J. Phys. Chem. 1990, 94, 1229-1232
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is supposed to be essentially the same as that proposed by Field Various artificial patterns of chemical waves can be reproducibly and N o y e ~ . ’ ~ JWhen ~ the light intensity of illumination was initiated on illumination of the oscillatory reaction system. The increased, the propagating wave disappeared immediately and the generation of propagating waves moving as straight lines is of great solution was returned entirely to the original reduced state. This importance as it applies new testing grounds for further mechainhibitory effect of strong illumination may result from excess nistic studies of the chemical waves. We are developing a more Br- production in the photochemical reaction of R ~ ( b p y ) ~ ~ + . ~quantitative *~ description of the wave behavior including photoA better understanding of the illumination effects on chemical chemical processes. waves will be furtheredwith information on the identity and the Acknowledgment. We thank Professor H. Shida for the use concentrations of intermediates. of the camera system. This work was suu~ortedby a Grant-in-Aid for Scientific Research (01540369) from-ihe Mini& of Education of Japan. We are very grateful to the anonymous referees for (12) Field, R. J.; Noyes, R. M. Nature 1972, 237, 390. (13) Field, R. J.; Noyes, R. M. J . Am. Chem. Sac. 1974, 96, 2001. careful readings of the manuscript and valuable suggestions.
Observation and Description of a New Type of Transient in Rotational Coherence Spectroscopy L. L. Connell, T.C. Corcoran, P. W. Joireman, and P. M. Felker*qt,t Department of Chemistry and Biochemistry, University of California, Los Angeles. California 90024- 1569 (Received: January 2, 1990)
We report the observation and explanation of a new type of rotational coherence effect that can occur in experiments on species having transition dipole vectors that deviate from the directions of their principal rotation axes. These newly observed transients arise due to the relaxed rotational selection rules in such species and occur at times almost proportional to * l 2 [ A ’ - I/2(B’+ C’)]-’. With the additional information available from such transients all three excited-state rotational constants of a species can be obtained. This point is illustrated for conformers of 3-indolepropionic acid and for tryptamine-water complexes.
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I. Introduction In the past few years rotational coherence spectroscopy (RCS) in various manifestations has been shownId to be a promising means by which to measure excited-state rotational constants of large species in cold molecular beams. In RCS, one excites a molecular beam (or gaseous) sample with a polarized pulse of light. One then monitors the evolution of the spatial anisotropy of the sample by some polarization-selective probe process. The result is a time-domain observable, [ ( t ) , which contains within it transients (or recurrences) that occur at times that are dependent in a straightforward way on the excited-state rotational constants of the species in the By measuring the positions of the transients, one obtains one or more of the species’ rotational constants. The transients in RCS arise from the thermal average of rotational quantum beats.l*2 As such, their existence depends on the ability to prepare coherent superpositions of rotational eigenstates. Furthermore, their characteristics and the information that they contain depend upon the composition of such superposition states. To date, two types of rotational coherence transients have been identified and observed.I” One type arises from superposition states of the form (in the symmetric top limit) I+) = alJ-l,K) @IJ,K) + -#+l,K), where J and K are used in the standard way as symmetric top rotational quantum numbers. Rotational superposition states of this form can be created in both parallel- and perpendicular-type vibronic transitions.” They give rise to rotational coherence transients of one polarity at times t = n/2B‘and of opposite polarity at t = (2n + 1)/4B’(n = 0, 1, 2, ...), where B’ ’lz(B’ C’)and A’, B’, and C’ denote the excited-state rotational constants. The second type of recurrence arises from superposition states that can be created in perpendicular-type vibronic transitions, those of the form I+) = culJ,K-l)
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‘Alfred P. Sloan Foundation Fellow 1988-1990. *Presidential Young Investigator 1987-1992.
