J . Phys. Chem. 1990, 94, 5786-5791
5786
Discrimination and Selective Reaction of Rotational Isomers of Jet-Cooled Substltuted Benzaldehydes As Studied by Sensitized Phosphorescence Excitation Spectroscopy Seiji Yamamoto, Takayuki Ebata, and Mitsuo Ito* Department of Chemistry, Faculty of Science, Tohoku University, Sendai 980, Japan (Received: December 19, 1989; In Final Form: March 2, 1990)
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The electronic transitions of the rotational isomers of nonfluorescent ortho- and meta-substituted (fluor0 and methyl) benzaldehydes have been investigated in supersonicjets. The Sl(n,r*) - S o and Tl(n,r*) S, transitions have been measured by the sensitized phosphorescence excitation method and partly by the resonance-enhanced multiphoton ionization (REMPI) method. The two rotational isomers ( 0 4 s and 0-trans isomers) arising from the orientation of the formyl group with respect to the ortho or metal substituent have been discriminated. The energy difference in the 0 4 transition between the two isomers is less than 40 cm-' for meta-substituted benzaldehyde but more than 500 cm-I for ortho-substituted benzaldehyde, showing the electronic excitations localized on the C = O group in Sl(n,r*) and T,(n,r*). It was found that the 0-cis isomer of emethylbenzaldehyde undergoes selectivechemical reaction in its triplet state in the isolated molecular condition. It is suggested that the reaction is an intramolecular hydrogen abstraction.
-
By using these spectroscopies, we observed the Sl(n,a*) So Introduction transitions of 0-,m-, and p-fluorobenzaldehydes and 0-,m-, and In a series of our papers, we have reported on the discrimination p-methylbenzaldehydes. For meta-substituted benzaldehydes and of the rotational isomers of ortho- and meta-substituted phenols emethylbenzaldehyde, the existence of the two rotational isomers and anisoles using the electronic transition in a supersonic free was confirmed. The difference in the electronic excitation energy j e ~ l - In ~ these molecules, the S,(T,T*) So excitation energy between the two isomers is large for ortho-substituted benzdifference between the rotational isomers amounts to 100-300 aldehyde but very small for meta-substituted benzaldehyde, incm-I. Such a large excitation energy difference indicated the dicating the electronic excitation localized on the C = O group potential of electronic spectroscopy in studying rotational isomers in the Sl(n,r*) state. Similar results were also obtained for the of a large polyatomic molecule. T,(n,r*) So transitions. In the present work, we extend the study to the rotational One of the interesting aspects of rotational isomers is their isomers of another type of aromatic molecule, that is, ortho- and characteristic dynamical behaviors including reactions. In a meta-substituted benzaldehydes. There exist two rotational iscprevious paper, we reported that the rotational isomers of omers for each molecule. They are 0-cis (syn) and 0-trans (anti) chlorophenol have different emission properties resulting from a isomers arising from orientation of the oxygen atom of the formyl large difference in the intersystem crossing rate.8 A similar but group with respect to the ortho or meta substituent (Figure 1). more remarkable difference was found for photochemical behaviors Similar to the cases of substituted phenols and anisoles, it is of the rotational isomers of emethylbenzaldehyde in their S,(n,r*) expected that the rotational isomers of substituted benzaldehyde and Tl(n,T*) states. One of the isomers was found to undergo will be easily discriminated by the electronic spectrum of the a photochemical reaction involving intramolecular hydrogen abjet-cooled molecule. However, fluorescence excitation spectrosstraction (photoenolization). copy, which was successfully applied