7311
J. Phys. Chem. 1995,99, 7311-7319
Rotational Coherence Spectroscopy of Aromatic-(Ar), Clusters: Geometries of Anthracene-(Ar),, 9,10-Dichloroanthracene-Ar, and Tetracene-Ar Shane M. Ohline, Joann Romascan, and Peter M. Felker* Department of Chemistry and Biochemistry, University of Califomia, Los Angeles, Califomia 90024-1569 Received: October 5, 1994@
Rotational coherence spectroscopy has been applied in a structural study of anthracene-(Ar), ( n = 0-3), 9,10 dichloroanthracene-(Ar), (n = 0, l), and tetracene-Ar. Geometries consistent with the experimental results have been determined for each of the species. These geometries are compared with previous structural predictions from minimum energy calculations and vibronic frequency shift rules and are considered in the light of other known aromatic-(&), structures.
I. Introduction Aromatic-(rare gas), clusters represent convenient model systems for the study of numerous important issues in chemistry, including solvation phenomena, surface adsorption, many-body interactions, multidimensional quantal dynamics, collective behavior in finite-size systems, intermolecularvibrational energy flow, and chemical reaction dynamics (e.g., see refs 1-8). The attractiveness of such species derives from the relative ease with which they can be studied experimentally due to advances in molecular-beam methods and spectroscopic techniques. It also derives from the relative simplicity of the rare-gas moieties, whose mutual interactions and interactions with aromatics can be modeled fairly effectively by using semiempirical atomatom potential energy functions.6 There is, however, a prominent limitation to using aromatic(rare gas), species as model systems for the types of studies mentioned above. This arises from the difficulty in obtaining experimental evidence for the structures of the species. Such difficulty arises from their size, which often renders them inaccessible to standard methods of rotational spectroscopy. In many cases, this situation has forced one to infer structures from the results of minimum energy calculations involving assumed potential energy functions6 or from experimental information relating to the frequency shifts of vibronic (or vibrational) bands as a function of cluster size (e.g., refs 7 and 8). Obviously, it is desirable, where possible, to check the inferences made from such information against other, more definitive structural results on the species. Such checks are particularly important given that even fully resolved rotational spectra on several isotopomers cannot provide one with enough information to deduce the geometries of clusters composed of more than several moieties. For such species, therefore, substantial reliance on assumed potential energy functions andor band-shift rules is likely to be required to decide on a structure. Clearly, knowledge of the limitations of these approaches is imperative if they are to be meaningfully employed. In a series of recent papers we have reported experimental results pertaining to the structures of several aromatic-(rare gas), clusters, including perylene-(Ar), and -Ne, (n = 1, 2)9 ~arbazole-(Ar)2,'~and diphenylacetylene-Ar.' I In these studies the use of the time-domain method of rotational coherence spectroscopy (RCS) made it possible to measure the rotational constants of the species, even though their large size makes study by frequency-domain rotational spectroscopies difficult, @
Abstract published in Advance ACS Abstracts, May 1, 1995.
0022-365419512099-7311$09.00/0
at best. The geometries deduced from the measured rotational constants allowed one to check both the qualitative (e.g., preferred isomeric form) and quantitative (e.g.,bond distances) accuracy of previous structural predictions based on assumed potential energy functions and band-shift rules. Mostly, our RCS results, like the structure results from other laboratories (e.g., refs. 13-20), confirm these predictions. Particularly notable is the confirmationgof (a) the prediction from minimum energy calculations of two structural isomers of peryIene-(Ar)*' and (b) the assignment by band-shift rules of two vibronic bands to those isomers.' However, for one species, carbazole-(Ar)2, there is a glaring difference between the RCS-deduced structure'O and the geometry found3 to be the minimum energy one on the atom-atom pairwise potential energy surface of ref 6. The calculated minimum energy geometry has both Ar atoms on the same side of the carbazole plane (a 2 0 structure), whereas the experimental geometry has them on opposite sides (a 1 1 structure). This qualitative discrepancy demonstrates the potential for error in relying on such calculations to deduce structures. It also highlights the need for even more experimental information on the geometries of aromatic-(rare gas), clusters, particularly ones that might be expected to exist in multiple isomeric forms. In this paper we report the results of further RCS experiments on, and structural studies of, aromatic-(rare gas), species. The work concentrates on some of the best-studied clusters of this type. The major part of the work pertains to anthracene-(Ar), (n = 0-3) clusters. Since the first report of experiments on this cluster series,2' several papers on them, including ones pertaining to calculated structures.22 complexation-induced vibronic band ~hifts,*,*~3~~ ultrafast and photoelectron spectra,23have appeared. The structural results that we report herein for these species are, therefore, not only valuable for evaluating the accuracy of potential energy functions and bandshift criteria for structure analysis, they are critical for the full interpetation of data pertaining to other important properties of the species. We also report results for 9,lO-dichloroanthracene(Ar),(n = 0, 1) and tetracene-Ar. (We abbreviate 9,lOdichloroanthracene as DCA henceforth.) These species are of interest because they are the first in two other series of wellstudied cluster^.^^.^^ Moreover, the tetracene-Ar complex is one that has been studied before by high-resolution frequencydomain rovibronic spectro~copy.~~ While spectra were successfully obtained in that work, they were too congested to yield to analysis, and no structural information could be deduced. In contrast, the RCS results that we report below are readily
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7312 J. Phys. Chem., Vol. 99, No. 19, 1995
Ohline et al.
analyzed to give rotational constants of, w d structural information on, the species. Thus, tetracene-Ar serves as a particularly compelling illustration of the value of RCS in the rotational spectroscopy of large species.
