J . Phys. Chem. 1988, 92, 7210-7216
7210
finity 183 kcal/mol). The 1:l complex is typical of mediumstrength hydrogen bonding, with a relative H I frequency shift with respect to monomer of 0.125. This complex (C), as several others involving weak bases such as CH3CN and H 2 0 interacting with HI, is sensitive to infrared irradiation and photodissociates into another non-hydrogen-bonded form (U) whose several vibrations have been identified. The interconversion processes between C and U have been examined. The conclusions are the same as those previously drawn for the CH,CN:HI pair, with minor differences tied to a change in barrier height between the C and U forms. This barrier is higher for E0:HI than for CH3CN:HI, and the most remarkable consequence is the weak photosensitivity of the E0:DI C species, the excitation of its us mode at 1400 cm-' being unable to overcome the barrier for the C U conversion. This conversion should then proceed through excitation of the C H stretching modes and, possibly, the first overtone of vg. Similarly, U conversion, as deduced from the barrier height for the C the temperature dependence of the kinetic rate constant, is 3 times greater for E0:HI so that no tunneling is observed for this system. Larger (EO),(HI), aggregates were also identified by low-frequency broad bands typical of proton transfer, with formation of ionic species such as [(EO),H]+ and possibly (IHI)-. Unfortunately, we were not able to determine the precise stoichiometries of these complexes, probably because any aggregate with m 2 2 gives rise to the same ionic species. Deuterium counterparts of these ions were not detected, which means that the breaking of the D-I bond does not occur. Instead, new bands were observed in the DI spectral region, some of them being photosensitive.
-
.,
Figure 6. Temperature development of k. Measurements were performed in the dark, the irradiation occurring from the laboratory thermal bath (295 K). EO/HI/Ar; +, EO/DI/Ar. experiments were carried for EO/DI/Ar mixtures. The values cf k according to T, are displayed in Figure 6. Since KDpis null, k( T,) is simply identified as k; use of eq 7 above 22 K allows one to get an activation energy Ec+" of 5 . 5 kJ/mol.
Conclusion In this paper we have examined the spectral properties of matrix-isolated hydrogen-bonded complexes between one of the strongest acids, hydrogen iodide, and ethylene oxide (proton af-
-
Acknowledgment. The authors are pleased to acknowledge the support of the research by CEE through Jumelage Grant 86200153FROlPUJUl. They also thank Mrs. Danielle Carrcre for technical assistance. Registry No. EO, 75-21-8;HI, 10034-85-2;DI, 14104-45-1; C2D4, 683-73-8; EO-d,, 6552-57-4; 1,2-dibromoethane-d4,22581-63-1; 2chloroethanol-d4,1 17067-62-6.
Shared Electronic Excitatlons in Clusters of Cyclopentanone. Mixed Isotope Studies Using OpticaUy Detected Magnetic Resonance W. Bryan Lynch and David W. Pratt* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: May 23, 1988; In Final Form: August 22, 1988)
We report zero-field optically detected magnetic resonance (ODMR) studies of neat, polycrystalline samples of cyclopentanone-do, cyclopentanone-d4,cyclopentanone-d8,and mixtures of these compounds cooled to 1.4 K and excited with a broad-band mercury arc. The ODMR spectra of the pure deuteriated compounds each exhibit nine lines, of different signs and unusually narrow widths, as in the case of the protonated compound [Shain, A. L.; Sharnoff, M. Chem. Phys. Left. 1973,22, 561. Concentration-dependent shifts in the frequencies, widths, and relative intensities of these lines are observed in the isotopically mixed samples. The results may be satisfactorily interpreted by assuming that the spectrum is that of a cluster in which the triplet excitation is shared by three or more translationally inequivalent molecules having well-defined orientations with respect to each other.
