J. phys. Chem. 1903, 87,4 7 8 1 4 7 8 3
two orders of magnitude difference in the reactivities of the radical cations of tetraphenylcyclobutane and of C, both dimerization reactions show similar dependence on the olefin concentration because k3 is also two orders of magnitude slower than the corresponding reaction in the dimerization of diphenylethylene. These differences result from the fact that PVE and C have similar oxidation potentials, whereas diphenylethylene is much more readily oxidized than its cyclobutane dimere5 The mechanism proposed by Mizuno,10which involves interception of the geminate ion pair, can be ruled out for the dimerization of PVE for several reasons. The near constant value of [T]/[C] when the concentration of PVE is ca. 0.1 M and higher would require that almost all the dimer is formed via the intercepted geminate pair even at 0.1 M PVE. Since the reaction ~ o n s t a n t ~ ~for ' ~ Jthe ' separation of geminate radical ion pairs in acetonitrile at room temperature is ca. 5 X lo8 s-', the reaction constant for interception of the pair would have to be higher than the (17) Schulten, K.;Staerk, H.; Weller, A,; Werner, J. J.; Nickel, B. 2. Phys. Chern. (Frankfurt a m Main) 1976,101, 371. Weller, A.Ibid. 1982, 130, 129.
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diffusion-controlled rate. On the other hand, the efficient quenching' at low ratios of quencher to PVE means that the dimerization is one to two orders of magnitude slower than the diffusion-controlled rate. Furthermore, if most of the product is formed via the intercepted pair at [PVE] > 0.1 M, then no chain reaction can take place, contrary to the findings reported earlier.I8 Registry NO. cis-1,2-Diphenoxycyclobutaneradical cation, 87638-53-7; phenyl vinyl ether, 766-94-9; trans-1,2-diphenoxycyclobutane, 35370-70-8. Supplementary Material Available: X-ray data of the trans-cyclobutane derivative (T): bond lengths, interatomic angles, positional and thermal parameters, observed and calculated structure factor amplitudes (32 pages). Ordering information is available on any current masthead page. (18) The chain reaction reported in ref 3 and 7 was confirmed again by measuring quantum yields of dimerization of 0.84 and 1.24 in acetonitrile a t PVE concentrations of 0.102 and 0.508 M, respectively. As shown in ref 7, the quantum yields and their dependence on [PVE] are very sensitive to traces of impurities in PVE and/or in the solvent. This is particularly so because of the small rate constant of the chain-propagation step (cf. text).
Direct Excitation of Triplet States in Supersonic Jets. Rotatlonally Resolved 3A, Laser-Induced Phosphorescence Spectrum of Glyoxal
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'A,
Lee H. Spangler, Yoshlyasu Matsumoto, and David W. Pratt' Department of Chemistry, University of Pinsburgh, Pittsburgh, Pennsylvania 15260 (Received September 19, 1983)
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By combining the techniques of free-jet expansion, high-power laser excitation, and phosphorescence detection, we have observed a rotationally resolved spectrum of the 0; band in the spin-forbidden 3A, 'A, transition of glyoxal. A preliminary analysis of this spectrum is given, together with a brief discussion of the advantages of this approach for the study of the static and dynamic properties of triplet states under collision-free conditions.
Introduction Laser spectroscopy, particularly in combination with supersonic jets, is a very powerful tool for the study of electronically excited states of polyatomic molecules. In most applications reported to date, the focus has been on excited singlet states, or at least on states which are connected to the ground state by spin-allowed transitions. But many photophysical processes that occur following the absorption of light depend, as well, on the properties of metastable, "dark" states. Chief among these are excited triplet states, the object of much attention since their existence was first demonstrated by Lewis and Kasha' nearly 40 years ago. Owing to their low oscillator strengths to the ground state, such states are usually accessed only indirectly.2 In this report, we show that triplet states of polyatomic molecules can also be prepared, with rotational state selection, by direct excitation of the spin-forbidden T1 So transition and detection of the resulting phos-
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(1) G. N. Lewis and M. Kasha, J. Am. Chem. SOC., 66, 2100 (1944). (2) Important exceptions do, of course, occur, both in the condensed phase and in the gas phase. See, for example, C. Michel and C. Tric, Chem. Phys., 50, 341 (1980); G.ter Horst, Ph.D. Thesis, R. U. Groningen, 1982.