for the measurements of the electronic spectra of jet-cooled phenols and anisoles, cannot be Experimental Section used for substituted benzaldehyde, because the latter is completely The sensitized phosphorescence excitation spectra of jet-cooled nonfluorescent. It is known however that they are usually highly molecules were measured with the same apparatus as that reported phosphorescent. Therefore, we can measure the electronic elsewhere.' The sample was heated to about 340 K to obtain spectrum by phosphorescence excitation spectroscopy. However, sufficient vapor pressure and was seeded in 3 atm of He gas. The the application of this technique to jet-cooled molecules is not so gas mixture was expanded into a vacuum chamber through a easy, because of a very weak phosphorescence signal gained by pulsed nozzle with an orifice of 800-pm diameter. The UV output a space-fixed photomultiplier from the molecules having a long of a dye laser (Lambda Physik FL 3002) pumped by a XeCl phosphorescence lifetime and moving with a high speed in the jet. excimer laser (Lambda Physik LPX100) was used as an exciting The difficulty has been solved by the development of a new light source. The laser light crossed the jet 15 mm downstream spectroscopy called sensitized phosphorescence excitation specof the nozzle. The triplet-state molecules produced by the introscopy, which uses sensitized phosphorescence emitted by a solid phosphor to which the triplet-state molecules in the jet ~ o l l i d e . ~ ~ ~tersystem crossing from the SIstate or by the direct laser excitation travel for about 40 ps further downstream from the excitation We used this spectroscopy for the measurements of the electronic position and collide with a liquid nitrogen cooled copper surface spectra of the substituted benzaldehydes. We also used resocovered with the solid sample serving as a sensitized phosphor. nant-enhanced multiphoton ionization (REMPI) spectroscopy for When the triplet-state molecules collide with the phosphor surface, o-methyl benzaldehyde. the energy transfer occurs and the sensitized phosphorescence is emitted. The sensitized phosphorescence was detected by a photomultiplier (HTV R585), and the signal was processed by ( I ) Oikawa, A,; Abe, H.; Mikami, N.; Ito, M. J . Phys. Chem. 1984,88, a gated photon counting system (Ortec 9302, 931 5, 9325, and a 5180. ( 2 ) Ito, M.; Oikawa, A. J . Mol. Spectrosc. 1985,126, 133. home-made gate generator). (3) (a) Oikawa, A.; et al., unpublished data. (b) Oikawa, A,; Abe, H.; The measurements of REMPI spectra were described elseMikami, N.; lto, M. Chem. Phys. Lett. 1W5, 116, 50. wherea9 For the one-color REMPI spectrum, the UV output of (4) Yamamoto, S.;Okuyama, K.; Mikami, N.; Ito, M. Chem. Phys. Lett. the XeCl excimer laser pumped dye laser was focused 10 mm 1986,125, 1 . (5) (a) Mizuno, H.; Okuyama, K.: Ebata, T.; Ito, M. J . Phys. Chem. 1987, downstream of the free jet by a 25-cm focal length lens, and the
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-
91, 5589. (b) Aota, T.; Ebata, T.; Ito, M. J . Phys. Chem. 1989,93, 3519. (6) Kamei, S.; Abe, H.: Mikami, N.; Ito, M. J . Phys. Chem. 1985,89, 3636. (7) Ohmori, N.; Suzuki, T.;lto. M. J . Phys. Chem. 1988.92, 1086.
0022-3654/90/2094-5786%02.50/0
(8) Yamamoto, S.; Ebata, T.; Ito, M. J . Phys. Chem. 1989, 93, 6340. (9) Mikami, N.; Suzuki, 1.; Okabe, A. J . Phys. Chem. 1987,91, 5242.
0 1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5787
Jet-Cooled Substituted Benzaldehydes
@rX
8"
0-cis (syn)
0-trans (anti)
11308)
Figure 1. Molecular structures of 0-cis (syn) and 0-trans (anti) rotational isomers of ortho- and meta-substituted benzaldehydes; X indicates substituent. 16
F
I
I
I
27600 28200 WAVENUMBER I cm-' Figure 2. Sl(n,r*) So sensitized phosphorescence excitation spectrum 27000
-
of jet-cooled m-fluorobenzaldehyde.
