhancement Factor of Chemically Induced Dynamic Nuclear Polarization in the Photolysis of Dibentyl Ketone: Determination of the Spin Multiplicity of the Precursor Tomomi Sakata,+ Satoshi Takahashi,* Masahide Terazima,l and Tohru Azumi* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: November 14, 1990; In Final Form: May 23, 1991)
The enhancement factor of chemically induced dynamic nuclear polarization (CIDNP) in the photolysis of dibenzyl ketone is measured by using the rf pulse technique and temperature dependence of the CIDNP intensity and of the consumption rate constant. The experimental results of the enhancement factor and the yield of the cage product are then compared with the theoretical value under high magnetic field approximation. On the basis of the result, we conclude that the main precursor of the photolysis of dibenzyl ketone especially in chloroform solvent is the singlet state.
1. Introduction Chemically induced dynamic nuclear polarization (CIDNP) of radical reaction products is based on nuclear spin dependent singlet-triplet transitions in spin-correlated pairs of radicals and their spin-selective reactivities. From the sign of CIDNP, the spin state of the precursor of the radical pair, the signs of Ag and hyperfine coupling constant (hfcc) can be deduced by using the Kaptein rule.' Among these parameters, the signs of Ag and hfcc of the intermediate radical pair can be determined either by the ESR method or by theoretical considerations. Therefore, the sign of CIDNP is frequently used to determine the spin state of the precursor of the reaction. In such a case, however, we would b$ careful that the absolute magnitude of the enhancement factor (EF) of CIDNP greatly differs between the singlet precursor and the triplet precursor. Sometimes, E F of the triplet precursor can be 1 or 2 orders of magnitude larger than that of the singlet precursor. Because of this large difference in the magnitude of EF, determination of the precursor encounters difficulty, as was pointed out earlier by Blank, Mennitt, and Fischer? when singlet and triplet precursors appear simultaneously. Thus, even in the case where the contribution of the triplet precursor is minor, there is a danger that the sign of CIDNP indicates the triplet precursor. In order to avoid such misdetermination of the reaction precursor, not only the sign but also the magnitude of E F of CIDNP should be measured and the value should be analyzed with the aid of the theoretical value. Quantitative comparisons of experimental and theoretical E F are, however, usually very difficult. The difficulty mainly comes from the fact that the experimentally observed CIDNP intensity depends not only on the intrinsic E F due to the chemical reaction but also on nuclear relaxation. Furthermore, in the case where the reactant and the product is the same species, the quantitative measurement of E F becomes more difficult, because the reaction product cannot be separated from the reactant and, thus, the reaction kinetics cannot be measured quantitatively by the conventional way. It is probably by this reason that few works have been carried out along this line except the pioneering work of Blank, Mennitt, and Fischere2 In this work, we overcome the above difficulties by using the pulse technique suggested by Lawler and Barbara,3 and by measuring the temperature dependence of the CIDNP signal intensity and of the consumption rate of the reactant. We applied this new method to the measurement of E F in the photolysis of dibenzyl ketone (DBK), whose cage product is the same as the reactant. The experimentally measured E F is then compared with the theoretical value calculated by the theory proposed by Pedersen Resent address: N l T Electrical Communication Laboratories, Musasino 180, Japan. tPresent address: Institute for Molecular Science, Okazaki 444, Japan. 1 Present address: Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan.
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and Freed: On the basis of the result, we conclude that the main precursor of the photolysis of DBK is the singlet state in chloroform-d, although the previous studies on CIDNP sign^,^*^-" CIDEP spectrum and magnetic field effectlo have indicated the triplet precursor. Our conclusion is further substantiated by the triplet sensitizer experiment.
