363
J. Phys. Chem. 1993,97, 363-373
Wavelength Effects in the Photolysis of Ketones: Stereoisomerization and Magnetic Isotope l3c/'W Separation. A Probe for Adiabatic versus Diabatic Trajectories during Bond Dissociation E. N. Step,fd V. F. Tarasov3 A. L. Buchachenko,*and N. J. Turro'$+ Department of Chemistry, Columbia University, New York, New York 10027, and Institute of Chemical Physics, Academy of Sciences of Russia, Moscow 11 7334, Russia Received: February 12. 1992; In Final Form: October 6, 1992
The I3C enrichment in the photolysis of three ketones, dibenzyl ketone (DBK), (S)-(+)-a-methyldeoxybenzoin (MDB), and d,l-2,4-diphenylpentan-3-one(DPP), along with the photostereoisomerization of the latter two ketones was studied in SDS micelles as a function of the wavelength of the exciting light. Following the radical pair paradigm, triplet radical pairs (RP) of the corresponding acyl and benzyl radicals are generated during the photolysis of these ketones and are the key intermediates responsible for the stereoisomerization of MDB and DPP and for the 13Cenrichment of starting ketones. Two parameters, the recombination probability of the primary RP (P,)and the efficiency of 13C isotope enrichment (a),which are not quantum yields, do not depend on the amount of light adsorbed by the ketones, and can be measured with high accuracy by measuring only chemical yields, were employed as mechanistic probes. The magnitude of both P,and a for the photolysis of MDB in micelles and the magnitude of a for the photolysis of DBK in micelles and in viscous glycerol solution decrease as the energy of the exciting light increases. On the other hand, for DPP no wavelength dependence is found for Pr and only a small wavelength dependence is found for a. The results are examined in terms of adiabatic or diabatic trajectories of the dissociation of the bond which produccs the radical pair. The wavelength dependencies of Pr and a for DBK and MDB are explained under the postulate that, for higher energy excitation, the triplet excited ketones follow 3 ~ , ~ * - 3 u ,trajectories w to a greater extent than for lower energy excitation during a-cleavage and that these trajectories produce (excited) linear acyl radicals. The 3 u , states ~ of 3RPs when produced by high-energy excitation do not correlate directly with the ground states of ketones and therefore cannot directly recombine to regenerate the parent ketone. The participation of 3u,r states of a 3RPcan cause a reduction of both Pr and a by introducing pathways which compete with recombination and which do not exist for a ground-state 3u,uradical pair, which in turn possesses an adiabatic orbital correlation with the parent ketone. One of these pathways is the release of the excess energy through enhanced chemical reactivity of excited acyl radicals (for example, by dwhrbonylation) which provides a m a n s of shortening the lifetime of primary RPs and also provides a possible contribution to the decrease of P, and a for DBK and MDB with decreasing wavelength. The absence of the wavelength effect on Prand the abservation of a small wavelength on a for DPP are postulated as the consequence of fast nonadiabatic 3r,w*-3u,u*trajectories of this molecule, resulting from an initial higher velocity of the representative point and a faster rate of passage of the representative point along the reaction coordinate.
IntrodPCtion The formation of a carbon-carbon bond between two carboncentered radicals represents one of the simplest and most significant "elementary" steps in all of organic chemistry. Investigations of magnetic field and magnetic isotope effects on carbon-carbon bond formation involving geminate radical pairs (RPs) have provided important information on the subtleties of the elementary step of recombination of carbon radicals.14 A "snip" and 'knit" strategy' has been devised to investigate the details of how carbon-carbon bonds are formed from geminate radical pairs that are initially in a triplet state. The strategy involves (1) the creation of a geminate radical pair by snipping or cleavage of a carbon-carbon bond of a precursor substrate molecule (typically a ketone, eq 1) by the absorption of a photon, n
- P,\
1
t Columbia University. t Academy of
Sciences of Russia.
All other processes
(2) the measurement of parameters (e&, recombination probabilitf and the efficiency of isotope enrichment3) which track the knitting or combination of re-forming the same bond that was originally snipped, and (3) the measurement of these parameters as a functionof experimentalvariables(e& magnetic field strength and environment) and then the fitting of the results to a quantitativetheory which considersthe snip and knit proass from cradle to grave? The overall snip and knit process can be confined exclusively to primary geminate pairs by adsorbing the substratein micellar aggregates, such as those formed from sodium dodecyl sulfate (SDS)in aqueous solution (for the remainder of this report, all the snip and knit chemistry refers exclusively to geminate reactions of primary pairs in SDS micelles, unless noted specifically). From the simpleparadigm of eq 1,the value P,,the probability of combination of a primary geminate pair, is a potentially useful and intuitive mechanistic parametcfl~~ which clearly represents the competition between the knit process and all other competing processes. When the primary geminate pair is created in a singlet state (IRP), the value of Pr is often close to unity and is relatively lacking in mechanistic information. On the other hand, when the primary geminate pair is produced in a triplet state ORP), the intersystem crossing step, )RP 'RP, requires a magnetic interaction and may ku" rate determining and, before the
-
0022-3654/58/2097-0363$04.00/0 (B 1993 American Chemical Society
364 The Journal of Physical Chemistry, Vol. 97, No.2, 1993
Step et al.
All other proceases
body of literature is available reporting such investigations.)-sJz--'8 In this report, we are concerned with the energy content and the initial electronic-vibrational state of the precursor triplet ketone on P r and u. Experimentally, we have measured the magnitudes of a and Pr as a function of the adsorbed energy for two substrates: (5')-(+)-a-methyldeoxybemin (*MDB) and d,l-2,4diphenylpentan-3-one (41-DPP). In addition, we have measured the magnitude of a as a function of excitation energy for dibenzyl ketone (DBK) and themagnitudeofthecageeffect oncombination of secondary RPs as a function of excitation energy for two substituted DBKs. In this report we show how the results of measurement of Pr, u,and the secondary cage effect arc consistent with a paradigm which interprets the observations in terms of the adiabaticor nonadiabatic (diabatic) trajectories of the a-cleavage step as a function of excitation energy and substrate structure.
