Photoinduced isomerization observed for ring-opened radical cations

Aug 17, 1988 - C. S. Parmenter and. Drs. A. E. W. Knight and S. H. Kable for many stimulating discussions concerning IVR in pDFB. This work was suppor...
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5388

J . Phys. Chem. 1989, 93, 5388-5393

has the lowest frequency. We thus expect that this effect will be observed for low-frequency modes in other molecules. This prediction should be readily subject to experimental verification and is an ideal test of the validity of our proposed mechanism.

Acknowledgment. W.D.L. thanks Prof. C. S. Parmenter and

Drs. A. E. W. Knight and S. H. Kable for many stimulating discussions concerning IVR in pDFB. This work was supported by the Australian Research Grants Scheme and the Flinders University Research Committee. Registry No. p-Difluorobenzene, 540-36-3.

Photoinduced Isomerization Observed for Ring-Opened Radical Cations of Methyl-Substituted Ethylene Oxides Kiminori Ushida,+Tadamasa Shida, * Department of Chemistry, Faculty of Science, Kyoto University, Sakyo- ku, Kyoto 606, Japan

and Kazuo Shimokoshi Department of Chemistry, Faculty of Science, Tokyo Institute of Technology, Meguro-ku. Tokyo, 152, Japan (Received: August 17, 1988; In Final Form: March 3, 1989)

Upon y-irradiation, unsubstituted and methylated ethylene oxides (hereinafter called epoxides) in Freon matrices exhibit optical absorption spectra with medium to strong intensity in the visible region, 400-600 nm. The spectra of most of the methylated epoxide systems changed reversibly upon photoexcitation that accompanied parallel ESR spectral changes. The observed changes were interpreted in terms of cis-trans isomerization of ring-opened radical cations of the epoxides. Subsidiary MO analyses and ESR simulation give support for the interpretation.

Introduction Since the discovery of the Freon matrix technique,'-3 a large number of radical cations have been studied by ESR. Some issues, however, still remain. For example, the structure of radical cations of three-membered cyclic compounds such as epoxides has been the subject of both e~perimental"'~and theoretical studies.16'* As for the prototype ethylene oxide (EO) a ring-opened structure was advocated on the basis of the observed hyperfine coupling (hfc) constants of proton^,^-^^ which are contrastingly small compared with the large hfc constants of radical cations of simple ethers.) However, there is also a group that seems reluctant to accept the ring Therefore, we have attempted to settle the issue by combined optical and ESR studies employing appropriately deuterated epoxides. As a result of the present study, it was confirmed that the y-irradiated epoxides in the Freon matrix exhibit a visible absorption band as has been reported for X-ray-irradiated ~ y s t e m s . ~ In addition, it was found that a reversible photoinduced optical and ESR spectral change takes place in methylated epoxide systems, which is most reasonably understood in terms of a cistrans isomerization of ring-opened radical cations. With the assumption of ring opening, the observed changes in ESR have been attributed to intramolecular motions of the methylene group in the propylene oxide (PO) systems. A standard simulation based on the two-site-jumping model approximately reproduced the general spectral features. However, detailed spectral changes remain to be accounted for, particularly for the butylene oxide (BO) systems studied. Both intramolecular and intermolecular steric hindrance on the molecular motion must be clarified in detail to explain the observed spectra completely. Experimental and Computational Section Undeuterated epoxides were purchased from Tokyo Kasei Kogyo, Ltd., except for ethylene oxide, which was synthesized from ethylene chlorohydrin through elimination of hydrochloric acid. 'Present address: Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan. *To whom correspondence should be addressed.

0022-365418912093-5388$01.50/0

Cis and trans isomers of 2,3-butylene oxide were separated on a gas chromatograph equipped with P,@'-oxydipropionitrile at 90 OC. Isolated trans-2,3-BO was also purchased from Aldrich Chemical Co., Inc. Deuterated epoxides were synthesized as follows: Monodeuterated PO, CH3CDOCH2,was obtained through the elimination of hydrochloric acid from the deuterated chlorohydrin, CH3CD(OH)CH2CI,which was synthesized from monochloro-

.

( I ) Shida, T.; Kato, T. Chem. Phys. Lett. 1979, 68, 106. (2) Shida, T.; Egawa, Y . ;Kubodera, H.; Kato, T. J . Chem. Phys. 1980, 77, 5963. (3) Kubodera, H.; Shida, T.; Shimokoshi, K. J . Phys. Chem. 1981, 85, 2583. (4) Bally, T.; Nitsche, S.; Haselback, E. Helv. Chim. Acta 1984, 67, 86. (5) Snow, L. D.; Wang, J. T.; Williams, F. Chem. Phys. Lert. 1983, 100, 193. (6) Symons, M. C. R.; Wren, B. W. Tetrahedron Lett. 1983, 24, 2315. (7) Symons, M. C. R.; Wren, B. W. J . Chem. Soc., Perkin Trans. 2 1984, 511.