2. Experimental Section The experimental apparatus consists of a JEOL FX-100A Fourier transform N M R spectrometer (100 MHz for 'H), which is equipped with a high-resolution 'H probe modified to allow the sideway irradiation to a sample tube (4 mm 0.d.). As a light source, a 500-Wsuper-high-pressure mercury lamp was used, and the excitation light was passed through a water filter and a UVtransmitting (Toshiba UV-D33S) glass filter in order to avoid the sample heating. Under this condition, DBK is excited mainly by 3 13-nm light. In CIDNP experiments, saturation recovery (SR) pulse sequence, (Paat-vPobs-tobs)N, proposed by Lawler and Barbara3 was used. Here the saturation pulse (Pmt)and the observation pulse (POb)keep a flip angle of 90°. The error of Pmtand Pobsdue to the imperfect 90° flip angle was reduced by using a 90°,-900, pulse sequence instead of a single 90° pulse.14 DBK (Tokyo Kasei) was purified by recrystallization from chloroform. All sample solutions were deoxygenated by bubbling with nitrogen before photolysis. TEMPO (Aldrich) and acetone-d6 (Wako) were used without further purification. The concentration of DBK in chloroform-d was adjusted to 0.05 M. The concentration of TEMPO was 0.10 M for the radical trap experiment.
(1) Kaptein, R. J . Am. Chem. SOC.1972, 94, 6251.
(2) Blank, B.; Mennitt, F. G.; Fischer, H. Spec. Lec?.XXIIIrd In?. Congr. Pure Appl. Chem. 1971, 4, 1. (3)Lawler, R. G.; Barbara, P. F. J . Maan. Reson. 1980, 40, 135. (4) Pedersen, J. B.; Freed, J. H. J . Chem.Phys. 1972,61, 1517: Pedersen, J . B. J . Chem. Phys. 1977,67, 4097. (5) Turro, N. J.; Chow, M. F.; Chung, C. J.; Tanimoto, Y.; Weed,G. C. J . Am. Chem. SOC.1981, 103, 4574. (6) Turro, N. J.; Krautler, B. J . Am. Chem. SOC.1978, 100, 7432. (7) Turro, N. J.; Krautler, 8.;Anderson, D. R. J . Am. Chem. SOC.1979, 101,7435.
( 8 ) Turro, N. J.; Krautler, B. Acc. Chem. Res. 1980, 13, 369. (9) Turro, N.J.; Chow, M. F.; Krautler. B. Chem. Phys. Le??.1980, 73, 545. (10) Baretz, B. H.; Turro, N. J. J . Am. Chem. Soc. 1983, 105, 1309. (11) Turro, N. J.; Mattay, J. J . Am. Chem. SOC.1981, 103, 4200. (12) Grant, A. I.; McLauchlan, K. A. Chem. Phys. Leu. 1983, 101, 120. (13) Turro, N. J.; Paczkowski, M. A,; Zimmt, M. B. Chem. Phys. Le??. 1985, 114, 561. (14) Takahashi, S.;Terazima, M.; Azumi, T. Chem. Phys. 1990,142,69. (15) Robbins, W. K.; Eastman, R. H. J . Am. Chem. Soc. 1970,92,6077.