@IJ,K+l). The RCS transients in this case are all of the same polarity, and they occur (in the prolate symmetric top limit) at times n/4(A’- 8’).2a In this Letter, we report the observation of a third type of rotational coherence transient. This new type obtains when the vibronic transition in a species is hybrid in nature,’ that is, when the transition dipole moment has components both parallel and perpendicular to the top axis (or to the near top axis in an asymmetric rotor). In such a situation, rotational superposition states involving adjacent K levels can be excited. As we show below, such states can give rise to RCS recurrences that occur (in the prolate top limit) at times n/(A’- 8’) and (2n + 1)/2(A’ - E’), the polarities of the recurrences at the two sets of times being opposite to one another and their amplitudes being sensitive to transition dipole direction. To demonstrate the existence of these new transients, we report the results of RCS experiments on four jet-cooled species: two of the conformers of 3-indolepropionic acid (IPA), the hydrogen-bonded complex of tryptamine (TA) and H 2 0 , and the complex of tryptamine-d, (DTA) and DzO. (Figure 1 shows molecular structures of IPA, TA, and DTA.) The results show that recognition of the existence of the transients can be essential to interpreting RCS results on asymmetric species. Just as important, they indicate the value that the information derivable ( 1 ) (a) Felker, P. M.; Baskin, J. S.; Zewail, A. H. J . Phys. Chem. 1986, 90,724. (b) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1986, 84, 4708. (2) (a) Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987,86,2460. (b) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J . Chem. Phys. 1987, 86, 2483. (3) Baskin, J. S.; Zewail, A. H. J . Phys. Chem. 1989, 93, 5701. (4) Scherer, N . F.; Khundkar, L.; Rose, T.; Zewail. A. H. J. Phys. Chem. 1987, 91, 6478.
(5) (a) Cote, M. J.; Kauffman, J. F.; Smith, P. G.; McDonald, J. D. J . Chem. Phys. 1989,90,2865. (b) Kauffman, J. F.; Cote, M. J.; Smith, P. G.; McDonald, J . D. J . Chem. Phys. 1989, 90, 2874. (6) Connell, L. L.; Corcoran, T. C.; Joireman, P. W.; Felker. P. M. Chem. Phys. Lett., in press.
0022-3654/90/2094-1229$02.50/00 1990 American Chemical Society
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Figure 1. Molecular structures for 3-indolepropionic acid (IPA), tryptamine (TA), and tryptamine-d3(DTA). The D in parentheses in the figure pertains only to the case of DTA.
from the transients can have in elucidating the structures of such species. 11. Theory and Simulations The theory of rotational coherence effects presented in ref 2a is sufficiently general to predict the new type of recurrence that is the subject of this Letter. Indeed, we were led to search for the recurrences experimentally after computer simulations based on eq 5.5 of ref 2a indicated their presence. Nevertheless, it is instructive to consider in somewhat qualitative fashion how the features arise. As mentioned in the Introduction, rotational coherence effects arise from the thermal average of rotational quantum beats. The thermal averaging of beats gives rise to recurrences in I(t) if all or a substantial fraction of the beat components arising from a sample have frequencies that are proportional (or nearly proprtional) to one another. In the case of a parallel-type transition in a near-prolate symmetric top, the rotational quantum number selection rules7 A J = 0, f l , AK = 0 allow for beat components that are all nearly multiples of 28’. The result of the thermal averaging of these components is rotational coherence transients at times t N n/28’and (2n 1)/48’. In a perpendicular-type transition of a near-prolate top the AK = f 1 selection rule’ can allow for further beat components, these having frequencies nearly proportional to 4(A’- B’). Upon thermal averaging these give rise to transients at times t N n/4(A’- 8’). In the case of a mixed parallel/perpendicular-polarizedtransition (a “hybrid” band), which can occur in species having point group symmetries Czh,C,, C,, C,, or Cl,7the rotational quantum number selection rules (in the symmetric top limit) are A J = 0, f 1 and AK = 0 and f 1. Thus, in the excitation from a single ground rovibronic state to the rotational manifold of an excited vibronic level by a polarized pulse of light, nine excited rovibronic states can be prepared coherently. Such a superposition state can give rise to 36 quantum beat components! some of them identical in frequency. Some of these beat components are just those that give rise to the types of RCS transients that have already been reported on. Others of the components do not survive thermal averaging because their frequencies are not integer multiples of one another. A final type of beat component that can arise from the excitation of a hybrid band represents (in the symmetric top limit) the beating of states having the same J quantum number but having K quantum numbers that differ by unity. Such beat components cannot arise in principal-axis-polarized transitions because K quantum number selection rules preclude the coherent
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(7) For example: Herzberg, G . Molecular Spectra and Molecular Structure; Van Nostrand Reinhold: New York, 1945; Vols. I1 and 111. (8) For example: Haroche, S. In High Resolution Laser Specrroscopy; Shimoda, K., Ed.; Springer: New York, 1976.