phorescence in a supersonic jet. Experimental Section The free jet was formed by expansion of helium through a 1-mm pulsed nozzle into a chamber evacuated by a 25-cm oil diffusion pump backed by a large mechanical pump. The carrier gas (Po= 1-5 atm) was saturated with glyoxal contained in a steel bottle maintained at 273 K. transGlyoxal was prepared by gently heating a mixture of the glyoxal trimer dihydrate, obtained from Aldrich, and P205 under vacuum. The seeded jet was crossed 2 cm downstream of the nozzle by a Nd3+:YAG pumped Coumarin 500 dye laser which delivered -225-mJ pulses of 10-ns duration at a 10-Hz repetition rate. The overall spectral width was 0.03 cm-l with an etalon in the laser cavity. The laser was pressure tuned over lO-cm-' intervals by introducing Nz into the cavity through a regulated flow valve. Phosphorescence was collected by f/1.2 optics and imaged on a filtered photomultiplier tube. The detection system was coaxial with the jet, the first lens being located 7 cm downstream of the nozzle. Under these conditions, molecules spend about 10 j ~ sin the viewing region. Signals were detected with a boxcar integrator with a delayed gate of 1-j~s duration. Relative line positions were determined 0 1903 American Chemical Society
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The Journal of Physical Chemisfty, Vol. 87, No. 24, 1983
Letters
Discussion The full power of the laser spectroscopy/supersonic jet technique is revealed, once again, by comparison of these results with an earlier high-resolution study of the 3A, 'A, system by Goetz et ala5 A reproduction of the absorption spectrum of the 0; band given in that work, which utilized a 1-m path length of glyoxal at 90 torr and was recorded photographically with a 7.3-m Ebert spectrograph AN in 11th order, shows hundreds of lines in the wavelength interval 520.0-521.8 nm, precluding a definitive rotational analysis. But, even with the elimination of vibrational sequence congestion and the cooling of rotational degrees of freedom made possible by the free-jet expansion, the spectrum is still complex. This is because of the important role played by electron spin in determining the rotational structure of singlet-triplet transitions in near-symmetric tops. Fortunately, Hougen6 has given an elegant treatment of this problem. The selection rules for the rotational 4 2 0 -2 branches are LW = 0, f l , f 2 and AK = 0, f l , f 2 , which Relative Frequency ( c r n - l ) differ from the selection rules for singlet-singlet transitions Figure 1. Rotationally resolved laser-induced phosphorescence by the occurrence of branches with AN = f 2 and AK = 'A, spectrum of a portion of the 0; band in the spin-forbidden 3A, f 2 . Here, N and K specify, respectively, the total angular transition of glyoxal, observed in a pulsed free jet. Lines are labeled momentum excluding spin and the projection of this anby their N" values and grouped together in the format AN (K",K'). gular momentum on the top axis. The relative contributions of transitions with different AN and AK depend on by comparison with frequency markers obtained from an the relative magnitudes of the transition moments between external etalon having a free spectral range of 1 cm-'. the singlet state and the three multiplet components of the triplet state. In general, there are nine such moments Results but four of these are zero by symmetry in C 2 h molecules. Glyoxal (HCOCHO) was chosen for our initial experiA fifth is zero in glyoxal because we know from solid-state ments on triplet states because the origin of the 3A, ODMR experiments' that the 3A, 'A, transition is potransition lies near 521 nm,3 a region conveniently acceslarized in the molecular plane, with T, (2' is the finesible with the fundamental of the dye laser. Indeed, the structure axis parallel to the C=O bonds) as the active laser-induced phosphorescence spectrum was readily obsublevel in the