J . Phys. Chem. 1985, 89, 2528-2533
2528
The Cyclohexadtene+./Hexatriene+* Case Revisited: Evidence for Flve Out of Six Possible Hexatriene Rotamers T. Bally,* S. Nitsche, K. Roth, and E. Haselbach Institut de Chimie Physique de ['Universitl, Plrolles, CH- 1700 Fribourg, Switzerland (Received: December 27, 1984)
A detailed matrix-isolation study of the photochemical interconversions between different hexatriene'. rotamers as well as the ring opening of cyclohexadiene'. is presented. Although earlier observations on the same systems embedded in Freon glasses are by and large confirmed, the accompanying spectral assignments to four of the six possible hexatriene'. rotamers must be reconsidered in the light of the new observation of a distinct spectrum for the cation originating from cis-hexatriene. A new set of assignments to five hexatriene'. rotamers is proposed.
Introduction
SCHEME I
The detailed investigation of light-induced 1,3,5-hexatriene+. (HT'.) interconversions published in 1977 by Shida and coworkers' marked a milestone in the growing field of radical ion photochemistry2 since it represented the first systematic study of this kind. These authors presented extensive spectral evidence for the photochemical formation of several different species from cyclohexadiene+-(CHD+.), some of which could be interconverted by selective illumination through different sets of optical filters. Accompanying experiments and theoretical calculations led them to propose that these species were different rotamers3 of HT+.. In Scheme I all possible HT rotamers are presented in such a way that their interrelation via individual bond-rotation processes becomes evident. Each rotamer is labeled according to the local configuration around the 2-3,3-4, and 4-5 bonds where c denotes cis and t t r a m 4 An assignment of the different optical absorption bands to individual HT'. rotamers was also proposed whereby the ctt and cct rotamers were, however, excluded from consideration. Furthermore, ionized ttt- and tct-hexatriene gave indistinguishable spectra upon ionization, a feature which was rationalized in terms of a quantitative tct ttt isomerization in the course of ionization. We were not satisfied with this explanation and the proposed spectral assignments and decided to reinvestigate the title problem using our recently developed method of X-ray ionization of matrix-isolated substrates, which gives better resolved spectra and hence more detailed information than the Freon-glass technique. The recent publication of a very similar study by Andrews and Kelsal15 prompts us to describe and discuss our results6 in some detail.
-
Results and Discussion
Production of Ions. We used the technique recently described in ref 7. Briefly, a mixture of the hydrocarbon, CH2C12(senling as an electron scavenger), and Ar (typcially 1:2:2000) is deposited (1) T. Shida, T. Kato, and Y . Nosaka, J . Phys. Chem., 81, 1095 (1977). (2) E. Haselbach and T. Bally, Pure Appl. Chem., 56, 1203 (1984). (3) In polyene cations, a clear distinction between single and double bonds can no longer be made. W e therefore propose to replace the terms 'conformers" (pertaining to rotation around the former) or "isomers" (formed by rotation around the latter) by the neutral term Yotamers" for such systems. (4) We follow the nomenclature introduced in ref 1 and used also in ref 5 but drop the hyphens between the three letters (the first and last letters can be interchanged without altering the identification). Note that we do not imply planarity of the species depicted in Scheme I. In fact, H-H contacts will force all but the ttt isomer to be twisted by varying degrees, which we believe to be at least one of the reasons for the observed spectral shifts. ( 5 ) B. J. Kelsall and L. Andrews, J . Phys. Chem., 88, 2723 (1984). (6) Presented in part by T.B. a t the 1983 Gordon Conference on the "Spectroscopy of Matrix Isolated Species". (7) T. Bally, S. Nitsche, K. Roth, and E. Haselbach, J . Am. Chem. SOC. 106, 3927 (1984).