excitation of adjacent K levels. However, in the excitation of a hybrid band they can exist. Moreover, the beat frequencies have the values uK+ N (2K l)(A’- 89 and uK- N (2K- l)(A’- 89: the former corresponding to the beating between (J,K+l) and (J,K) states and the latter to that between (J,K-1) and IJ,K). Upon thermal averaging, these components interfere constructively at times t N n/(A’- 8’)to produce transients of one polarity (at these times cos 2auK+t = cos f?fuK-t = +I for all J, K ) , while when t N (2n + 1)/2(A’- 81, they interfere to produce transients of opposite polarity (at these times COS 2wK+t = COS 2rVK-t = -1 for all J, K ) . These are the new RCS “hybrid-band transients”, the observation of which we report herein. To facilitate the interpretation of experimental RCS traces and to make apparent the existence of the hybrid-band transients, we have performed calculations of RCS traces by using the results of ref 2a. In these simulations a set of rotational constants and a set of transition dipole components were assumed for a given species. A trace was then calculated by (1) finding the rotational eigenfunctions and eigenvalues corresponding to the assumed rotational constants, (2) evaluating eq 5.5 of ref 2a, and (3) convoluting the result with a response function approximating that of our experimental apparatus (a Gaussian function with a width of 25-ps fwhm). To perform step 2 above, several assumptions and approximations were made. We assumed a temperature of 7.5 K for the sample. We performed Boltzmann averages over rotational levels by including only those levels up to a maximum J value of 30. Beat components having frequencies above about 150 GHz were neglected. And, finally, excited-state lifetimes were not accounted for in the calculations.
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111. Experimental Section Our experimental apparatus has been briefly described elsewhere6 and will be described in full at a later date. Briefly, RCS experiments were performed by employing the picosecond pumpprobe scheme known as time-resolved fluorescence depletion (TRFD).5 The frequency-doubled, linearly polarized output of a cavity-dumped dye laser (rhodamine 590 in ethanol as dye, about 3-cm-I bandwidth after doubling), synchronously pumped by a mode-locked, Q-switched, CW Nd:YAG laser (1-kHz repetition rate), was used as the excitation source in the scheme. The ultraviolet light was directed through a Michelson interferometer, the output of which was focused (30-cm focal length lens) into a continuous, supersonic free-jet expansion. The free jet was formed by passing helium at 50-70 psig over the compound of interest, which was heated to 140-150 OC. (In the case of the studies of tryptamine-water complexes, the He carrier gas was bubbled through water or deuterated water at 0 or 25 OC prior to encountering the sample.) The gaseous mixture then expanded through an orifice (about 0.1-mm diameter) into a vacuum chamber maintained at less than 0.001 Torr. Spectrally and temporally integrated laser-induced fluorescence from the free jet was collected through long-pass filters and detected by a photomultipler tube, the output of which was processed by a boxcar integrator. The fluorescence signal was normalized to laser power (measured by a photodiode/boxcar combination) and monitored as a function of interferometer (Le., pumpprobe) delay. As shown by McDonald et al.? this scheme gives rise to fluorescence intensity vs delay traces that are essentially the “inverses” of corresponding fluorescence decay