3431
J . Phys. Chem. 1984,88, 3431-3435 TABLE V: Rotational Correlation Times at 33 "C in D,O Expected from the Prolate Model (Figure 7B) of M,X and M,X, Aggregates (M = [Ru(phen),] ,+,X = SO,,-)
2rl,a A 2r,,' A
22 11
6.0 1.2
:::} 2.7
~ O " ~ M M , C H ( ~ ) , s (i= 2, 4, 5, 6, 7, and 9)
3.0 5.0
1 0 1 0 7 ~ ~ ( 0s ) , C
3.8, 2.2 3.4
a r1 (= 2rM) and rs (= YM) represent the radii of the longer and shorter axes of the prolate, and TM denotes the radius of the [Ru(phen),] '+ ion listed in Table I. See Figure 7 and ref 25. b The rotational correlation time to be measured for each C-H vector is the average value vector over three phenanthroline ligands of the complex ion. Observed.
the value of 7MM,CH(j) obtained from the observed R 1values showed no appreciable differences between different positions. The fact that anisotropy was hardly detected in the rotational motion of the complex ion in the aggregate suggests that the configuration of the aggregate as shown in Figure 8 is not the unique one. For example, the A,A and A,A aggregates take configurations different from those of the A,A aggregate. The C-H(i) vector at the 3and 8-positions probably has a large 7MM,CH(i) value in the A,A and h,A aggregates in contrast to the case of the A,A aggregate listed in Table V. The ~ ~ ~value ( 0determined ) from the observed relaxation rate is an average of the ~ ~ ~values ( 0for) the aggregates of different configurations, since the observed relaxation rate of 13C in each position of the ligand showed a single value. If such a situation is taken into account, the agreement between the calculated TMM,CH(~)values and the experimental rMM(0)value is reasonably good. This indicates that an aggregate with a definite configuration is maintained for the time interval of at least 3 X s, the same order of magnitude as TMM, although the aggregate does not survive so long that a 7 value characteristic of the particular aggregate can be obtained.
Acknowledgment. This work was carried out as a cooperative research (1982) of the Institute for Molecular Science. We thank the Ministry of Education, Science and Culture of Japan for the results are summarized in Table V.32 support of this work (Grant-in-Aid for Scientific Research No. While the calculated 7MM,CH(i) value for the 56470039). Registry No. R~(bpy),~',15158-62-0; R~(phen),~+, 22873-66-1; (32) In the D20 solution of ruc-[Ru(phen),]SO4, two kinds of the aggreCo(bPY)33+,19052-39-2; Co(phen),'+, 18581-79-8; 2[Ru(phen),12',gate with different combinations of the optical isomers of [Ru(phen),]*', h,AL O > - , 90641-15-9; [ R ~ ( p h e n ) ~ ] S O 41745-79-3; ~, [Ru(bpy),]SO,, and A,A (and its antipode A,A), exist (ref 30). The calculation here was made 50989-45-2; [C0(bpy)J~',S04~-,74436-63-8; [C~(phen)~]~',SO>-, on the A,A aggregate. 76229-12-4.
C-H(i) vector of the phenanthroline ligands were calculated in
Intermediate-Sized Deuterium Effect on T,
+.So Intersystem Crossing in Acetaldehyde
Warren F. Beck,' Merlyn D. Schuh,* Mark P. Thomas,2 and T. John Trout3 Department of Chemistry, Davidson College, Davidson. North Carolina 28036 (Received: September 12, 1983)
Reciprocal lifetimes, extrapolated to zero pressure ( 7 f 1 ) , and rate constants for self-quenching (k,) have been measured for flash-excited T,-state acetaldehyde and its deuterated analogues. Both 70-l and k decrease with increasing deuteration and the ranges of values are 70-1 = 3400-29 000 s-l and k, = 0.52 X 107-1.8 X lo7 s-l. Reductions in 70-1and k, are very specific with respect to which hydrogens are deuterated, and the largest deuterium effect is found for the aldehyde hydrogen. The deuterium effect for acetaldehyde is intermediate in value between those for formaldehydeand larger ketones and aldehydes. Experimental evidence is presented in support of the activity of the out-of-plane deformation mode, ~ 1 4 as , the dominant accepting mode for T I So intersystem crossing.
k-l
-
Introduction Studies of the factors and structural features that control the partitioning of energy between the various decay channels of excited electronic-state molecules are of fundamental importance to chemical dynamics. In particular, the loss of electronic (vibronic) energy through radiationless decay has been of considerable experimental and theoretical interest for several decades and a general theory has evolved4 which explains the most important phenomenological aspects of radiationless transitions. For example, the lengthening of lifetimes for triplet-state (T,) aromatic hydrocarbons caused by deuterium substitution is well understood (1) Present address: Department of Chemistry, Yale University, New Haven, CT 06520. (2) Present address: Department of Chemistry, Emory University, Atlanta, GA 30322. (3) Present address: Department of Chemistry, Pennsylvania University, Philadelphia, PA 19104. (4) P. Avouris, W. M. Gelbart, and M. A. El-Sayed, Chem. Reu., 77, 793 (1977). and references therein.