+
+
molecules were ionized by a 1 2 or 1 3 process. For the two-color REMPI measurement, laser light of about 200-240 nm was necessary for the ionization step. The laser light of this wavelength was generated either by third harmonic generation using the fundamental and the second harmonic of a Nd:YAG laser (Quantel YG 581-10) pumped dye laser (Quantel TDL 50) or by second harmonic generation using the fundamental of the XeCl excimer laser pumped dye laser. The two unfocused laser beams were introduced coaxially into the supersonic jet, and the delay time of the ionization laser was changed from -50 to 300 ns by a digital delay generator (BNC Model 7030 A). The molecular cations generated by one-color or two-color REMPI via the SIstate or the TI state were repelled by a positively charged plate from the ionization region into the entrance of a quadrupole mass filter (Extranuclear 4-270-9) and detected by an electron multiplier (Murata Ceratron). The ion signals was amplified by a current amplifier (Keithley 427) and averaged by a boxcar integrator (PAR 4402/4420) system. The samples, 0-, m-, and p-methylbenzaldehydes and 0-,m-, and p-fluorobenzaldehydes (Tokyo Kasei), were used without further purification. Results A. Meta-Substituted Benzaldehydes. Figure 2 shows the Sl(n,r*) So sensitized phosphorescence excitation spectrum of Jet-COOled m-fluorobenzaldehyde. It is seen from the figure that all the main bands consist of two close peaks. In the region of the 0-0 band, there exist two strong bands (26 71 5 and 26 731 cm-I), which are separated by 16 cm-I. Similar band pairs can be seen in the entire spectral region. A pair of bands separated by 43 cm-' appears at about 1330 cm-l above the 0-0 band, and the frequency is assigned to the excited-state C=O stretching vibration. These bands have their relative intensity almost equal to that of the pair of the band origin. Since such a pair is not found in the spectra of benzaldehyde or p-fluorobenzaldehyde measured under the same condition, the two peaks at 26 715 and 26 73 1 cm-' are attributed to the 0-0 transitions of the two ro-
-
-
and of C=O stretching vibrational band (b).
1 1
I
2j100 28'400 WAVENUMBER I cm-' WAVENUMBER I cm" Figure 3. Sl(n,?r*) So sensitized phosphorescence excitation spectrum of jet-cooled m-methylbenzaldehyde in the regions of the 0-0 band (a)
tational isomers (0-cis and 0-trans) of m-fluorobenzaldehyde. Although the stabilities of the isomers were discussed from dipole moment,'"'* IR,l3-l4NMR,I5 and theoretical c a l ~ u l a t i o n s , ' ~a J ~ clear conclusion has not been obtained and almost equal stability is suggested. Recently, Haque et al. reported from the study of the phosphorescence spectrum that the 0-trans isomer is more populated than the 0-cis isomer in the ground state.'* Accepting their result, the stronger band at 26 731 cm-I is assigned to the 0-0 band of the 0-trans isomerI9 and the weaker one at 26715 cm-I to that of the 0-cis isomer. The difference in the Sl(n,a*) Soelectronic transition energy between the two rotational isomers is only 16 cm-I, which is much smaller than the difference found for the SI(*,**) So transition of meta-substituted phenol or anisole. The Sl(n,a*)state of benzaldehyde arises by the excitation of a nonbonding electron localized on the oxygen atom of the C=O group to an antibonding a* orbital. Although the a* orbital spreads over whole molecule, the contribution of the C=O group to the a* orbital is predominant as evidenced by a large frequency change of the C-0 stretching vibration in going from So (- 1700 cm-I) to SI (- 1300 cm-I). Therefore, the electronic excitation is considerably localized in the C=O bond. As a result, the orientation of the formyl group with respect to the m-fluorine atom being located far from the C=O group gives only a small effect on the excitation energy. This situation is in contrast to the case of meta-substituted phenol or anisole, where the electronic excitation spreads in the Sl(a,n*) state over the whole molecule, and a fairly large excitation energy difference arises from the orientation of the OH or OCH3group. A small excitation energy difference was also found for jet-cooled m-methylbenzaldehyde, whose SI So sensitized phosphorescence excitation spectrum is shown in Figure 3. The longest wavelength band clearly forms a doublet (26910 and 26918 cm-I) with a frequency interval of 8 cm-I. These two bands can be assigned to the 0-0 bands of the rotational isomers of this molecule in accordance with the case of m-fluorobenzaldehyde. The stronger band at 26918 cm-' is tentatively assigned to the 0-0 band of the 0-trans isomer and the weaker band at 26910 cm-I to that of the 0-cis isomer from their relative intensities similar to those of m-fluorobenzaldehyde. A much smaller excitation energy dif+-
+ -
+ -
(10) Bruce, E. A. W.; Ritchie, G.L. D.; Williams, A. J. Ausf. J . Chem. 1974, 27, 1809. ( 1 1 ) Bock, E.; Tomchuk, E. Con. J . Chem. 1972, 50, 2890. (12) Aw, C. T.: Huang, H. H.; Tan, E. L. K. J. Chem. Soc., Perkin Trans. 2 1972, 1638. (13) Green, J. H. S.;Harrison, D. J. Specfrochim. Acta 1976,324 1265. (14) Miller, F. A.; Fateley, W. G.;Witkowski, R. E. Spectrochim. Acta
__
1967. 23A. . ._ . , _ _ _891 __ , .