(8) Qin, X.-Z.; Snow, L. D.; Williams, F. J . Am. Chem. SOC.1985, 107, 3366. ( 9 ) Qin, X.-Z.; Snow, L. D.; Williams, F. J . Phys. Chem. 1985,89, 3602. ( I O ) Rideout, J.; Symons, M. C. R.; Wren, B. W. J . Chem. Soc., Faraday Trans. I 1986, 82, 161. (11) (a) Rhodes, C. J.; Symons, M. C. R. Chem. Phys. Lett. 1987, 140, 61 1. (b) Symons, M. C. R.; Wyatt, J. L. Chem. Phys. Lett. 1988, 146,473. (12) (a) Snow, L. D.; Williams, F. Chem. Phys. Leu. 1988, 143, 521. (b) Williams, F.; Dai, S.;Snow, L. D.; Qin, X.-2.; Bally, T.; Nitche, S.; Haselbach, E.; Nelsen, S. F.; Teasley, M. F. J . Am. Chem. SOC.1987, 109, 7526. (13) (a) Qin, X.-Z.; Williams, F. Chem. Phys. Lett. 1983, 100, 198. (b) Qin, X.-Z.; Snow, L. D.; Williams, F. J . Am. Chem. SOC.1984, 106, 7640. (c) Qin, X.-Z.; Williams, F. Tetrahedron 1987, 42, 6301, (14) Ushida, K.; Shida, T.; Walton, J. C. J . Am. Chem. SOC.1986, 108, 2805. (15) (a) Qin, X.-2.; Williams, F. J . Phys. Chem. 1986, 90, 2292. (b) Qin, X.-Z.; Williams, F. J. Am. Chem. Soc. 1987, 109, 595. (c) Qin, X.-Z.; Meng, Q.-C.; Williams, F. J . Am. Chem. SOC.1987, 109, 6778. (16) (a) Bouma, W. J.; Poppinger, D.; Saeba, S.; Macled, J. K.; Radom, L. Chem. Phys. Lett. 1984, 104, 198. (b) Nobes, R. H.; Bouma, W. J.; Macled, J. K.; Radom, L. Chem. Phys. Lett. 1987, 135, 78. (17) Clark, T.J . Chem. Soc., Chem. Commun. 1984, 666. (18) Feller, D.; Davidson, E. R.; Borden, W. T. J . Am. Chem. SOC.1983, 105, 3347; Ibid. 1984, 106. 2513.

0 1989 American Chemical Society

The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5389

Methyl-Substituted Ethylene Oxides

h

n

/cFC13

, 77

K

460 nm

Figure 1. Optical spectra of y-irradiated ethylene oxide in CFCI, at 77 K. (A) Immediately after y-irradiation. (B) Same as (A) after photobleaching with X > 690 nm. (C) Same as (A) after subsequent photobleach with A < 350 nm. The quintet ESR spectrum in the inset did not change through the stages A to B, but decreased homogeneously in parallel with the decrease of the optical spectra from B to C.

-

-

acetone and LiA1D4. Dideuterated PO, CH3CHOCDzwas synthesized by the method of Fickett et al. using LiA1D4.I9 Monodeuterated 2,3-BO, CH3CHOCDCH3 (denoted D1) was also obtained from commercial 2-chloro-3-butanone and LiAID., by the same method as for monodeuterated PO. Dideuterated 2,3-

.

BO, CH3CDOCDCH3 (denoted Dz) was synthesized by the method of Lucas et al. from deuterated 2,3-butanediol, CH3CD(OH)CD(OH)CH,, which was obtained by the method of Loewus et a1.20*z1The purity of all the synthesized samples was checked by gas chromatograph and NMR. CFC13and CFClzCF2Clfrom Daikin Kogyo Co., Ltd., were used as received. Experimental procedures for the optical and ESR measurements were the same as described b e f ~ r e . I - ~A Cary 14RI spectrophotometer at the Institute of Physical and Chemical Research (IPCR) was used for the optical measurement of light-scattering samples. The simulation program based on the extended Bloch equation was essentially the same as described p r e v i o u ~ l y ~but ~ ~was ~ 2 coded ~ for a FACOM M382 computer and a microcomputer PC-9801E of NEC.