The paradigm of eq 2 represents one of the simplest sequences necessary to analyze the results of spin-selective carbon-carbon formation, Sequential to light absorption, each reactive intermediate possesses an "escape" pathway that competes with the motion along the reaction coordinate which includes the snip and knit processes: (1) Photochemical excitationof a ketone produces a single n,r* state, which intersystem crosses to a triplet n,r* state (efficiency &T) or undergoes other competitive processes such as fluorescenceor radiationless deactivation. (2) The triplet n,r* state undergoes a-cleavage (primary efficiency ipm) to produce a primary, geminate radical pair, 'PRP, in a primary cage, or undergoes other competitive processes such as phosphorescence or radiationless deactivations. (3) The 'PRP undergoes a diffusional excursion to yield a separated geminate radical pair upon which magnetic perturbations act to produce a spin-mixed separated pair, 1.3PRP,in competition with processes such as decarbonylation and scavenging. (4) The separated primary geminate pair 'v3PRP undergoes return to a secondary cage. The singlet component of the 's3PRP is assumed to react in the secondary cage with unit efficiency to form the originally cleaved carbon-carbon bond, in competition with other processes such as scavengingor decarbonylation. We assume, as isjustified by considerable results in the literature,8p9 that steps 1 and 2 occur near unit efficiency, and we have designed experiments6to determine the parameter Pf (which directly monitors the probability of the recombination step in q 2) and to determine other parameters which indirectly monitor the probability of the recombination step. The recombination probability, Pr, may be monitored directly by measuring the extent of photoisomerization1-6Jov1 (photoenantiomerization6Jo or photodiastereomerizationlJ1)of the precursor ketone which results from the knitting of the originally snipped carbon-carbon bond. Magnetic interactions operating on the triplet geminate pair may be related to Pf through the paradigm of the magnetic isotope effect (MIE)'t4 and the measurement of the selectivity of isotopic enrichment, a. The hyperfine induced intersystem crossing of the 'RP (by, e.&, 13C) increases the recombination probability Pr* of this "magnetic" 'RP, compared to the probability, Pr, of a "nonmagnetic" pair ( I T ) . The key parameter for MIE is the efficiency of isotope selection, a,which can be quantitatively related to Pr and Pf* through the paradigm of q 2. The relationshipof the measurable parameters u and Pr and Pr* allows for an indirect evaluation of Pr* or any third parameter if two others are known.7 The operation of MIE and the efficiency of isotope selection are determined by a number of variables such as the restriction and rates of diffusion of the the rates of competing spin-selectivelsor spin-nonselective16processes, the initial spin multiplicity, the initial radical pair separation, the extent of hyperfine coupling in the RP,17J8etc. A general theory exists which allows for the investigation of carbon-carbon bond formation through measurements of u (13C/12Cisotope separation) and P r as a function of these variabk2 Indeed, a large
Experiwncrl Section Dibenzyl ketone (DBK, Aldrich) was recrystallized from ethanol. DBKJ'C was prepared from phenylacetic acid19 (l3COOH, K h i i k t i v , Russia). Methyldeoxybenmin(MDB) was prepared by methylation of deoxybenzoinZO(Aldrich). (S)-(+)a-Methyldeoxybenzoin was prepared as described in the literatureF1 (S)-(+)-a-Methyldeo~ybenzoin-carbonyl-~~C was synthesized according to the same procedure from ~-a-alanine-'~C (Cambridge Isotop). d,l-2,4-Diphenylpentan-3-one (DPP) was prepared as described in the literature,'." as were o-MeDBK and ~ M C D B K Sodium . ~ ~ dodecyl sulfate (SDS, Bio-Rad, electrophoresis purity), glycerol (Aldrich, spcctrograde),and rert-butanol (Fisher, certified) were used as received. The concentration of SDS in water was 0.1 M, and the concentrations of DBK and otherketoneswere4.8 X lt'and(1.7-3.4) X lt3M,respectively. The solutions were bubbled with Ar before and during the photolysis. Photolysisof DBK was carried out in a thermostabilized quartz cell with stirring at 270 & 1 K in a 1:1 volume mixture of glycerol with tert-butanol or at room temperature in SDS micellar solution withtheemission of a Xe 300-W high-pressurelamp (ILC) passed through a Kratos GM-252 double monochromator. Photolysis of MDB in SDS aqueous solution was performed at room temperature in a quartz cell with the emission of a Xe-Hg 1-kW lamp (Hanovia) passed through a LP-345 glass filter (A > 345 nm) or on a Rayonet photochemical reactor with a different set of lamps for 254-, 300-, and 350-nm emission. Although some experiments were conducted under conditions such that two ketones occupied a micelle, because of the short lifetime of the radical pairs in the systems studied, excitation of a ketone in a micelle containing a pair has a very low probability and can be safely neglected. Photolyses of DBK, p and o-MeDBK, and d,l-DPP in SDS aqueous solution were carried out with the emission of a Xe-Hg 1-kW lamp (Hanovia) passed through a LP-320 glass filter (A > 320 nm) or on a Rayonet photochemical reactor with sets of lamps emitting 254- or 300-nm emission. Aqueous micellar solutions of ketones (before and after photolysis)were extracted with an equal volume of ethyl acetatemethylenechloridsmixture( 4 1 by volume) containingan internal standard (typically hexadecane). The organic phase was used for measuring conversion, the diastereomeric excess, and isotope enrichment. CD spectra of optically active *MDB were recorded in aqueous SDS solution without extraction. To ensure the appropriate optical density, the solutions were diluted by 4 times with 0.1 M aqueous SDS solution. The extent of conversion of ketones was measured by gas chromatography employing a Hewlett Packard 5890 capillary GC with a 25-m HP-1 column. In the case of d,I-DPP, the conversion as well as the diastereomeric excess was measured on a 25-m Carbowax 20 M capillary column. Isotope enrichment was measured by mass spectrometry employing a HP-5988A GC-MS (also 25-m HP-1 capillary
knit stepcan occur, an interesting sequenceinvolving the interplay of spin dynamics, molecular dynamics, and chemical dynamics is required. When this is the case, since the 'RP is assumed to be inert to carbon-carbon bond formation and since the 'RPis assumed to be very reactive toward carbon-carbon bond formation, the simple paradigm of q 1 must be extended to eq 2, which is the working 'chemical" paradigm employed in this paper.
'PRP
Wavelength Effects in Ketone Photolysis column) in the E1 mode. The starting ketones for the isotope enrichment experiments contained about 25% 13Cin the carbonyl group. In the mass spectra the ratio b,, = @+- 13C)/(M+ I T ) was measured as an experimental parameter. For the calculations of the efficiencies of isotope separations the 13C enrichment was considered to occur only in the carbonyl position; Le., the I3C/W isotope ratios in all positions, except for carbonyl position, were assumed to be constant. The CD spectra of *MDB were recorded on a JASCO 5-500 A CD spectrometer in the region of 280-400 nm. The shapes of the CD spectra were constant through all experimental conversions of the photolyses. The signal intensities were measured at the maximum of the absorptive peak (regularly at 320.2 nm).