(IS) Drankenberg, T.; Jost, R.; Sommer, J. M. J . Chem. SOC.,Perkin
Trans. 2 1975, 1682. (16) Wasylishen, R.; Schaefer, T. Con. J . Chem. 1971,49, 3216. (17) Lakshmi, A.; Walker, S.;McClelland, B. J.; C a l d e r w d . J. H. J . Mol. Struct. 1982, 95, 249. (18) Haque, M. K.; Thakur, S. N. Chem. Phys. Lett. 1979, 66, 561. (19) Haque, M. K.; Thakur, S. N. Indian J . Phys. 1981, 55E,477.
5788 The Journal of Physical Chemistry, Vol. 94, No. 15. 1990
Yamamoto et al.
(b)
(a)
I
b/b ~I
ZO?C
2siaa
-
2&QO
WAVENUMBER I c n i '
WAVENUMBER /
c d
Figure 4. Tl(n,r*) So sensitized phosphorescence excitation spectrum of jet-cooled m-fluorobenzaldehyde in the regions of the 0-0 band (a) and of C=O stretching vibrational band (b).
ference in m-methylbenzaldehyde than that in m-fluorobenzaldehyde is readily understood because the CH3 group is a much weaker perturber than the fluorine atom. The weak perturber of the CH3 group is evidenced by the frequency of the 0-0 band of m-methylbenzaldehyde (26910 and 26918 cm-' for 0-cis and 0-trans, respectively), which is almost identical with that of benzaldehyde (26919 cm-l).' In contrast, the 0-0 band frequency of m-fluorobenzaldehyde (26 73 1 and 267 15 cm-' for 0-trans and 0-cis, respectively) differs by about 200 cm-l from that of benzaldehyde. Figure 4a shows the Tl(n,r*) SOsensitized phosphorescence excitation spectrum around the band origin of jet-cooled mfluorobenzaldehyde. The spectrum again exhibits a doublet feature. The band origin consists of the stronger band at 24981 cm-I and the weaker one at 24950 cm-I, the interval being 31 cm-I. The stronger band is tentatively assigned to the 0-0 band of the 0-trans isomer and the weaker one to that of the 0-cis isomer for a reason similar to that for the S I So transition. From the electronic transition energies obtained for the SIand T I states, the energy separation between the Sl(n,r*)and Tl(n,r*) states is determined to be 1750 and 1765 cm-' for the 0-trans and 0-cis isomers, respectively. Figure 4b shows the TI So spectrum of jet-cooled m-fluorobenzaldehyde in the C-0 stretching vibrational region. The C--O stretching vibrational frequency in the TI state is 1314 and 1334 cm-I for the 0-cis and 0-trans isomers, respectively. In the case of the TI So transition of m-methylbenzaldehyde, the pair of 0-0 bands was also observed. The stronger band at 25 190 cm-l is tentatively assigned to the 0-0 band of the 0-trans isomer and the weaker one at 25 156 cm-' to the 0-0 band of the 0-cis isomer, the frequency difference being 34 cm-'. For both m-fluorobenzaldehyde and m-methylbenzaldehyde, the frequency difference between the isomers is larger in TI than that in S I , especially for m-methylbenzaldehyde. We also observed the SI So sensitized phosphorescence excitation spectrum of jet-cooled p-fluorobenzaldehyde. The spectrum is rather simple since there is no isomer in this case. The strong band at 27 207 cm-' was assigned to the 0-0 band, whose frequency agrees well with the reported value in the absorption spectrum.I9 From the observation of the TI So sensitized phosphorescence excitation spectrum, it is found that its 0 band is located at 25484 cm-'. Therefore, the &-TI energy gap is 1723 cm-' for this molecule. B. Ortho-Substituted Benzaldehyde. Figure 5 shows the S I S, sensitized phosphorescence excitation spectrum of jet-cooled o-fluorobenzaldehyde. Only one peak of the 0-0 band was observed at 26 331 cm-' for this molecule in spite of the possibility of the two rotational isomers. This result suggests that one of the isomers has a dominant amount. It has been reported from the measurements of dipole moment" and NMRI6 spectra that the 0-trans isomer is much more stable than the 0-cis isomer by the ratio of about 4:l. The stability of the 0-trans isomer is due to the intramolecular hydrogen bonding formed between the fluorine atom and the hydrogen atom of the formyl group. Therefore, most of the bands in the spectrum in Figure 5 are