(a) hIo-Anthracene
.b
-c v)
a,
11. Experimental Section RCS experiments were performed by using the method of time-resolved fluorescence depletion (TRFD).28 The apparatus and general procedure have been described e l s e ~ h e r e . *We ~,~~ describe here the specifics pertinent to the anthracene-, 9,lOdichloroanthracene-, and tetracene-(Ar), experiments. The excitation pulses for TRFD were produced by a picosecond laser system composed of a Q-switched, mode-locked continuous wave (CW)Nd:YAG laser and a cavity-dumped dye laser. For the anthracene-(Ar), and DCA-(Ar), experiments the fundamental of the dye laser (LDS 750 and LDS 765 in methanol, respectively, as dye) was frequency-doubled in @-bariumborate to produce excitation pulses in the vicinity of the SI SO0; transition of each system (27 695 cm-I for anthra~ene,~’ 27 764 cm-’ for anthracene-dl~,~~ and 25 950 cm-’ for DCAZ5). For the tetracene-Ar experiments, the fundamental of the dye laser (LDS 750 in methanol as dye) was mixed in @-bariumborate with that of the Nd:YAG laser to generate light by sumfrequency generation at the SI SO0; band of tetracene-Ar (22354 ~ m - l ) . The ~ ~ excitation light produced by these methods was directed through a Michelson interferometer to produce the pump and variably delayed probe pulse trains. The weakly focused output of the interferometer intersected the supersonic molecular beam sample at right angles and at a distance of several millimeters from the expansion orifice of the beam. The molecular beam was formed by entraining heated anthracene, DCA, tetracene (all from Aldrich Chemical Co.), or anthracene-dlo (Cambridge Isotope Labs) in a carrier gas and expanding the mixture continuously through a small orifice (-50 pm in diameter) into vacuum. The carrier gas consisted primarily of helium with a small amount of argon mixed in by means of a needle-valve arrangement. The total preexpansion pressure was about 90 psig. Time-integrated fluorescence was collected with an elliptical mirror, filtered spatially and spectrally (with sharp-cut, red-pass filters) to eliminate excitation-light scatter, and detected with a photomultiplier. The photomultiplier signal was averaged with a boxcar integrator, whose output was stored by a computer as a function of the delay between the pump and probe pulses to yield a TRFD trace. All traces were measured by tuning the excitation frequency to that band assigned to the SI SO 0; of the species of interest. For the anthracene-dlo-(Ar), complexes, argondependent vibronic resonances were found at frequency shifts from the 0; band of anthracene-dlo that were the same as the shifts found for the perprotonated clusters. We assign these bands to the perdeuterated analogs of the perprotonated species and refer to them as such below. Measured RCS-TRFD traces were manipulated numerically to eliminate any delay dependence in background fluorescence levels due to lifetime effects,28delay-line misalignment, laserbeam divergence,etc. Ultimately, for determination of rotational constants from the data, the measured traces were compared with simulated traces by using the theoretical results of refs. 32 and 33. These simulations were performed by assuming a temperature of 5 K and a Gaussian temporal response of 28-ps fwhm, unless otherwise noted.
c
a,
2 a, 5: F 0 a
ii
1wO
500
0
Delay ( p g W
2 m
25W
(b)dlo-Anthracene
-
-
111. Results A. Anthracene. The structure of anthracene is well-known. Indeed, RCS results pertaining to the SI Oo level of the
J
i
I
C
I
500
Delay
(PAY
I
1500
Figure 1. (a) Top: Measured RCS-TRFD trace corresponding to the 0; band of anthracene-hlo. Features labeled with J’s, K’s, or C’s have been assigned as J-, K-, or C-type rotational coherence effects, respectively (see text). Bottom: Calculated trace that best fits the experimental trace above. The calculated trace is the sum of two traces, one with the SIrotational constants 2.159, 0.4445, and 0.3683 GHz, respectively, for A, E , and C,’* and one with constants calculated from the crystal structure of anthracene32 and equaling 2.151, 0.4538, and 0.3747 GHz. The transition dipole was taken to be along the b principal axis (short in-plane axis) of the molecule. (b) Top: Measured RCSTRFD trace corresponding to the 0; band of anthracene&. Bottom: Calculated trace that best fits the experimental trace above. The rotational constants for the calculated trace were taken as 1.820, 0.408, and 0.336 GHz for A, E , and C, respectively. The transition dipole was taken to be along the b principal axis (short in-plane axis) of the molecule.
perprotonated species have been reported previo~s1y.I~Nevertheless, we include here our results on anthracene bare molecule for two reasons. First, the perdeuterated species has not yet been studied with rotational spectroscopy,and we require its rotational constants in order to perform analyses of the structures of Ar clusters involving it. Second, the TRFD method samples rotational coherences in both of the vibronic states involved in an experiment, not just the excited state.33 When the rotational constants in the two states are sufficiently close to one another that the ground- and excited-state rotational coherence effects occur at unresolvably similar times, then the rotational constants derived from a RCS-TRFD trace are averages of the ground- and excited-state values. Such is the case for all but one of the species that we address herein. Thus, in order to derive structures of anthracene-containing clusters from RCS-TRFD-measured rotational constants, one would ideally like to know the average of SI and SOconstants for the anthracene bare molecule. Since the SOconstants have not been measured for either anthracene isotopomer, our TRFD results on the species are pertinent. Parts a (top) and b (top) of Figure 1 show measured RCSTRFD traces for the SI SO 0; bands of anthracene-hlo and anthracene-& respectively. The traces exhibit the three types of RCS transients to be expected30for a b-axis polarized (short
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J. Phys. Chem., Vol. 99, No. 19, 1995 7313
RCS of Aromatic-(Ar), Clusters TABLE 1: RCS-Derived Rotational Constants (GHz) of Anthracene-, Tetracene-, and DCA-(Ar). Clusters species 24-B-C B+@ B-Cb c“ anthracene hio dio
anthracene- Ar hio dio
3.488 2.896
0.821 0.744
0.921 0.875
0.707 0.645
1.076
0.552 0.510 0.365
anthracene-(Ar)z hio dio
tetracene- Ar species anthracene -(Ar)3 hio
40 DCA hs DCA-(Ar) hs
(A
+ B)”
I
(a) Anthracene-Ar
0.3715 0.336 0.036 0.024
,
0.070 0.053 10.015 (A - B)b
0.622 0.584
‘0.025 ‘0.030
1.001
C
0.7 12
‘0.054
C”
0
1500
Dela\).’$.s) (b) Anthracene-(Ar),
0.247d
Estimated uncertainty of &0.5%. Estimated uncertainty of 115%. The assumption of planarity for DCA allows the extraction of A - B, as well. See text. Represents an average of the measured SO (C = 0.2485) and S I (C = 0.246) values. See text for details. a
in-plane axis) transition in a near-prolate asymmetric top. That is, there are K-type transients, all occurring with the same polarity at t = m44A - 2B - 2 0 (m being an integer), J-type transients, occurring with alternating polarity at t m42B 2 0 , and C-type transients, occurring with no definite polarity at t = d ( 4 0 . These transients are labeled accordingly in Figure 1. The existence of these three types of transients in the RCS results means that three independent principal rotational constants can be derived from the data for each isotopomer. These constants, derived by the detailed comparison of the measured traces with ones calculated while varying the values of the rotational constants, are given in Table 1. As a check on the reported rotational constants, we have calculated the average of the SI rotational constants reported in ref 17 for the hlo species (A = 2.159, B = 0.4445, and C = 0.3683 GHz) and those for SO computed from the crystal structure of the species34(2.151, 0.4538, and 0.3747 GHz). These average values agree very well with the rotational constants reported in Table 1. Moreover, RCS-TRFD traces obtained by taking the sum of traces calculated by assuming the ground- and excited-state constants, respectively, match the measured traces very well. Such comparisons are shown in Figure la,b for the hlo and d1o