0 1991 American Chemical Society
8672 The Journal of Physical Chemistry, Vol. 95, No. 22, 1991
OCHptH@
Sakata et al. 9cand 9,denote the yield of the cage product (DBK) and that of escaped product, respectively. Since only two reaction paths exist from the intermediate radical pair, the sum of aCand @e should be unity. (i) The Apparent Enbancement Factor. As mentioned above, the enhancement factor (EF) of the reaction system in which geminate product is the same as the reactant cannot be determined because 610 cannot be measured. Therefore, we define the apparent enhancement factor as the CIDNP signal intensity normalized by the decrease of the reactant. Under the condition of 7 is very similar and supports the negligible contribution of the temperature dependence of 8. If we assume that the ratio of @ : to @pET can be calculated correctly by
the fraction of the triplet precursor in RP (f) can be estimated from eqs 24 and 25. In chloroform-d solution at 27 OC, @:/@.I is calculated as 20.3 and the f value is 0.240. The smallf’and f value clearly show that the main precursor of this photocleavage reaction is the singlet state. The same method is applied to the photocleavage reaction in toluene in order to investigate the solvent effect. The c value in toluene-d8 are -0.0282, and the quantum yield of the recombination and t h e y value a t each temperature are summarized in Table 111. Although the measured t value is larger than that in chloroform, the value is still one-tenth of the theoretical value tT. Furthermore, thef’value is, again, less than 0.128. Even though the estimated f value in toluene is rather large (f=0.747) under from eq 25 is valid, the assumption that the calculated the small t andf’indicate that the singlet precursor is considerably mixed in the photochemical reaction of DBK. The origin of the solvent dependence off is not clear at this moment. 5.3. Measurement of e and aCunder Triplet Sensitization Condition. For further support of the singlet precursor, a triplet sensitizer experiment was carried out. If the photolysis of DBK occurs mainly from the singlet state and the triplet precursor is slightly mixed, the enhancement factor under the triplet sensitization conditions should be close to cT. Since acetone is reported to be a triplet sensitizer of DBK,20e is measured in acetone-d6 solvent. Figure 5 shows that the e* vs 1 / K plot in acetone-d6 is also linear. t determined from the slope and the intercept is -0.1 10. Although the observed t factor is still smaller than the theoretically calculated one (eT), z in acetone-d6 is about 100 times as large as that in chloroform-d. This result supports that the main precursor in chloroform-d is the singlet state. Assuming tT and ts are valid again, the determined value of f’in acetone solution is 0.492. In acetone solution at 27 OC, the calculated value of aJ,s/O> is 26.8 and this leads to f = 0.963. The calculatedfvalue of about unity shows that acetone is a good triplet sensitizer of DBK. (20) Engel, P. S.J . Am. Chem. SOC.1970, 92, 6074.
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5.4. Relationship to the Previous ExperimentalWorks. Let us first compare our results with those reported by Blank, Mennitt, and Fischer.2 These authors obtained the enhancement factor E of -1080 in benzene solution. If we assume that the experiments were carried out at 300 K, the Boltzmann factor corrected enhancement factor t would be -9 X This value is not very different from our value (c = -3 X lV3)determined for chloroform solution. However, the conclusions are entirely different. Blank, Mennitt, and Fischer somehow considered that t was very large and thus concluded that the precursor was the triplet state. The reason why they considered so is not explicitly stated in the paper. As is discussed above, we regard the observed c value too small for the triplet precursor based on the theory of Pedersen and Freeda4 Numerous other studies have referred to the precursor of the photocleavage reaction of DBK. Turro et aI.l3 and Grant and McLauchlanI2 concluded the triplet precursor from the polarization of the CIDEP spectrum (E/A pattern; E, emission and A, absorption) in 2-propanol. Several groups also explained the observed magnetic isotope effect on the recombination yield in terms of the triplet p r e c ~ r s o r . ~Baretz et a1.I0 observed the magnetic field effect on the recombination process in micellar solution and the effect has been interpreted by the triplet precursor. Our conclusion that the singlet precursor mixes with a small amount of the triplet precursor disagrees with the conclusions of the previous works. We consider that the discrepancy between our results and the previous works might come from the medium effect. The CIDEP, isotope effect, and the magnetic field effect experiments have all been carried out in viscous solvents or micelles. In those conditions, the recombination probability from the singlet precursor (a:) is predicted to be very large and the main part of the escaped radicals, which is detected by the CIDEP or magnetic field effect, is expected to be from the triplet precursor, which cannot recombine without T S intersystem crossing. On the other hand, the created radical pair even from the singlet precursor can separate quickly in the nonviscous solvent (chlo-
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roform and toluene). Recently, Arbour and Atkinson concluded that the photocleavage reaction of DBK (methanol and cyclohexane solvents) is initiated from the excited singlet state on the basis of the results of picosecond transient absorption experiments.*' We consider that the very fast and spin insensitive detection of the intermediate radical is the reason why they can detect the radical pair of the singlet precursor. 6. Conclusions The enhancement factor of CIDNP in the photolysis of DBK is measured by using the rf pulse technique and temperature dependence of the CIDNP signal intensity and of the consumption rate constant. Although the sign of the CIDNP indicates that the precursor of the reaction is the triplet state, the experimentally determined enhancement factor in chloroform-d is 2 orders of magnitude as small as the theoretically calculated one under the assumption of the triplet precursor. The large discrepancy is due to the singlet precursor (-80%) mixed with a small amount of the triplet precursor in chloroform-d. This interpretation is confirmed by using the triplet sensitizer. This conclusion is not consistent with the conclusions or interpretations of most of the previous works. The inconsistency is tentatively explained by the medium effect. The assignment of the singlet precursor, however, agrees with the recent investigation of the picosecond transient absorption spectroscopy.