Results and Discussion EO and PO Systems. Upon y-irradiation of ethylene oxide (EO) in CFC13 an absorption spectrum (curve A) and a quintet ESR spectrum appeared as shown in Figure 1. By a brief photobleach with X > 690 nm the optical spectrum changed from curve A to B, but the ESR spectrum did not change appreciably. The persisting 460-nm band was weak compared with similar absorption bands of methylated epoxides shown in Figure 2. The 460-nm band in Figure 1 appears to be associated with the quintet ESR spectrum because photobleach with X C 350 nm caused a homogeneous decrease in both the optical and ESR spectra as shown in curve C of Figure 1. Visually, the sample at the stages of B and C was pale yellow which may be due to the strong absorption at X C 400 nm. The initial photosensitive absorption at X > 550 nm is similar to the absorption observed for y-irradiated neat CFCl, and is attributed to the residual matrix-trapped hole.24 The same absorption at X > 550 nm was observed immediately after y-irradiation for all the epoxides, but the intensity for the same radiation dose was always negligibly small for these epoxides. This may be attributed to the fact that the ionization potential (19) Fickett, W.; Garner, H. K.; Lucas, H. J. J . Am. Chem. SOC.1951, 73, 5063. (20) (a) Willson, C. E.; Lucas, H. J. J . Am. Chem. SOC.1936, 58, 2396. fb) . , Lucas. H. J.: Garner. H. K. J . Am. Chem. Soc. 1948. 70.990. (21) Lckwus, F. A.; Westheimer, F. H.; Vennesland, B. i.A h . Chem. SOC. 1953. 75. 5018. (22) McDowell, C. A.; Shimokoshi, K. J . Chem. Phys. 1974, 60,1619. (23) Heinzer, J. Mol. Phys. 1971, 22, 167. (24) Shida, T. Electronic Absorption Spectra of Radical Ions; Physical Sciences Data 34; Elsevier: Amsterdam, 1988. ~

350 400 450 500 600 800.m Figure 2. Optical spectra of y-irradiated epoxides in CFCI,a t 77 K. The spectrum at the bottom corresponds to curve B of Figure 1. The radiation dose was the same for all the samples. For all the epoxides except EO the initial spectra immediately after y-irradiation indicates only a trace of residual matrix-trapped hole which showed absorption a t X > 550 nm. The absorptions were easily photobleached with the light of A > 690 nm (we call this procedure “photocleaning” in the text). Subsequent to the photocleaning, the photobleach with X > 500 nm (for 2,3-BO a t the top of this figure, 550 nm) caused the spectral change from the solid curve to the dotted for propylene oxide and 2,3-BO, but the spectrum in the solid curve was recovered when the sample was successively photobleached with X = 440 nm (for 2,3-BO, 520 nm instead of 440 nm). The change between the dotted and the solid curves was reproducible without noticeable decrease in the total intensity of absorption. The reversible optical changes accompanied corresponding ESR spectral changes a s discussed in the text. As for 1,2-butylene oxide there was no discernible spectral change compared with the above two systems, but there was an indication of the interconversion between heavily overlapping optical absorptions corresponding to the solid and the dotted curves in the PO and 2,3-BO systems. However, in the case of 2-methylpropylene oxide (2,2-dimethylethylene oxide) there seemed no spectral shift as in the case of the prototype EO system.

of EO is significantly higher than those of the methylated epoxides so that the intermolecular hole transfer from CFCl, to the solute molecule may be inefficient in the case of EO, whereas almost all the holes are scavenged by the other epoxides to give the absorption spectra in Figure 2. The determination of the extinction coefficient of the 460-nm band in Figure 1 was not made because of this incomplete scavenging of the hole and of the light-scattering property of the polycrystalline matrix, but we feel that the observed relative weakness of the 460-nm band is due to both the incomplete scavenging and the intrinsic extinction coefficient of a relatively small value. A previous statement of the absence of a violet color in the EO systemlo may be correct because the weak absorption peak at 460 nm superimposing on the tail of the absorption at X < 400 nm should not appear violet. Figure 2 demonstrates collectively the optical spectra of various y-irradiated epoxides in CFCI, a t 77 K. All the spectra were observed for approximately the same irradiation dose. The absorption spectra immediately after y-irradiation were not shown

5390 The Journal of Physical Chemistry, Vol. 93, No. 14, 1989

Ushida et al. A+

H

H?\&yo,i

H

'H "Cis"

"TRANS"

,,,E,,

=-191,5654A , U ,

E,,,,,=-lE1.5663

*.d,

Figure 4. Optimized geometries for the cis and trans isomers. The GRADIENT method in GAUSSIAN 80 was used in combination with the UHF approximation and Huzinaga and Dunning's [9s5p/4s2p] basis set. The symbol of the double bond emphasizes the significant shortening of the Cz-0 bond.