0022-3654/85/2089-2528$01.50/0
HT
ctc
ccc
It
It
=+c
ctt
cct
11
11 5
6
CHD 32-3
3
W
ttt
'
3
4-5
=J-
'
3
tct
3-4
on a cold sapphire window to form a clear matrix after which the sample is exposed for 2 h to intense X-rays. The resulting spectra were worked up as illustrated in Figure 1 for the example of tct-HT: After ionization, the UV absorptions of the neutral have diminished and new bands due to ionic products have risen in the visible. Through digital subtraction the fraction of the converted neutral can be determined (about 37% in this particular case) and a corresponding fraction of the spectrum recorded prior to ionization is then subtracted from all subsequent spectra to reveal the "pure" ion spectra. The figures in this paper show such difference spectra* except where otherwise noted. Cyclohexadiene'. (CHD'.) and Hexatriene'. ( H P . ) Spectra. Figures 2a and 3a show the spectra obtained after ionization of CHD and pure tu-HT, re~pectively.~Apart from the weak shoulders at 423 and 403 nm (marked D and C, respectively), ionized C H D shows the spectral features of a conjugated diene cation, Le., an intense near-UV and a weak visible band. The two lie much closer than for butadiene cation which can be rationalized by following the diagrams given in ref 7, taking into account the differences in geometry and substitution between the two hydrocarbons. In ionized ttt-HT, on the other hand, the 423- (D) (8) In order to fully compensate for scattering effects, we preferred to subtract the preionization spectrum with a factor of 1.0 in all cases where no spectral information is sought for in the region where the neutral precursor absorbs. (9) The ion yield is considerably lower in the case of CHD, presumably due to its higher ionization energy.
E9 1985 American Chemical Societv
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2529
Cyclohexadiene/Hexatriene Radical Cations
I""""'I""""'I""""'I
1
b 1.0 A
a 111111111i111111111IIIIIIIIIII
IIIIIIIIIIIIIIIllllllllllIIIl
nm Figure 1. Spectrum of tct-HT before (a) and after (b) 2 h of X-ray irradiation (same scale). Trace c (expanded 5 times) shows the result of a scaled digital subtraction ( b - 0.63a) which represents the spectrum of the ion(s) formed during X-ray irradiation. 588
408
300
200
608
500
\
480
R
llllllrrlrlll
I
IIIIII l l IIIIII
300 nm Figure 3. (a) Spectrum of the ions formed by x-ray irradiation of rtt-HT in argon. (b) Spectrum after 30 min of irradiation at 380 nm.
700
C
& ,I"\'. ii I\
a I l l l l l l l l l l l l IlllllllllllllI l l l l l l l l l l l l l l
700
600
500
400
308
nm
Figure 2. (a) Spectrum of the ions formed by X-ray irradiation of C H D in argon. (b) Spectrum after 30 min of irradiation at 372 nm.
and 403-nm (C) bands are much more prominent after ionization while the remaining spectrum matches well with what is expected for a conjugated triene cation.' Photolysis at 380 nm or, alternatively, at 500 nm in the case of C H D produces the spectra in Figures 2b and 3b, respectively. Although these are now quite similar in appearance we note that the band pair E at 462/443 nm is significantly s t r o n g e r for C H D while the 423- (D) and 403-nm (C) peaks dominate if one starts with ttt-HT. The different species C-E absorbing between 400 and 500 nm can be bleached by illumination within this wavelength range which eventually leads to spectra which are essentially empty in this spectral region. If this 400-500-nm bleaching is done immediately after ionization of ttt-HT the spectrum displayed in Figure 4a is obtained. In accord with earlier work'q5 we assume that this represents the parent cation. This assignment is supported by a detailed analysis of the band system at 600 nm (Table I) which
IIIIIIIIIIIIIIIIIIIIlIIII1I1IIII1IIII1III
700
Ism
588
488
300
nm
Figure 4. (a) Spectrum obtained after ionization of ttt-HT and subsequent bleaching between 400 and 500 nm. (b) Same as (a) for tct-HT. (c) Same as (a) for a CHD (a very similar spectrum is obtained when a 3:2 mixture of ttt- and tct-HT is subjected to the same procedure). (d) Arithmetic mean of (a) and (b).
shows that the vibrational progressions are identical within experimental error to those observed earlier in a laser excitation spectrum of ttt-HT'. in neonlo as well as in a laser photodissociation spectrum obtained in the gas phase." The entire band (IO) V. E. Bondybey, J. H. English, and T. A. Miller, J. Mol. Spectrosc., 80, 200 (1980).
Bally et al.