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 365
TABLE I: Expcriwslbl Data for W I Z C Isotope Separation dpriao DBK Photolydr io I1:l Mixture of c ~ dtb~tert-Buc.nol8t d 270 K WMIM f f m t Wavekqths of Excitation wavelength, 1log -log nm sample S foxp S (1 -f) 260
260-1 260-2 260-3 260-4 260-5
1.0783 1.1159 1.1560 1.2286 1.2309
0.774 0.640 0.462 0.338 0.319
0.0327 0.0476 0.0629 0.0894 0.0901
0.0854 0.1592 0.2899 0.4070 0.4316
286
286- 1 286-2 286-3 286-4 286-5
1.0300 1.1080 1.1604 1.2537 1.3197
0.793 0.603 0.458 0.310 0.221
0.0128 0.0445 0.0646 0.0981 0.1205
0.09 14 0.1876 0.2928 0.4391 0.5709
308
308-1 308-2 308-3 308-4 308-5
1.0496 1.0917 1.1620 1.3522 1.6375
0.798 0.653 0.490 0.249 0.105
0.0210 0.0381 0.0652 0.1310 0.2142
0.0828 0.1576 0.263 1 0.5120 0.8336
325
325-1 325-2 325-3 325-4 325-5
1.1107 1.2521 1.4329 1.5704 1.7598
0.646 0.420 0.250 0.151 0.111
0.0456 0.0976 0.1562 0.1960 0.2455
0.1568 0.3073 0.4932 0.6860 0.7890
RdtS
MersurementS of tbe Isotopic Selectivity Parameter, a. The assumed initial photochemical act in the photolysisof all ketones under investigation is a-cleavage of the C 4 0 bond of the triplet excited state of the molecule to form the 3PRPof related acyl and benzyl radiixls11*u-2s(see eq 2). Due to the MIE, RPs which contain the magnetic isotope 13C recombine faster than those which contain 12C. As a result, ketone molecules regenerated by recombination of RPs during the photolysis are enriched with isotope 13C. For RPs formed in the photolysis of MDB, DBK, and DPP the acyl radicals containing a 13Cin the carbonyl group possess a very large HFI constant A = 120-1 30 G which justifies the assumption that the 13Cenrichment takes place mainly at the carbon of carbonyl group.17J8 The measured extent of 13Cisotope enrichment as a function of conversion was analyzed in terms of the established equation for isotope log s = (1 - a) log (1 -f*) (3) In eq 3, the extent of enrichmenet, S = 6co/bco0,bco = [13C/ W ] , bcoo = [13C/12C]~ (all for the carbonyl group of ketones); a n d p is the conversion of magnetic ketone where From the experimentaldata (fa,is the experimentally determined conversion of the starting mixture of magnetic and nonmagnetic ketones, tic0 and bcoo are measured by GC-MS, assuming that all changes in baupare caused only by the 13Cenrichment in the carbonyl group and the 13C/12Cisotopic ratios for all carbons besides carbonyl the same as in [12C]ketone)and eqs 3 and 4,the important parameter, a,the coefficient (the efficiency) of isotope selection, may be evaluated. Table I contains the experimental data for the 13C/12Cseparation during DBK photolysis with diferent wavelengths of excitation in a 1:l mixture of glycerol with tert-butanol, and Table 11, that in SDS micellar aqueous solution. The measured I3Cenrichment, S,is plotted as a function of conversions,f, for the photolysis of DBK in glycerol and SDS micelles at different wavelengths, according to eq 3 as shown in Figure 1. The excellent fits of the data to eq 3 allow the extraction of the efficiencies of 13Cenrichment, a. The results presented in Tables I and I1 and Figure 1 establish that a (a logarithmic parameter), in the photolysis of DBK, shows a significant sensitivity to the energy of the exciting radiation. Decreasing the wavelength of excitation from 325 to 260 nm decreases a from 1.302f 0.012 to 1.195f 0.018 and from 1.212 f 0.006 to 1.137 f 0.005 for the glycerol-tert-butanol and SDS micelle systems, respectively. The experimental data of 13C/W isotope separation during MDB photolysis with different wavelengths of excitation in SDS aqueous solution are presented in Table 111. Photolysis of MDB in SDS micelles with different wavelengths, as in the case of DBK, results in wavelength-dependent values of a: for X > 345 nm a = 1.196 f 0.003,for X = 300 nm a = 1.117 f 0.006, and
a
1.195
0.018
1.219
0.009
1.258
* 0.005
1.302
0.01 2
TABLE II: Experimental Data for W/W Isotope sCp.rattOn during DBK Wotolysis in SDS Aqwom Solution dtb D i f i m t WIVelenpthS Of hCihti00 wavelength, 1log -log nm sample S fap S (1 -f) a 260
260- 1 1.0548 0.656 260-2 1.0191 0.492 260-3 1.1630 0.324 260-4 1.2692 0.146
0.0232 0.0379 0.0656 0.1035
0.1662 0.2805 0.4422 0.7620
283
283-1 283-2 283-3 283-4
1.0466 1.1001 1.1574 1.2476
0.717 0.502 0.342 0.193
0.0198 0.0414 0.0635 0.0961
0.1301 0.2693 0.4204 0.6462
308
308-1 308-2 308-3 308-4
1.0542 1.1039 1.1762 1.2736
0.734 0.562 0.385 0.239
0.0229 0.0429 0.0705 0.1050
0.1 176 0.2191 0.3638 0.5468
325
325-1 325-2 325-3 325-4
1.0610 1.1264 1.2103 1.2928
0.749 0.526 0.258 0.222
0.0257 0.0517 0.0829 0.1115
0.1068 0.2417 0.3869 0.5747
1.137 A 0.005
1.149
0.005
1.192
0.004
1.212
* 0906
for A = 254 nm a = 1.094 f 0.004,respectively (Figure 2). For MDB the relative difference between values of a under different wavelengths is even larger than in the case of DBK. In Table IV one can see the experimental data for 13C/1*C isotope separation during the photolysis of DPP in SDS micella. The value of a for the photolysis of DPP (Figure 3), in contrast to that for DBK and MDB, shows only a small dependence on wavelength: for A > 320 nm a = 1.1 10 f 0.004, for X 300 nm a = 1.088 f 0.003,and for X = 254 nm a = 1.079 f 0.004, respectively. (Generally, smaller a values for DPP under similar conditions of photolysis are the result of a fast decarbonylation reaction of sec-phenethylacyl ~ C = I4.9 X lo7 s-*, which effectivelycompeteswith recombination of a primary RF'.) Thus, the wavelength effect on 13Cenrichment in DPP is less than that for DBK and MDB. Mtof tbe Recombhation Probability Parameter, Pp Photoraccmization6Joas well as photodiastereomerizationl,ll (see Scheme I for *MDB and Scheme I1 for 41-DPP) are related to
-
Step et al.
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 0.20
0.30 DBK in glycerd
325
f1
MOB in SDS
0.25
0.20 0.15 0.10 0.05
0.00 0.0
0.2
-
0.6
0.4
0.0
1 .o
0.8
log (1-r)
0.2
0.4 0.6 -log (1-f')
1 .o
0.8
Figure 2. 1sC/12C isotope selection during the photolysis of MDB with different wavelengths of excitation in SDS micellar solution in terms of eq 3.
TABLE Iv: Experimental Data for lJC/W Isotope Separation d U D P P Photolysis in SDS Aqseope Mution with Di e m t Wavelengths of Excitation
3
o.oc 1
wavelength, nm sample S 254 254-1 1.0695 254-2 1.0995 254-3 1.1281 254-4 1.1622 254-5 1.2101
0.04 0.0:
1-
j &
log S
0.394 0.289 0.206 0.131 0.079
0.0299 0.0412 0.0523 0.6530 0.0828
0.3826 0.5083 0.6482 0.8350 1.0450
-log (1 -f)
U
1.079 0.004
O.O( 0.0
0.2
0.4
0.6
0.8
300
300-1 300-2 300-3 300-4 300-5
1.0841 1.1107 1.1322 1.1759 1.2181
0.410 0.291 0.207 0.147 0.0958
0.0351 0.0456 0.0539 0.0704 0.0857
0.3611 0.5020 0.6440 0.7807 0.9557
>320
320-1 320-2 320-3 320-4
1.0291 1.0592 1.1949 1.3222
0.755 0.589 0.186 0.063
0.0125 0.0250 0.0773 0.1213
0.1127 0.2115 0.6648 1.1122
-log (1-f') Figure 1. lac/'% isotope selection during the photolysis of DBK with different wavelengths of excitationin terms of cq 3; (a, top) in glyceroltert-butanol mixture; (b, bottom) in SDS micellar solution.