curves. That is, transients that are positivegoing in fluorescence decays are negative-going in TRFD traces, and vice versa. The temporal response of our apparatus was estimated to be 25-35-ps fwhm, based on autocorrelation measurements and the observed widths of RCS transients. All of the experimental data were obtained with the pump and probe pulses polarized in the same direction. TA, IPA (both from Sigma Chemical Co.), and D 2 0 (Cambridge Isotope Laboratories) were used without further purification. In experiments on TA-D,O complexes we observed be(9) uK+ and uK. are the approximate energy differences between the 1J.K) and (J,K+I ) states and the 1J.K) and (J,K-I) states. respectively, as discussed in ref 7, for example
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The Journal of Physical Chemistry, Vol. 94, No. 4, 1990 1231
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cence vs delay obtained by assuming A’= 1373, E’= 415, and C’= 372 MHz and by taking the transition dipole of the vibronic resonance to be 18’ from the A principal axis. (bottom) Same as the middle trace except that the vibronic transition was taken to be A-axis-polarized. In the top and middle plots arrows point to hybrid transients.
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Figure 4. (top) Measured fluorescence intensity vs pump-probe delay
obtained upon exciting the jet-cooled hydrogen-bonded complex trypt: band of the species (ref 12). (middle) amine-H,O at 34959 cm-I, the 0 Calculated fluorescence vs delay obtained by assuming A’ = 1465, E’ = 479, and C’ = 397 MHz and by taking the transition dipole of the vibronic resonance to be 23’ from the A principal axis. (bottom) Same as the middle trace except that the vibronic transition was taken to be A-axis-polarized. In the top and middle plots arrows point to hybrid transients.
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obtained upon exciting jet-cooled 3-indolepropionic acid at 34 918 cm-I, : band of the B conformer (ref 11). (middle) Calculated fluoresthe 0 cence vs delay obtained by assuming A’ = 1441, E’ = 349, and C’ = 300 MHz and by taking the transition dipole of the vibronic resonance to be 15’ from the A principal axis. (bottom) Same as the middle trace except that the vibronic transition was taken to be A-axis-polarized. In the top and middle plots arrows point to hybrid transients.
havior consistent with the tryptamine sample quickly exchanging its labile amine protons (three per molecule; see Figure 1) for deuterons. In particular, we observed the bare tryptamine vibronic resonances to shift in a way consistent with deuteration.Ih Also, by RCS we found that the rotational constants associated with these resonances were significantly different from those determined’Ob.cfor fully protonated tryptamine but were consistent with those of DTA.’Oa The point is that although we started with protonated TA, the tryptamine-D20 measurements correspond to DTA-D20 species.
IV. Results Figures 2-5 show experimental and calculated RCS-TRFD traces for conformers A and B of IPA, TA-H20, and DTA-D20. The experimental traces were obtained by setting the excitation as resonances of these wavelength to values previously ~bserved~l-’~ (IO) (a) Wu,Y. R.; Levy, D. H. J. Chem. Phys. 1989, 91, 5278. (b) Philips, L. A.; Levy, D. H. J. Phys. Chem. 1986, 90, 4921. (c) Philips, L. A,; Levy, D. H. J. Chem. Phys. 1988, 89, 85. ( I 1 ) Park, Y. D.; Rizzo, T. R.; Peteanu, L. A,; Levy, D. H. J . Chem. Phys. 1986, 84, 6539. (12) Sipior, J.; Sulkes, M. J. Chem. Phys. 1988, 88, 6146.