2530 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 A
700
600
500
400
300 nm
Figure 5. Results of selective bleaching. All traces show digitally obtained differences between spectra recorded before and after narrow-bandwidth irradiaiton at (a) 700 nm (30 min), (b) 680 nm (90 min), (c) 665 nm (30 min). Bands pointing downward correspond to species lost during irradiation, bands pointing upward to species arising concomitantly. TABLE I Comparison of the Vibrational Frequencies of ttt-HT+.in the First Excited State”
vibrational assignmentb
excitation in Neb 15 868 (0)
16211 (343) 16311 (443) 16 558 (690) 16654 (786) 16939 (1071) 17311 (1245) 17 302 (1434) 17354 (1486)
photodissociation‘ absorption (gas phase) in Ard 15480 (-350) 15 670 (-160) 15 320 (-220) 15 830 (0) 15 540 (0) 16 180 (350) 15 895 (355)
f f 16530 (700) 16245 (705) f 16350 (810) 16890 (1060) 16610 (1070) f 16790 (1250) 17240 (1410)
16980 (1440) 17010 (1470) ‘All numbers are in cm-I. Figures in parentheses denote displacements relative to the spectral origin. bReference 10. CReference11. dPresent work. CPresumablyhot bands in the case of the room temperature gas-phase experiment. Assignment uncertain in the 20 K matrix experiments (see text). fUnresolved bands, appears as shoulders in some cases.
f
system is red-shifted by some 300 cm-’ in Ar relative to the gas phase which can be ascribed to a solvation effect. Note that the spectrum shows a weak band on the low-energy side of the main 643-nm peak which appears even more strongly in the Ar resonance ionized sample^.^ Its presence has been explained in terms of a site effect but attempts to selectively “bleach” this alleged site12 by careful narrow-bandwidth photolysis at 655 nm (see Experimental Section) always resulted in the concomitant destruction of the entire group of bands. Although the general insensitivity of the first excited-state energy of polyene radical cations to medium effects makes it unlikely that different sites in a given medium should induce such pronounced spectral shifts, we can at present offer no better explanation for this phenomenon. In the C H D experiments, the spectra after 400-500-nm bleaching (Figure 4c) differ slightly in appearance from that of ttt-HT+. in that there is less resolved fine structure and both the red and the blue bands are displaced by small but signifcant amounts (a very similar spectrum is obtained if one starts from (11) R. C. Dunbar and H.H.-I. Teng, J. Am. Chem. SOC.100, 2279 (1978). (12) Our experimental techniques seem to be well suited for this type of experiment because very selective site bleaching could be induced in the related case of octatetraene+.; T. Bally and K. Roth, to be submitted for publication.
the commercially available 3:2 mixture of ttt- and tct-HT). The difference between the two spectra can be explained by considering the corresponding spectrum obtained if one starts from pure tct-HT. The result is shown in Figure 4b (spectrum after 400500-nm bleaching). We note in particular that the red band is comparatively broad and slightly blue-shifted relative to ttt-HT+and the near-UV band is completely featureless and peaks at slightly longer wavelengths. Thus, we submit that tct- and ttt-HT upon ionization in argon give distinct species with similar, yet significantly different spectra. Such a similarity is in fact to be expected in view of the near identity of the two photoelectron spectra in the 8-12-eV range.13 Consequently, we propose that the spectrum obtained from CHD+. after 400-500-nm bleaching (Figure 4c) represents a mixture of tct- and ttt-HT+.. Simple addition of the lower two traces of Figure 4 gives the spectrum in Figure 4d which is-not surprisingly-almost superimposable to that displayed immediately below it. Selective Bleaching Experiments. The spectra shown in Figures 2b and 3b obviously contain optical absorptions of several species next to those of the parent cations. We found that these species can be selectively bleached by irradiation with narrow-bandwidth light (see Experimental Section) starting at 700 nm and moving cautiously toward the blue while monitoring carefully the spectral changes. The result of this procedure is illustrated in Figure 5 which shows differences between spectra taken before and after illumination at the respective wavelength indicated by the open arrow. Thus, irradiation at X > 700 nm (Figure sa) causes species E, absorbing at 462 and 443 nm, to disappear with concomitant formation of new bands at 423 (D), 403 (C), and 380 nm (B), respectively. Incident light of 680 nm (Figure 5b) leads to the decay of the sharp 423-nm peak (D) and the growth of a broad band B at 382 nm, strongly reminiscent of tct-HT+. (compare Figure 4b) plus a small peak at 403 nm (C). With 665-nm irradiation we finally monitor the disappearance of the band system C originating at 403 nm with simultaneous formation of a structured absorption A peaking at 378.4 nm resembling the band of ttt-HT+. (compare Figure 4c). Similar sets of spectra are obtained by monochromatic illumination at 470-400 nm although the fact that all the observed species absorb in this wavelength range makes it impossible to monitor individual interconversions as cleanly as with the strategy (13) M. Beez, G. Bieri, H.Bock, and E. Heilbronner, Helu. Chim. Acta,
56, 1028 (1973); M. Allan, J. Dannacher, and J. P. Maier, J . Chem. Phys., 73, 3114 (1980).