-
-
-
-
-
-
+-
0-trans 26125
113171
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990 5189
Jet-Cooled Substituted Benzaldehydes (a)
TABLE I: Spectroscopic Data (cm-I) of the S,(n,r*) and T,(n,r*) States of Fluorobenzaldehydes; Data for Benzaldehyde Also Listed for Comparison' o-fluoro
0-trans SI so 0; 26331 A" shiftb -588 TI So 08 24599 A. shiftb -584 SI-TI gap 1732 C = O stretching SI 1339 TI 1334 +
0: 0-cis 24861
-
0: 0-trans 24401
I
I
I
I
m-fluoro
0-cis 0-trans p-fluoro benzaldehyde 26715 26731 16 -204 -188 24950 24981 31 -233 -202 1765 1750 1316 1314
1343 1334
27207
26919
+288 25484
25 183
+301 1723
1736
1330 1322
1313 1316
0
0
a A indicates the frequency difference of the 0transition between the two rotational isomers. bThe frequency difference of the 0-0 band from that of benzaldehyde. The minus and plus signs indicate red- and blue-shifts, respectively.
TABLE II: Spectroscopic Data (cm-I) of the Sl(n,r*)and Tl(n,r*) States of Metbylbenzaldebydes;Data for Benzaldehyde Also Listed for comparison' o-methyl m-methyl pbenz0-cis 0-trans 0-cis 0-trans methyl aldehyde
-
SI so 0: A" shift*
-
T, A0
So 0 :
shiftb %-TI gap C=O stretching SI
26641 26125 516 -278 -794 24867 24401 466 -316 -782 1774 1724 1332
1317
26910 26918 8 -9 -I 25156 25190 34 -27 +7 1754 1728 1308
1341
27081
26919
+162 25352
0 25183
+169 1729
0 1736
1331
1313
A indicates the frequency difference in the 0-0 transition between the two rotational isomers. *The frequency difference of the 0-0 band from that of benzaldehyde. The minus and plus signs indicate red- and blue-
shifts, respectively.
-
methods. Figure 7a shows the TI Sosensitized phosphorescence excitation spectrum around the band origin. Only a single band appears at 24401 cm-I, and this frequency disagrees again with the 0-0 band at 25 160 cm-I reported for the solution absorption So one-color REMPI spectrum.20 Figure 7b shows the T , spectrum in the same region as that of Figure 7a. A newly appearing band at 24867 cm-' is stronger than the band at 24401 cm-'. Therefore, the result shows again that one of the rotational isomers having the 0-0 band at 24 867 cm-I has a short lifetime even in the lowest triplet state (less than 40 ps). This is quite different from the normal benzaldehyde derivative, whose triplet lifetime is usually much longer than 40 ps. In both the SI So and TI Soelectronic excitations, the band at shorter wavelength could be observed only in the REMPI spectrum. Therefore, the bands at 24401 and 26 125 cm-l are the band origins of TI So and S, So transitions of one of the two rotational isomers, and the bands at 24867 and 26641 cm-l are those of another isomer. As will be discussed later, the bands observed on the longer wavelength side are assigned to the 0-trans isomer and those observed on the shorter wavelength side to the 0-cis isomer.
-
-
-
-
Discussion The spectroscopic data obtained in the present work are summarized in Tables I and 11. It is seen from the tables that both the Sl(n,?r*) So and T,(n,?r*) So transitions are shifted to the blue in para-substituted benzaldehyde relative to those of benzaldehyde. On the other hand, red shift is generally seen for all the isomers of ortho- and meta-substituted benzaldehyde except for the TI So transition of the 0-trans isomer of m-fluorobenzaldehyde. It is interesting to compare this direction of the shift with that of the S1(?r,r*) So transition of substituted SO phenol. For example, in p-fluorophenol, the S,(r,n*) transition exhibits a large red-shift relative to phenol,3a and all the rotational isomers of 0- and m-fluorophenols show a blue-
-
+
-
+
(22) Schaefer. T.; Salmon, S. R.;Wildman, T. A. Can.J . Chem. 1980,58, 2364.