Introduction Photoexcited cyclopentanone and its derivatives have been the subject of several studies using the optically detected magnetic resonance (ODMR) technique, especially by Sharnoff and cow o r k e r ~ . ] - ~Among the results obtained, perhaps the most in( 1 ) Shain, A . L.; Chiang, W.-T.; Sharnoff, M. Chem. Phys. Lefr. 1972, 16, 206. (2) Shain, A. L.; Sharnoff, M. Chem. Phys. Left. 1972, 16, 503 (3) Shain, A. L.; Sharnoff, M. Chem. Phys. Len. 1972, 17, 95. (4) Shain, A. L.; Sharnoff, M. Chem. Phys. Left. 1973, 22, 56
0022-3654/88/2092-72 lO$O 1.50/0
teresting was the finding4that pure, polycrystalline samples cooled to 1.4 K exhibit a zero-field (zf) ODMR spectrum consisting of nine lines rather than the three lines expected for an isolated molecule in its lowest triplet state. The nine lines, which are both positive and negative in sign, can be grouped into three triads, suggesting the simultaneous existence of three different triplet states. The observed ODMR line widths are also unusually small, 5-10 MHz, which is surprising for a polycrystalline sample. Similar results were obtained for cyclopentanone dissolved at low (5) Mak, G . S.-K., Ph.D. Thesis, University of Delaware, 1977
0 1988 American Chemical Society
Electronic Excitations in Cyclopentanone Clusters concentrations in hexane or benzene. Shain and Sharnoff4 also found that the amplitude of a zf ODMR signal belonging to one triad changes when the sample is additionally irradiated with a second microwave field, resonant with an ODMR transition of a second triad. Some sort of rapid interconversion between the different states, perhaps via a tunneling mechanism, was proposed to account for this finding. But Tarrasch et aL6 subsequently showed, using optically detected spin-locking techniques, that the postulated exchange process, if it exists, occurs on a time scale longer than the phosphorescence lifetime. Intrigued by these results and the lack of a completely satisfactory explanation for them, we have recently reinvestigated the zf ODMR spectrum of cyclopentanone. Our first report’ described studies of the effects of concentration and sample cooling rate on the appearance of the spectrum. Slowly cooled samples of cyclopentanone in 3-methylpentane exhibit spectra at 1.4 K that are identical with those observed in neat cyclopentanone or in other hydrocarbon solvents. But we also found that the ODMR lines of the guest broaden significantly with decreasing concentration and/or increasing cooling rate, both unexpected results for an isolated molecule. To account for these observations, we proposed that cyclopentanone forms clusters of structurally unique aggregates when frozen slowly and that the sharp ODMR spectrum containing nine lines is the spectrum of these aggregates rather than isolated molecules. We suggested that exchange of the triplet excitation between different monomer units comprising the cluster (intracluster exchange) gives rise to the extra lines and their different signs. Line narrowing results from intercluster exchange; disruption of this process by solvent molecules causes line broadening and ultimately, at very low concentrations, the disappearance of the spectrum. Clusters, or small aggregates of weakly bound atomic or molecular units, have attracted a great deal of attention in recent years. Part of this interest stems from a desire to understand the structure and dynamic behavior of such species, especially in the gas phase. The size and shape of metal clusters have also figured prominently in discussions of the surface catalytic activity of different transition elements. Clusters are thought to be important in the nucleation and growth of crystals, in biological systems, and in molecularly based electronic devices. There is also much interest in both ordered and disordered systems, which may show diffusion, percolation, and/or Anderson localization. Typically, both optical and magnetic resonance techniques have been used to study such effects, particularly in isotopically mixed crystals.8 In this report, we describe a zf ODMR study of polycrystalline cyclopentanone-do, cyclopentanone-cu-d4,cyclopentanone-dE,and samples containing mixtures of the different isotopically labeled compounds. Like cyclopentanone-do, the deuteriated species also exhibit ODMR spectra with nine lines having different signs and unusually narrow widths. But both the frequencies of these lines and the triplet state excitation energies in cyclopentanone-do,-a-d4, and -dsare different. Thus, by mixing these compounds, we create a sample that is energetically disordered, one in which the magnetic and/or optical energy varies from site to site. However, whatever spatial order exists in the polycrystalline samples is preserved in these mixtures. Unlike the glass samples, a cyclopentanone-do molecule dissolved in such a mixture should have a microscopic environment that is identical with that which it experiences in a pure dosample. Thus, if clusters form in the pure samples, they should also form in the mixed samples. But if these clusters are responsible for the observed ODMR spectra of pure samples, then the corresponding spectra of mixed samples might be different, owing to the energy disorder caused by deuterium substitution. We report here the observation of such differences, manifest as concentration-dependent frequency shifts, line broadenings, and intensity variations in the zf ODMR spectra of the isotopically (6) Tarrasch, M. E.; Chen, C.-R.; Harris, C. B. Chem. Phys. Lett. 1977, 48, 519. (7) Lynch, W. B.; Pratt, D. W. J . Phys. Chem. 1985, 89, 890. ( 8 ) For a review, see: Organic Molecular Aggregates; Reineker, P., Haken, H., Wolf, H. C., Eds.; Springer-Verlag: New York, 1984; Springer Series in Solid State Sciences, Vol. 49.
The Journal of Physical Chemistry, Vol. 92, No. 26, 1988 7211 mixed samples. These results can be satisfactorily interpreted within the framework of the cluster model. This provides further evidence that shared electronic excitations exist in both pure and isotopically mixed polycrystalline samples of cyclopentanone. We also obtain order-of-magnitude estimates for the intra- and intercluster exchange rates from an approximate analysis of the data.