nonrotating molecule. Given this inforserved in the free jet. The spectrum shows a relatively mation, the intensity factors tabulated by Hougen,a the strong (S:N 1OO:l) 0; band at 19 197 cm-', 2776 cm-' to full symmetry species of the ground- and excited-state the red of the much stronger lA, 'A ~ y s t e m .Further ~ rovibronic levels, and the appropriate nuclear spin stato the blue of the origin of the 3A, 'A, transition, weak tistics, we have been able to make assignments of most of vibronic bands could be detected at 0: + 466,503, and 963 the lines by comparison of the observed spectra with cm-l. The vibrational structure of this system is very computer-simulated ones. Some of these assignments, similar to that of the corresponding singlet-inglet system, which were also aided by studies of the dependence of both in the magnitudes of the frequency displacements and relative intensities on the temperature of the jet, are shown in the intensity distribution of the bands. The bands of in Figure 1. The majority of the intensity is contained in the former transition can be distinguished from "hot" AK = 0, LW = 0, f 2 type branches originating in K" = 0 bands of the latter by their behavior under different exbecause of rotational cooling in the jet and the aforepansion conditions, by their appearance in magnetic romentioned intensity factors. The rotational temperature tation ~ t u d i e s ,and ~ , ~by their rotational structure. in Figure 1,assumed to be the same for both N - and KAssuming reasonable values for the absorption cross type structure in our calculations, is about 5 K. The triplet section (lo4 A2) and molecule density in the jet (10l2~ m - ~ ) , state rotational constants required for this "fit" are not very we estimate that the number of singlet-triplet excitations different from those of the 'A, state (A = 1.9648, B = per 25-mJ laser pulse is lo8. But we detect only a small 0.1549, and C = 0.1435 cm-'),8 as expected from the simfraction of these because of the (expected) low phosilarity of the Franck-Condon profiles in the 3A, 'A, and phorescence quantum yield and long triplet lifetime (>>1 'A,, 'A&ansitions. Note that lines in the 0 ( A N = -2) ps). Further improvements in signal-to-noise ratio can be and S ( = +2) type branches have a spacing of about anticipated by rather straightforward modifications to our 4B [B = (B + C)/2]. These lines also exhibit intensity apparatus, making possible experiments under truly colalternation effects owing to differences in the statistical lision-free conditions. Nonetheless, even under the present weights of A, (1)and B, (3) rotational levels. conditions, we were able to observe the rotational structure With refinements in our fitting procedure, we can anof the 0; band. Figure 1 shows a high-resolution scan of ticipate obtaining new structural information about glyoxal a portion of this band, recorded at a relatively high backing in its lowest triplet state from an accurate determination pressure (5 atm) in order to simplify the spectrum. Many of its rotational constants. The fine-structure splittings more lines appear in this region at higher gain and/or lower of the isolated molecule may be measured directly with backing pressures. further improvements in resolution. And the spin-orbit coupling routes may be deduced from the values of the 3Au
- 'As
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-'$
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--
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(3)J. C. D.Brand, Trans. Faraday Soc., 50, 431 (1954). (4)W. H. Eberhardt and H. Renner, J.Mol. Spectrosc., 6,483 (1961). (5) W. Goetz, A. J. McHugh, and D. A. Ramsay, Can. J. Phys., 48, 1 (1970).
(6) J. T. Hougen, Can. J. Phys., 42, 433 (1964).
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(7)I. Y.Chan and K. R. Walton, Mol. Phys., 31, 1137 (1976). (8)J. Paldus and D. A. Ramsay, Can. J. Phys., 45, 1389 (1967).