species, respectively. The rotational constants reported in Table 1 for the anthracene isotopomers are the ones that we use below (section IV) in the structure analyses of the anthracenecontaining clusters. B. Anthracene-Ar. Figure 2a (top) shows a RCS-TRFD trace measured for the S I SO0; band of anthracene-hlo-k that occurs at a shift (dv) of -42 cm-I from the 0; band of bare a n t h r a ~ e n e .Two ~ ~ types of transients are clearly visible in the trace. The first set, with transients spaced by -545 ps, can be assigned as K-type. The second, including the prominent negative-going feature at 1409 ps and the barely discemible positive-going feature at 704 ps, we assign as J-type. These assignments are based on the characteristics of the transients and on knowledge of the most likely gross geometries of the complex. The presence of these two types of transients, along with the assumption that the species is a prolatelike asymmetric top (Le., K 0, where K is Ray’s asymmetry ~ a r a m e t e r ~allows ~), one to extract values for the rotational constants 2A - B - C and B C from the data. A less precise measure of a third independent constant, B - C, can also be obtained from the experimental trace by detailed comparison of the results with
.-,
+
500
0
+
1500
Figure 2. (a) Top: Measured RCS-TRFD trace corresponding to the BY = -42 cm-’ band assigned to anthracene-hlo-Ar. Features labeled with K’s (J’s) have been assigned as K-type (J-type) rotational coherence effects (see text). Bottom: Calculated trace that best fits the measured trace. The rotational constants for the calculated trace were taken as 0.814,0.3715, and 0.3355 for A, B , and C, respectively. The transition dipole was taken to lie along the c principal axis of the species. (b) Top: Measured RCS-TRFD trace corresponding to the BY = -100 cm-’ band assigned to anthracene-hlo-(Ar)Z. Features labeled with J’s have been assigned as J-type rotational coherence effects (see text). Bottom: Calculated trace corresponding to the geometry of the (2 0) anthracene-Arz isomer C (see Table 2 and Figure 7c). The rotational constants for the calculated trace were 0.575, 0.308, and 0.246 GHz for A, B, and C, respectively. The transition dipole was taken to lie along the y axis of the anthracene moiety.
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traces simulated as a function of B - C. These values are all given in Table 1. The simulated trace shown in Figure 2a (bottom) was calculated by assuming these values for the rotational constants and by taking the relevant transition moment to be C-type. One sees that the match between the measured and the simulated traces is quite good. Experimental results similar to those shown in Figure 2a were also obtained for the perdeuterated anthracene-Ar complex. The rotational constants found by the analysis of those results are also given in Table 1. C. Anthracene-(Ar)z. Figure 2b (top) shows a RCS-TRFD trace measured upon excitation of the anthracene-h10-(Ar)2band at dv = -100 The trace exhibits two prominent features. There is a positive-going transient at 892 ps and a negative-going one at 1791 ps. Results similar in form to these were also obtained for the perdeuterated anthracene-(k)2 cluster. The transients observed for these species have .the characteristics expected of J-type transients, and we assign them as such. Their positions therefore provide information pertaining to B C if the cluster is prolatelike or A B if it is oblatelike. Unfortunately, one cannot a priori rule out either type of geometry in the case of this cluster. Given this, we refrain at this point from quoting any rotational constants for the species. Instead, since the ultimate aim is to obtain structural results, our approach will be to check (by comparison of measured and
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7314 J. Phys. Chem., Vol. 99, No. 19, 1995
Ohline et al.
Anthracene-(Ar),
I
J.--,
I 0
I
I
I
I
loo0
2000
3wo
4000
Delay (PS) Figure 3. Top: Measured RCS-TRFD trace corresponding to the dv = -142 cm-I band assigned as the 0; of anthracene-hlo-(Ar)3. Features labeled with J's have been assigned as J-type rotational coherence effects. Bottom: Calculated trace corresponding to the geometry of the (2 1) anthracene-Ar3 isomer A (see Table 3 and Figure 9). The rotational constants for the calculated trace were 0.314, 0.309, and 0.182 GHz for A , B , and C, respectively. The transition dipole was taken to lie along the y axis of the anthracene moiety.
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simulated RCS traces) which likely geometries for the cluster22 can reproduce the detailed form of the RCS results. This analysis is presented below in section IVC. D. Anthracene-(Ar)J. Figure 3 (top) shows RCS-TRFD traces corresponding to the BY = -142 cm-' vibronic band assigned to anthracene-h10-(Ar)3.~~ The clear, prominent transients at 804, 1608, and 3219 ps are readily assigned as J-type on the basis of their equal spacings, their altemating polarity, and their relative magnitudes (the first being smaller than the second). Similar results were also obtained for the perdeuterated isotopomer of the cluster. As with the anthracene-(Ar)2 species, one cannot rule out a priori either the possibility that anthracene-(Ar)3 is prolatelike or that it is oblatelike. Therefore, we refrain at this point from quoting any rotational constants derived from the data of Figure 3. Instead, we shall proceed below (section IVD) by checking likely geometries for the cluster against the RCS results. From such analysis, not only will definite values for rotational constants emerge but so will the geometry of the cluster. E. 9,lO-Dichloroanthracene. An RCS-TRFD trace resulting from excitation of the SI SO 0; band of DCA25is shown in Figure 4a (top). One notes two kinds of transients entering into this trace. There are two broad negative-going features at 968 and 1960 ps that are of the same type and two narrower negative-going features spaced by 1015 ps that are of another type. Since the gross geometry for DCA is known, the analysis of these results is fairly straightforward. One starts with a rough geometry for DCA in which the species is planar with all C-C bond distances being 1.40 A, all C-H band distances being 1.08 A, the C-C1 bond distances being 1.74 A, and all bond angles being 120". One then calculates the rotational constants associated with such a geometry, which are A = 0.555, B = 0.461, and C = 0.252 GHz. With these constants and the assumption of a short-axis-polarized transition moment (which corresponds to the b principal axis of the assumed geometry), one then computes a simulated RCS-TRFD trace. The trace so computed looks very similar to the measured one in Figure 4a. Moreover, an analysis of the rotational coherences responsible for the transients in the simulated trace reveals that the broad, more closely spaced features in the experimental data are J-type, whereas the narrower, wider-spaced features are C-type. Having identified the types of transients observed by this means, it is then easy to fit the experimental data and extract rotational constants by nonlinear least-squares methods. However, in the DCA results there is one further complication that
-
2m
2803
3000
3400
3200
Delay (PS)
Figure 4. (a) Top: Measured RCS-TRFD trace corresponding to the
0; band of 9,lO-dichloroanthracene (DCA). Features labeled with J's (C's) have been assigned as J-type (C-type) rotational coherence effects (see text). Bottom: Calculated trace that best fits the experimental trace above. The calculated trace is the sum of two traces, one calculated with the rotational constants 0.545, 0.456, and 0.2485 GHz and the other with 0.568, 0.434, and 0.246 GHz for A , B, and C, respectively. In both cases the transition dipole was taken to lie along the b principal axis (short in-plane axis) of the molecule. (b) Top: Measured RCS-TRFD trace for DCA at the position of the third J- and C-type recurrences. The C-type transient is split with the two components appearing at 3016 and 3048 ps. Bottom: Calculated trace that best fits the trace above. The constants and the transition dipole direction used are the same as those used to obtain the calculated trace appearing in a.