Figure 3. ESR spectra of y-Irradiated PO in CFCI, and the simulation spectra. (A) immediately after y-irradiation at 77 K and recorded at the same temperature. (B) Same as (A) after photobleach with X > 690 nm to eliminate a small amount of the matrix-trapped hole ("photocleaning") and the subsequent photobleach with X > 500 nm at 77 K and measured at 140 K. The sample is associated with the optical spectrum in the dotted curve of Figure 2 and is attributed to the cis isomer (see text). (C) Same as (A) after photobleach with X = 440 nm at 77 K and measured at 140 K. The sample is associated with the optical spectrum in the solid curve of Figure 2 and is attributed to the trans isomer (see text).

in Figure 2, but they were roughly similar to those in the figure except for EO, for which Figure 1 is given. The solid and dotted curves for the two epoxides were recorded as a result of a reversible photwonversion of the spectra as described in the caption of Figure 2. The relation between the absorption maxima in Figure 2 and the degree of the alkylation is somewhat irregular, but the whole spectral feature seems similar. The spectrum for X-ray-irradiated 2,3-dimethyl-2,3-butylene oxide in CFC13 and CFCI2CF2C1may also be consistently included in Figure 2 whose absorption maximum at 560 nm should be responsible for the observed violet A representative study of the optical and ESR spectra will be discussed for propylene oxide (PO); the PO system showed a reversible photoinduced change in the optical spectrum (cf. Figure 2). The ESR spectrum also showed a reversible change between curves B and C in Figure 3 after the initial "photwleaning" (see the captions for Figures 2 and 3 for photocleaning). The temperature of the ESR measurement was 140 K rather than 77 K as in the case of the optical measurement to improve the ESR spectral resolution. However, we are confident that the ESR samples giving spectra B and C of Figure 3 correspond to the optical absorption bands in the dotted and the solid curves in Figure 2, because the slightly different but unmistakably discernible tints of the samples corresponding to the dotted and the solid curves of Figure 2 were preserved during the ESR measurement at 140 K of spectra in curves B and C of Figure

The reversibility in both the Optical and the ESR is accounted for by assuming a cis-trans isomerization of the ring-opened radical cation of Po. As for the initial

spectrum immediately after y-irradiation, it may be a mixture of both isomers, or it may be that the nascent structure of the ring-opened radical cation is slightly different from both the cis and the trans conformation. However, we will focus on the photoconvertible stages only. Assuming the cis-trans isomerization of the ring-opened structure, we have carried out subsidiary MO calculations. Figure 4 shows optimized geometries of the assumed isomers which were obtained by restricting to C, symmetry and using the GRADIENT method incorporated in the GAUSSIAN 80 programz5with the U H F approximation and Huzinaga and Dunning's [9s5p/4s2p] basis set. In deference to the "state of the art" calculation carried out for the radical cation of the geometries in Figure 4 should be regarded as only a guide for our arguments in the following. (See the discussion in ref 18 on the extremely subtle feature of the molecular geometry of the radical cation of EO.) The geometries of Figure 4 were used for a CNDO/S calculation: the first electronic transitions were predicted at 2.62 eV (473 nm) with f = 0.043 and at 2.61 eV (474 nm) withf= 0.041 for the cis and the trans isomers, respectively, and the second transitions were predicted to be deep in the UV region for both. If this result is of significance, the slightly blue- and red-shifted absorption bands in the dotted and solid curves in Figure 2 should be associated with the cis and the trans isomers, respectively. A standard INDO calculation based on the geometries in Figure 4 gives the following hfc constants: a( 1 H, methine) = -5.8 G, 4 3 H, methyl) = 8.1 G (the arithmetic average), and a(2 H, methylene) = -21.1 and -21 .O G for the cis isomer; and a(l H, methine) = -4.3 G, 4 3 H, methyl) = 7.6 G (the arithmetic average), and 4 2 H, methylene) = -21.6 and -21.4 G for the trans isomer. Since the observed spectra in curves B and C of Figure 3 appear almost isotropic, we ignored the effect of the anisotropy of a and g tensors in the simulation. However, despite numerous attempts referring to the above INDO calculation, we failed to reproduce the observed spectra satisfactorily. Therefore, we conjectured that some molecular motion such as two-site jumping in each isomer may have affected the ESR line shape and attempted to analyze the observed spectra by use of the extended Bloch equation. In the ring-opened structures such as those in Figure 4 there may be various intramolecular motions that could affect the observed ESR line shape. In order to search for the part in the radical cation that is effective in affecting the line shape, it should be efficient to use appropriately deuterated PO to simplify the experimental spectra. First, we guessed that the methylene group is the most plausible part that is executing (25) Binkley, J. S.; Whiteside, R. A,; Krishnan, R.; Seeger, R.; DeFrees, D. J.; Schlegel, H. B.; Topiol, S.; Kahn, L. R.; Pople, J. A. QCPE 1981, 13, 406. Program Library GAUSSIAN 80 (No. 482). Computer Center of the Institute for Molecular Science, Okazaki, Japan, 1982.