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Delay (psec) Figure 5. (top) Measured fluorescence intensity vs pump-probe delay obtained upon exciting the jet-cooled hydrogen-bonded complex tryptamine-d3-D20 at 34965 cm-I. (middle) Calculated fluorescence vs delay obtained by assuming A’= 1382, E‘= 455, and C’ = 377 MHz and by taking the transition dipole of the vibronic resonance to be 23’ from the A principal axis. (bottom) Same as the middle trace except that the vibronic transition was taken to be A-axis-polarized. In the top and middle plots arrows point to hybrid transients.
species and then following the TRFD procedure described in the previous section. Excitation frequencies are given in the figure captions. In each of Figures 2-5 there are two calculated traces in addition to the experimental one. These are meant to illustrate the effect of dipole moment direction on the RCS results. For each species one calculated trace corresponds to an assumption of A-axis polarization while the other corresponds to assuming an off-axis dipole direction consistent with what is known about transition dipoles in indoleI4 and tryptamine.IO To facilitate comparison between the three traces for a given species, the exponential rise time of t h e experimental results5 was subtracted away. V. Discussion From the results of Figures 2-5 one can clearly see that all four of the species studied give rise to RCS recurrences that can be assigned as “hybrid transients”. Consider first the comparison between the A-axis-polarized and hybrid-polarized simulations. (13) Peteanu, L. A.; Levy, D. H. J. Phys. Chem. 1988, 92, 6554. (14) Philips, L. A.; Levy, D. H. J . Chem. f h y s . 1986, 85, 1327.
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It is apparent that transients exist in the hybrid traces which do not exist in the others. Inspection of these additional transients (marked by arrows in the figures) reveals that they occur with positive polarity at times t N (2n + 1)/2(A’ - B’) and with opposite polarity at t N n / ( A ’ - 8’). Referring to section 11, this is just the behavior that one expects hybrid transients to exhibit. Now consider the comparison between experimental and theoretical results. In particular, note the close similarity between the observed results and the simulations corresponding to hybrid polarizations. On the basis of this similarity, one can assign with confidence features marked with arrows in the experimental traces as hybrid transients. Recognition of the existence of RCS transients occurring at t = n / ( A ’ - 8’)in the case of off-axis-polarized transitions is a very significant step in expanding the usefulness of RCS as a gas-phase structure determination technique. This is so because the additional information provided by such transients renders it feasible to obtain all three excited-state rotational constants of a species by straightforward, computationally efficient analysis of RCS results. Specifically, one first obtains approximate values for B’and ( A ’ - 8’) from the experimental positions of the n/2B’ and hybrid transients, respectively. With these values one guesses approximate values for A’, B’, and C’. These initial values of the rotational constants, together with a reasonable guess for the transition dipole direction (based, in part, on the amplitudes of the hybrid transients), are then used as input in generating a calculated RCS trace to compare with experiment. The traces are compared with respect to the positions of the major transients-that is, the ones at t n/28’ and the hybrid transients. From this comparison more accurate values for 8’and ( A ’ - 8’) are obtained. With these values, another set of rotational constants is obtained and used to generate a second calculated trace. At this point the comparison between experimental and calculated results pertains to the positions of all the transients, but particularly to those in the vicinity o f t = ( 2 n + 1)/48’: simulations reveal that the positions of these latter transients are especially sensitive to the asymmetry of the molecule15(i.e., to the difference between B’ and C’ in a near-prolate top). Now the deviation between calculated and experimental results provides a measure of how accurate the individual B’ and C’ constants are. By comparing the calculated and experimental traces for different B’ and C’ values while keeping B‘and ( A ’ - 8’)constant, one rapidly converges to a set of rotational constants consistent with the experimental results. (In this procedure, at least as applied to RCSTRFD results, quantitative comparison of the intensities of transients is not very useful since a variety of factors, such as pump-probe misalignment and the presence of natural abundance isotopic species in the sample,, can cause the experimental intensities to deviate from the simulated ones. In contrast, the shift in the positions of transients due to these factors is within our experimental error.) The procedure outlined above was used in order to generate the calculated traces of Figures 2-5. The rotational constants corresponding to the simulations are our best estimates of the excited-state constants of the species. (These values have uncertainties of about f 1%, which derive primarily from the precision with which the positions of RCS features were measured.) While it is not possible here to discuss in very much detail the molecular structures consistent with these values, it is pertinent to ask whether (15),Connell; L.L.;Corcoran, T.C.; Joireman, P. W.; Felker, P. M. To be published.