must be addressed first. The complication is shown in Figure 4b, where one sees that the third C-type transient in the TRFD results for DCA is split into a doublet. This splitting indicates that the C constants of the molecule are different enough in the ground and excited states that resolvable C-type transients, arising from SO and SIcoherences, respectively, are observable. The upshot is that analysis of the DCA data cannot be performed by varying a single set of rotational consants. Both SOand SI rotational coherences must be explicitly included. Ultimately, by taking account of this and by matching measured and simulated TRFD traces, one obtains the values for A B and C that are reported in Table 1 for DCA. Making the assumption that the molecule is planar in both vibronic states then yields A - B values as well. Comparison of the simulated traces in Figure 4 with the measured ones shows that the sets of rotational constants given in Table 1 are, indeed, consistent with the RCS results. F. 9,lO-Dichloroanthracene-Ar. RCS-TRFD experiments pertaining to the SI So 0; excitation of DCA-Ar at BY = -50 cm-' 25 yielded traces in which only a single J-type transient was identified. From the position of the transient and the fact that only oblatelike geometries are likely for this complex, a value for A B was determined from the data. An upper limit on A - B could also be fixed by detailed comparison of simulated traces with the experimental results. The experi-
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J. Phys. Chem., Vol. 99, No. 19, 1995 7315
RCS of Aromatic-(Ar), Clusters Tetracene-Ar
.-
I_ v)
!??
s
ii
0
I
I
I
1
I
I
500
1000
1500
2000
2500
3000
Delay (PS)
Figure 5. Top: Measured RCS-TRFD trace corresponding to the 0; band of tetracene-Ar. Features labeled with K’s (J’s) have been assigned as K-type (J-type) rotational coherence effects (see text). Bottom: Calculated trace that best fits the experimental trace above. The rotational constants used in the calculation were 0.721,O. 186, and 0.179 GHz for A, B, and C, respectively. The transition dipole was taken to lie along the c principal axis of the species.
mentally derived rotational constants for this complex are given in Table 1. G. Tetracene-Ar. Figure 5 (top) shows a measured RCSTRFD trace corresponding to the SI SO 0; excitation of tetracene-Ar at 6v = -43 cm-l. The trace clearly exhibits two types of transients. One type comprises the four negativegoing features that are spaced by about 470 ps. The other comprises the negative-going transient appearing at 2725 ps. The only reasonable assignmentsfor these two types, given their spacings and polarity characteristics, as well as consideration of the reasonable possibilities for the geometry of the complex, are as K-type and J-type transients, respectively, from a prolatelike asymmetric top. Their positions, therefore, yield values for the rotational constants 2A - B - C and B C, respectively. The values obtained from these constants by fitting the data of Figure 5 are given in Table 1. In addition, an upper limit to B - C was obtained by detailed comparison of simulated traces with measured ones. This value is also given in Table 1. In Figure 5 (bottom) we show a trace calculated by using the experimentallyderived rotational constants and by assuming a c-axis-polarized transition dipole (consistent with the dipole being along the short, in-plane axis of the tetracene moiety). One can see that the simulated trace matches the measured one quite well.
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IV. Analysis of Structures In this section we use the RCS results reported above to ascertain effective zero-point geometrical parameters for the anthracene-, DCA-, and tetracene-(Ar), species. In doing this we make the usual assumption that the geometry of the aromatic molecule does not change upon complexation. We also adopt an axis-labeling convention in which the aromatic’s out-of-plane axis is z, its in-plane short axis is y, and its inplane long axis is x (e.g., see Figure 6a). Note, again, that in this work the rotational coherence effects observed for each of the complexes and clusters represent the unresolved contributions of ground- and excited-state coherence~.~~ Because of this, the structural parameters determined from the RCS-TRFD data represent averages of the parameters corresponding to the two states. A. Anthracene-Ar. The best candidate for the geometry of anthracene-Ar is one in which the Ar atom is directly above the central ring of the anthracene molecule (Figure 6b). Such a geometry is predicted by minimum energy calculations22and would fit with the known structures of other, similar species
Figure 6. (a) The anthracene-fixed axis convention used herein. The same convention is used for DCA and tetracene, as well. (b) Two views of an anthracene-Ar structure that is consistent with the RCSmeasured rotational constants for the species. R was determined to be 3.43 f 0.03 %, for both anthracene-hlo- Ar and anthracene-dlo-Ar.
(e.g., ref 13). The RCS results on the complex provide strong support for this type of structure. First, for both isotopomers the value of B for anthracene-Ar is the same as the value of C for the anthracene molecule, within experimental uncertainty. This is strong evidence that the b principal axis of the complex is coincident with the c principal axis (i.e., z axis) of the anthracene and that the Ar atom lies on the z axis. Second, by taking the Ar atom to be centrally bound, one can compute the effective zero-point distance, R, between the atom and the anthracene plane by using RCS-determined rotational constants. One uses = l(bA) pR2, where is the moment of inertia corresponding to the c principal axis of the complex (parallel to the y axis of anthracene), I(bA) is the moment of inertia corresponding to the b principal axis of anthracene (the y axis), and ,u is the pseudodiatomic reduced mass of the complex. One finds R = 3.43 f 0.03 8, for both the protonated and deuterated species, the uncertainty representing the results of a propagation of errors analysis. This value for R agrees with the calcdated value reported in ref 22 and matches analogous distances determined for similar complexes. Finally, simulated RCSTRFD traces computed by using the rotational constants and the c-axis transition-dipole direction corresponding to the centrally bound, R = 3.43 8, geometry for the complex (see Figure 6b) agree very well with the traces observed for both isotopomers of anthracene- Ar. We have assessed how large deviations from the geometry of Figure 6b can be while remaining consistent with the RCSderived rotational constants. We find that displacements of the Ar atom by f 0.4 8, along the x axis, f 0 . 2 along the y axis, and f0.03 A along the z axis yield structures whose rotational constants agree with the measured ones to within experimental error. B. Dichloroanthracene- Ar. The limited results on DCAAr do not permit a structure analysis as unambiguous in its outcome as that for anthracene-Ar. Nevertheless, one can show that the RCS results on the species are consistent with a geometry that is very similar to anthracene-Ar. In particular, by assuming a centrally bound structure, one can calculate a
6‘’
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6‘’
Ohline et al.