The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 5391

Methyl-Substituted Ethylene Oxides n

“CIS”

FRDfl

CH3-CD-O-CH1

and less pronounced temperature changes of the reduction of the hfc constants of the methylene group. This was, in fact, confirmed by experiment although the spectra are not shown to avoid redundancy. In this context we have reinvestigated the PO/CFCl2CFZCl system also which has been studied previo~sly:~The optical spectrum appearing after photobleaching with X = 440 nm of the sample immediately after y-irradiation was almost identical with the solid curve for PO/CFC13 in Figure 2 and the subsequent photobleach with A > 500 nm caused the spectral shift from ,A, = 470 nm to A, = 450 nm, quite similar to the shift from the solid curve to the dotted curve in Figure 2. Therefore, we presume that the same cis-trans isomerization takes place in the CFC12CF2Cl matrix at 77 K also. However, contrary to the PO/CFC13 system, the ESR spectra during the above photoinduced optical changes remained almost the same as that reported previo~sly,~ i.e., only a poorly resolved “triplet” of 4 2 H ) 21 G persisted. We interpret this to mean that in the matrix of CFC12CF2Cl,which is considered to be “softer” than CFC13,9the methylene group in both the cis and the trans isomers undergo a free rotation around the C2-0 axis and the hyperfine structure due to the methyl and the methine protons is not resolved enough so that only a broad “triplet” due to the rotating methylene protons becomes apparent. However, since the optical spectra are quite discernible as mentioned above, the general structural features of the cis and the trans isomers are considered to be retained in the CFC12CF2Cl matrix at 77 K also. In both Freon matrices studied the total hyperfine splitting of the assumed methylene protons amounts to 42 f 2 G, that is, 26 17 G and 22 18 G for the cis and the trans isomers in CFCI3 and 21 X 2 G for both in CFCl2CFZC1,which indicates a substantial localization of the odd electron on the methylene carbon. This should be due to the inequivalence in the C , - 0 and Cz-0 bonds as suggested in Figure 4. Two serious questions remain open: What is the reason for the significant differences in the hfc constants of the two methylene protons in both cis and trans isomers in CFC13, and what is the reason for the apparent lower activation energy of the assumed two-site jumping for the cis than for the trans isomer in CFC13? At present we have no answer, but in reference to the contrasting results of the two matrices studied, we conjecture that in the “rigid” CFC139the molecular motion of the two isomers is likely to be modified by the surrounding matrix molecules. Whatever the detailed nature of the effect of the matrix may be, it is emphasized that we have detected two discernible isomers by the combined study by optical and ESR spectroscopy. Although we will not discuss in detail the other PO derivatives, Le., 1,Zbutylene oxide and 2-methylpropylene oxide, the ring opening and the localization of the odd electron on the methylene carbon seem to be characteristic of the family of PO derivatives. As for 2-methylpropylene oxide the absence of any indication of photoinduced reversible conversions in the optical spectra (see the caption of Figure 2) and in the ESR spectrum (not shown) can be understood because there is no possibility of having distinguishable cis and trans isomers for this 2,2-dimethylated species. As for the prototype EO, whose optical spectra in Figures 1 and 2 are not greatly different from those of the other derivatives, there seems to be no motivation to regard the prototype as behaving differently from the others unless strong experimental evidence is presented to support the contrary. The smaller hfc constant of a(4 H ) = 16 G for EOS may indicate that the odd electron is shared rather evenly in the ESR time scale even if some inequivalence in the bond length of the two C-0 bonds may be real as suggested by Davidson.18 2,S-Butylene Oxide System. We have extended a similar study to the system of 2,3-butylene oxide (Le., 2,3-dimethylethylene oxide) to find spectral changes induced by the photo and the thermal treatments. This is anticipated if the ring is opened upon ionization. The optical spectra are already shown in Figure 2. The initial optical spectra immediately after y-irradiation were slightly different depending upon whether a mixture of cis- and trans-

=

.

Figure 5. Observed and simulated spectra of the assumed cis isomer originating from CH3CDOCH2in CFCI,. The jumping frequencies used in the standard two-site-jumping model to reproduce the observed spectra are given in MHz. The simulation parameters are, a(1 H, methine) = 12 G, a(3 H, methyl) = 12 G, and a = 26 and 17 G for the methylene protons.

sin. 6.0 llhz

.

Figure 6. Observed and simulated spectra of the assumed trans isomer Originating from CH3CDOCH2in CFCI3. The simulation parameters are a(l H, methine) = 12 G, a(3 H, methyl) = 12 G, and a = 22 and 18 G for the methylene protons.