-
579(
The Journal of Physical Chemistry, Vol. 94, No. 15, 1990
-..+
bCH3OCH3 h v'
J, 3*
1st
O\ / H
'F
HO\t/H I
(yHz L
Figure 8. Reaction scheme of the rotational isomers of o-methylbenzaldehyde after excitation to the S , state in solution. ISC stands for intersystem crossing.
shift.3b Therefore, the direction of the shift is entirely different between substituted phenol and substituted benzaldehyde. The difference reflects the difference in the nature of the SIstate, that is, the (n,x*)state for a substituted phenol and the (n,x*) state for a substituted benzaldehyde. As mentioned before, the characteristic nature of the SI!",:*) or Tl(n,n*) state is also reflected in the difference in the excitation energy between the two rotational isomers, that is, the difference is very small for meta-substituted benzaldehyde but very large for ortho-substituted benzaldehyde. This can be ascribed to the electronic excitation localized on the C=O bond. In the case of the S,(n,n*)of a substituted phenol, the difference in the excitation energy between the rotational isomers is comparable between the ortho- and meta-substituted phenols, showing the spread of the electronic excitation over the whole molecule. The Sl(n,r*)-Tl(n,n*) energy gap is nearly constant at 1730 cm-I for all the molecules and isomers studied, although a slightly larger value is found for m-fluorobenzaldehyde and for the 0-cis isomers of 0-and m-methylbenzaldehyde. The constant energy gap implies that the electronic structures of the S,(n,r*) and Tl(n,n*) states of each molecule are very similar except for spin, and the energies of the both states are perturbed by almost equal amounts irrespective of the nature of the perturbation. As shown in Figures 6 and 7, one of the two isomers of omethylbenzaldehyde is detected by the sensitized phosphorescence excitation method and another isomer can be detected only by the REMPI method. This result means that the latter isomer has a fast nonradiative process in its triplet state. As a result, the triplet-state lifetime is much shorter than 40 M, which is the limit of the detection in our sensitized phosphorescence experiment. o-Methylbenzaldehyde is known to be a typical molecule exhibiting photoenolization like o-methylacetophenone or o-methylbenzop h e n ~ n e . ~ ~The - ~ *oxygen atom of the formyl group abstracts
-
(23) Haag. R.; Wirz, J.; Wagner, P. J . Helv. Chim. Acta 1977, 60, 2595. (24) Wagner, P. J.; Chen, C. P. J . Am. Chem. SOC.1976, 98, 239. (25) Porter, G.;Tchir, M . F. J . Chem. Soc. A 1971, 3772. (26) Sammes, P. G. Tetrahedron 1976, 32, 405. (27) Findlay, D. M.; Tchir, M. F. J . Chem. Soc., Faraday Trans. I 1976, 72. 1096.