such structures are reasonable ones. Consider the IPA conformers first. We have previously reported B’values for these species and from these values made conclusions about their gross structures.6 The additional values reported here for all three of the excited-state rotational constants of each species narrow considerably the range of possible structures. Briefly, the structure of the B conformer is such that x2 (the C,C,C,C6 dihedral angle;16 see Figure 1) is near 90°, x, (the C’C,C,C, dihedral angle) is near 160°, and (the O,C’C,C, dihedral) 1s near 20’. Such a structure is quite consistent with what one would expect given the structures of other tryptophan analogues in crystal^^'-^^ and in the gas phase.I0 The rotational constants for the A conformer are consistent with two possible structures. One such structure has x2 = 40’ and xi 80°, with several possible values. The other has x2 = 120’ and x1 = -80°, also with several possible values for While further experiments involving isotopic species and/or molecular mechanics theoretical studies are necessary to further specify the A conformer’s structure, these possible structures are certainly not unreasonable ones. The structures consistent with the rotational constants obtained for the TA-H20 and DTA-D20 complexes are also quite reasonable. Assuming that water bonds to the lone pair electrons of the TA side-chain amine group via a linear hydrogen bond having an 0-N distance of 2.98 A (the distance in the HOH-NH, complex20), one can vary the x2, x i , (xi is now the NC,C,C, dihedral), and 4 dihedral angles (4 is the ONC,C, dihedral) to find structures consistent with the measured constants. Doing so, one finds that such structures have x2 = 95’ and xl -60°, values similar to those of the most abundant (Le., A and B) conformers of the free TA molecule.i0 Furthermore, in these structures the oxygen atom of the water is in proximity to H6on the indole ring, allowing for an additional hydrogen-bonding interaction. (For example, when x2 = 95’, x1 = -60°, and 4 = -30°, the 0-H6 distance is about 2.2 A.) This additional interaction and the favorable conformation of the TA moiety in the structures are probable reasons for the apparent dominance of a single conformation of TA-H20,I2,l3a dominance that is in contrast to the behavior of bare TA, which exhibits numerous stable conformers in seeded molecular Preliminary RCS results on TA-methanol complexesi5are also consistent with this picture of conformation “ l ~ c k i n g ” ~inl -one-to-one ~~ complexes of TA and -OH-containing species. In conclusion, we have reported the prediction and observation of a new kind of RCS transient, one which can occur upon excitation of hybrid-polarized vibronic bands. The information available from such transients, together with that contained in other types of RCS features, can allow one to obtain all three of the excited-state rotational constants of a species by means of a straightforward data analysis procedure. This capability significantly enhances the usefulness of RCS in structural studies of asymmetric species.
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Acknowledgment. This work was supported by the National Science Foundation (Grant CHE 88-13470), by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the UCLA Academic Senate. (16) We use the convention for dihedral angles recommended by the IUPAC-IUB commission in: Biochemistry 1970, 9, 3471. (17) Pasternak, R. A. Acta Crystallogr. 1956, 9, 341. (18) Karle, I. L.;Britts, K.; Gum, P. Acta Crystallogr. 1964, 17, 496. (19) Takigawa, T.;Ashida, T.;Sasada, Y.; Kakudo, M. Bull. Chem. SOC. Jpn. 1966, 39, 2369. (20) Herbine, P.; Dyke, T. R. J . Chem. Phys. 1985, 83, 3768.