TABLE Ilk Experimental Data for '3C/lY: Isotope Seprntion during MDB Photolysis in SDS Aqueous Solution with Different Wavelength of Excitation wavelength, nm samDle
1-
S
L.
log S
-log (1 -P)
0.0184 0.0249 0.0355 0.0467 0.0538 0.0662
0.2339 0.3062 0.3935 0.4865 0.5905 0.7250
1.088i 0.003
1.110 0.004 U
~
254
254-1 254-2 254-3 254-4 254-5 254-6
1.0433 1.0591 1.0853 1.1136 1.1318 1.1646
0.566 0.474 0.381 0.302 0.235 0.169
/ 1.094& 0.004
300
300-1 300-2 300-3 300-4 300-5
1.0697 1.1003 1.1089 1.1460 1.1797
0.593 0.423 0.366 0.286 0.226
0.0293 0.0415 0.0449 0.0592 0.0718
0.2059 0.3447 0.4044 0.5016 0.5951
350
350-1 350-2 350-3 350-4
1.0987 1.2322 1.3647 1.5332
0.598 0.297 0.161 0.085
0.0410 0.0907 0.1350 0.1856
0.1921 0.4592 0.6931 0.9373
0
1.117 0.006
0'04 5ic 0.00 0.0
0.2
0.4
0.6
0.8
1.0
1.2
-log (1.1.) 1.194 i 0.004
lacenrichment in that the photoisomerizationsand the isotopic enrichment both involve a critical RP recombination step. The efficiency of photoracemization is measured by a parameter 6, which, in analogy to the parameter a, may be derived from experimental data by measurement of the extent of photoisomerization as a function of conversion6 according to q 5 , whose
analogy to eq 3 is apparent. In q 5, 2 and ZOare the optical or diastereomeric purity of the ketone before and after the photolysis (analogous to the extent of lacenrichment, S, in q
F'jgure 3. 13C/12Cisotope selection during the photolysis of DPP with different wavelengths of excitation in SDS micellar solution in terms of cq 3.
3), f is the extent of conversion, and 6 is the efficiency of racemization or diastereomerization. The value of 0 is directly related to the probability of recombination, Pr,of RP under our experimental conditions by q 6.6
The extent of photoenantiomerization was measured directly by circular dichroism (CD) of the solution during the photolysis. Due to the decrease of the concentration of the starting ketone, the measured intensity of the CD signal of the solution duMg
Wavelength Effects in Ketone Photolysis
The Journal of Physical Chemistry, Vo1. 97, No. 2, 1993 367
SCHEME I: wotorrcemizrtioa of (R)-d (S)-MethyMeoxybenzoin (*MDP) hv
p'+m
O
-
1s'
3s'
0
1
SI
i U
0
c
1.5
-log (1-9 Figure4. Photorecemization of (S)-(+)-MDB duringthe photolysis with different wavelengths of excitation in SDS micellar solution in terms of eq 5.
t
'R'
hv
1 .o
0.5
0.0
'R'
J
RI 0.15
SCHEME Ik Photodiistereomeriutionof
-
2,+DipbenylPeat~1-3--0~ (WDPP)
hv
Phc&(&ph
0.10-
'(d.l-DPP)*
&I-DPP
0.05
-
1
1I&
0.00 0.0
0.2
0.4
0.6
1
1 .o
0.8
-log (1-9 Figure 5. Photodiastereomerizationof $I-DPP during the photolysis in
SDS micellar solution with different wavelengths of excitation in terms of eq 5.
TABLE VI: Experimental Data for the Diastereomeriution during W D P P Photolysis in SDS Aqueous Solution with Different Wavelengths of Excitation wavelength, -log -log nm sample 2/20 1-/ (Z/Zo) (1-A B
mcwo-DPP
~~
TABLE V Experiment.l Data for the Enrntiomeriution during *MDB Photolysis in SDS Aqueous Solution with Different Warelenetas of Excitation wavelength, -log -log nm sample A/Ao 1-f (A/&) (1-fl b+ 1 254
254-1 254-2 254-3 254-4
0.387 0.150 0.0543 0.0144
0.580 0.298 0.160 0.062
0.412 0.823 1.242 1.842
0.236 0.525 0.795 1.205
300-1 300-2 300-3 300-4
0.317 0.591 0.499 0.200 0.461 0.699 0.119 0.362 0.924 0.0406 0.204 1.392
0.228 0.336 0.441 0.059
254
254-1 254-2 254-3 254-4 254-5
0.986 0.943 0.920 0.886 0.855
0.896 0.727 0.641 0.533 0.419
0.0067 0.0255 0.0362 0.0526 0.0680
0.048 0.1385 0.193 0.273 0.378
300
300-1 300-2 300-3 300-4 300-5 300-6
0.951 0.928 0.890 0.860 0.828 0.708
0.764 0.648 0.537 0.431 0.339 0.156
0.0218 0.0325 0.0506 0.0655 0.0820 0.150
0.117 0.188 0.270 0.366 0.470 0.824
>320
320-1 320-2 320-3 320-4 320-5
0.963 0.927 0.899 0.867 0.823
0.830 0.657 0.497 0.362 0.261
0.0166 0.0327 0.0458 0.0620 0.0846
0.081 0.182 0.304 0.441 0.583
*
1.560 0.054
300
0.181
*
2.026 0.035 350
350-1 350-2 350-3 350-4
0.372 0.245 0.157 0.0853
0.626 0.516 0.410 0.328
0.429 0.611 0.805 1.069
0.203 0.287 0.387 0.484
*
2.174 0.061
the photolysis, A, is related to @ by eq 7. Table V contains the experimental data for *MDB photoenantiomerization during
1%
(44) = (B + 1) 1%
(1 -A
*
0.186 0.006
(7)
photolysis in SDSmicelles with differentwavelengthsof excitation. Figure 4 shows that a plot of the efficiencyof photoracemization of (S)-(+)-a-methyldeoxybenmin(*MDB) under photolysis in SDS follows eq 7 very well and that the value @ (and therefore P,) as the value of a is wavelength dependent.
* 0.003
0.177 h 0.005
In complete analogy to the use of photoenantiomerization to measure the extent of recombination of the 3RP (through the parameters @ and P,) for *DMB, the extent of recombination of the 3RP for DPP may be evaluated for the photodiastereomerization of d,l-2,4-diphenylpentan-3-one (DPP) in terms of eq 5.' Table VI contains the experimental data, and Figure 5 shows that a plot of the efficiency of photodiastereomerizationof DPP during photolysis in SDS follows eq 5 very well and that the value of @ (and therefore P,) is wavelength independent, although the
Step et al.
368 The Journal of Physical Chemistry, Vol. 97, No.2, 1993
F
TABLE M: R M b h t i O O Rd#bilitia (PI)and tbe EffiCicnCia Of Isotope M a t (a)Of Photolvsis of +MDB and U D P P in SDS Miak ketone MDB
P,
DPP
a Pr
300 nm
350 nm9
0.359 0.017 1.094 0.004 0.157 h 0.007 1.079 h 0.004
0.506 h 0.012 1.117 h 0.006 0.153 h 0.003 1.088 h 0.003
0.540 0.022 1.196 h 0.003 0.151 0.006 1.110 0.004
*
a 9
254 nm
u r ,
433.5
d dprinl tk
%
* 7.3)
Aa,% -(52.0
* 3.5)
-(28.0
7.3)
0
*
For DPP X > 320 nm.