0022-3654/84/2088-3431$01.50/0
and is attributed primarily to a reduction in the Franck-Condon factors for C-H stretching modes that are efficient acceptors of electronic energy.4 In general, deuterium substitution also increases the lifetimes of TI-state carbonyl containing compounds, but the causes for this important phenomenon are not as well understood. Vapor-phase alkyl ketones and aldehydes seem to belong to two classes with respect to the effect of deuterium substitution on their T1-statelifetimes. Formaldehyde alone forms one class for which deuterium substitution causes a dramatic increase in lifetime of greater than 40-f0ld.~ The second class consists of acetone, propynal, glyoxal, and biacetyl for which the deuterium effect is much smaller and lengthens the vapor-phase phosphorescence lifetimes by less than a factor of 2.6-9 (5) A. C. Luntz and V. T. Maxson, Chem. Phys. Lett., 26, 553 (1974). (6) (a) J. C. Miller and R. F. Borkman, J . Chem. Phys., 56,3727 (1972); (b) W. A. Kaskan and A. B. F. Duncan, ibid.,18, 427 (1950). (7) U. Bruhlmann, P. Russegger, and J. R. Huber, Chem. Phys. Lett., 75, 179 (1980).
0 1984 American Chemical Society
3432 The Journal of Physical Chemistry, Vol. 88, No. 16, 1984
Beck et al.
TABLE I: Kinetics Data for Acetaldehyde and Deuterated Analogues
compd CH3CHO CH3CHO CH3CDO CD3CDO
To-’, S-’
29000 f 27 000“ 42 OOOb 27 000 f 5 100 f 3400 f
r,,-’(CH3CHO)/rC1
.o
1500
1
1500 300 200
1.1 5.7 8.5
10’kq, M-’
9pH/4pD
kq(CH3CHO)/kq
S-’
1.8 f 0.1 1.7a 1.7b 1.7 f 0.1 0.52 f 0.03 0.52 f 0.03
kPH/kpDd
.o
1.o
1
1.o
1.1 3.5 3.5
0.72 f 0.04 0.22 f 0.001 0.096 f 0.006
0.77 1.1 0.64
“These are corrected values obtained by Gandini and Hackett of ref 19 but reported in ref 18. bReference 18. “Ratio of the areas under the phosphorescence intensity vs. time plots discussed in the Experimental Section. dThese ratios were obtained from eq 2 for relative quantum yields and lifetimes of phosphorescence at 8.0 torr. kODrefers to each deuterated analogue. It is intriguing that no alkyl aldehydes or ketones have yet been reported to have an intermediate-sized deuterium effect on their TI-state lifetime. For several reasons the discovery and study of such molecules would be important and help in the further development and application of radiationless transition theory to compounds other than aromatic hydrocarbons. First, it will be valuable to determine whether the existence of the aforementioned classes is related, as is the case for “small”, “intermediate”, and “statistical” limit molecules with respect to S1 T, intersystem crossing, to the number of atoms and vibrational modes in a molecule. Second, such studies might reveal an important relationship between the magnitude of the deuterium effect and molecular structure. The possibility for such a relationship is suggested by the large difference between molecular geometries of the excited electronic states of some molecules from the two aforementioned classes. For instance, the geometries of the S, and TI state of formaldehyde, which has a large deuterium effect, are pyramidal.lOJ1 However, the geometries of SI glyoxal’* and SI and TI propynal are planar.’3-14 Thus, molecules with an intermediate-sized deuterium effect might have an out-of-plane deformation angle that is between those formed in formaldehyde and propynal. Third, molecules with an intermediate-sized deuterium effect may bridge the gap between those with small and large deuterium effects, and a comparison of molecules from all three classes may allow identification of the structural features and other factors (such as particular classes of vibrational modes) that are essential for the characteristic behavior of each class. Fourth, molecules with an intermediate-size deuterium effect may also show other types of intermediate behavior that may form a link between extremes of behavior exhibited by larger and smaller molecules. Acetaldehyde would seem to be an excellent molecule in which to observe an intermediate-sized deuterium effect since its structure is intermediate between those of formaldehyde and acetone, which have large and small deuterium effects, respectively. We have used flash photolysis to measure the phosphorescence lifetimes of acetaldehyde and its deuterated analogues as a function of pressure and report acetaldehyde to be, to our knowledge, the first example of an alkyl aldehyde with an intermediate-sized deuterium effect on its TI-state lifetime.