Experimental Section Cyclopentanone-do (Aldrich, Gold Label) was vacuum distilled before use. Cyclopentanone-cu-d4was prepared by base-catalyzed exchange of the do compound with DzO and vacuum distilled. Percent deuteriation (98% a-d) was determined by NMR. Cyclopentanone-dE (98.9% d) was obtained from Merck Stable Isotopes and vacuum distilled. Samples of these compounds, either neat or mixed in specific mole ratios, were sealed off in quartz ampules containing -0.5 atm of He gas after repeated freezethaw cycles on a vacuum line. Each sample was slowly cooled to liquid H e temperatures over a period of 1 h by immersing it in a liquid-Nz-cooled H e exchange gas just prior to use. The zf ODMR apparatus has been described e l ~ e w h e r e . ~An appropriately filtered 100-W Hg lamp was used for optical excitation of the frozen sample, at -313 nm. The phosphorescence wavelength monitored during the ODMR experiment was the PMDR’O emission maximum for each triad.4 Kinetic parameters were determined by using the MIDP” and MIPl2 techniques employing an optical shutter with 1-ms closure time. Of crucial importance in our experiments are the ODMR line positions, widths, and relative intensities in mixed isotope samples. Line positions could be determined in all cases to within f10% of the line widths by amplitude modulating the microwaves and using phase-sensitive detection techniques to measure the changes in phosphorescence intensity induced by the scanning microwave field. But ODMR line widths and intensities are a sensitive function of incident microwave power levels, relative phosphorescence quantum yields, and electron spin sublevel lifetimes. All spectra were recorded in the low power limit; Le., at microwave power levels well below those that produce detectable linebroadening effects. To compensate for differences in quantum yields, we monitored the average PMDR maximum of the two isotopically labeled components for each triad, using a slit of 100 Fm (6-cm-’ resolution). We also adjusted the modulation frequency so as not to exceed one-third the decay rate of the pumped state, so that steady state was reached during the off cycle of the microwave^.'^ Typically, normal d4 and dEtransitions were modulated at lower frequencies than those for normal do transitions, and satellite and weak transitions were modulated at higher frequencies than normal transitions. Careful control of the microwave power level during the sweep then resulted in line widths and integrated relative intensities (measured with a planimeter) that are accurate to *lo%.
-
-
Results Figure 1 shows representative examples of the data obtained for (a) pure cyclopentanone-do (do, hereafter), (e) pure cyclopentanone-dE (dg), and three isotopically mixed samples, (b) 16.1/23.3% do/dE, (c) 50.9/49.1% do/d,, and (d) 26.8/13.2% do/ds. We obtained qualitatively similar results for pure cyclopentanone-a-d4 (d4) and a series of do/d4 samples (5.9, 12.5,25.0, 48.8, and 74.8% d4). All pure samples exhibit nine lines of similar relative intensities. The set of nine lines for each compound may each be grouped into three sets of three lines, termed the “normal”, “satellite”, and “weak” triads by Shain and Shamoff.* The zf ODMR frequencies and relative integrated intensities of six of these lines for each compound are listed in Table I. The tran(9) Kothandaraman, G.; Pratt, D. W.; Tinti, D. S . J . Chem. Phys. 1975, 63, 3337. (10) Tinti, D. S.; El-Sayed, M. A. J . Chem. Phys. 1971, 54, 2529. (1 1) Schmidt, J.; Antheunis, D. A,; van der Waals, J. H. Mol. Phys. 1971, 22, 1. (12) Winscom, C. J.; Maki, A. H. Chem. Phys. Lett. 1971, 12, 264. Yamauchi, S.; Pratt, D. w . Mol. Phys. 1979, 37, 54. (1 3) Kothandaraman, G. Ph.D. Thesis, University of California, Davis, 1913.