TABLE WI: Cage Effect during Photolysis of o and pMeDBK in SDS at Different Waveleugtb8 of Excitation cage effect >320 nm Acage, 5% 300 nm 254 nm ketone &MeDBK 0.543 i 0.022 0.410 0.008 0.389 f 0.018 39.6 i 10.4 pMeDBK 0.378 i 0.005 0.343 & 0.006 0.303 i 0.031 24.8 i 12.1
*
value of a is slightly wavelength dependent. Table VI1 summarizes the values of P, for RPs of benzoyl-sec-phenethyl and sec-phenethylacyl-sec-phenethylradicals in SDS micelles along with values of a and the wavelength effect on Pr and a. For a, the wavelength effect is defined as a parameter Aa = [(a1- 1) - (ao- l)]/(d- 1) where a0is the efficiency of isotope selection at the long wavelength, X > 345 nm for MDB or X > 320 nm for DBK, and al is the efficiency of isotope selection at 254 nm. The value (a- 1) has a clear physical sence according to eq 3, being the slope of the dependence of enrichment versus conversion in a logarithmic scale. For Pr, the wavelength effect is defined as a parameter AP, = (Prl - P,O)/PF, where P,O refers to P r at long-wavelength photolysis and Prl to the photolysis at 254 nm. Analysis of the data in this fashion shows that there is a much larger wavelength effect on Pr in the case of the RP from MDB, and virtually no effect of wavelength on P, in the case of the RP from DPP. Similarly,for DPP there is a much smaller wavelength effect on the value of a as a function of wavelength than in MDB. Meururement of the Cage Effect for Secondary Pairs. Since theDBKmoleculedoesnot possessachiralcenter in thea-position to the carbonyl group, it is not possible to measure directly P, of RPof phenacyl and benzyl radicals in the manner that is possible for 'MDB and DPP. We have therefore sought an indirect parameter that might be related to P, and to the nature (orbital and electronic state) of the RP, formed during the photolysis of DBK, that could be measured as a function of excitation wavelength. For this parameter, we selected the cage effect for diphenylethane formation from secondary radical pair recombination from photolysisof the nonsymmetrical molecules p and o-MeDBK (eq 8, Ar = pmethylphenyl or o-methylphenyl, AKH~~
fi0
~
hv ~ pArCHzCHzAr h + ArCH2CHzF'h + PhCHzCHzPh AA
Cage effect =
AB
-
AB (AA
BB
+ BB)
AB + AA + BB
respectively). It has been shown that in the case of micellar solution the cage effect of the secondary aryl-benzyl RP from the photolysis of 0- or pMeDBK is related to the probability of recombination of the primary arylacyl-benzyl RP within the micellar cage.22.28The CIDNP spectra of pMeDBK in SDS micelles showed polarized signals of AB, but not AA and BB products28(see eq 8), which supports the relationship between P, of primary and secondary RPs (cage effect measured by eq 8). For the photolysis of p- and o-MeDBK in SDS solution the wavelength dependence of the cage effect according to q 8 was measured (Table VIII). From Table VI11 it can be seen the trend for the wavelength effect on the cage effect is opposite that for P,of the primary RP of M D B Le., the cage effect for the secondary RP increases as the wavelength of photolysis decreases. The relative increase in the cage effect (Acage) is of the same order of magnitude as the
decrease of Pr (AP,)for MDB. From the data in Tables I and I1 one can calculate A a for the photolyses of DBK with wavelengths 325 and 260 nm, which are similar in SDS and glycerol, and A a = -35.3%. Discussion
Pa", Purmetem,and Variables. The working paradigm which servesas the basis for discussionof the results is the radical pair This model emphasizesthe interplay of chemical, spin, and molecular dynamics in determining the probability of recombination, P,, of geminate triplet radical pairs and has been extended to include the combination of triplet radicat pairs in micelle~.l*~ The mode1.b characterized by a strong intuitive physicalbasis and a tremendous experimentalsupportin the fslds of CIDNP,29*30 magnetic field effects,' and magnetic isotope Within theradical pairmodel wenow applythesurface correlation paradigm for the mechanism of photochemical a-cleavage of k e t ~ n e s . ~This l * ~ ~model emphasizes the orbital correlation of the a-cleavage reaction through the trajectories of a representative point on energy surfaces and assumes that only triplet states are involved in the primary a-cleavage of ketones. As discussed above, the direct measurement of P, is possible by simple measurement of photoenantiomerization (of MDB) or photodiastereomerization (of DPP) and application of eqs 5-7. Although, for DBK, recombination cannot be monitored by isomerization, it is possible to obtain indirect information on P, through the measurement of the experimental parameter for 13C isotopic selectivity,' a,and through the measurement of the cage effect of secondary pairs. The parameter a is related to P, as shown in eq 9, where Pr' is the probability for recombination of a=-=1 - P r
1 - P,'
1/P, - 1 l/Pr - P,*/P,
(9)
magnetic radical pairs, Le., pairs containing a 13Catom (for this manuscript we shall consider that this atom is located only at the carbonyl carbon position), and P r is the probability for recombination of nonmagnetic radical pairs, i.e., pairs containing only 12Catoms. Through the agency of eq 9, we obtain only indirect information on Pr under the conditions when only the ratio P,*/ Pr is known or e~timated.3~ Another indirect method for the evaluation of trends in the magnitude of Pr for DBK is the measurement of recombination of the geminate secondary radicals and the application of q 8. Measurement of the secondary cage effect as a function of the experimental variable of exciting wavelength should provide a qualitative guide to the influenceof wavelengthon Pr. Although these measurements require the use of methyl-substituted DBKs as surrogates for DBK, the relationship between Pr and the secondary cage effect should be preserved. The salient results of Tables I, 11, VII, and VIII which we seek to interpret may be summarized as follows. For MDB, the values of P, and a both decrease significantly with increasing excitation energy (decreasing wavelength of excitation), but for DPP the value of P r is independentof excitation energy; however a slightly decreases with excitation energy at shorter wavelengths. For DBK the values of a decrease with increasing excitation energy, and the value of the secondarycage effect increaseswith incrsaSing
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 369
Wavelength Effects in Ketone Photolysis
CHART I: Cornhtioa Dhg"e for o-Cleavage of Ketones (Top)Ceaerd Di8gr8m .ad the Case of MethyMCOxyknzoin (MDB) rad DIbenzyl Ketone (DBK) d e r Law .ad High Energy of Excibtios; (Bottom)c.(K of Diphenylpcdu-3-one (DPP) E
I
r c
=
I
37c,x+ T
I
0,o
ICs
3'u
r
I
excitation energy. We see immediately that the results do not fit an obvious pattern. Nature of the Wavelength Effect. According to the radical pair paradigm, under the assumption that a primary geminate triplet radical pair is produced with high efficiency (q2), the probability of recombination will depend on a number of parameters that can be associated with the spin, molecular, and chemical dynamics. In order to simplify further analysis and to provide a common reference paradigm for each system investigated,we consider the results in terms of the Salem correlation diagrams31.32 for photochemical cleavage of ketones. Under the assumption that only triplet surfaces are involved in the conversion of electronically excited ketonesto thegeminate triplet radical pairs that eventually recombine (we shall examine this assumption later in the discussion), the pertinent Salem diagram can be described in terms of the adiabatic surfaces shown in Chart I (top). The diagram pog~essesthe following characteristic features: (1) an initial r,r*energy surface that is higher in energy than the n,r* surface in the region of the diagram correspondingto the starting ketone, (2) an avoided crossing region (labeled X in Chart I (top)) of the n,r*-u,u and the r,r*-u,r surfaces which occurs as one proceeds along the reaction coordinate, and (3) the final radical pair products consisting of a bent acyl radical and benzyl radical partner (u.0) as a ground state and a linear acyl radical and benzyl radical partner (u,r) as the first excited state. Let us first consider the possible effects of excitation energy on the point representing the reaction coordinate, as electronic excitation places it on the singlet spectroscopic energy surface. For the lower energy excitation, the initial orbital configuration of the singlet will be mainly Sl(n,r*), whereas for the highest energy excitation, the orbital configuration will be Sz(r,r*) or a vibrationally excited level of Sl(n,r*) significantly mixed with r,r*character. Since the discussion is qualitative, we shall not