-
Experimental Section Chemicals. Acetaldehyde, with stated purity of 99+%, and acetaldehyde-& with a stated isotopic purity of 99+%, were obtained from Aldrich. Acetaldehyde-d, (isotopic purity 99%) and acetaldehyde-d3 (isotopic purity 98%) were obtained from Merck. All samples were used as received after several deoxygenations. Samples were prepared as described previo~sly.’~Gas pressures were established with a Wallace and Tiernan gauge as described previously. (8) J. T. Yardley, J . Chem. Phys., 56, 6192 (1972). (9) R. F. Borkman, Chem. Phys. Lett., 9, 77 (1971). (10) J. C. D. Brand, J . Chem. SOC.,858 (1956). (11) G. W. Robinson, Can. J . Phys., 34, 699 (1956). (12) G. W. King, J. Chem. SOC.,5054 (1957). (13) J. C. D. Brand, J. H. Callomon, and J. K. G.Watson, Discuss Faraday SOC.,35, 175 (1963). (14) (a) C. T. Lin and D. C. Moule, J . Mol. Spectrosc., 38, 136 (1971); (b) C. T. Lin and D. C. Moule, ibid., 37, 280 (1971). (15) W. F. Beck, M. D. Schuh, and I. R. Williams, Chem. Phys. Lett., 81, 435 (1981).
I
1
1.2 time (msec)
0.6
I
1.8
Figure 1. Typical oscilloscope trace of phosphorescence intensity, Ip,vs. time. The sample was 2.0 torr of CD3CDO. The trace consists of 256 points.
Instrumentation. The vacuum system has been described p r e v i ~ u s l y . ~ ~Excited-state ~’~ molecules were produced by a repetitively pulsed Xenon Corp. flashlamp (Model N-734C), operated at a flash energy of 6.4 J. The excitation pulses had a duration of 1.7 p s (fwhm) and were filtered through a Corning CS 7-54 filter to produce a band of excitation that coincides with a So SI absorption of acetaldehyde between 2500 and 3500 A.18 Phosphorescence was focused into a 1/4-m Jarrell-Ash grating monochromator, which was operated in first order with a bandpass of approximately 20 nm and centered at 430 nm, the peak of the phosphorescence s p e c t r ~ m . ’ ~ Phosphorescence signals were amplified by a thermoelectrically cooled EM1 Model 9798 QB photomultiplier with S-20 response characteristics and were recorded and displayed on a Tektronix Model 7854 digital, data-processing oscilloscope. For each lifetime 1000 oscilloscope traces were averaged to produce a signal that had a sufficently large signal/noise ratio to permit analysis of phosphorescence intensity over two to three orders of magnitude. Averaged signals were stored on diskette by an Apple I1 microcomputer interfaced to the oscilloscope and were processed by a nonlinear least-squares regression/graphics display routine loaded onto the computer. Relative quantum yields of phosphorescence, I+$,, were evaluated from the areas under the oscilloscope traces of phosphorescence intensity vs. time, after correction for scattered light and the small differences between extinction coefficients of the deuterated analogues.
-
Results All phosphorescence signals displayed single-exponential decay over two to three orders of magnitude as is demonstrated by the typical oscilloscope trace in Figure 1. Reciprocal lifetimes, T-,, were plotted vs. acetaldehyde pressure, (A), in accordance with eq 1. The reciprocal lifetime extrapolated to zero pressure is T ~
(16) M. D. Schuh, J . Phys. Chem., 82, 1861 (1978). (17) K. W. Holtzclaw and M . D. Schuh, Chem. Phys., 56, 219 (1981). (18) R. J. Gill, W. D. Johnson, and G.H. Atkinson, Chem. Phys., 58,29 (1981).
- ~
Deuterium Effect on Ti
-
I
The Journal of Physical Chemistry, Vol. 88, No. 16, 1984 3433
So Crossing
I
I
I
I
deuterated compounds. In contrast, deuteration of the aldehyde hydrogen reduces r0-l by a much larger but approximately same amount for perprotonated and methyl-deuterated acetaldehydes. To interpret these results, is assumed to equal the sum k , k, kTS,where kpdand kTs are the rate constants for photodissociation and T, So intersystem crossing, respectively; and the effect of deuteration on each rate constant is considered. The mechanism for photodissociationis still not fully understood and is the subject of active r e s e a r ~ h . ~ However, * ~ ~ ~ , ~the ~ activation barrier for the principal T1-statephotodissociative step was recently estimated experimentally to be 14 kcal/mol>2 and recent ab initio calculation^^^ show that photodissociationof the C-C and aldehyde C-H bonds occurs from the TI state with a large activation energy of about 60 kcal/mol. Excitation pulses from our broad-bandpass flashlamp may create T1-state molecules with vibrational energy in excess of the photodissociation energy. However, at the pressures used, several thousand hardsphere collisions occur before the T,-state molecules phosphoresce. These collisions should lower the vibrational energy content below the photodissociation energy, and hence photodissociation makes only a negliglble contribution to rO-I. Equation 2 shows that the effect of deuteration on the radiative rate constant can be determined only if the ratios &TD/$sTH and $pH/+pD are known. The latter values were taken from Table I, but the former have not been reported. However, the value of $sT for carbonyl compounds with small S1-TI energy gap (e.g.,