Cyclohexadiene/Hexatriene Radical Cations
The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 2531 TABLE II: Comparison of Band Positions for Different HT+Rotamen in Freon Glass and Argon Matrices (in nm)
h E
D
spectrum' (assignment) A (ttt)
_-/---i/ "
Freon glassb
392
-J\
B (tct)
c (ctt)
655
655 633 61 1 597 582 570 403 398 390
659
675 630 590 432
670 630 590 423 404
680
59P 465 448
60W 462 443
413
A 600
500
400
308
nm
Figure 6. Collection of spectra for species A-E (see text). Spectra C, D, and E (Corresponding to spectra D, C, and B, respectively, in ref 1)
E (ccc)'
were obtained by the subtraction technique described in the text. The exact peak positions are listed in Table I1 along with the assignments. followed above. By adding a properly scaled fraction of the ttt-HT+. spectrum to the negative of the difference spectrum in Figure 5c we can contstruct the spectrum of pure C. Similarly, the spectrum of D can be extracted from Figure 5b by adding those of ttt-HT+. and species C with appropriate respective scaling factors. Finally, fractional addition of all the above spectra to inverted Figure 5a gives the spectrum of E. The result of this procedure is summed up in Figure 6 which displays the entire set of HT+. rotamers observed in our experiments. Although there are indications that the band at 423 nm consists actually of two peaks at 423 and 419 nm, the corresponding complete spectra could not be fulling resolved and we therefore hesitate to assign them to different HT+. rotamers. Table I1 lists the exact peak positions and juxtaposes them to the data obtained earlier in Freon glasses' or more recently in an Ar m a t r i ~ Agreement .~ with the latter is satisfactory for ttt-HT+. and somewhat poorer for the other rotamers observed (note that species E was not detected in these experiments). In comparing our results to the Freon glass data we note that the first excited ionic state energy of all HT+-rotamers is rather insensitive to the change in environment while the second excited state is more strongly affected. This is in line with the above-mentioned observation of a very small shift between the gas-phase and the Ar-matrix spectra in the first band (a feature pertaining also in the case of other polyene radical cations') and a large shift for the second band in the case of octatetraene+. relative to the third PE band of the neutral.' These findings can be rationalized in terms of a general increase in polarizability of states with increasing energy, leading to increasing effects of the medium. Assignments. The similar appearance of the spectra A-E in Figure 6 suggests that they belong to a common class of compounds, i.e., conjugated triene cations. The formation of other C6H8+. (valence) isomers such as 3-vinycyclobutene+. or bicyclo[2.2.0] hexene+. by photolysis cannot be rigorously excluded but these ions would not be expected to show any prominent
Ar matrixd 644 603 561 384 319 357
638 623 598 583 554 541 382
613
D (cct)
Ar matrixC 643.5e 602.0 559.2 383.2 378.4 357.2 ~~
642 588 555
612
405 398
423
'Refer to Figure 6. bReference 1. 'Current work. dReference 5. #For a complete list of all vibrational progressions see Table I . f
Assignment tentative, see text. guncertain evidence for band.