attempt to place significanceon which of these situations is most accurate but refer to either as S2(r,r*). The results of S1 n,r* excitation of ketones have been studied in detail and with the following general c o n ~ l u s i o n s : * ~ ~ ~ ~ ~ ~ ~ * ~ ~ (1) For alkyl aryl ketones SI does not undergo significant a-cleavage. (2) Intersystem crossing from SIto the lowest triplet surface is rapid. (3) a-Clevage occurs adiabatically on the lowest triplet energysurface. (4) Due to a surface avoiding at the position of the reaction coordinate, X, the typical trajectory of the representative point upon excitation to the lower vibration levels of S1 is adiabatic, n,r*-X-u,u (termed TA'", where the subscript refers to adiabaticity of the trajectory and the superscript refers to the radical pair generated by the trajectory, see Chart I (top)). The possible nonadiabatic tiajectory n,r*-X-u,r (termed TN'B', where the subscript refers to nonadiabaticity of the trajectory and the superscript refers to the radical pair generated by the trajectory, Chart I (bottom)) can be generally ruled out as an accessible trajectory from thermochemical considerations; i.e., the energy of the thermally relaxed n,r* state is lower in energy than the u,r radical pair but is higher or comparable in energy to the u,u pair. Now let us imagine that the higher energy excitation places the representative point of the electronic system on the S2(r,r*) surface and consider the possible pathways it may follow in terms of the postulated surface diagram. The representative point has several possible trajectories: (1) a-cleavage; (2) intersystem crossing to the T2(r,r*) surface; (3) internal conversion to the Sl(n,r*) state. Provisionally, we rule out trajectory 1 because the rate of a-cleavage of singlet ketones is generally orders of magnitude smaller than that of triplet ketones34and does not competewith intersystem crossing. In fact, formationof T2(r,r*) after ISC of an unrelaxed excited singlet state of benzophenone in solution has been proposed as an explanation of the absorption spectra changes in the picosecond time domain.36 Starting from the Tz(r,r*), movement of the representative point along the coordinate for a-cleavage may either follow the adiabatic trajectory T2(r,r*)-X-u,r (termed TA',', Chart I (top)) or the nonadiabatic trajectory T~(r,r*)-X-u,u (termed TN'", Chart I (bottom)). Possibility 3, internal conversion of S2(r,r+) to Sl(n,r*),isexpected tobecompetitivewitheitherofthese'higher energy trajectories" but is equivalentto photoexcitationwith lower energy excitation, i.e., the trajectory TA'*~. Dependeweof Molecular Dynamics on Trajectories. Possible Microscopic Heating Effects. We now ask how the trajectories TA',', TA"', and TN"" might influence the value of Pr and a. We seek answers by considering how the trajectories influence the spin, chemical, and molecular dynamics which determine the magnitude of Pr and a. First, we consider that it is very unlikely that the molecular dynamics along any of these trajectories is significantlyinfluenced by different excitation energies employed in our studies. The reason for this conclusion is that the pertinent molecular dynamics are the mechanical motions (diffusion, translation, and rotational) of the radical pair and these molecular motions are not expected to be strongly influenced by the nature of the electronic surface except for strong electronic interactions, which appear to be absent along the reaction coordinate until intersystem crossing to a singlet occurs. Although the molecular dynamics is not expected to be significantly influenced by the surface trajectories of Scheme 111, the relaxation of the excess energy of initial electronicvibrational excitation of the precursor molecule (350 nm = 82 kcal/mol and 254 nm = 114 kcal/mol) might cause the heating of the molecular environment of the RP (solvent cage) and decrease the local microviscosity; this in turn could result in an increase in the diffusion coefficient of the radicals as well as the value of the initial distance between radicals, both important parameters in determining the magnitude of Pr. For the reactions of RPs in viscous liquids a critical parameter is the diffusion
370 The Journal of Physical Chemistry, Vol. 97, NO. 2, 1993
Step et al.
coefficientDof radicals in the liquid, which is determined mainly by the viscosity v of the media. For RPs formed in the photolysis of DBK in the media of variable viscosity, there is an optimum value of V , and consequentlyD, where u has a maximum value.13J7 Let us now consider the consequence of the possibility that "local heating" is responsible for the wavelength effects on Pr and u for the systems investigated. In principle, the vibrational relaxation of the molecules excited with the excess of energy must cause the local heating of the microenvironment.37,38 For example, radiationless deactivation of optically excited molecules is known to heat and "softenm Thus,one expects that theexcess energyofan excited molecule, beyond that rquired [or photodissociation could microscopically heat the "solvent cagem.The extinctioncoefficient of DBK is very small for wavelengths A > 325 nm, the longwavelength absorption edge of most dialkyl ketones. Therefore, light, whosewavelengthisca.325nm,correspondstotheminimum energy which can cause photolysisand which will cause minimum heating of the solvent. However, if light of wavelength A = 260 nm is employed for excitation, molecules excited by absorption of such photons will possess ca. 20 kcal/mol energy in excess of that needed to cause dissociation. The injection of this large amount of excess vibrational energy into the area of the solvent cage will tend to reduce the viscosity of the microenvironment of the RP. In addition, this excess heating may increase the initial interradical separation, ro, of the primary caged RP as well as cause scrambling of the mutual orientation of the radical pair. The increase of ro should decrease the probability of the encounters of RP and cause a reduction of Pr. Therefore, the increase of energy of exciting light should result in a decrease of Pr and a. These factors, discussed in terms of homogeneoussolventsshould also pertain to the environmentof radical pairs in micellar solution. The possibility that vibrational energy dissipation into the matrix induces local heating is unlikely, on the basis of thermal conductivitymeas~rements'~ which show that thermal equilibrium in organic media for separations of 1&15 A occurs generally in a time period shorter than 100 ps. We rule out the local heating mechanism as a general means of explaining the observed results because of the absence of the wavelength effect on Prfor DPP. Since the lifetime of the secphenethylacyl radical is only 22 nsZ7 and the initial reactive encounters are very important in determining Pr, we expect that Pr for this RP is very sensitive to the initial separation of the radicals within the micelle. However the experimental results show no wavelength dependence on Pr in DPP (see Table VI). Dependence of Cbemid D y ~ m i cpad ~ Spin D y ~ m i con~ Surface Trajectories. It is much easier to envision how either chemical dynamics or spin dynamics of a RP can be influenced by an excess energy of excitation. For example, when initiation of the u-cleavage occurs from the relaxed %,x* state the trajectory TA'.' is followed and the structure of the acyl radical is bent (sp2 hybridized at the carbon atom). After passing adiabatically through the avoidance region, the final radical pair has the u,u configuration. On the other hand, when initiation of the a-cleavage occurs from a 3r,x* state, two trajectories are possible: (1) If the nonadiabatic trajectory TN"'"is followed, the structure of the acyl radical is bent after passing through the avoidance region and the final radical pair has the u,u configuration, the same configuration resulting from the adiabatic trajectory TA"". (2) If the trajectory TA"'* is followed, the structure of the acyl radical is linear (sp hybridized at the carbon atom) after passing through the avoidance region and the final (electronically excited) radical pair has the u,r configuration. Clearly, the value of Pr will be sensitive to which trajectory is traversed. Let us now consider some energetic factors. Salem3Icalculated that for acetyl radical CH3C0, which has a bent 2A' ground state, the linear 2A't excited state39 lies about 28 kcal/mol above