J. Phys. Chem. 1903, 8 7 , 4783-4790
singlet-triplet transition moments which are required to fit the relative intensities of the lines in the laser-induced phosphorescence spectrum. However, the major consequences of the discovery that excited triplet states of polyatomic molecules can be prepared with rotational state selection under collision-free conditions is likely to be dynamic, rather than structural, in nature. Glyoxal is only one of several different molecules in which triplet states appear to participate in, and in some cases dominate, the internal “radiationless” energy All previous experimental studies of these processes have involved the initial preparation of singlet states followed by “intersystem crossing” into isoenergetic states high in the triplet manifold,1° and therefore suffer from various types of perturbations that may obscure and/or modify the true intramolecular behavior. To be sure, such studies are valuable; they have revealed, for example, an important discrepancy between the gas-phase triplet decay rate extrapolated to zero excess energy and that measured in crystals at low temperatures.ll But detailed information (9) For a review, see Ph. Avouris, W. M. Gelbart, and M. A. El-Sayed, Chem. Reo., 77, 793 (1977). (10)See. for examDle. Y. Mataunoto, L. H. SDander, and D. W. Pratt. Chem: Phys. Lett., 98,’333 (1983),and refer&& contained therein;
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about the origins of these effects can only be obtained if state-selected, “pure” triplets are first prepared and then studied as a function of vibrational and rotational energy content. Direct access to the triplet manifold will also facilitate studies of other dynamic processes, including Tl/So and Tl/T, interstate couplings, vibrational and rotational relaxation, and photodissociation, all on a much longer time scale than is possible with “pure” singlet states. Experiments of this type will be difficult, but not impossible, using the approach described in this Letter.
Acknowledgment. Many persons have contributed to the success of this experiment, both directly and indirectly. Ms. Wendy Butler synthesized glyoxal and Ms. Leanne Henry wrote the program which aided us in the interpretation of the spectrum. We thank them both. We are also grateful to Professors K. K. Innes, J. Kommandeur, M. E. Michel-Beyerle, R. E. Smalley, A. Trambr, and J. H. van der Waals for encouragement. This work was supported by the Research Corporation, the North Atlantic Treaty Organization (31.80/DI), and the National Science Foundation (CHE-8021082). (11) R. E. Smalley, J . Phys. Chem., 86, 3504 (1982).
FEATURE ARTICLE Symmetry Breaking in Polyatomic Molecules: Real and Artifactual Ernest R. Davldson‘ and Weston Thatcher Borden Chemistry Department B G IO, University of Washington, Seattle, Washington 98 195 (Received: May 3 1, 1983)
A review is presented of recent work by the authors and their collaborators on molecules with broken symmetry. Examples are given in which the symmetry breaking arises from Von Neumann-Wigner or Jahn-Teller avoided intersections or from pseudo-Jahn-Teller effects. Examples are also cited in which approximate wave functions show broken symmetry at symmetrical arrangements of the nuclei. Such calculations necessarily, and often incorrectly, predict distorted equilibrium geometries.
Potential surfaces are of importance in chemistry for explaining geometries, spectra, and chemical reactions. Potential surfaces for molecules which could be of high symmetry may display minima of lower symmetry that are induced by degeneracies or near degeneracies. The effects of avoided crossings in diatomic molecules are fairly well characterized, but intersections of polyatomic potential surfaces are still only vaguely understood. While the general type of features to be expected are well-known, the actual appearance of these structures in potential surfaces is still often regarded with surprise and is frequently overlooked. Although the potential surfaces for the ground states of closed-shell molecules seldom display much structure near the equilibrium geometry, the potential surfaces for excited states and for open-shell molecules often display large amounts of structure because of the closer spacing of the electronic states. Examples of some of these highly
structured surfaces, gleaned from recent work done in this laboratory, will be presented in this review. The structure of the polyatomic potential surface and related wave function in the neighborhood of an intersection has been described in the papers of Von Neumann and Wigner, Jahn and Teller, and Herzberg and Longuet-Higgins. The surface in every case exhibits a double cone structure when plotted against an appropriate pair of coordinates. The locus of nuclear coordinates over which degeneracy persists is of dimensionality 3N - 8 in the 3N - 6 dimensional space of internal coordinates. Davidson has discussed the locus of degeneracy on the ground-state surface for several triatomic radicals. The paper of Von Neumann and Wigner’ described the case of degeneracy due to “accidental” crossing of surfaces, while Jahn and Tellel.2 described the structure near a point (1) Neumann, J. V.; Wigner, E. P. 2. Phys. 1929, 30, 467.
QO22-3654/83/2Q87-4783$01.5Q/Q0 1983 American Chemical Society