7316 J. Phys. Chem., Vol. 99, No. 19, 1995 TABLE 2: Possible Structures for Anthracene4 IO-(Ar)2 and Their Corresponding Rotational Constants isomer“
2
0.00 3.80 B(1+1) 0.00 0.00 C(2+0)n 1.88 -1.88 D(2+0) 0.95 -0.95 measured values
A(2+0)
y”
Z”
A“
B+C
B-C‘
0.00 0.00 0.00 0.00 0.00 0.00 1.88 -1.88
3.43 3.43 3.43 -3.43 3.43 3.43 3.43 3.43
0.662
0.482
0.033
0.431
0.617
0.126
0.575
0.554
0.062
0.455
0.566
0.012
0.552
0.070
0
(b)
See Figure 7 for pictorial depictions of the isomers. Coor$nates refer to the Ar positions in the anthracene-fixed axis system (A) and are taken from ref 22. The coordinates of the first argon atom are on the first line; those of the second are on the second line. Rotational constants are in gigahertz. dThe rotational constants for the perdeuterated anthracene-(Ar);! isomer C are A = 0.528, B C = 0.512, and B - C = 0.054 GHz.
0
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value for R by using the equation 6 ‘’= FA’ + pR2, where 6‘’and I(hDCA) are the b moments of inertia of the complex and DCA, respectively (both correspondingto principal axes parallel to y ) , or the equation 1‘6’ = I(RCA’ + pR2, where 4 ‘’and
,‘,“c“’
are the a moments of inertia of the two species, respectively (both corresponding to axes parallel to x). From the first equation, by using the measured rotational constants of DCA-Ar to determine r(,c’ and the calculated B constant quoted in section IIIE to determine one finds R = 3.42 A. From the second equation, by using the measured rotational constants of DCA-Ar to determine and the calculated A constant quoted in section IIIE to determine one also finds R = 3.42 A. Clearly, the two values are consistent with one another. They also are what one would have guessed given the structure deduced for anthracene-&. The upshot is that the RCS results are consistent with a geometry for DCA-Ar that is very similar to the centrally bound anthracene-Ar structure, as shown in Figure 6b. C. Anthracene-(Ar)z. The RCS results on anthracene( A r ) 2 are not sufficient for determining unambiguously the structure of the cluster. However, they can serve to eliminate some of the four geometries that have been proposed for the species 21-22 based on the results of miminum energy calculations and band-shift considerations. The details of the four geometries (A, B, C, and D), together with the principal rotational constants associated with them, are given in Table 2. Pictures of the isomers are presented in Figure 7. Note that three of these are (2 0) structures and one, B, is a (1 1) structure. RCS-TRFD simulations corresponding to the four geometries of Figure 7 and Table 2 are shown in Figure 8. In each of these simulations the transition dipole was taken to be parallel to the anthracene y axis. One point that is immediately obvious upon comparison of these traces with the data of Figure 2b is that the (1 1) structure does not characterize the species. Not only are the transients in the simulated trace (Figure 8b) too closely spaced, the simulated feature at about 1250 ps is much too prominent. Therefore, the RCS results lead to the conclusion that the cluster is a (2 0) isomer. Of the three (2 0) possibilities, comparison of the simulated and measured traces shows that the symmetric one (C) is the one that best characterizesthe cluster. Structure A gives rise to transients at positions significantly different than those observed, and on that basis can be ruled out. Structure D gives rise to J-type transients that are close in position and identical in polarity to the observed ones. However, it also produces a very prominent K-type transient at -1450 ps that is not present in the measured trace. The simulated trace corresponding to structure C, on the other hand, matches all aspects of the experimental results quite well.
e‘*’, 4‘’
rCA’,
8
6
8
Figure 7. Two views each of the four possible anthracene-(Ar);! isomers considered by Henke et aL2* Following Henke et al., we label the isomers depicted in a-d as the A, B, C, and D isomers of the species. The Ar coordinates and rotational constants of these species are given in Table 2.
V
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I
1
1
I
0
500
loo0
1
I
2000
1 2500
Delay (ps) l5O0
Figure 8. Calculated RCS-TRFD traces corresponding to each of the four fully protonated anthracene-(Ar);! isomers depicted in Figure 7: (a) isomer A, (b) isomer B, (c) isomer C, and (d) isomer D. The rotational constants used are given in Table 2. The transition dipole in each case was taken to be along the short in-plane 01) axis of the anthracene moiety.
Given that a geometry similar in form to C is the one that characterizes the observed anthracene-(Ar)2 species, one knows that the species is prolatelike. Thus, one can now extract rotational constants from the experimental results and use those values in a fitting procedure to obtain quantitative estimates of geometrical parameters. The constants so extracted are given in Table 1. The values for B C follow directly from the positions of the observed J-type transients. The B - C values were derived by detailed comparison of the observed traces with ones simulated as a function of B - C. The constants quoted
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J. Phys. Chem., Vol. 99, No. 19, 1995 7317
RCS of Aromatic-(Ar), Clusters TABLE 3: Possible Structures for Anthracene-hlo-(Ar)J and Their Corresponding Rotational Constants xh
y" z" A" B" C' 1.88 0.00 3.47 0.314 0.309 0.182 -1.88 0.00 3.47 (A B = 0.623, A - B = 0.008) 0.00 0.00 -3.43 B(3+0) -3.75 0.00 3.50 0.457 0.203 0.162 0.00 0.00 3.50 3.75 0.00 3.50 C(3+0) 4.60 0.00 3.45 0.368 0.195 0.144 0.85 0.00 3.45 -2.90 0.00 3.45 measured values (A B = 0.622, A - B = 0.025)
isomer" A(2+l)' best fit
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See Figure 9 for pictorial depictions of the isomers. Coor$nates refer to the Ar positions in the anthracene-fixed axis system (A) and are taken from ref 22. The coordinates of the first argon atom are on the first line, those of the second are on the second line, and those of the third on the third line. Rotational constants are in gigahertz. The rotational constants for the perdeuterated anthracene-(Ar)3 isomer A are A = 0.304, B = 0.283 (A B = 0.587, A - B = 0.0219), and C = 0.174 GHz.