-

motion, so we employed CH3CDOCH2to minimize unnecessary complexity that could originate from the methine proton. By the same photochemical treatment described for the spectra in Figures 2 and 3, we obtained spectra in Figures 5 and 6 for the assumed cis and trans isomers, both of which showed temperature dependence. The agreement of the observed spectra in Figures 5 and 6 with the accompanying simulated spectra is fair. However, the vulnerable part in the simulation is that we were forced to use the following hfc constants: a( 1 D, methine) = 2 G, a(3 H, methyl) = 12 G, and a = 26 and 17 G for the two methylene protons for the cis isomer; and a(1 D, methine) = 2 G, a(3 H, methyl) = 12 G, and a = 22 and 18 G for the two methylene protons for the trans isomer arising from the above deuterated PO. With these parameters and the jumping frequencies in Figures 5 and 6, the activation energy for the assumed motion of the methylene group was calculated as 0.45 and 2.65 kcal/mol for the cis and trans isomers, respectively. If the methylene group is the most influential in determining the ESR line shape as suggested by the above choice of the hfc constants, the cis and the trans isomers of another deuterated PO, I CH3CHOCD2,should give more or less similar looking spectra

+

+

5392

The Journal of Physical Chemistry, Vol. 93, No. 14, 1989 U%

/ CFC13,

Ushida et al.

77 K

Figure 9. Do: ESR spectra of the isomer that has been photobleached to give the spectrum in Figure 7A at 77 K, but measured at 80 and 40 K. D,: Same as Do, but one of the methine protons in the starting BO is deuterated. D2:Same Do, but both of the methine protons in the starting BO are deuterated.

* 30 G

Figure 7. ESR spectra of 2,3-BO in CFC1, at 77 K. (A) After photobleaching of the irradiated sample at 77 K with X > 550 nm. (B) Same as (A) after subsequent photobleach with X = 520 nm. (A) and (B) correspond to the dotted and the solid curves in Figure 2. The photoconversion between (A) and (9) was repeatable as in the case of the PO systems.

Figure 8. Observed and simulated ESR spectra of 2,3-BO and its mono(D,) and dideuterated (Dz) isotopes in CFCI, at 120 K. The assignment of 16 and 14 G to the methyl and the methine protons with the assumption of a complete averaging of the anisotropy by rotation gives the fair simulation spectra at the bottom (see text).

2,3-BO or the isolated trans isomer was used as the solute. (In the cis isomer two methyl groups are on the same side of the plane of the ethylene oxide framework whereas in the trans the two are on different sides of the plane.) However, after the initial “photobleaching” (see the caption for Figure 2) of the y-irradiated sample, the familiar reversible photoconversion was observed as shown in Figure 2. If the ring opening occurs in the 2,3-BO system, three topologically different isomers, all-trans, cis-trans, and all-cis, are expected. However, we exclude the last one in the following discussion because the experimental results do not require the assumption of the occurrence of the three isomers but only two suffice and because the steric hindrance due to the two methyl groups id all-cis isomer will probably disfavor its stable persistence at the instance of the ring opening upon ionization. Figure 7 shows the ESR spectra of 2,3-BO/CFC13 at 77 K. The upper and the lower spectra correspond to the optical spectra in the dotted and the solid curves in Figure 2. The reversibility of the optical change shown in Figure 2 parallels with the reversible change of ESR spectra between Figure 7A and Figure 7B. In order to analyze the observed ESR spectra, we first tried to get a rough estimation of the isotropic hfc constant of the methyl and the methine protons by raising the temperature of the sample exhibiting the spectrum in Figure 7A to 120 K. Similar treatments were also made for the two isomers arising from CH3CDOCHCH3 (denoted as D1) and CH3CDOCDCH3(D2) to obtain a nine-line,

-

.

Figure 10. Do: ESR spectra of the isomer that has been photobleached to give the spectrum in Figure 7B, but measured at 80 and 40 K. D,: Same as Do, but one of the methine protons in the starting BO is deuterated. D2: Same as Do, but both of the methine protons in the starting BO are deuterated.