Yamamoto et al. the hydrogen atom of the methyl group at the ortho position after photoexcitation. The reaction scheme proposed for the solution is shown in Figure 8.28 The 0-cis and 0-trans isomers are in equilibrium and thermally distributed in the solution (for example, in H20/CH3CN). In solution these two isomer cannot be discriminated, both isomers are excited by nitrogen laser to their own SIstates. They immediately relax to their own triplet states by intersystem crossing. In the triplet state, the hydrogen abstraction occurs selectively for the 0-cis isomer and it forms an enol type biradical. Due to the collision in the solution, the initially excited 0-trans isomer isomerizes to the 0-cis isomer and also produces the same biradical. The existence of the biradical was confirmed by time-resolved ESR,29and its lifetime is known to be 1500 ns by time-resolved absorption method using electron transfer from biradical to paraquat dication.2s This reaction mechanism in the solution can also be applied to the molecule in a jet. By supersonic expansion, the two vibrationally cooled rotational isomers are generated with their relative populations equal to those in the sample housing. They do not have enough time to become equilibrated during the expansion, because of a large barrier to the 0-trans-0-cis isomerization (that is, torsional barrier of the formyl group). Since the two rotational isomers have different electronic transition energies, we can selectively excite each isomer to its single vibronic level in SI (also in TI). The molecule that is excited to the SI state would not isomerize, unless more excess energy than the barrier height is supplied. Though the barrier of the isomerization in the SIstate is not known, that in the So state is reported to be about 30 kJ/mol (2500 c ~ - I ) . ~ O Both the SIstate isomers relax immediately to their triplet states via intersystem crossing. The 0-trans isomer is expected to have a long triplet-state lifetime similar to a normal benzaldehyde derivative, while the 0-cis isomer will have a short lifetime because the hydrogen abstraction, namely, the enolization reaction, will selectively occur. Therefore, the isomer (having the 0-0 band at 26 125 cm-I) detected by the sensitized phosphorescence excitation method is assigned to the 0-trans isomer, and that (26641 cm-I) detected only by REMPI is assigned to the 0-cis isomer. Now, we will consider the ionization mechanism of the two isomers. In the two-color (1 1') REMPI measurement shown in Figure 6c, the 0-cis isomer was ionized, but the 0-trans isomer was not when the ionization laser frequency (v2) is set at 49 570 cm-' (201.67 nm). Since the ionization occurs from the triplet levels isoenergetic to initially pumped SIstate, the total energies ( v l f v 2 ) put to the 0-cis and 0-trans isomers are 76211 and 75 695 cm-l, respectively, when the pumping laser frequency (vI) is tuned to the band origin of each isomer. Moreover, the ionization of the 0-cis isomer could be observed even when the ionization laser frequency was reduced to about 45 000 cm-l. This means that the 0-cis isomer can be ionized with a total energy of about 71 600 cm-I. From these results, it is concluded that the total ionization energy differs by more than 4500 cm-I between the two isomers. However, such a large difference in the ionization energy is hard to believe for the two rotational isomers in their original forms. The fact that the phoshorescent 0-trans isomer cannot be ionized even by a total energy of 75 695 cm-I (9.42 eV) seems to be reasonable considering the ionization potential of benzaldehyde of about 9.6 eV.)I This means that the energy needed for the ionization of the 0-cis isomer (8.88 eV) is surprisingly small for a benzaldehyde derivative itself, suggesting that an intermediate species resulting from the reaction of the 0-cis isomer in its triplet state is responsible for the ionization. Referring to the reaction scheme proposed for the solution, the most probable intermediate is the
+
(28) Das, R. K.; Encinas, M. V.; Small, R. D., Jr.; Scaiano, J. C. J . Am. Chem. Soc. 1979, 101,6965. (29) Ikegami, Y .; et al., private communication. For o-methylacetophenone, the observation of time-resolved ESR spectrum was reported by the same group: Ikoma, T.; Akiyama, K.; Tero-Kubota, S.;Ikegami, Y . J . Phys. Chem. 1989, 93, 7087. (30) Dankenberg, T.; Jost, R.; Sommer, J. M . J . Chem. Soc.. Chem. Commun. 1974, 1011. (31) Turner, D. W.; Baker, C.; Baker, A. D.; Brundle C. R. Molecular Photoelectron Spectroscopy, A Handbook of He 584 A Spectra; Wiley: London. 1970.
J. Phys. Chem. 1990, 94, 5791-5795
I
I
hv2
5191
that for the triplet state of the 0-trans isomer (see Figure 9). The shortest lifetime limit of triplet-state molecule that we can detect by our apparatus of sensitized phosphorescence excitation spectroscopy is 40 ps. From the absence of the 0-cis band in the sensitized phosphorescence excitation spectrum, the triplet-state lifetime of the 0-cis isomer should be much shorter than 40 ps. The enol type biradical is also in its triplet state. If it lives more than 40 ps, it should also appear in the sensitized phosphorescence excitation spectrum. The absence of the 0-cis isomer in the sensitized phosphorescence excitation spectrum means that the lifetime of the radical also should be less than 40 ps. We measured the (1 + 1’) REMPI spectrum by taking the delay time of about 100 ns to the ionization laser and found the strong appearance of the band of the 0-cis isomer. It is concluded that the lifetime of the biradical is longer than 100 ns but shorter than 40 ps. This agrees with the reported value of 1500 ns in the solution.28 In the above, we assumed that the enolization occurs in the triplet state of the 0-cis isomer. This can be confirmed by the comparison between the sensitized phosphorescence excitation spectrum and the one-color REMPI spectrum due to the direct Tl(n,.lr*) Sotransition shown in Figure 7. While the 0 band of the T1 So transition of the 0-cis isomer is missing in the former spectrum (Figure 7a), it appears strongly in the latter (Figure 7b). This provides the evidence that the enolization reaction occurs in the triplet state. The selective chemical reaction of the 0-cis isomer of omethylbenzaldehyde found in this work has a profound implication in the photochemistry of rotational isomers. A similar selective reaction is also expected for other carbonyl compounds whose studies are now in progress in our laboratory.