3
a
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in Table 1 were then fit by varying one of the geometrical parameters characterizing geometry C, namely, R (see Figure 7c), the distance of the Ar dimer from the plane of the anthracene. In this fit the Ar-Ar distance was fixed at 3.76 8, (the equilibrium internuclear distance of gas-phase argon dimer),2 and the species was assumed to be symmetric with respect to reflection in the y - z plane, leaving only R as a free parameter. A value of 3.48 f 0.03 8, was found to fit the observed rotational constants best. Figure 2b (bottom) shows a simulated trace corresponding to the resulting structure. Comparison with the measured trace clearly shows that the derived structure is consistent with the observed RCS results. D. Anthracene-(Ar)3. Just as for the anthracene-(Ar)z cluster, the RCS data on anthracene-(Ar)3 are not sufficient to fix an unambiguous geometry for the species. Thus, we again take the approach of checking simulated TRFD traces corresponding to proposed geometries against the RCS results. Table 3 summarizes the characteristics of three structures that have been proposed22as possibilities for the geometry of anthracene(Ar)3. One of the geometries, A, is a symmetrical (2 1) structure that is simply geometry C for anthracene-(Ar)2 together with a centrally bound Ar atom added on the other side of the anthracene plane. The two other possibilities, B and C, are both (3 0) species in which all three Ar atoms are on a single side of the anthracene. All three of the geometries are displayed pictorially at the bottom of Figure 9. At the top of Figure 9 we show simulated RCS-TRFD traces corresponding to each of the three proposed geometries for anthracene-@& One notes the pronounced difference between the trace corresponding to the (2 l) structure and those corresponding to the (3 0) structures. The former geometry is a near-oblate top ( K = 0.927) with the y-axis transition moment along the c principal axis. This situation gives rise to the very prominent J-type transients at t = m/2A 2B) that one sees in Figure 9a. The other isomers, on the other hand, are significantly asymmetric, prolatelike species with y-axis transition dipoles parallel to their c principal axes. As a result, the simulated traces corresponding to them (Figure 9b,c) exhibit rather weak, somewhat ill-defined features. A comparison of the traces in Figure 9 with the experimental results of Figure 3 leaves little doubt that the species studied in the experiment is of the same form as geometry A. Further, the amplitudes of the transients in Figure 3 are so large as to preclude the possibility that the experimental trace represents the sum of signals from two or more isomers having similar vibronic transition frequencies. In short, the RCS results pertain to a
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IsomerA
0
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Ra
-
I
Isomer C
Isomer B 0
0
0
JRb
..
I
0
0
J
-1
- . .. ....
-
b
6
O
i I
I
Rc
b
Figure 9. Top: Calculated RCS-TRFD traces corresponding to 1) different possible structures of anthracene-(Ar)3: (a) the (2 species, isomer A; (b) the (3 0) cluster, isomer B; and (c) a second (3 0) cluster, isomer C. The rotational constants used to calculate the traces are given in Table 3. In each case the transition dipole was taken to be along the y axis of the anthracene moiety. Bottom: Two views each of the anthracene-(Ar)3 isomers A, B, and C. The Ar atom positions and rotational constants of the clusters are given in Table 3.
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single, dominant anthracene-(Ar)s isomer and that isomer is of the (2 1) form. Having identified the anthracene-(Ar)3 structure as a nearoblate (2 1) geometry, one can try to deduce quantitative geometrical parameters from the RCS data. To do this we first note that, since one now knows that the species is a near-oblate top, rotational constants can now be extracted from the RCS results. Values for A B can be obtained directly from the positions of the J-type transients. Moreover, upper limits for the values for A - B can be obtained by detailed comparison of the measured traces with ones calculated as a function of A - B, A B, and C being held constant. The rotational constants so derived are given in Table 1 for both the hlo and dlo isotopomers of athracene-(Ar)3. Next, one can use a weighted nonlinear least-squares procedure to fit the measured rotational constants by varying the geometry of the cluster. In this fitting it is necessary to make several assumptions, given that there are only four measured rotational constants, whereas nine parameters are required to specify the structure. We have chosen to fix all parameters except one. In particular, we have assumed the general form of geometry A-one Ar atom centrally bound at a distance of 3.43 8, from the anthracene plane, the other two forming an Ar dimer on the other side of the plane with the dimer oriented along the x axis and symmetrically placed with respect to the y - z plane-but we have allowed the distance of the Ar dimer from the anthracene plane to float in the fit. Under these constraints one finds that a distance of 3.47 8, fits the measured rotational constants very well. More significant, a simulated RCS-TRFD trace correspondingto this geometry agrees with the experimental results, as one can see
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7318 J. Phys. Chem., Vol. 99, No. 19, 1995 TABLE 4: Rotational Constants (GHz) Corresponding to Possible Structures of Tetracene-Ar 2A-B-C centrally bounda (Figure 1Oc); R = 3.43 A off centeln (Figure 10d); best fit RCS-TRFD determined rotational constants
B+C
B-C
1.056
0.370
0.006
1.076 1.076
0.365 0.365
0.006 c0.015
a R represents the distance of the argon atom from the tetracene plane. If R is increased, 2A - B - C and B C decrease. The x, y, and z coordinates of the Ar atom in this geometry are 1.10, 0.15, and 3.43 A, respectivel y .