absorptions above 300 nm. Thus, an assignment of all the spectra in Figure 6 to different HT+. rotamers seems logical and has in fact been attempted earlier. However, the spectral resolution attainable in Freon glasses did not permit a distinction between the very similar spectra arising from rtt- and tct-HT+, a finding which was explained by invoking a quantitative conversion of tct to (presumably more stable) ttt-HT+- during ionization driven by the excess energy contained in the former ion immediately after its formation. Instead, spectrum C in Figure 6 was identified with the (photochemically formed) tct rotamer. However, the 13-nm (" 309-cm-') shift between the first maxima of A and C in the Freon glass (corresponding to E to D in ref 1) is difficult to reconcile with the nearly identical first excited-state energies for ttt- and tct-HT+. observed in the gas phaseI3 which makes this assignment questionable. Since we also consider it unlikely that tct-HT undergoes ionization without leaving any spectral trace of the parent cation, we prefer to attach enough significance to the subtle but distinct and reproducible spectral differences observed after ionization of ttt- and tct-HT to support the claim that traces A and B in Figure 6 actually represent spectra of the two parent cations. Thus, we decided to reexamine also the other assignments given in ref 1 where the choice of observable HT+.rotamers had been narrowed to four on the basis of INDO potential energy surface calculations. According to these, a concerted disrotatory twisting of the 2-3 and 4-5 bonds of the initially formed ccc rotamer leading to ctc-, tct- and ttt-HT+. had to be enforced in order to visualize a simultaneous formation of these three rotamers on a two-dimensional potential energy surface. Since the remaining two rotamers (cct and ctt) could not be formed via such double bond rotation processess, it was concluded that they were inaccessible altogether. One could argue whether such calculations
2532 The Journal of Physical Chemistry, Vol. 89, No. 12, 1985 form a valid basis for this c o n c l ~ s i o nbut , ~ ~it turns out-in view of our subsequent discussion-to be more rewarding to analyze the evidence that led the authors in ref 1 to propose a simultaneous formation of three HT+. rotamers (i.e,, species C, D, and E in Figure 6) in a single photochemical step. They had observed that even during the initial stages of CHD'. photolysis at 500 nm the three optical absorptions increased synchronously which led them to (legitimately) rule out consecutive photochemical reactions. However, they did not consider the possibility of initial photoinduced electrocyclic CHD'. ring opening followed by ground-state isomerizations of the incipient, not fully thermalized HT+. rotamer (presumably ccc-HT+.). We therefore propose that a fraction of the primary CHD+. photoproduct will dissipate its excess energy by crossing over to more stable HT'. rotamers (via rotations around one bond at a time) rather than by direct energy transfer to the matrix.16 This model explains equally well the observed simultaneous rise of several optical absorptions while it does not require the primary photochemical step to involve both ring opening and simultaneous rotation around two bonds, Le., a one-step rearrangement of extreme complexity. Furthermore it restricts the choice of possible primary HT'. isomerization products to rotamers which are attained by twisting of one bond at a time, Le., which are connected horizontally or vertically in Scheme I. The above propositions allow us now to identify also products C and D on the basis of the observation that the former can be converted cleanly to ttt-HT+. (Figure 5c) while the latter forms predominantly tct-HT+. plus a small amount of C upon selective photolysis (Figure 5b). Hence C corresponds to ccc-HT+. and D to CC~-HT+.,~', two rotamers which were in fact disregarded in previous assignment^.^^^ This leaves us with species E in Figure 6 and the two remaining rotamers ccc- and ctc-HT+- in Scheme I . From Figure Sa we can deduce that D (Le., ccc-HT'. is the predominant and C (Le., ccc-HT+.) a minor photoproduct of E which leads us to propose that species E is a direct neighbor of the former in Scheme I, i.e., corresponds to ccc-HT+., the presumed primary product of CHD+. photolysis. This would entail that ctt(C) and tct-HT+. (B), but not ttt-HT+. (A) are secondary thermal products of ccc-HT+. (E) photolysis of 700 nm, in accord with the position of the corresponding HT+.rotamers in Scheme I. The assignment of spectrum E in Figure 6 to ccc-HT+. also finds support from the observation that the bands at 462 and 443 nm are only observed upon photolysis of CHD+. and-contrary to those at 423 and 403 nm-cannot be re-formed photochemically once they have been bleached. Interestingly, these bands were not observed in the recent matrix-isolation study of Kelsall and AndrewsS which is probably due to the fact that under their conditions of CHD+. irradiation the corresponding species had no chance to accumulate in the photostationary equilibrium. This emphasizes again the need for very selective conditions of illumination when dealing with such multicomponent systems having overlapping spectra. It should be noted at this point that all reuerse photoreactions (i.e., A C, B D, and D E) probably take place as well but the corresponding products may not be observable because they absorb too strongly at the respective irradiation wavelength (indicated the open arrows in Figure 5). Furthermore +