an eight-line, and a seven-line spectrum, respectively as shown in Figure 8 (the notation Do corresponds to undeuterated BO). The simulation assuming a set of a(methy1 H ) = 16 G and a(methine H) = 14 G reproduced fairly well the observed spectra shown at the bottom of Figure 8. Upon cooling the sample exhibiting the spectra in the upper panel of Figure 8 to 80 and 40 K, the spectra in Figure 9 were obtained. Since the spectral changes with temperature were completely reversible between 120 and 40 K, the changes must be associated with some molecular motions in one of the two isomers assumed to be all-trans and cis-trans, as in the case of the PO systems. The spectra designated Do in Figure 10 were obtained at 80 and 40 K for the sample exhibiting the spectrum in Figure 7B at 77 K. Likewise, spectra D, and D2 in Figure 10 are the corresponding spectra for mono- and dideuterated BO treated in the same way as undeuterated BO. It was found that the three spectra Do through D2 in the upper part of Figure 10 became almost identical with spectra Do through D2 in Figure 9 by raising the temperature to about 90 K and then recooling to 80 and 40 K. This result indicates that the isomer originally exhibiting the ESR spectrum in Figure 7B and the optical spectrum in the solid curve of Figure 2 has isomerized to the other by surmounting a potential barrier upon warming to about 90 K. Due to the increased complexity of the BO systems, the complete spectral analysis in relation to the molecular motion is difficult. However, several remarks can be made for the spectra in Figures 8-10. ( I ) The general pattern of the spectrum for D2 at 80 K in Figure 10 seems to be a septet with a(6 H ) = 16 G and a quintet with 4 4 H ) = 24 G at 40 K. This may imply that the two methyl groups are rotating rapidly enough at 80 K but that at 40 K they are immobilized and only four out of the six methyl protons are out of the plane defined by the oxygen and the two adjacent carbon atoms. At both temperatures the total hfc splitting due to the methyl protons becomes equal as 16 G X 6 = 24 G X 4, as it should be. (2) The spectrum for D2 at 40 K in Figure 9 also appears to be a quintet with a hfc constant of 24 G, similar to the spectrum for D2 at 40 K in Figure 10. Comparison of D I and D2 at 40 K in Figure 9 suggests that D1 gives a 10-line spectrum resulting

J . Phys. Chem. 1989, 93. 5393-5400 from the splitting of each of the quintet lines into a doublet of an equal intensity with a hfc constant of 14 G. This value coincides with that assigned to the methine protons in the discussion for Figure 8. One of the two methine protons that has been deuterated in Dl, thus, seems to contribute only to broaden the 10 lines with a hfc constant of a ( l D) = 2.2 G. Furthermore, the spectrum for Doat 40 K in Figure 9 can be regarded as a quintet-triplet, the triplet splitting being due to the two methine protons with the same hfc constant of 14 G. (3) The above interpretation for Doto D2at 40 K in Figure 9 seems applicable to Do through D2 at 40 K in Figure 10 also, although the additional small splittings in the latter hampers the comparison between the two sets of Do-D2. The small splittings are probably due to the superhyperfine interaction with fluorine atoms in the matrix molecules as proposed for the EO ~ y s t e m . ~ (4) The above assumption that the methine proton possesses a common hfc constant of 14 G irrespective of the temperature leads to the conclusion that the observed spectral change with temperature shown in Figures 8-10 is associated with dynamic behavior of the methyl groups. As mentioned above, the detailed analysis of such a dynamic effect on the line shape is suspended at the moment, and the definite identification of the two assumed rotational isomers is reserved. However, we emphasize again the existence of two

5393

rotational isomers in the BO systems, too, which reinforces the hypothesis of the ring opening of the radical cations of epoxides. Conclusion We have surveyed the radical cations of EO, PO, and BO in Freon matrices at various temperatures below the melting point of the matrices and found reversible photoconversion of the optical and ESR spectra that can be understood in terms of a cis-trans isomerization of ring-opened radical cations. As for the prototype ethylene oxide, we suggest that the ring opening is a natural consequence extrapolated from the results of the other epoxide systems studied.

Acknowledgment. M. C. R. Symons, F. Williams, and T. Bally have kept us informed of their published and unpublished results. W e acknowledge Dr. A. Kira of the IPCR for his general assistance in performing the present work. Dr. T. Kat0 at Kyoto University helped us in our daily activity. The study was supported by the subsidies for scientific research of the Ministry of Education in Japan, Grants 59430004, 60790030, and 62606006. Registry No. D,, 120743-09-1; Dz, 120743-10-4; CHpCDOCH2,

.

.

41452-33-9; CHpCHOCDz, 8903 1-89-0; CFClp, 75-69-4; CFC12CF2C1, 76-13-1; ethylene oxide, 75-21-8; propylene oxide, 75-56-9; cis-2,3-butylene oxide, 1758-33-4; trans-2,3-butylene oxide, 21490-63-1.