Acknowledgment. The present research was supported by a Grant-in-Aid for Scientific Research (No. 62430001 and 03640422) from the Japanese Ministry of Education, Science and Culture. Registry No. DBK, 102-04-5;TEMPO,2564-83-2; acetone-& 66652-4. (21)
Arbour, C.; Atkinson, G. H.Chem. Phys. f e r r . 1989, 159, 520.
SitcSpecHic Branching Ratios for H Atom Production Resulting from the 193- and 248-nm Photolysis of 1-Iodopropane Jeffrey L. Brum, Subhash Deshmukh, and Brent Koplitz* Department of Chemistry, Tulane University, New Orleans, Louisiana 701 18 (Received: December 27, 1990; In Final Form: June 10, 1991) This work systematicallyexplores "site-specific" H atom production in I-iodopropane photolysis experiments conducted under collisionless conditions. H and D atoms are used as labels to investigate the carbon site at which C-H (or C-D) bond cleavage occurs in the following series of selectively deuterated 1-iodopropanes: ICDzCH2CH3,ICH2CD2CH3,and ICH2CH2CD3. By measuring relative H/D ratios via two-photon (121.6 + 364.7 nm) ionization, competition among a-,&, and y-positions with respect to carbon-hydrogen bond cleavage is quantitatively compared. For excimer laser photolysis at 193 nm, no dominant site is identified. In contrast, 248-nm excimer laser photolysis clearly results in preferential bond cleavage at the @-carbon site, the estimated contributionsbeing 0.2 f 0.1,0.7 f 0.1, and 0.1 0.1 for the a,b, and y sites, respectively. Power dependence studies suggest that a two-photon mechanism contributes significantly to this site-specific behavior. A critical step appears to be 248-nm photon absorption by an intermediate, quite possibly the propyl radical.
*
Introduction Understanding and influencing photochemistry that involves competition between H and D atom reactive channels has been the subject of much recent research.'-' Efforts involving (I) (2) 803. (3) (4)
Hartke, B.; Manz, J.; Mathis, J. Chem. Phys. 1989, 139, 123. Vander, Wal, R. L.; Scott. J. L.; Crim, F.F. J . Chem. Phys. 1990, 92,
Sinha, A.; Hsiao, M. C.; Crim, F. F. J . Chem. Phys. 1990, 92, 6333. Sharer, N.; Satyapal, S.;Bersohn, R. J . Chem. Phys. 1989,90, 6807. (5) Bar, 1.; Cohen, Y.;David, D.; Rosenwaks, S.;Valentini, J. J. J . Chem. Phys. 1990. 93, 2146. (6) Cool,T. A.; Goodwin, P. M.; Otis, C. E. J . Chem. Phys. 1990, 93, 3714.
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HCzD,6and CHzCD2 systems have produced important results that show excellent agreement with theory. In the majority of these experiments, the focus is on the actual H versus D competition that takes place at chemically equivalent sites. In other words, the chemistry differs only by the presence of either a D or H atom. In a related but somewhat different approach, experiments in our laboratory have utilized H and D atoms to label chemically distinct reactive sites within a m o l e c ~ l e . ~ *By ~ (7) Satyapal, S.;Johnston, G.; Bersohn, R.; Oref, 1. J. Chem. Phys. 1990. 93, 6398. (8) Brum, J. L.; Deshmukh, S.;Koplitz, B. J . Chem. Phys. 1990, 7504.
0 1991 American Chemical Society