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from the comparison shown in Figure 3. The upshot is that the RCS results are consistent with a geometry for anthracene( A r ) 3 that looks like anthracene-Ar on one side of the anthracene plane and like anthracene-(Ar)2 on the other side. E. Tetracene-Ar. In order to obtain a structure for tetracene-Ar from its measured rotational constants we require the rotational constants for bare tetracene. These are available from ref 27 and equal 1.630, 0.2134, and 0.1888 GHz for the SO A, B, and C values, respectively, and 1.647, 0.2116, and 0.1876 GHz for the corresponding SI values. As mentioned above, the averages of these values are pertinent here since the measured constants for the Ar complex are averaged over its SO and SI vibronic states. Thus, we have used the constants A = 1.639, B = 0.2125, and C = 0.1882 GHz for bare tetracene. By allowing the x, y, and z coordinates of the Ar to float as fitting parameters, the measured rotational constants for tetracene-Ar were fit in a weighted, nonlinear least-squares procedure. The best-fit geometry is one in which the argon resides 3.43 8, above one of the two inner rings, at a distance of 1.10 A in the x direction from the tetracene center and with essentially no displacement in the y direction. Simulated RCSTRFD traces correspondingto this geometry and to one in which a centrally bound argon atom was assumed are shown in Figure 10 along with measured traces. One sees clearly the close match between the trace corresponding to the best-fit geometry and the experimental results. One also sees that the centrally bound structure is inconsistent with experiment. V. Conclusion We have presented the results of rotational constant measurements leading to the structure analysis of anthracene-(Ar), (n = 1-3), DCA-Ar, and tetracene-Ar. We now consider the experimentally derived structures in the light of prior results pertaining to them. Regarding the one-to-one complexes, we have shown, first, that the spectroscopically dominant isomer of anthracene-Ar (that with dv = -42 cm-l) is one in which the argon atom is centrally bound to anthracene with a bond distance of 3.43 f 0.03 A. This structure is virtually identical to that predicted from minimum energy calculations22employing the atom-atom pairwise potential of ref 6. While there have been suggestions that a second, less stable isomer of anthracene-Ar absorbing at dv = -47 cm-' also we were not able to observe the -47-cm-' band and therefore cannot make any structural conclusions relating to it. Second, our results show that the dominant isomer of the DCA-Ar complex also has a centrally bound geometry and has a van der Waals bond distance of 3.42 f 0.03 A. This result, too, is consistent with the results of minimum energy calculations employing the potential energy surface of ref 6. Moreover, the similarity of the DCA-Ar geometry to that of anthracene-& is what one would expect, especially given that the dv values for the two species are close. Finally, in contrast to the findings for the Ar complexes of anthracene and DCA, the RCS results for tetracene-Ar indicate that the species does not have a
-Figure 10. (a) Comparison of measured RCS trace for tetracene-Ar (middle) with traces calculated by assuming central binding (top) and off-center binding (bottom) for the Ar atom. The transients present are K-type. (b) An analogous comparison at longer delay times. In this case the transient present is J-type. The rotational constants assumed to obtain the traces in a and b are given in Table 4. For both traces the transition dipole was taken to lie along the short in-plane axis of the tetracene moiety. (c) Two views of the centrally bound geometry assumed to calculate the top-most traces in a and b. (d) Two views of the off-center geometry assumed to calculate the bottom-most traces in a and b. The structure in d is the one that best fits the RCSdetermined rotational constants for the complex. See Table 4 for specific geometrical parameters. centrally bound Ar. The geometry is, however, entirely consistent with the computed minimum energy structure from ref 6. Thus, tetracene-Ar represents another species for which the semiempirical atom-atom potential energy function of ref 6 correctly and quantitatively predicts geometry. The results on tetracene-Ar are also significant in the light of the prior failure of high-resolution frequency-domain experiments to yield any structural information on the species.27 Such failure was due to the complexity, inherent congestion, and consequent unassignability of the pertinent spectra. In contrast, as the results of Figure 5 and the discussion pertaining to those results clearly demonstrate, the time-domain, RCS data on the same species are rather easy to analyze and derive structural parameters from. In short, the case of tetracene-Ar is a clearcut example of the fact that there is an ease of interpretation associated with RCS datal2 (particularlydata on large species) that is often lacking in frequency-domain data on the same species. The determination made herein that the spectroscopically dominant, dv = -100 cm-I band of anthracene-(Ar)2 is due to a (2 0) isomer (isomer C) of the cluster is interesting in several respects. First, the application of band-shift criteria has led others to conflicting assignments for the isomeric form giving rise to the vibronic band. On one hand, the fact that the band does not occur at twice the dv value of the 0; band corresponding to centrally bound anthracene-Ar is evidence against it arising from a (1 1) anthracene-(Ar)2 isomer. Henke et a1.22and Cockett and K i m ~ r have a ~ ~correspondingly assigned the band to a (2 0) species. On the other hand, the theory of Shalev et al.* on vibronic band shifts induced by rare-
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J. Phys. Chem., Vol. 99, No. 19, 1995 7319
gas complexation of an aromatic chromophore predicts that 1dvI for a (2 0) isomer of anthracene-(Ar)2 should be less than, not more than, twice the 1BvI value corresponding to the centrally bound anthracene-& complex. Consequently, this group assigned the Bv = - 100 cm-’ band to a (1 1) isomer. Since the RCS results support the (2 0) interpretation, it is clear that further refinement of the theory of ref 8 is required for it to be used with confidence in relating band shifts to structural forms. Second, the dominance of the (2 0) isomer C of anthracene-(Ar)2 verifies the results of minimum energy calculations2* employing the semiempirical potential energy surface of ref 6. Such calculations predict isomer C to be about 60 cm-’ more stable than the next stable isomer. Thus, one has yet another example of an aromatic-(rare gas), species in which the atom-atom potential proves accurate in predicting geometry. Finally, it is noteworthy that while the favored isomer of anthracene-(Ar)2 is a (2 0) structure, that of carbazole(Ar)210,19 and of fl~orene-(Ar)2~~ are both of the centrally bound (1 1) form. This different behavior is surprising given the rather similar sizes and shapes of the three aromatics, characteristics which should predominate in determining the favored isomers of species whose intermolecular binding is governed primarily by dispersion and exchange repulsion. Indeed, on the atom-atom pairwise potential of ref 6, (2 0) structures are the most stable isomers of all three of these clusters. Why the fluorene- and carbazole-(Ar)2 clusters exhibit “anomalous” behavior is not clear, but the “normal” behavior of anthracene(Ar)2 suggests that the reason is a subtle one. One suspects that the eventual understanding of this subtlety may prove very illuminating in regard to the nature of intermolecularinteractions in aromatic-(rare gas), clusters and to the limits of applicability of atom-atom potential energy functions in predictions of structure. Finally, the RCS results presented herein clearly show the dominant anthracene-(Ar)s species (BY = -142 cm-’) to be a (2 1) isomer in which there is a centrally bound Ar on one side of the anthracene plane and, on the other, Ar bonding that is the same or very similar to that characterizing the dominant anthracene-(Ar)z isomer. This finding c o n f i i s the predictions of the minimum energy calculations of Henke et a1.22 for anthracene-(Ar)s. It also verifies the assignments of the BY = -142 cm-’ band from refs 8, 22, and 23. Lastly, since the BY for the band equals the sum of the BY values for the centrally bound anthracene-Ar species and the (2 0) anthracene-(Ar)2 isomer, the RCS-derived structural results provide confirmation for the validity of using additive band-shift rules in the structural analysis of anthracene-(Ar), species.