Spectroscopy of Tryptophan in Supersonic Expansions: Addition of Solvent Molecules Chin Khuan Teh, Jeffrey Sipior, and Mark Sulkes* Chemistry Department, Tulane University, New Orleans, Louisiana 70118 (Received: October 18, 1988; In Final Form: January 4 , 1989)

Solvent complexes with the amino acid tryptophan have been studied spectroscopically in the So-SI origin region by using supersonic free jet techniques. The major spectroscopic method employed was laser-induced fluorescence excitation spectra; individual features were also characterized by use of fluorescence lifetime determinations or dispersed emission spectra. Addition of proton-donating solvents results in the appearance of a slightly blue-shifted band in the vicinity of 3 4 960 cm-I. Evidence suggests that this band results from interactions analogous to those that produce a similar band upon proton solvent addition to tryptamine. Unlike the case of tryptamine, other site addition possibilities in tryptophan bring about slightly red-shifted solvent bands as well.

Introduction Spectra of jet-cooled bare molecules and solvent complexes of indoles,'-8 analogues of t r y p t ~ p h a n , ~and - ' ~ bare molecule tryptophanI4-l7 have been reported in recent years. More recently, (1) Montoro, T.; Jouvet, C.; Lopez-Campillo, A.; Soep, B. J. J . Phys. Chem. 1983,87, 3582-3584. (2) Nibu, Y.; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1983, 87, 3898-3901. (3) Hays, J. R.; Henke, W. E.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1983, 97, 347. (4) Philips, L. A.; Levy, D. H. J . Chem. Phys. 1986, 85, 1327-1332. (5) Bersohn, R.; Even, U.; Jortner, J. J . Chem. Phys. 1984, 80, 1050. (6) Hager, J. W.; Wallace, S. C. J . Phys. Chem. 1983, 87, 2121. (7) Hager, J. W.; Wallace, S. C. J . Phys. Chem. 1984, 88, 5513-5519. (8) Hager, J. W.; Demmer, D. R.; Wallace, S. C. J . Phys. Chem. 1987, 91, 1375-1382. (9) Park, Y. D.; Rizzo, T. R.; Peteanu, L. A.; Levy, D. H. J . Chem. Phys. 1986, 84,6539-6549. (10) Philips, L. A.; Levy, D. H. J. Phys. Chem. 1986, 90, 4921-4923. (1 1) Sipior, J.; Sulkes, M.;Auerbach, R.; Boivineau, M. J . Phys. Chem. 1987, 91, 2016-2018. (12) Philips, L. A.; Levy, D. H. J. Chem. Phys. 1988, 89, 85-90. (13) Philips, L. A.; Webb, S. P.; Martinez, S.J.; Fleming, G. R.; Levy, D. H. J. Am. Chem.Soc. 1988, 110, 1352-1355. (14) Rizzo, T. R.; Park, Y. D.; Peteanu, L.; Levy, D. H. J . Chem. Phys. 1985, 83, 4819-4820. (15) Rizzo, T . R.; Park, Y. D.; Peteanu, L.; Levy, D. H. J. Chem. Phys. 1986,84, 2534-2541. (16) Rizzo, T . R.; Park, Y. D.; Levy, D. H. J. Am. Chem. SOC.1985,107, 277-278.

0022-3654/89/2093-5393$01.50/0

detailed work on solvent complexes with tryptophan analogues has begun.18-20 An ultimate goal is to understand better the solution fluorescence of tryptophan. In solution, tryptophan fluorescence decay is known to have multiple-exponential components,21 and it was plausibly proposed that this was due to multiple emitting As refined by Fleming and cow o r k e r ~ , the ~ ~ ,conformer ~~ theory proposes that tryptophan fluorescence quenching in solution takes place as a result of intramolecular charge transfer from the indole to side-chain electron acceptors at the 3-indole position, principally the carbonyl group (see Figure 1). Since quenching will vary as a function of the orientation and distance of the indole relative to electron acceptors, the existence of different conformers should make for different emission lifetimes. The possibility of excited-state complexation with solvent molecules, placing the tryptophan IL, state below the ILbstate, has also been brought up.24,25Supersonic expansion (17) Rizzo, T. R.; Park, Y. D.; Levy, D. H. J. Chem. Phys. 1986, 85, 6945-6951. (1 8) Sipior, J.; Sulkes, M. J. Chem. Phys. 1988, 88, 6 146. (19) Sulkes, M.; Sipior, J. SPIE Proc. 1988, 910, 204-208. (20) Peteanu, L. A,; Levy, D. H. J . Phys. Chem., in press. We are grateful to Prof. Levy for a preprint of this work. (21) Szabo, A. G.; Raynor, D. M. J. Am. Chem. SOC.1980,102,554-567. (22) Chang, M. C.; Petrich, J. W.; McDonald, D. B.; Fleming, G. R. J . Am. Chem. SOC.1983, 105, 3819-3824. (23) Petrich, J. W.; Chang, M. C.; McDonald, D. B.; Fleming, G. R. J . Am. Chem. SOC.1983, 105, 3824-3832.

0 1989 American Chemical Society