-
-+
(14)It has been pointed out repeatedlyI5 that simplifying assumptions concerning the course of a reaction (such as maintenance of symmetry elements) can lead to serious misjudgements and that complete geometry optimization (apart from one or two judiciously chosen reaction coordinates) is paramount if the resulting potential energy surface is to be of any relevance. This is especially true for radical cations where isomerizations are frequently accompanied by changes in state symmetry which entails a lowering of molecular symmetry during the course of the reaction. (15) See, for example, M. 3. S . Dewar and S . Kirschner, J . Am. Chem. Soc.. 96,5244 (1977). (16) Especially in the case of rare gas matrices, such vibrational energy transfer can be quite inefficient. See, for example, V. E. Bondybey, Adu. Chem. Phys.,47, 521 (1981). and references cited therein. (17)As mentioned also in ref 5 , methylene-l,3-cyclohexadienemay serve as a model for cct-HT. The corresponding ion absorbs strongly at 430 nm and weakly at 638 nm.I8 Althodgh these values lie within the range of HT+absorptions, they cannot serve to support the present assignments due to the inductive and hyperconjugative influence of the bridging C H 2 group.
Bally et al. there seems to be a tendency for formation of the (presumably most stable) ttt- and tct-HT+- rotamer in the secondary thermal reactions which also puts the back-reactions at a disadvantage. The same arguments may also help to explain our failure to observe the sixth HT+. rotamer, Le., ctc-HT+.: Either it absorbs at a longer wavelength than all the other rotamers and is immediately photolyzed to ctt-HT+. (which would explain the formation of the latter in the photolysis of CHD'.) or it undergoes an efficient thermal reaction to a more stable rotamer, Le., again to ctt-HT+.. Conclusion
The use of very narrow-bandwidth light to induce selective photochemical conversions in ionized HT and CHD coupled with computer-assisted spectrum workup to give digital difference spectra yielded the spectra of five different HT+. rotamers. Four of them compare reasonbly well with the results of similar experiments conducted earlier in Freon glasses' while only three of them were detected in a recent matrix-isolation study.5 The newly observed spectrum of tct-HT+. requires a revision of the s+ctral assignments given in ref 1 and reiterated in ref 5. The results of selective photochemical interconversions among different HT'. rotamers lead to a new set of assignments which is discussed on the basis of a complete scheme of possible rotamers and their connections via one-bond rotations. These new assignments avoid some of the inconsistencies with related experimental observations and with chemical intuition inherent in the previously proposed scheme of isomerizations. Experimental Section
Materials. ttr-HT was prepared by Pd-catalyzed AcOH elimination from 1-acetyl-2,4-hexadiene according to Yamamoto et aI.l9 This material contained 95% ttt- and 5% tct-HT according to analytical VPC and was used without further purification. Pure tct-HT was separated by preparative VPC on B,P'-oxydipropionitrile (ODP) at room temperature20 from a mixture prepared according to Woods et aL2' by pyrolysis of 1,3-hexadiene-6-01over A1203conditioned for improved yield according to Alder.22 Some early experiments were carried out with a hexatriene sample from Aldrich which according to analytical VPC contained ttt- and tct-HT in a 60:40 ratio. This sample and CHD obtained from Fluka were used without further purification. After thorough degassing in several freeze-thaw cycles the hydrocarbons were mixed in the gas phase with twice the molar quantity of CH2CI2 and a 1000-fold excess of 99.995% pure Ar. This mixture was deposited for 4 h at a rate of 1 mm of Ar/h on a sapphire window held at 20 K. Apparatus. The techniques for preparation of matrix-isolated radical cations outlined in ref 7 were followed. Spectra were recorded on a Perkin-Elmer Lambda 9 UV/VIS/NIR instrument and worked up digitally on a PDP 11/34 computer. Typically, the spectra were run at a spectral resolution of 1 nm and two data points were recorded per nanometer in order to keep the digital resolution at the same level. For photochemistry, the light of an argon plasma arc from GAT23run at 1 kW was focussed into an f = 3.4 monochromator (grating blazed at 500 nm) whose entrance and exit slits were et at 1 mm corresponding to a spectral fwhm of 4.5 nm. In the selective bleaching experiments the slit width was reduced to 0.5 mm (spectral fwhm = 2.25 nm). The exiting light was admitted via a silica window to the sample which was otherwise carefully shielded from other sources of light. This (18) B. J. Kelsall and L. Andrews, J . Am. Chem. SOC.,105,1413 (1983). We have recently confirmed these results via an independent route: T. Bally, D. Hasselmann, and K. Loosen, Helu. Chim. Acta, 68,345 (1985). (19) K. Yamamoto, S.Suzuki, and J. Tsuji, Bull. Chem. SOC.Jpn., 54, 254 (198I ) . (20) J. C.Hwa, P.L. Bennesville, and H. J. Sims, J . Am. Chem. SOC.,82,
2537 (1960). (21)G.F. Woods, N. C. Bolgiano, and D. E. Duggan, ._ J . Am. Chem. Soc., 77, 1800 (1955). (22) K. Alder and H. v. Brachel, Ann. Chem., 608, 195 (1957) (23)Gamma Analysen Technik, D-2850Bremerhaven, F.R.G.