the ground state. This energy is very close to the activation energy, EA, for the decarbonylation of benzoyl radical PhC0.40 This coincidence makes physical sense since the transformation from sp2 to sp hybridization is expected to be close to the transition state for decarbonylation of a bent acyl radical. Therefore, the rate of decarbonylation of a linear, excited acyl radical should be much higher than for a bent acyl radical. For phenacyl radical PhCHzCO, the activation energy of decarbonylation, EA, is substantially smaller than that for the benzoyl radical, i.e., only ca. 7 k c a l / m ~ l . ~ This ~ , ~ Ifact suggests that the wavelength effect for DBK and DPP should be more sensitive to the energy of exciting light than it is for MDB. In the latter case the effect should manifest itself only when hE > 25-30 kcal/mol. As any electronicallyexcited state, the linear 2A"excited state of acyl radicals in addition to chemical deactivation via decarbonylation or enhanced hydrogen abstraction, may undergo radiationless deactivation (relaxation) to the ground-state bent acyl radicals.3g The lifetimesof 2A"excited states of acyl radicals areunknown, but typically for electronicallyexcited radicals these lifetimes may be in a time scale up to tens or even a few hundreds of nano~econds,4~*~~* which is enough to compete with primary recombination of RPs and to reduce Pp Since the u,x radical pair is an electronically excited state of the u,u radical pair, the chemical properties of the two radical pairs will in general be quite different. Excited radicals generally possesses very different reactivities compared with the radicals in the ground state.42.43bFor instance, the rates of decarbonylation and intermolecular hydrogen abstraction of the u,r pair are expected to be much higher than those for the u,u pair. Thus, it is expected that the value of Pr and u will be reduced for the u,w RP relative to the u,u RP because of the more efficient competition of chemical pathways which compete with recombination. The formation of a linear acyl radical as the result of trajectory TA"*' could also influence the spin dynamics because of the fast electronicspin relaxation due to spin-orbital and spin-rotational interactions in a linear acyl radical.u These types of electron spin relaxation are nonselective to nuclear spin and provide additional and specific mechanisms for intersystem crossing to a singlet RP for the u,w RP relative to the u,u RP. This would tend to increase Pr but would tend to decrease u, which is a nuclear spin selectiveprocess. Since an odd electron in an excited linear acyl radical is located in a pure p orbital (w radical) compared to the sp2 orbital in a bent radical (u radical), the former also should have a smaller value of HFI constants for the carbonyl carbon. For a triplet acyl-benzyl RP this may not change Pr but should significantly decrease Pr* and u. In summary, the working model assumes one likely trajectory, TA"."only for lower energy excitation and either (or both) of the trajectories TA".' or TN"" for higher energy excitation. Now let us consider how the values of A?', and Au are expected to vary for thethreeselectedtrajcctories. Ifessmtially thesametrajectory were followed at higher and lower energiesof excitation, we expect that both APr and Au would q u a l zero; i.e., there would be either a small or no energy effect on the probability of recombination or the efficiency of isotope enrichment. If trajectory TA""were followed at higher excitation energies, A P r would be negative if new chemical reactions of the pair or electron relaxation of excited acyl radical competed favorably with recombination of the pair; on the other hand, APr may be positive if spin relaxation of the linear acyl radical causes a favorably competing (additional) mechanism for intersystem crossing of )RP to *RP. Although the conclusion concerning Pris ambiguous, it is exptxted that if trajectory TAU,' were followed, Au would be negative (smaller efficiency at higher energy) if either a new favorablecompetition for the chemical or spin dynamics occurred at higher excitation energies. If trajectory TN"*'were followed at higher energy excitation, APr would be expected to be close to zero or slightly
The Journal of Physical Chemistry, Vol. 97, No. 2, 1993 371
Wavelength Effects in Ketone Photolysis
SCHEME IIk Secondary Cage Effect in the Photolysis of o and pMethyldibenzyl Ketone (eand pDBK) in SDS Micelles PhCH,CH,Ph + PhCH,CH,Ar + Ar CH2CH2Ar
: 0
(
PhCH2!CH,Ar
t
I
escape
[PhCH,k- CH,Ai
)"
PhCH, CH,Ar
\
\
escape
I
1
lSc
recombination Cage
'
1
PhCH2CH,Ar
positive, since after leaving the avoided crossing region, the trajectory coincides with that on the lower surface. The value of Aa however should decrease slightly if the pair 'feels" the increased chemical reactivity or intersystem crossing before passing through theavoided region. Wearenow ready tointerpret the data. Discussiooof the ExperimentalData in Terms of the Trajectory Modd Let us examine how the experimental results fit the proposed trajectory model. Photochemical excitationof the three ketones under investigation-MDB, DBK, and DPP (for our puwe can consider o- and p-MeDBK as mechanistic surrogates for DBK)-yields three RPs of substituted acyl and benzyl radicals which possess some structural similarities. While MDB and DBK show significant wavelength effects on the measured parameters, DPP shows no measurable effect on Pr and a relatively small effect on a. For MDB, the experimental results are that AP, = -33% and Aa = -52% is., both parameters decrease significantly with increasing excitation energy. We thus conclude that at higher excitation energies MDB follows T A ' ~and ~ that new chemical (e.g. decarbonylation or reaction with surfactant molecule) or electron relaxation dynamics are important for this trajectory relative to TA".'. For DPP, the results are that APr = 0 and Aa = -28%; i.e., Pr is independent of excitation energy, and a decreases (the absolute ha value in the case of DPP is very small, -0.03) with increasing energy of excitation. We thus propose that at higher excitation energies, DPP follows TN"" and that intersystem crossing is now important as the avoidance region is approached, but new competing chemical processes may occur in competition with the formation of the u,u radical pair. For DBK we do not have a direct measure of Pr, but the secondarycageeffectofthesurrogates,o-MeDBKandpMeDBK, increaseswith increasing excitation energy (see Table VIII). This result is somewhat antiintuitive,since a higher cageeffect suggests higher organization, whereas higher energy usually decreases organization. From the paradigm of Scheme 111,there are two micellar escape pathways which could lead to formation of free radicals and decrease the cage effect from its maximum value of 100%. At the primary radical pair stage, there might be a competition between the process of escape of the more hydrophylic phenacyl radical into theaqueous phase and the processes of decarbonylation and intersystem crossing to form singlet geminate pairs, which are assumed to combine with 100%efficiency. At the secondary radical pair stage, there is a well-establishedcompetitionbetween escape of the benzyl radical fragments into the aqueous phase and intersystem crossing to form a singlet geminate pair which undergoes efficient recombination?* A decrease in the competition of escape pathways at either stage wiff lead to the observed increase in the cage effect.