7212 The Journal of Physical Chemistry, Vol. 92, No. 26, 1988
Lynch and Pratt
L
5
-41
"
s
5
Satellite
4
i
0-
E lL n I
O
0
1
1
0 I
,
I
do d e Satellite
,
Yo
do d, Normal 'M W
TABLE I: Frequencies (MHz) and Relative Intensities of Lines in the zf ODMR Spectra of Pure Cyclopentanone-d, Cyelopentanone-a-d,, and Cyclopentanone-d8 T.-T...* Tx-Ty [ 100.01" 1624.8 do normal 3394.6 - 1
d4 normal satellite weak d, normal satellite
weak
3591.2 343 1.2 3223.4 3642.8 3423.3 3220.3 3638.0
(36.6)' (35.6) (51.6) (28.7) (32.5) (46.8)
[24.7] [ 1.531 [ lOO.O] [I351 [2.24] [100.0] [15.1] [2.45]
1616.3 1483.1 1618.0 1611.9 1467.8 1621.9 1614.8 1474.2
d,
decrease by a factor of 3 upon deuteriation; d4 substitution produces changes that are nearly as large as those produced by d, substitution. N o microwave-induced transients could be detected in time-resolved MIDP or MIP studies of the satellite or weak transitions. Apparently, the lifetimes and/or spin-lattice relaxation (SLR) times of these sublevels are less than the response time (- 1 ms) of our shutter. There are also "anomalies" in the zf ODMR spectra of the isotopically mixed samples. First, we note that the mixed samples exhibit spectra with sharply reduced signal-to-noise ratios when compared to the corresponding ratios in the spectra of pure samples. This effect, though difficult to quantify, was definitely reproducible from sample to sample, providing that the samples were similarly prepared. For example, the do normal transition is reduced in intensity by at least an order of magnitude on going from pure do to -75% do,Figure la,b, despite the fact that there is little or no change in the phosphorescence intensity. Second, the mixed isotope samples produce just two sets of spectra having zf splittings close to those of the pure samples. For example, there are six lines present in the mixed T,-T, spectra; three are found very near neat do frequencies, and the other three very near neat d8 (d4)frequencies, at all mole ratios. The normal lines are positive and the satellite and weak lines are negative, as in the neat spectra. Similar observations were made in the Tx-Ty spectra, but the lines are not as well resolved (cf. Table I). But the Tz-Ty spectra are well enough resolved to demonstrate, unambiguously, that no lines having frequencies lying between those of the pure doand d8 (d4)compounds are found in the mixed samples. Third, and finally, we find that the mixed sample spectra are not simple superpositions of the spectra of their isotopic components, scaled according to their mole ratios. There are small but reproducible changes in the frequencies, widths, and relative intensities of the ODMR transitions that occur on going from the pure samples to samples containing more than one isotope. For example, Figures 2 and 3 show the frequency shifts observed with increasing d4 and d8 concentrations. Consider, for example, the Tz-Ty normal transitions of do and d4 (Figure 2), which occur at
K).
weak
100
75
Figure 2. ODMR frequency shifts of do ( 0 )and d4 (A) transitions in mixed samples of do and d4,with respect to those of the corresponding neat samples.
Figure 1. Normal and satellite T,-T, zero-field ODMR spectra of (a) pure cyclopentanone-do,(b) do with 23.3% cyclopentanone-d,, (c) do with 49.1% d,, (d) dowith 73.2% d8, and (e) pure cyclopentanone-d, (T= 1.4
satellite 3187.8
50
25
(-6.8) (-4.4) (-15.3) (-2.9) (-1.5) (-8.9)
" Relative intensities in brackets. Normal and satellite intensities accurate to 110%; weak intensities accurate to +20%. 'Frequency shifts from do in parentheses. All frequencies accurate to 10.5 MHz. sitions shown in Figure 1 are the "TZ-Ty" satellite and normal transitions, at 3187.8 and 3394.6 MHz for pure do and 3220.3 and 3423.3 MHz for pure d8,respectively. Note that deuterium substitution produces significant frequency shifts in the positions of these (and other) lines in the zf ODMR spectrum (cf. Table I). These shifts are typically larger than the ODMR line widths, which in all cases are less than 10 MHz in the pure samples. Also note that in all cases the normal lines are positive and the satellite (and weak) lines are negative. (Here, the terms "positive" and "negative" refer to the sign of the change in the microwave-induced phosphorescence intensity). Thus, the zf ODMR spectrum of a pure, polycrystalline sample of cyclopentanone-do, -a-d4, or -d, is "anomalous" in three respects; it exhibits nine lines rather than the expected three, the lines are unusually narrow, and they have different signs. The triplet-state kinetic properties of the three different labeled compounds are given for the normal triad in Table 11. Overall, the total spin sublevel decay rates of the protonated molecule
TABLE II: Normal Sublevel Decay Properties of Cyclopentanone-dmCyclopentanone-a-d,, and Cyclopentanone-ds" do" d4
d8
775 273 255
79.4 24.7 24.6
'All values are accurate to +IO%.
725 288 268
0.47 0.41 0.43
0.017 0.021 0.026
1 1 1
0.28 0.10
bShain, A. L.; Sharnoff, M. Chem. Phys. Lett.
1
1
1972, 16, 503.
0.46 0.28
0.65 0.30
0.24 0.31
1 1
Electronic Excitations in Cyclopentanone Clusters
The Journal of Physical Chemistry, Vol. 92, No. 26, 1988 7213
8
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