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Acknowledgment. This work was supported by the US. National Science Foundation through Grant No. CHE 91-15656. S.M.O. acknowledges the support of a UCLA Office of the President, Dissertation Year Fellowship. References and Notes (1) Even, U:; Amirav, A.; Leutwyler, S.; Ondrechen, M. J.; BerkovitchYellin, 2.;Jortner, J. Faraday Discuss. Chem. SOC.1982, 73, 153. (2) Leutwyler, S.; Jortner, J. J. Phys. Chem. 1987, 91, 5558. (3) Leutwyler, S.; Bosiger, J. Chem. Rev. 1990, 90, 489.
(4) See: Topp, M. Int. Rev. Phys. Chem. 1993,12, 149, and references cited therein. (5) Mandziuk, M.; Bacic, Z. J. Chem. Phys. 1993, 98, 7165. ( 6 ) Ondrechen, M. J.; Berkovitch-Yellin, A,; Jortner, J. J. Am. Chem. SOC.1981, 103, 6586. (7) Doxtader, M. M.; Gulis, I. M.; Swartz, S. A.; Topp, M. R. Chem. Phys. Lett. 1984,112,483. Doxtader, M. M.; Topp, M. R. J . Phys. Chem. 1985, 89, 4291. (8) Shalev, E.; Ben-Horin, N.; Even, U.; Jortner, J. J. Chem. Phys. 1991, 95, 3147. (9) Ohline, S. M.; Joireman, P. W.; Connell, L. L.; Felker, P. M. Chem. Phys. Lett. 1992, 191, 362. (10) Ohline, S. M.; Connell, L. L.; Joireman, P. W.; Venturo, V. A,; Felker, P. M. Chem. Phys. Lett. 1992, 193, 335. (1 1) Ohline, S. M.; Romascan, J.; Felker, P. M. Laser Chem. 1994, 14, 45. (12) For reviews, see: Felker, P. M. J. Phys. Chem. 1992, 96, 7844. Felker, P. M.; Zewail, A. H. In Femtosecond Chemistq Manz, J.; Woste, L., Eds.; VCH Verlagsgesellschaft: Weinheim, 1995; p 193. (13) Meerts, W. L.; Majewski, W. A,; van Herpen, W. M. Can. J. Phys. 1984, 92, 1293. (14) Weber, Th.; Riedle, E.; Neusser, H. J.; Schlag, E. W. Chem. Phys. Lett. 1991, 183, 77. (15) Haynam, C. A.; Brumbaugh, D. V.; Levy, D. H. J. Chem. Phys. 1984, 80, 2256. (16) Baskin, J. S.; Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1986, (17) Baskin, J. S.; Zewail, A. H. J. Phys. Chem. 1989, 93, 5701. (18) Neusser, H. J.; Sussmann, R.; Smith, A. M.; Riedle, E.; Weber, Th. Ber. Bunsen-Ges. Phys. Chem. 1992, 96, 1252. (19) Sussmann, R.; Neusser, H. J. Chem. Phys. Lett. 1994, 221, 46. (20) Kaziska, A. J.; Shchuka, M. I.; Topp, M. R. Chem. Phys. Lett. 1991, 181, 134. Troxler, T.; Smith, P. G.; Topp, M. R. Chem. Phys. Lett. 1993, 211, 371. Troxler, T.; Smith, P. G.; Stratton, J. R.; Topp, M. R. J. Chem. Phys. 1994, 100, 797. (21) Hays, T. R.; Henke, W.; Selzle, H. L.; Schlag, E. W. Chem. Phys. Lett. 1981, 77, 19. (22) Henke, W. E.; Yu, W.; Selzle, H. L.; Schlag, E. W. Comm. Phys. 1985, 92, 287. (23) Cockett, M. C. R.; Kimura, K. J. Chem. Phys. 1994, 100, 3429. (24) Heikal, A.; Banares, L.; Semmes, D. A,; Zewail, A. H. Chem. Phys. 1991, 156, 231. (25) Amirav, A,; Sonnenschein, M.; Jortner, J. Chem. Phys. 1984, 88, 199. (26) Ben-Horin, N.; Even, U.; Jortner, J. J. Chem. Phys. 1992,97,5988. (27) van Herpen, W. M.; Meerts, W. L.; Dynamus, A. J. Chem. Phys. 1987, 87, 182. (28) C6t6, M. J.; Kauffman, J. F.; Smith, P. G.; McDonald, J. D. J. Chem. Phys. 1989, 90, 2864. Kauffman, J. F.; C6t6, M. J.; Smith, P. G.; McDonald, J. D. J . Chem. Phys. 1989, 90, 2874. (29) Connell, L. L.; Corcoran, T. C.; Joireman, P. W.; Felker, P. M. J. Phys. Chem. 1990, 94, 1229. Felker, P. M.; Connell, L. L.; Corcoran, T. C.; Joireman, P. W. Proc. SOC.Photo-Out. Instrum. Ena. 1990, 1209, 53. Connell, L. L.; Ohline, S. M.; Joireman, P. W.; Corcoran, T. C.; Felker, P. M. J. Chem. Phys. 1992, 96, 2585. (30) Joireman, P. W.; Connell, L. L.; Ohline, S. M.: Felker, P. M. J. Chem. Phys. 1992, 96, 4118. (31) Lambert, W. R.; Felker, P. M.; Syage, J. A,; Zewail, A. H. J. Chem. Phys. 1984, 81, 2195. (32) Felker, P. M.; Zewail, A. H. J. Chem. Phys. 1987, 86, 2460. (33) Hartland, G. V.; Connell, L. L.; Felker, P. M. J. Chem. Phys. 1991, 94, 7649. (34) Cruickshank, D. W. J.; Sparks, R. A. Proc. R. Soc. London A 1960, 258, 270. (35) We use the band-shift values from ref 23 because they are calibrated with respect to a Feme hollow cathode lamp. Similar values were reported first by the Schlag group in refs 21 and 22. (36) No reasonable geometry for the species is consistent with an oblatelike asymmetric top. (37) Ray, B. S. Z. Phys. 1932, 78, 74. (38) Sussman, R.; Neuhauser, R.; Zitt, U.; Neusser, H. J. Faraday Discuss. Chem. SOC.1994, No. 97, poster abstract. JP942707B