According to our working paradigm based on trajectories, trajectory TA'.~ will cause an increase in both the rate of decarbonylationand therateof intersystemcrossingof the primary pair. An increase in the rate of decarbonylation will tend to decreasePr,whereas an increase in the rate of intersystem crossing will tend to increase P,. However, borh rate increases will tend to increase the observed cage effect of the secondarypair because an increased rate of decarbonylation will reduce any contribution from escape of the primary pair to form free radicals and because an increased rate of intersystem crossing will produce a higher fraction of singlet secondaryradicals upon decarbonylation. Thus, the paradigm is consistent with the observationsbut is mute with respect to the magnitude of the wavelength effect on Pr, Although not quantitative, results from the literature suggest that escape of the primarypair is not a significantpathway at lower excitation energy, so that we provisionally conclude that the increase in the singlet character is produced by the increased intersystemcrossing of the primary pair or that the loss of spin correlation during the process of decarbonylation is carried over to the secondary pair and leads to the higher observed secondary cage effect. One final piece of evidence can be brought to bear concerning the details of the trajectories followed by DBK. Photolysis of o-MeDBK in SDS micelles yields 2-benzyl-2-indano1, the product of the &hydrogen abstraction reaction, in addition to the diphenylethane products of a-cleavage. The relative yield of 2-benzyl-2-indanol decreases by a factor of 2 (from 14 to 7%) upon going from an excitation of X = 300 to 254 nm;4s i.e., more diphenylethane products are produced at higher energies. This result is conaistentwith a decrease in P r for the primary phenacylbenzyl )RP,since according to Scheme I11 a lower value of Pr effectively decreases the yield of 2-benzyl-2-indanol relative to the diphenylethanes;Le., more decarbonylation occurs at higher energies, reducing P r , increasing the yield of diphenylethanes, and decreasing the yield of 2-benzyl-2-indanol. Therefore, the available data on the secondary cage effect as well as the yield of indanol under photolysis of o-MeDBK is consistent with the conclusionthat Prof primary RPs of phenacyl and benzyl radicals decreases with high excitation energy and that this decrease is due to an increase in the rate of decarbonylation of the phenacyl radical. These results fit the hypothesis that DBK follows trajectory TA'.", with an enhanced rate of chemical reaction causing a lower efficiency of recombination. As is expected from this conclusion, the value of a decreases significantly with increasing excitation energy. We now consider the reasonableness of the conclusions and consider some alternative possibilities. In the case of MDB we concluded that the results (AP,< 0, Aa < 0) are consistent with the increase of adiabatic trajectories TA'S~ at higher excitation energies and mainly TA',' at lower excitation energies, i.e. adiabatic trajectories in both cases. Looking at the results more closely, we note that there is no energy dependence of P r or a in the region of 300-350-nm excitation, but a large effect on each is observed for 254-nm excitation. The energy difference between 300- and 350-nm photons is ca. 14 kcal/mol, while the difference between 300and 254-nm photons is ca. 32 kcal/mol. The activation energy for decarbonylation of the benzoyl radical is ca. 29 kcal/m~l.~* The reason for the greater effectivenessof the 254-nm excitation is therefore attributed either to a greater access to the Tz(r,r*) surface or to the higher energy trajectories toward the u,r radical pair being more thermodynamically favored. The formation of the u,rradical pair impliesa different reactivity and more efficient competitive relaxation and chemical processes, such as dearbonylation, which would reduce Pr. Of course, it should be noted that our discussion should not be interpreted to imply that we consider TA'*~ trajectories to be exclusively followed under 254nm excitation but rather that these trajectories become more
372 The Journal of Physical Chemistry, Vol. 97, No. 2, 1993
Step et al.
favored relative to TAU+'trajectories on the lower surface at higher excitation energies. In the case of DPP, AP,= 0 for the energy range investigated and a slightly decreasca. What is the basis of the difference in behavior of the DPP and MDB systems at higher excitation energy? We look to the structural difference in the starting ketones and in the incipient radical pair for an answer to this question. The act of a-cleavage is considerably more exothermic for DPP than MDB because of the greater stability of the radicals produced by the bond breaking event of DPP. The rate of a-cleavageof ketones isdependent on the thermodynamic stability of theradicals prod~ccd;"~ i.e., themorestable the formingradicals (thelarger theexothermicity of the radicaldissociation reaction), the faster the rate constant of a-cleavage. For~nosinho~~ showed that in the series of deoxybenzoins each substitution of hydrogen to methyl group in benzyl radical increases the rate constant of a-cleavage by about 1 order of magnitude. Photolysis of DPP generates a sec-phenethyl radical which is more stable than a benzyl radical. In comparison with MDB, whose triplet energy ET is about 73 kcal/mo1,26 ET for DPP should be larger and close to that for DBK, ET = 79 kcal/mol. The rate constant for a-cleavageof MDB from its lowest triplet is 2.1 X lo7s-Y6 DBK has a triplet state lifetime, TT, 1 1 0 0 p ~ . *Therefore, ~ TT for DPP should be on the order of several picoseconds and a-cleavage with such a ratecould lead toa nonadiabatic trajectory, TN'", because of the rapid bond cleavage. The greater exothermicity of the cleavage of DPP suggests a steeper initial slope of the energy surface correspondingto a-cleavage for this ketone than that for DMB or DBK. The steeper slope in turn implies a higher initial velocity of the representative point so that as it moves toward the avoided crossing region it might be carried nonadiabatically through the avoidance region. As a result, the representative point would find itself on the lower u,u surface and the resulting radical pair would have a similar probability for recombination as a RP that follows TA'"'. Thus we expect that AP, 0 for such a situation. To a first approximation one expects the energy dependence of a to scale as P,. However, as can be seen from eq 9, a tracks simultaneously two probabilities, P,and P,*,the probabilities of recombination of nonmagnetic and magnetic pairs, respectively. Thus, the interpretation of the edergy dependence of a is ambiguous, although Aa decreases with increasing excitation energy to a much less significant extent than observed for MDB. In the case of DBK, one might expect a qualitatively similar behavior with respect to energy dependence, as was found for DPP. However, since P, cannot be measured directly for this system, we must analyze the two indirect parameters; the sewndary cage effect and a. The result that the secondary cage effect increases with increasing excitation energy according to thediscussion above suggests that P,of the primary RP decreases with increasing excitation energy (more rapid decarbonylation), in contrast to the actual results for DPP. The most obvious structural factor that can explain the difference between DBK and DPP is the strength of the carbon-carbon bond undergoing a-cleavage and the strength of the carbonyl-carbon bond of the acyl fragment which undergoes decarbonylation. Indeed, DBK is intermediate to MDB and DPP with respect to those bond energies. It would not be unreasonable, therefore to expect 'mixed" behavior in terms of the trajectories followed by DBK upon higher energy excitation, i.e., a mix of TA'.' and TN"'" trajectories. The TA',' trajectories would tend to decrease P,and to decrease a, whereas the TN'*' trajectories would tend to have small differences in Pr and a as a function of excitation energy. Finally, given the many parameters involved in our interpretation of the results, we should consider whether simpler interpretations exist or whether some of our initial critical assumptions are still tenable in light of our analysis. First, let us consider again our rejection of the heat effect which could lead
to a transiently higher diffusion coefficient at higher excitation energies. This effect is a property of the environment, should be general and apply to all three ketones in the same type of micelles, and should lead to a general reduction of P,independent of ketone structure. However, for DPP there is no effect of ex- excitation energy on the value of P,and for DBK we deduce that Pr probably decreases with increasing excitation energy. Thus, the rejection of the heat explanation is justified. Next let us consider our rejection of u-cleavage in an upper singlet level at higher excitation energies. If this were the case, then the excitation spectra of the fluorescence would be different from the absorption spectra at short wavelengths. In the case of DBK and DPP, for which the possibility of upper singlet cleavage is most likely, we found that the fluorescence excitation spectra are identical to that for acetone, for which there is no upper-state ph~tochemistry.~~ The suggestion of a substantial contribution to a-cleavage from a singlet state for DBK was reported by Ammi et a1.& and Arbour and Atkinson." In addition, from CIDNP enhancement factors and quantum yield measurements inside the probe of an NMR spectrometer, Azumi et a1.& concluded that singletDBKa-cleavageoccursin tolueneatroomtemperature to the extent of about 25%. Using transient absorption spectroscopy Arbour and A t k i n ~ o nprovided ~~ support for one DBK excited state which gives benzyl radicals with a characteristic time of about 2 ns. These authors conclude that either (1) a-cleavage occurs directly from SIof DBK or (2) rate-limiting ISC to TI occurs followed by extremely rapid (