--
0-trans
0-cis
enol
Figure 9. Relaxation scheme of the rotational isomers of o-methylbenzaldehyde. v1 and v2 are the frequencies of the pumping and probing laser lights, respectively. The waving arrow indicates phosphorescence.
enol type biradical, namely, triplet state enol resulting from the intramolecular hydrogen abstraction of the 0-cis isomer. Therefore, the appearance of the 0-cis isomer in (1 1’) REMPI is interpreted as due to the ionization of the biradical produced from the 0-cis isomer in the triplet state by assuming a much smaller energy is needed for the ionization of the biradical than
+
Acknowledgment. We are grateful to T. Niwa for her experimental assistance. Registry No. o-Methylbenzaldehyde, 529-20-4; m-methylbenzaldehyde, 620-23-5; p-methylbenzaldehyde, 104-87-0; o-fluorobenzaldehyde, 446-52-6; m-fluorobenzaldehyde, 456-48-4; p-fluorobenzaldehyde, 459-57-4.
Nuclear Spin Relaxation and Dipolar Interactions in Maiononitrile/Dichioromethane Solutions L. Foucat> M. T. Chenon,*f and L. Werbelowt*§ LASIR, CNRS, 94320 Thiais, France and Laboratoire des Methodes Spectroscopiques, Centre de St. Jerome, Boite 541, 13397 Marseille, Cedex 13, France (Received: December 21, 1989)
A low-temperature (-20 “C)NMR relaxation study of malononitrile dissolved in dichloromethane or dichlorodideuteriomethane was performed. Using a multispin formalism, both intermolecular and intramolecular dipolar relaxation rate constants were determined. Cross-correlation between various intramolecular dipolar interactions yielded a very exacting microdynamical description of malononitrile in the solution state. The plausibility of the relative ratio of intermolecular dipolar relaxation rates in the protonated and deuterated solvents was demonstrated. It is suggested that multispin NMR relaxation may provide a very sensitive probe of liquid-state structures. Further extensions of this work are proposed.
Introduction The study of nuclear spin relaxation in coupled multispin systems provides a very powerful method for determination of solution-state local structure and site-specific dynamics.lV2 In many applications, the relaxation characteristics of a multispin system are modeled as being affected by time-dependent random-filed and intramolecular dipolar couplings. The temporal ‘Current address: Department of Chemistry, Institute of Mining and
Technology, Socorro, New Mexico 87801.
*
LASIR, CNRS. (Centre de St. Jerome.
0022-3654/90/2094-579 1$02.50/0
correlation (interference) between various dipolar interactions leads to creation of various degrees of multispin order, each possessing an informationally rich unique Inclusion of random-field interactions acknowledges the fact that “other” relax(1) Werbelow, L. G.; Grant, D. M. Ado. Mugn. Reson. 1977, 9, 189. (2) Canet, D. Prog. NMR Spectrosc. 1989, 21, 237. (3) Hartzell, C. J.; Lynch, T. J.; Stein, P.C.; Werbelow, L.G.;Earl, W. L. J. Am. Chem. SOC.1989, 111, 51 14, and references cited therein. (4) Oschkinat, H.; Limat, D.; Emsley, L.; Bodenhausen, G. J. Mugn. Reson. 1989,81, 13. Wimperis, S.; Bodenhausen, G. Mol. Phys. 1989, 66, 897. Dalvit, C.; Bodenhausen, G. Adu. Mugn.Reson., in press.
0 1990 American Chemical Society