J. Phys. Chem. 1985,89,2533-2540 measure proved necessary after we found that a sample of ttt-HT+left unshrouded for 30 min had undergone considerable photoisomerization induced apparently by diffuse ambient light.
Acknowledgment. We thank Ms. Chantal Monney for her help in the preparation of the hexatriene precursors. This work is part
2533
of project No. 2.219-0.84 of the Schweizerischer Nationalfonds zur Foerderung der Wissenschaftlichen Forschung. Registry No. CHD cation radical, 62697-69-2; trans-l,3,5-hexatriene cation radical, 62015-34-3;cis-1,3,5-hexatrienecation radical, 6201535-4.
Dynamics of Intermolecular Vibrational Energy Transfer between COFp and NO Jyothi Subbiah and George Flynn* Department of Chemistry and Columbia Radiation Laboratory, Columbia University, New York, New York 10027 (Received: January 7, 1985)
-
-
The technique of C 0 2 laser-induced IR fluorescence has been used to measure the rate of intermolecular vibrational energy transfer between COF2and NO. The laser was used to excite the u = 1 u = 2 (u2 2 4 transition in the u2 (C-F stretch) mode of COF2. IR fluorescencewas observed at 5 pm from the (2u2,uI)Fermi mixed levels. Energy transfer from the (2u2,ul) levels of COF2 to the u = 1 level of NO was found to occur in approximately 68 collisions.
Often the study of vibrational energy transfer between two polyatomics is complicated because of the difficulty of unequivStudies of the dynamics of vibrational energy transfer are ocally determining the specific states of the two molecules involved important for developing propensity rules for molecular relaxain the energy transfer and the possible existence of multiple ret i ~ n l and - ~ testing or improving current theoretical models for these laxation pathways in larger molecules.Is This paper reports p r o c e s ~ e s . ~ ,Vibrational ~ relaxation data can be used for the laser-induced infrared fluorescence studies of the intermolecular development of laser systems6s7and to provide an improved unvibrational energy transfer from COF2 to NO following initial derstanding of chemical relaxation processes. Intramolecular excitation of the u2 mode of COF2 in COF2-NO mixtures. For vibrational energy transfer has been studied extensively by a this case, the energy-transfer path is unambiguous, and the rate number of techniques such as time-resolved IR f l u o r e s c e n ~ e , ~ ~ ~of the intermolecular crossover is found to be fairly rapid. The phase-shift experiments,I0 and thermal-lensing methods."-I4 probability for the intermolecular energy transfer has also been In general, a molecule can be significantly excited by a laser calculated based on a model of long-range attractive forces. only if there is a chance overlap of its absorption profile with a Open-shell species like O(jP) and NO are known to exhibit laser line and if the absorption is strong. However, for a molecule anomalous relaxation b e h a ~ i o r . ' ~ - ~The l relaxation behavior of in which this is not the case, excitation by efficient energy transfer NO has long been of interest because of its nonsinglet Z ground from a suitable sensitizer which does strongly absorb the laser state. The vibrational self-relaxation rate of NO(A2Z+,u=1) is line and which has a very slow vibration-translation/rotation 50 times larger than its relaxation rate by N2.22 In addition, the (V-T/R) relaxation rate may be possible. Although information vibrational self-relaxation rate of NO is 6 orders of magnitude on intermolecular relaxation rates would be interesting in this larger than the self-relaxation rate of CO which has a comparable context, relatively little work has been done on intermolecular molecular weight and vibrational frequency.23 One possible energy explanation for this anomalous behavior is based on the splitting of the potential surface for a pair of NO molecules into singlet and triplet surfaces, with curve crossing resulting in net vibrational r e l a x a t i ~ n . ~Another ~ . ~ ~ explanation is based on strongly attractive (1) J. T. Yardley, "Introductionto Molecular Energy Transfer", Academic interactions between two NO molecules, resulting in the formation Press, New York, 1980, p 162. of a collision complex.26 During the lifetime of a few rotational (2) G. W. Flynn, Acc. Chem. Res., 14, 334 (1981). periods, vibrational energy may be efficiently redistributed in the (3) E. Weitz and G. W. Flynn, Ado. Chem. Phys., 47, 185 (1981). complex, resulting in efficient self-relaxation of NO. Furthermore, (4) I. Procaccia and R. D. Levine, J . Chem. Phys., 63, 4261 (1975). unusually efficient intermolecular energy transfer from NO to (5) A. D. Wilson and R. D. Levine, Mol. Phys., 27, 1197 (1974). triatomic hydrides like H 2 0 , H2S, D 2 0 , and D2S has been ob(6) R. W. F. Gross and J. F. Bott, 'Handbook of Chemical Lasers", Wiley, served.27 It was therefore of interest to study the relaxation New York, 1976. behavior of NO with COF2. (7) I. Shamah and G. W. Flynn, Chem. Phys., 55, 103 (1981). (8) G. W. Flynn in "Chemical and Biochemical Applications of Lasers", (17) R. C. Slater and G. W. Flynn, J . Chem. Phys., 65, 425 (1976). Vol. 1, Academic Press, New York, 1974, p 163. (18) R. K. Huddleston and E. Weitz, J . Chem. Phys., 74, 2879 (1981). (9) L. 0. Hocker, M. A. Kovacs, C. K. Rhodes, G. W. Flynn, and A. (19) G. West, R. Weston, Jr., and G. Flynn, J . Chem. Phys., 67, 4873 Javan, Phys. Rev.Lett., 17, 233 (1966).
Introduction
(10) J. T. Yardley and C. B. Moore, J. Chem. Phys., 45, 1066 (1966); J. T. Yardley and C. B. Moore, J . Chem. Phys., 48, 14 (1967); J. T. Yardley and C. B. Moore, J . Chem. Phys., 49, 1111 (1965). (11) T. Aoki and M. Katayama, Jpn. J . Appl. Phys., 10, 1303 (1971). (12) F.G. Gebhardt and D. C. Smith, Appl. Phys. Lett., 20, 129 (1972). (13) F. R. Grabiner, D. R.Siebert, and G. W. Flynn, Chem. Phys. Lerr., 17, 189 (1972). (14) L. Sica, Appl. Phys. Lett., 22, 396 (1973). (15) M. I. Lester and G. W. Flynn, J . Chem. Phys., 72, 6424 (1980). (16) S. M. Lee and A. M. Ronn, Chem. Phys. Lett., 26, 497 (1974).
0022-3654/85/2089-2533$01.50/0
(1977). (20) G. P. Quigley and G. J. Wolga, J . Chem. Phys., 63, 5263 (1975). (21) K. Glanzer and J. Troe, J . Chem. Phys., 63,4352 (1975). (22) A. B. Callear, Appl. Opt. Suppl., 2, 145 (1965). (23) E. Weitz and G. W. Flynn, Annu. Reu. Phys. Chem., 25,294 (1974). (24) E. E. Nikitin, Opt. Spekrrosk., 9, 16 (1960). (25) E. A. Andrew, S.Ya. Umansky, and A. A. Zembekov, Chem. Phys. Lett., 18, 567 (1973). (26) J. T. Yardley in "Introduction to Molecular Energy Transfer", Academic Press, New York, 1980, p 121. (27) A. B. Callear and G. J. Williams, Trans. Furuduy Soc., 62, 2030 (1966).
0 1985 American Chemical Society
